Patent Description:
There has been commercialized an interdental cleaning tool including a base portion made of a synthetic resin and a soft portion made of an elastomer, in which the base portion includes a handle base portion and a core base portion having an elongated shaft shape provided at a tip end portion of the handle base portion, the soft portion includes at least a cleaning soft portion covering the core base portion, the handle base portion forms a handle portion as a grip, and the core base portion and the cleaning soft portion form a cleaning portion for cleaning a space between teeth (see, e.g., Patent Literatures <NUM> to <NUM>).

A method widely used to manufacture the interdental cleaning tool includes: filling a synthetic resin material into a first molding space of a first die to form a base portion; placing, in a second molding space of a second die, the base portion molded in the first die; and filling an elastomeric material into the second molding space to form a soft portion in a state of positioning and holding a core base portion at a central part of the second molding space by plural sets of holding pins provided in a first mold and a second mold of the second die at an interval in a length direction (axial direction of the core base portion) of the second molding space, so that the interdental cleaning tool is obtained.

In general, a plurality of interdental cleaning tools are also molded simultaneously by a method that includes: providing a first die having a plurality of first molding spaces and a second die having the same number of second molding spaces as the first molding spaces; in manufacturing the interdental cleaning tools, supplying a synthetic resin material to the first molding spaces to simultaneously form a plurality of base portions so that the plurality of base portions can be connected to one another by a runner portion; placing, in the second molding spaces of the second die, a primary molded product made of the plurality of base portions connected to one another by the runner portion; and then filling an elastomeric material into the plurality of second molding spaces , so that a plurality of interdental cleaning tools are simultaneously formed.

Patent Literature <NUM> describes a method for manufacturing an interdental cleaning tool. The interdental cleaning tool comprises: a base part made of a synthetic resin, the base part having a handle base and an elongated shaft-like core base connected to a leading end of the handle base; and a flexible part made of an elastomer and covering at least a portion of the base part, the flexible part having at least a cleaning flexible part covering the core base, the handle base constituting a handle part, and the core base and the cleaning flexible part constituting an interdental cleaning part.

Patent Literature <NUM> describes a brush, in particular an interdental brush, which comprises a bristle-carrying stem <NUM> defining a longitudinal direction <NUM> and having an elongate support core <NUM>, a bristle area <NUM> having bristles <NUM> projecting from the bristle-carrying stem <NUM>, and a neck element <NUM> connecting the support core <NUM> to a handle <NUM>', wherein the handle <NUM>', the support core <NUM> and the neck element <NUM> are injection moulded integrally from a first plastics component, and wherein a second plastics component in the form of an integral layer and the bristles <NUM> projecting therefrom is mounted on the support core <NUM>.

When the cleaning portion is inserted into the interdental space, a large compressive force acts on the axial direction. In addition, when spaces between molars, particularly between large molars are cleaned, it is necessary to insert, into these spaces, a portion from a tip end portion to an intermediate portion of the interdental cleaning tool that is inserted between the molars into an oral cavity in the anterior-posterior direction, while curving the portion from the tip end portion to the intermediate portion by <NUM>° to <NUM>°. Moreover, during interdental cleaning, it is necessary to put the cleaning portion in and out from a gap between the molars in the curved state, so that a large bending load acts on the cleaning portion. For this reason, in the interdental cleaning tool in which the base portion is made of a synthetic resin material, countermeasures against breakage of the cleaning portion at the time of insertion between teeth or during interdental cleaning are one of the important problems.

As one of the countermeasures against the breakage of the cleaning portion, there is also proposed one in which glass fiber is added to the synthetic resin material forming the base portion. When the glass fiber is added by an amount enough to prevent the elongated shaft-shaped cleaning portion from being broken is added, then strength and rigidity of the core base portion in the axial direction of the core is increased. This improves the insertability into an interdental portion. However, a stronger force is required to insert the interdental cleaning tool since the core base portion has difficulty in warping. This causes large bending load be easily act on the core base portion during cleaning spaces between the molars. This has caused such a problem that the core base portion might be broken at an intermediate portion in a length direction thereof or gums might be pierced, upon being used.

On the other hand, when super engineering plastics such as polyether sulfone is adopted as the core base portion, the core base portion becomes easy to warp and the breakage of the cleaning portion can be suppressed. However, for molding of the super engineering plastics, a high injection temperature is essential, and a cooling time after injection molding becomes long. This causes the productivity of the interdental cleaning tool to be lowered, and material cost is also high. This considerably increases total manufacturing cost. There has been such a problem. There has been such a problem.

The inventors of the present invention have found out the following as causes of the breakage of the core base portion. First, a recess is likely to be formed at a position where the holding pin for positioning the core base portion in the second die during molding of the cleaning soft portion and the core base portion are in contact with each other. Second, a cross-sectional area of the core base portion becomes smaller at the position of this recess, and a stress generated by application of bending force to the core base portion becomes higher. Third, a structural change is likely to occur due to the existence of the recess in a region to which such a high stress concentrates, and the core base portion is broken in the vicinity of a shaft position where the recess is formed due to the bending force acting upon insertion of the interdental cleaning tool into the interdental space or during interdental cleaning.

Although a mechanism in which the recess is formed in the core base portion by the holding pin is not clear, it can be presumed that the recess is formed by the following mechanism. That is, the base portion molded by the first die is placed in the second die after being cooled, and in order to shorten a molding time, the base portion is placed in the second die in a relatively high temperature state. In addition, the base portion placed in the second die is softened during molding of the cleaning soft portion since the base portion is exposed to the high temperature elastomer filled in the second molding space. In the second die, the plural sets of holding pins are allowed to protrude into the second molding space by a preset protruding length, and the core base portion is positioned at the central part of the second molding space. Here, variations in molding dimensions of the core base portion may occur due to a molding shrinkage difference by shortening the molding time. The core base portion is exposed to the high temperature elastomer causing thermal expansion thereof, and vibrations of the core base portion at the time of filling the elastomer. It can be presumed that, in this way, the tip end portion of the holding pin bites into the softened core base portion and the recess is formed in the core base portion.

In addition, in a so-called I-type interdental cleaning tool in which a center line of the handle portion and an axis of the core base portion are disposed coaxially with each other, the handle portion is often formed in a flat shape in order to facilitate gripping with fingers. In this case, the handle portion is generally formed to be flat with respect to a mold opening and closing direction of the first die and the second die, and the holding pins are provided so that an axial direction of the holding pins is disposed in the mold opening and closing direction of the second die in order to make a die structure as simple as possible. In view of the above, in the <NUM>-type interdental cleaning tool, when the space between the molars is cleaned with the handle portion being gripped, such recesses of the core base portion, which are formed by the holding pins, are arranged on an outer peripheral side of the curved core base portion and an inner peripheral side thereof. The inventors of the present invention have found out that, when this interdental cleaning tool is used in this state, a concentration of a large stress occurs in the vicinity of each of the recesses on both the inner and outer peripheral surfaces of the core base portion, and thereby the core base portion is easily broken at that region.

An object of the present invention is to provide an interdental cleaning tool which has a simple configuration and can effectively prevent the occurrence of the breakage of the core base portion at the time of inserting the interdental cleaning tool into interdental spaces or during interdental cleaning without reducing the productivity of the interdental cleaning tool.

The present invention is related to an interdental cleaning tool as defined in claim <NUM>. Embodiments of the invention are recited in the dependent claims.

(<NUM>) Provided is a method for manufacturing an interdental cleaning tool that includes: a base portion made of a synthetic resin; and a soft portion made of an elastomer that covers at least a part of the base portion, the base portion including a handle base portion and a core base portion having an elongated shaft-shape connecting continuously to a tip end portion of the handle base portion, the soft portion having at least a cleaning soft portion that covers the core base portion, the handle base portion constituting a handle portion serving as a grip, and the core base portion and the cleaning soft portion constituting a cleaning portion for interdental cleaning, the method including: a base portion molding step of supplying a synthetic resin material into a first molding space of a first die to form a base portion; and a soft portion molding step of placing the base portion molded in the base portion molding step in a second molding space of a second die for molding the soft portion, holding the core base portion on a substantially central part of a cleaning soft portion molding portion so that core base portion recesses are formed in the core base portion by a plurality of holding pins, at least one of which has a cross section formed into a shape elongated in the length direction of the second molding space, the plurality of holding pins being individually provided at intervals in the length direction of the second molding space in a first mold and second mold of the second die, and filling the second molding space with an elastomeric material to mold the soft portion in a state of holding the core base portion on the substantially central part of the cleaning soft portion molding portion.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to (<NUM>), a cross-sectional area of the holding pins elongated in the length direction of the second molding space is set to <NUM><NUM> or more and <NUM><NUM> or less, more preferably <NUM><NUM> or more and <NUM><NUM> or less, still more preferably <NUM><NUM> or more and <NUM><NUM> or less.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to (<NUM>) or (<NUM>), the cross sections of the holding pins elongated in the length direction of the second molding space has a maximum length along the length direction of the second molding space, which is set to <NUM> or more and <NUM> or less, more preferably <NUM> or more and <NUM> or less, still more preferably <NUM> or more and <NUM> or less, most preferably <NUM> or more and <NUM> or less.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), the holding pins having the shape elongated in the length direction of the second molding space are individually formed in the first mold and second mold of the second die so as to correspond to predetermined spots within a range of <NUM> to <NUM> from the tip end of the core base portion.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), three or more holding pins are provided in at least either one of the first mold and second mold of the second die, among the holding pins having the shape elongated in the length direction of the second molding space and other holding pins if the other holding pins are present. With such a configuration, three or more cleaning portion recesses are formed on at least one of the first side portion and second side portion of the cleaning portion by the holding pins. Accordingly, in the interdental cleaning tool manufactured by this manufacturing method, unevenly distributed portions of stress caused by the bending force acting on the cleaning portion at the time of the interdental insertion or during the interdental cleaning can be dispersed at three or more positions of the cleaning portion, at which the cleaning portion recesses are formed, and the breakage of the core base portion due to the local application of a large bending force can be prevented effectively. Moreover, when three or more holding pins are provided, a total contact area of the holding pins with respect to the core base portion is increased and the vibrations of the core base portion can be suppressed more surely. Accordingly, it is easy to perform control to make the depth of the core base portion recesses shallow. Therefore, the depth of the core base portion recesses can be made shallower, so that the occurrence of such stress concentration at the positions of the core base portion recesses at the time of the interdental cleaning can be prevented more surely. It should be noted that the number of the holding pins of the first mold and the second mold in the second die can be the same or different.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), arrangement intervals of the holding pins in the length direction of the second molding space are substantially identical, among the holding pins having the shape elongated in the length direction of the second molding space and other holding pins if the other holding pins are present.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), arrangement intervals of the holding pins in the length direction of the second molding space are made narrower as approaching to the tip end side of the second molding space, among the holding pins having the shape elongated in the length direction of the second molding space and other holding pins if the other holding pins are present.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), among the holding pins having the shape elongated in the length direction of the second molding space and other holding pins if the other holding pins are present, an area of a tip end portion of the holding pin located on a most tip end portion of the cleaning portion is set to be substantially identical in size to one another as or smaller than areas of tip end portions of the holding pins at other positions. The core base portion has a shape that contracts continuously or stepwise, and a thickness of the cleaning soft portion is substantially constant within a range of about <NUM> to <NUM>. Accordingly, the passage area of the second molding space for molding the cleaning soft portion decreases as approaching to the tip end side. Therefore, the area of the tip end portion of the holding pin located at the most tip end portion of the cleaning portion is reduced, whereby the passage area can be set as large as possible. This setting allows the flow resistance of the elastomeric material to be set as small as possible. In addition, the influence of the Karman vortex, which is generated in the vicinity of each holding pin, on the molded product and the holding pin, can be further suppressed. Then, the holding of the core base portion can be improved, and the poor filling of the elastomeric material with respect to the cleaning soft portion molding portion can be prevented.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), the elastomeric material is filled from the tip end side of the second molding space.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), the elastomeric material is filled from the base end side of the second molding space.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), a specific or all the holding pins of the first mold in the second die and the corresponding holding pins of the second mold are arranged at intervals in the length direction of the second molding space so as not to overlap each other in the circumferential direction of the second molding space, among the holding pins having the shape elongated in the length direction of the second molding space and holding pins having other shapes if the holding pins having the other shapes are present. With such a configuration, the holding pins of the first side portion and the second side portion can apply forces to different positions in the axial direction of the core base portion. Accordingly, a vibratory motion of the core base portion during molding of the soft portion can be suppressed, as compared with the case where the holding pins are arranged to face each other. This vibratory motion is suppressed, thereby allowing the depth of the core base portion recess formed during the molding of the soft portion to be made shallower. Accordingly, the occurrence of the stress concentration on the core base portion recess at the time of the interdental cleaning can be suppressed, and the occurrence of the breakage of the core base portion can be prevented. Furthermore, the interval between the holding pins in the length direction of the core base portion becomes substantially uniform or narrower as approaching to the tip end side of the core base portion. Accordingly, it is possible to hold the core base portion satisfactorily stably. It should be noted that the holding pin of the second mold, which corresponds to the holding pin of the first mold, means a holding pin in the same order counted from the tip end of the second molding space. In addition, "not overlapping in the circumferential direction of the second molding space" means that two holding pins making a set are in a positional relationship of not colliding with each other even when the cleaning portion recesses of the holding pins are moved in the circumferential direction of the second molding space. It is preferable that an interval between at least one set of the holding pins arranged at an interval in the length direction of the second molding space with respect to the length direction of the second molding space is set to a length of not less than one fourth of a maximum axial length of the holding pins. It should be noted that "arranging the holding pins to face each other" means that a pin tip position of the holding pin of the first side portion and a pin tip position of the holding pin of the second side portion overlap each other in the center line direction (the direction of DL in <FIG>) of the core base portion. In other words, a perpendicular line (DL in <FIG>) is first set, which is drawn to the center line (CL in <FIG>) in the length direction of the core base portion from an arbitrary point of a straight line (UL in <FIG>) that connects contact points (points B and T in <FIG>) of a plane (BS in <FIG>) passing through the CL and ends of the core base portion recess to each other. The "arranging the holding pins to face each other" means that the DL of the core base portion recess of the first side portion and the DL of the second recess are in a relationship of partially or entirely overlapping each other, in this case.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), at least one set of the holding pins out of plural sets of the holding pins paired with each other between the first mold and the second mold in the second die has a central segment of the at least one set of holding pins is formed with an angle in the circumferential direction of the second molding space with respect to a mold opening and closing direction of the second die, among the holding pins having the shape elongated in the length direction of the second molding space and holding pins having other shapes if the holding pins having the other shapes are present. With such a configuration, the degree of freedom in the arrangement layout when the protrusions are provided in the cleaning soft portion can be improved. In other words, the soft portion is molded in a state where the core base portion is held in the central part of the second molding space by the plurality of holding pins. The positions of the holding pins can be adjusted in the length direction and the circumferential direction with respect to the second molding space so that the holding pins do not interfere with the forming positions of the cleaning protrusions. Accordingly, the degree of freedom in the arrangement layout of the cleaning protrusions can be improved. Note that "forming the central segment of the holding pin with an angle in the circumferential direction of the second molding space with respect to the mold opening and closing direction of the second die" means that the holding pins of the first side portion and the second side portion are not in such a relationship of being arranged to face each other. That is, the above-described phrase means that the DL of the core base portion recess of the first side portion and the DL of the second recess never overlap each other when the perpendicular line (DL in <FIG>) is set, which is drawn to the center line (CL in <FIG>) in the length direction of the core base portion from an arbitrary point of the straight line (UL in <FIG>) that connects the contact points (points B and T in <FIG>) of the plane (BS in <FIG>) passing through the CL and the ends of the core base portion recess to each other.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), at least either one of a fibrous material and talc is added to the synthetic resin material constituting the base portion. With such a configuration, the strength and rigidity of the core base portion against the bending force can be enhanced. Moreover, since the rigidity of the core base portion can be increased, the holding pin can be further suppressed from biting into the core base portion, the depth of the core base portion recess formed in the core base portion is hardly deepened. This is also preferable in preventing the occurrence of the stress concentration at the position where the core base portion recess is provided.

(<NUM>) In the method for manufacturing an interdental cleaning tool according to any one of (<NUM>) to (<NUM>), the cross section of the holding pin having the shape elongated in the length direction of the second molding space, and an opening shape of the core base portion recess elongated in the axial direction and formed by the holding pin have such a shape elongated in a spiral direction of the cleaning portion shaft, as an elliptical shape, an oblong shape, a rectangular shape, an egg shape, an oval shape and a bale shape (rectangular shape with curved short side portions, rectangular shape with rounded corners), a teardrop shape, and a parallelogram shape.

In accordance with the interdental cleaning tool according to the present invention and the method for manufacturing the same, the opening shape is configured to be elongated in the axial direction of the cleaning portion. Accordingly, the width of the core base portion recess along the circumferential direction of the cleaning portion of the cleaning portion recess is avoided from being increased, thereby improving the degree of freedom in design of the arrangement layout of the protrusions when the protrusions are provided in the cleaning soft portion. In addition, the holding pins of the first side portion and the second side portion with respect to the core base portion can apply forces to different positions in the axial direction of the core base portion. Accordingly, a vibratory motion of the core base portion during the molding of the soft portion can be suppressed, as compared with the case where the holding pins are arranged to face each other. This vibratory motion is suppressed, thereby allowing the depth of the core base portion recess formed during the molding of the soft portion to be made shallower. Accordingly, the occurrence of the stress concentration on the core base portion recess at the time of the interdental cleaning can be suppressed, and the occurrence of the breakage of the core base portion can be prevented.

An interdental cleaning tool of the present invention includes a base portion made of a synthetic resin and a soft portion made of an elastomer.

Examples of synthetic resin materials that can be used to form the base portion include thermoplastic synthetic resin materials, such as polypropylene (PP), polybutylene terephthalate (PBT), polyethylene, polyethylene terephthalate, polycyclohexylene dimethylene terephthalate, saturated polyester resins, polymethyl methacrylate, cellulose propionate, polyurethane, polyamide, polycarbonate, acrylonitrile butadiene styrene (ABS), and. In addition, as a synthetic resin material constituting the base portion, it is preferable to adopt a thermoplastic synthetic resin material having crystallinity with a melting point of <NUM> or more in order to improve the productivity. In particular, polypropylene (PP), polybutylene terephthalate (PBT), and polyamide (PA) are preferred, which can prevent a base portion <NUM> from be broken. Polypropylene is most preferred, which can be molded at low temperatures, can reduce cycle time and improve the productivity, and imposes less heat load on molding equipment.

For the synthetic resin material constituting the base portion, in order to prevent breakage of a cleaning portion at the time of insertion into the interdental space or during interdental cleaning, powders such as plate-like or granular glass flakes, mica and talc, and fibrous materials such as glass fibers, carbon fibers and aramid fibers can be added.

Examples of the elastomer that can be used to form the soft portion include thermoplastic elastomers such as styrene elastomers, olefin elastomers, and polyamide elastomers, and thermosetting elastomers such as silicone rubbers, urethane rubbers, fluoro rubbers, natural rubbers, and synthetic rubbers. Particularly preferred are materials having compatibility with the synthetic resin material constituting the base portion. For example, when the base portion is made of polypropylene, the soft portion should preferably be made of a polyolefin-based elastomer or a styrene-based elastomer. As the elastomer, one to which an additive is added can also be adopted.

Next, a specific shape of an interdental cleaning tool <NUM> will be described with reference to the drawings.

As shown in <FIG>, the interdental cleaning tool <NUM> includes a cleaning portion <NUM> for interdental cleaning and a handle portion <NUM> as a grip, which are distinguishable in terms of function, and also includes the base portion <NUM> made of a synthetic resin and a soft portion <NUM> made of an elastomer, which are distinguishable in terms of material. As shown in <FIG>, such interdental cleaning tools <NUM> are manufactured in the form of an interdental cleaning tool connected body <NUM>, which includes a plurality of interdental cleaning tools <NUM> separably connected in parallel to one another. A user disconnects the interdental cleaning tools <NUM> one by one at connecting portions <NUM> from one side of the interdental cleaning tool connected body <NUM>, so as to sequentially use them. Although <FIG> illustrates that ten interdental cleaning tools <NUM> are connected in parallel to form the interdental cleaning tool connected body <NUM>, any number of interdental cleaning tools <NUM> may be connected.

The base portion <NUM> is made of a thermoplastic synthetic resin. As shown in <FIG>, the base portion <NUM> includes: a handle base portion <NUM> that has a flat elongated plate shape and forms the handle portion <NUM>; a core base portion <NUM> that is connected to a tip end portion of the handle base portion <NUM> and has an elongated shaft shape; and the connecting portions <NUM> separably connecting the adjacent handle base portions <NUM>.

The handle base portion <NUM> is formed in a flat elongated plate shape. However, the handle base portion <NUM> may have any shape other than the flat elongated plate shape as long as the shape facilitates gripping by fingers and interdental cleaning. For example, the handle base portion <NUM> can be formed into a rod shape, a plate shape, or a continuous or stepwise curved shape, in which a cross-sectional shape is formed to be a circular shape, an oval shape (an elliptical shape, an oblong shape, a rounded rectangular shape, an egg shape, an oval shape (Japanese old coin shape), a bale shape (rectangular shape with curved short side portions, rectangular shape with rounded corners), etc.), a teardrop shape and a polygonal shape. The handle base portion <NUM> may be provided with a curved portion or a recess for improving ease of holding. The tip end portion of the handle base portion <NUM> becomes narrower in width as approaching to the core base portion <NUM> side, and is smoothly connected to the core base portion <NUM>. The handle base portion <NUM> may have any dimensions capable of facilitating the gripping by fingers and the interdental cleaning. For example, the handle base portion <NUM> illustrated in <FIG> and <FIG> has a length L1 of <NUM> to <NUM>, a width W1 of <NUM> to <NUM>, and a gripping portion thickness t1 of <NUM> to <NUM>. In this manner, since the handle base portion <NUM> is made thin, there is less unevenness in dimension due to shrinkage of the handle base portion <NUM> when molding the base portion <NUM>. In addition, occurrence of sink marks is prevented, and loading defects of the base portion <NUM> into second dies <NUM> and <NUM> for molding the soft portion <NUM> can be prevented.

The core base portion <NUM> is formed in a substantially linear elongated shaft shape, and the handle base portion <NUM> and the core base portion <NUM> are disposed substantially in the identical axis line, and the core base portion <NUM> and the handle base portion <NUM> are disposed in the identical plane. An exposed portion 12a exposed to the outside is formed on a gripping portion side of the core base portion <NUM>. A core main body 12b which is covered with an elastomer and is insertable between the teeth is formed at a tip end side portion of the core base portion <NUM>. At least a portion of the core main body 12b, which is covered with the soft portion of the core base portion <NUM>, is formed in a gentle tapered shape to decrease in diameter as approaching to the tip end side. In addition, the portion which is not covered with the soft portion may not necessarily be linear, and for example, a shape bent continuously or stepwise may be adopted.

In view of operability, a length L2 of the exposed portion 12a of the core base portion <NUM> from an end point of a round portion (curved portion) on a side surface of the tip end portion of the handle base portion <NUM>, which is configured to be narrow, to a base end portion of a covering portion 21a of the soft portion <NUM> is set to be, for example, <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, most preferably <NUM> to <NUM>. In view of interdental cleaning performance, a length L3 of a cleaning soft portion <NUM> is set to be, for example, <NUM> to <NUM>. From viewpoints of insertability and relaxation of stress concentration, a cross-sectional shape of the core base portion <NUM> is preferably circular, but may be a cross-sectional shape such as an oval shape, a teardrop shape or a polygonal shape.

In view of such insertability into the space between the teeth, the tapered shape of the outer surface of the core base portion <NUM> makes an angle θ1 of <NUM>° to <NUM>° with a center line of the core base portion <NUM>. The tip end portion of the core main body 12b has a diameter of <NUM> to <NUM>. The base end portion of the core main body 12b has a diameter of <NUM> to <NUM>. A curved surface end portion of the tip end portion of the covering portion 21a of the cleaning soft portion <NUM> has a diameter D of <NUM> to <NUM>. The core main body 12b is so formed that the tip end portion with a length by at least <NUM> from the tip end is reliably insertable between the teeth.

In this case, the angle θ1 of the tapered shape of the core base portion <NUM> is constant over the entire length of the core base portion <NUM>. Alternatively, the angle θ1 may also be continuously or stepwise reduced toward the tip end side of the core base portion <NUM>. In addition, the exposed portion 12a may be formed in a shaft shape with a constant diameter over its entire length, and only the core main body 12b may be gently tapered to decrease in diameter as approaching to the tip end side. Alternatively, the exposed portion 12a may also be omitted, and the core main body 12b may be connected directly to the handle base portion <NUM>.

In the present embodiment, the present invention is applied to the I-type interdental cleaning tool <NUM> in which the handle base portion <NUM> and the core base portion <NUM> are disposed substantially in the identical axial line. The present invention can also be applied to a so-called L-shaped interdental cleaning tool <NUM> that includes the core base portion <NUM> a center line of which is inclined at an angle of, for example, <NUM>° with respect to a center line of the handle base portion <NUM>, and to a curve-shaped interdental cleaning tool in which a handle portion connected to a cleaning portion has a smooth curved shape of about <NUM>° to <NUM>°.

As shown in <FIG>, each of the connecting portions <NUM> between the adjacent handle base portions <NUM> is integrally formed with the handle base portions <NUM>. A pair of the connecting portions <NUM> are provided at the base end portion side and the tip end portion side of each of the handle base portions <NUM> with a certain spacing therebetween in the length direction. The connecting portions <NUM> are elongated in the length direction of the handle base portion <NUM>, and are formed into a trapezoidal shape (isosceles trapezoidal shape in <FIG>) in front view. The number of the connecting portions <NUM> can be arbitrarily set, and only one can be provided. However, with such a configuration, when the interdental cleaning tool <NUM> is manufactured, connecting strength of the adjacent base portions <NUM> cannot be sufficiently ensured. Then, when the mold is opened after the molding of the base portions <NUM>, the connecting portions <NUM> are ruptured, the base portions <NUM> may be broken, and the soft portions <NUM> cannot be molded. In addition, the connecting portions <NUM> may be bent, and the base portions <NUM> cannot be loaded in an appropriate position of a second molding space <NUM> (see <FIG>) for molding the soft portions <NUM>, and molding defects may occur. Therefore, two or more of the connecting portions <NUM> are preferably provided at intervals in the length direction of the handle base portion <NUM>.

The connecting portions <NUM> are configured in the following manner. A cross section of the connecting portions <NUM> is formed in a trapezoidal shape or a triangular shape (isosceles trapezoidal shape or isosceles triangular shape in <FIG>). Then, as shown by a virtual line in <FIG>, bending force is concentrated on boundary portions 13a by allowing the interdental cleaning tool <NUM> to pivot in such a direction that the interdental cleaning tools <NUM> adjacent to each other overlap each other, around the boundary portions 13a. In addition, each circular-arc side surface 11a on a side edge of the handle base portion <NUM> comes into contact with the outer surfaces of the connecting portions <NUM>. In this way, large force in a direction to pull apart the side surface 11a from the boundary portion 13a is applied by the lever principle. As described above, the interdental cleaning tool <NUM> can be completely separated at the boundary portions 13a without large deformation of the connecting portion <NUM>. Here, the shape of the connecting portions <NUM> can be formed arbitrarily as long as the connecting portions <NUM> are configured to be capable of easily and completely separating the interdental cleaning tools <NUM> from each other by allowing the interdental cleaning tool <NUM> to pivot in such a direction that the interdental cleaning tools <NUM> adjacent to each other overlap each other, around the connecting portions <NUM>.

When fibrous materials are added to the synthetic resin material constituting the base portions <NUM>, the length direction of the fibrous materials is preferably oriented in a direction along the length direction of the base portion <NUM>. This configuration makes it possible to improve the bending strength or axial buckling strength of the base portion <NUM> and to effectively prevent the core base portion <NUM> from breakage or buckling during use of the interdental cleaning tool <NUM>. Further, by adding the fibrous materials as described above and powders such as plate-like or granular glass flakes, mica and talc, a biting amount of a holding pin which bites into the core base portion <NUM> can be reduced, and a recess 14a of the formed core base portion <NUM> can be made shallow.

As shown in <FIG>, the soft portion <NUM> is molded so as to be integrated with the base portion <NUM> using an elastomeric material, and includes the cleaning soft portion <NUM> externally mounted on the core base portion <NUM>. Here, for the soft portion <NUM>, it is possible to provide an insertion restriction portion having an annular shape for restricting the interdental insertion at the base end portion of the core main body 12b, or to provide a non-slip portion in which all or a part of the surface of the handle base portion <NUM> is covered with an elastomer. Although it is also possible to mold the insertion restriction portion and the non-slip portion independently of the cleaning soft portion <NUM>, it is preferable to form the insertion restriction portion and the non-slip portion so as to be continuous with the base portion of the cleaning soft portion <NUM> since a structure of the dies becomes complicated.

The cleaning soft portion <NUM> includes the covering portion 21a with which the core base portion <NUM> is covered and a plurality of cleaning protrusions 21b protruding outward from the covering portion 21a at intervals in the length direction.

If a thickness of the covering portion 21a is too large, it is necessary to reduce a diameter of the core main body 12b covered with the covering portion 21a. This undesirably reduces the rigidity of the cleaning portion <NUM> significantly during the insertion between teeth, and raises the possibility of occurrence of Karman vortex at the time of molding the cleaning soft portion <NUM>, which causes the cleaning portion <NUM> to be subject to a significant influence of the Karman vortex. If the thickness of the covering portion 21a is too small, the elastomeric material cannot be filled up to the base end portion of the cleaning portion <NUM> undesirably. Therefore, the thickness of the covering portion 21a is preferably set at <NUM> to <NUM>.

The cleaning protrusions 21b are formed apart from one another in the length direction of the covering portion 21a, and are also arranged at intervals from one another in the circumferential direction of the covering portion 21a. More specifically, in order to make it possible to mold the cleaning protrusions 21b with the second dies <NUM> and <NUM> which are described later, in the circumferential direction of the covering portion 21a, totally six types of the cleaning protrusions 21b are arranged at intervals from one another in the length direction of the covering portion 21a. The six types include: which are: a set of the two cleaning protrusions 21b protruding from the covering portion 21a to one side in the mold opening and closing direction; a set of the two cleaning protrusions 21b protruding from the covering portion 21a to the other side in the mold opening and closing direction; one cleaning protrusion 21b protruding to one side along mating surfaces 40a and 41a from the covering portion 21a; and one cleaning protrusion 21b protruding to the other side along the mating surfaces 40a and 41a from the covering portion 21a. Here, it is also possible to form the cleaning protrusions 21b in an arrangement pattern other than the above-described one. For example, it is also possible to provide a set of four cleaning protrusions 21b protruding outward from the covering portion 21a in a cross shape, at intervals in the axial direction.

A cross-sectional area, length, number, and arrangement interval of the base end portion of the cleaning protrusions 21b can be arbitrarily set, but in view of moldability and cleaning performance, the cross-sectional area of the base end portion of the cleaning protrusion 21b is preferably set to <NUM><NUM> to <NUM><NUM>, more preferably <NUM><NUM> to <NUM><NUM>, and most preferably <NUM><NUM> to <NUM><NUM>. The length of the cleaning protrusions 21b is preferably set to <NUM> to <NUM>, more preferably <NUM> to <NUM>, and most preferably <NUM> to <NUM>. The number of the cleaning protrusions 21b is preferably set to <NUM> to <NUM>, and the arrangement interval of the cleaning protrusion 21b is preferably set to <NUM> to <NUM>. In addition, although conical protrusions are adopted as the cleaning protrusions 21b, protrusions having a tapered axially flat plate shape can also be adopted. Further, as the cross-sectional shape of the cleaning protrusions 21b, an arbitrary sectional shape such as an oval shape, a teardrop shape and a polygonal shape can be adopted, in addition to the circular shape.

When the soft portion <NUM> is molded, in order to position and hold the core base portion <NUM> within the central part of the second molding space 42with a plurality of holding pins provided in the second dies <NUM> and <NUM>, as described later, cleaning portion recesses <NUM> which penetrate the covering portion 21a and form core base portion recesses 14a in the core base portion <NUM> are formed at positions corresponding to the holding pins, in the cleaning portion <NUM> of the interdental cleaning tool <NUM>. The core base portion recesses 14a are formed such that the base portion <NUM> with relatively high temperature immediately after being molded by first dies <NUM> and <NUM> is softened by exposure to heat of the elastomer filled in the second dies <NUM> and <NUM>, and that tip end portions of the holding pins abut against the softened core base portion <NUM>.

As shown in <FIG>, a maximum depth d of each of the core base portion recesses 14a from an outer peripheral surface of the core base portion <NUM> is set to <NUM> or less, preferably <NUM> or more and <NUM> or less in order to prevent the core base portion <NUM> from being broken due to an occurrence of a concentration of a large stress at the forming position of the core base portion recess 14a when bending force is applied to the cleaning portion <NUM> at the time of inserting the cleaning portion <NUM> into the interdental space or during interdental cleaning by the cleaning portion <NUM>. Note that the depth of the core base portion recess means a degree of deformation (distance within which the core base portion is compressed and deformed) of the core base portion, the deformation being caused by the fact that the holding pins abut against the core base portion during molding in the second die. With respect to the cross-sectional area of the core base portion <NUM> at a position adjacent to the core base portion recess 14a, a maximum cross-sectional area of the core base portion <NUM> at the position corresponding to the core base portion recess 14a is preferably set to <NUM> to <NUM>%, more preferably <NUM> to <NUM>%, further more preferably <NUM> to <NUM>%, and most preferably <NUM> to <NUM>%. With such a configuration, the stress concentration in each of the core base portion recesses 14a during use is reduced, and the breakage of the core base portion <NUM> at the time of the insertion into the interdental space or during the interdental cleaning can be prevented far more effectively.

A front of the cleaning portion recess <NUM> has a shape elongated in the axial direction of the cleaning portion. In each of the examples of <FIG>, a pin tip shape of the holding pin is formed into a bale shape (rectangular shape with curved short side portions, rectangular shape with rounded corners), whereby the cleaning portion recess <NUM> having substantially the same shape is formed between cleaning protrusions 21b adjacent to one another in the circumferential direction so that a longitudinal direction thereof can coincide with the axial direction of the cleaning portion <NUM>. Besides the bale shape (rectangular shape with curved short side portions, rectangular shape with rounded corners), for example, it is possible to adopt a rectangular shape shown in <FIG>, an elliptical shape, or other shapes. All of the plurality of cleaning portion recesses <NUM> provided in the interdental cleaning tool <NUM> can be formed in the same shape as in this embodiment, or those in different shapes according to distances from the tip end portion of the cleaning portion <NUM> can also be arbitrarily combined and mixed with one another. It is preferable to provide the recesses <NUM> having the above shape with a depth of <NUM> or less, which are shown in this embodiment, at least at a predetermined spot within a range of <NUM> to <NUM> from the tip end of the core base portion. In this embodiment, such spots become the recesses <NUM> having the core base portion recesses 14a formed by intermediate portion holding pins <NUM>. These spots may be broken at the time of use, due to the stress concentrated in the recesses <NUM> at these spots. Therefore, it is preferable to form the recesses <NUM> having the above shape at this spot.

In this way, if each of the cleaning portion recesses <NUM> is configured to have an elongated shape in the axial direction, a width of cleaning portion recess <NUM> (the width along the circumferential direction of the cleaning portion) is small as compared to a circular cleaning portion recess formed by a circular holding pin having the same cross-sectional area as the cleaning portion recesses <NUM>. As a result, the concentration of the stress in the vicinity of the cleaning portion recess when the same force is applied is alleviated, so that the effect of preventing the breakage of the core base portion is obtained. Further, the pin tip shape of the holding pin having an elongated shape in the axial direction has a longer length in the cleaning axis direction than the circular holding pin having the same cross-sectional area. During the molding of the soft portion, the holding pin having an elongated shape in the axial direction has stronger holding force for the core base portion <NUM> than the circular holding pin, and accordingly, vibrations of the core base portion during the molding of the soft portion are suppressed, and a depth of each of core base portion recesses 14Aa becomes difficult to deepen. In this way, the stress generated at the position where the core base portion recess 14Aa is provided can be alleviated, and the effect of preventing the breakage of the core base portion is obtained. As in the case of a cleaning portion <NUM> shown in <FIG>, instead of the cleaning portion recess <NUM>, a cleaning portion recess <NUM> having a long front shape can also be formed at an angle in an axial direction of the cleaning portion <NUM> (in a spiral direction of the cleaning portion shaft). The cleaning portion recess <NUM> can be formed by a holding pin <NUM> having a plate shape as shown in <FIG>.

A bottom surface of the core base portion recess 14a of the cleaning portion recess <NUM> reflects a shape of tip end supporting surfaces of holding pins <NUM> to <NUM> which are described later. As shown in <FIG>, the bottom surface is constituted by a flat surface in a direction perpendicular to the depth direction of the core base portion recess 14a. However, the bottom surface may be configured in a recessed shape having a central part that rises as compared with both side portions as viewed from the axial direction of the cleaning portion <NUM>. For example, in a state in which the holding pin bites into the entire surface of the tip end supporting surface and abuts against the same like a core base portion recess 14Ca of a cleaning portion recess 14C shown in <FIG>, the bottom surface can be formed into an inverted V-shaped mount shape in which the shape of the tip end supporting surface is directly transferred and the center is raised, and so that a maximum depth of the recess 14Ca is set so as not to be at the central part of the recess 14Ca but to be at side positions sandwiching the central part, i.e., at the side end positions in this embodiment. Here, the bottom surface may have a shape that gently rises from both ends to the central part, and may have various other shapes are possible, by changing the shape of the tip end supporting surface of the holding pin, for example, as shown in a core base portion recess 14Ba of a cleaning portion recess 14B in <FIG>. It is to be noted that, when the tip end portion of the holding pin to be described later does not completely bite into the core base portion and the central part of the tip end supporting surface of the holding pin does not contact the outer peripheral surface of the core base portion during the elastomer molding and maintains a gap therebetween, each of the core base portion recesses 14Ba and 14Ca is separated and formed into two parts with the central part interposed therebetween.

An opening area of the core base portion recess 14a is set to <NUM><NUM> to <NUM><NUM>, preferably <NUM><NUM> to <NUM><NUM>, more preferably <NUM><NUM> to <NUM><NUM>. In particular, the core base portion recess 14a which is long in the axial direction is set to preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, still more preferably <NUM> or more and <NUM> or less, and most preferably <NUM> to <NUM> or less, in maximum in the axial direction. The opening area of the core base portion recess 14a is set to preferably <NUM><NUM> to <NUM><NUM>, more preferably <NUM><NUM> to <NUM><NUM>, and most preferably to <NUM><NUM> to <NUM><NUM>. Further, it is preferable that a total area of the core base portion recesses 14a formed in one interdental cleaning tool <NUM> be set to <NUM><NUM> to <NUM><NUM>. The opening areas of all the core base portion recesses 14a can be set to the same size, but it is preferable to set the opening areas so that the core base portion recess 14a closer to the tip end side of the core base portion <NUM> has a smaller opening area. Note that the opening area of the core base portion recess 14a means an area surrounded by a projected figure of a boundary line between the core base portion <NUM> exposed in the core base portion recess 14a, and the cleaning soft portion <NUM>. The opening area is defined with respect to a plane perpendicular to the depth direction center line DL (see <FIG>) of the core base portion recess 14a.

A plurality of the cleaning portion recesses <NUM> are individually provided on a first side portion and a second side portion in the cleaning portion <NUM> at intervals in the axial direction of the cleaning portion <NUM>. Here, the first side portion and the second side portion of the cleaning portion <NUM> refer to a portion of the cleaning portion <NUM>, which is molded by the second die <NUM>, and a portion of the cleaning portion <NUM>, which is formed by the second die <NUM>. The first side portion and the second side portion mean a front half portion and back half portion of the cleaning portion <NUM>, which correspond to the front side and back side of the handle base portion <NUM> having a flat shape.

In the cleaning portion <NUM> shown in <FIG>, the cleaning portion recesses <NUM> of the first side portion and the second side portion are disposed so as to face each other with the core base portion <NUM> interposed therebetween, and to have the depth direction that coincides with the mold opening and closing direction of the second dies <NUM> and <NUM>. Three sets of the cleaning portion recesses <NUM> are provided at intervals in the axial direction of the cleaning portion <NUM> when a pair of the cleaning portion recesses <NUM> arranged to face each other is defined as one set. It is preferable that the number of pairs of the cleaning portion recesses <NUM> be two or more, desirably three or more. In this way, the number of sets of the cleaning portion recesses <NUM> is set to three or more or four or more, whereby the stress generated by the bending force acting on the cleaning portion <NUM> at the time of the interdental insertion or during the interdental cleaning can be dispersed. In addition, as described later, the depth of the core base portion recesses 14a is made shallow while ensuring holding properties for the core base portion <NUM> by the holding pins <NUM> to <NUM>, whereby the breakage of the core base portion <NUM> due to local concentration of a large stress can be effectively prevented during the interdental cleaning.

Such arrangement intervals of the cleaning portion recesses <NUM> in the axial direction of the cleaning portion <NUM> may be entirely set to the same, but may partially or entirely be set to be different from one another. For example, the arrangement intervals may be set so as to become substantially uniform or narrower as approaching to the tip end side of the cleaning portion <NUM>.

Although it is preferable to arrange one set of the cleaning portion recesses <NUM> so as to face each other, the cleaning portion recesses <NUM> can also be provided at position shifted from each other in the axial direction of the cleaning portion <NUM> within a range where the cleaning portion recesses <NUM> partially overlap each other in the circumferential direction, that is, within a range of the length of the cleaning portion recesses <NUM> with respect to the axial direction of the cleaning portion <NUM> ("Not overlapping in the circumferential direction" means that two cleaning portion recesses of the two holding pins defining a pair are in a positional relationship of not overlapping each other even when the cleaning portion recesses are moved in the circumferential direction of the cleaning portion). Further, though it is preferable to arrange the cleaning portion recess <NUM> on the first side portion and the cleaning portion recess <NUM> on the second side portion on the identical axial line as shown in <FIG>, a cleaning portion recess 14D can also be formed so that a center line of the cleaning portion recess 14D of the first side portion and a center line of the cleaning portion recess 14D of the second side portion are disposed in parallel so as to be shifted by a predetermined distance in a radial direction of the cleaning portion 2D, like a cleaning portion recess 14D of a cleaning portion 2D shown in <FIG>.

In addition, arrangement positions of the cleaning portion recesses <NUM> with respect to the cleaning portion <NUM> can also be configured as follows. Here, cleaning portion recesses 14E and 14F and core base portion recesses 14Ea and 14Fa are formed by changing only forming positions thereof with respect to the cleaning portion <NUM> from the cleaning portion recesses <NUM> and the core base portion recesses 14a, and the front shape, depth, and opening area thereof can be configured similarly to the cleaning portion recesses <NUM> and the core base portion recesses 14a.

Like the cleaning portion recess 14E and the core base portion recess 14Ea of a cleaning portion 2E shown in <FIG>, the cleaning portion recess 14E of the first side portion and the corresponding cleaning portion recess 14E of the second side portion, which is in the same order counted from the tip end of the cleaning portion 2E, can be formed at an interval in the axial direction of the cleaning portion 2E so as not to overlap each other in the circumferential direction of the cleaning portion 2E. In <FIG>, the cleaning portion recesses 14E of the second side portion are disposed at the substantially central part between the adjacent cleaning portion recesses 14E of the first side portion. However, the cleaning portion recesses 14E of the second side portion can also be disposed at positions biased to the tip end side or the base end side between the adjacent cleaning portion recesses 14E in the first side portion. Further, all the cleaning portion recesses 14E on the first side portion and all the cleaning portion recesses 14E on the second side portion can also be formed at intervals in the axial direction of the cleaning portion 2E so as not to overlap each other in the circumferential direction of the cleaning portion 2E. Alternatively, the specific cleaning portion recess 14E on the first side portion and the corresponding cleaning portion recess 14E on the second side portion, which is in the same order counted from the tip end of the cleaning portion 2E, can also be formed at an interval in the axial direction of the cleaning portion 2E so as not to overlap each other in the circumferential direction of the cleaning portion 2E. It is preferable to set the interval between at least one set of the cleaning portion recesses 14E formed at an interval in the axial direction with respect to the axial direction of the cleaning portion 2E to a length of one fourth or more of a maximum axial length of the cleaning portion recesses 14E. Further, the number of the cleaning portion recesses 14E of the first side portion and the number of the cleaning portion recesses 14E of the second side portion are preferably the same, but can be differentiated from each other. For example, the number of the cleaning portion recesses 14E of the first side portion can be reduced by one than the number of the cleaning portion recesses 14E of the second side portion. The "interval in the axial direction of the cleaning portion 2E" of the two recesses which do not overlap each other in the circumferential direction refers to a shortest distance when ends of two recesses are connected to each other in the axial direction of the cleaning portion. Specifically, the interval is obtained by the following procedure. First, a plane (BS in <FIG>) including UL and a center line (CL in <FIG>) of the core base portion is set. Next, perpendicular lines are drawn to CL from the intersections (B, T in <FIG>) of the respective recesses of the first side portion and the second side portion and the BS, and intersections of the perpendicular lines and CL are obtained. Further, intersections of CL and the perpendicular lines obtained from the first side portion are obtained. One is selected from the obtained two points on the CL in the first side portion, one is selected from two points on the CL in the second side portion, and a length between both of the points on the CL is obtained. Since there are four combinations, the number of lengths obtained above is four. A shortest length among the obtained four lengths is defined as the "interval in the axial direction of the cleaning portion 2E" between the two recesses which do not overlap each other in the circumferential direction. In the present application, there are two types of recesses, which are cleaning portion recess and a core base portion recess. The recesses are obtained by replacing the above-described "recess" by the recesses desired to be obtained.

In such a manner, the core base portion recesses 14Ea are formed alternately on the first side portion and second side portion of the core base portion <NUM>. In this way, the pair of core base portion recesses can be prevented from being formed at the same position in the axial direction of the core base portion <NUM>. Therefore, the cross-sectional area of the core base portion <NUM> at each of the positions corresponding to the core base portion recesses 14Ea can be increased to prevent the core base portion <NUM> from being broken. In addition, the positions in the axial direction of the core base portion, where the respective holding pins apply force to the core base portion, do not overlap each other, so that a portion in the axial direction of the core base portion, where the core base portion is subjected to the force, is longer than in the case of disposing the holding pins which overlap each other in the circumferential direction. As a result, the core base portion is more firmly held, and the core base portion recesses 14Ea formed during the molding of the soft portion <NUM> is suppressed from being deepened. Hence, the cross-sectional area of the core base portion <NUM> at each of the positions corresponding to the core base portion recesses 14Ea is increased, and the occurrence of breakage of the core base portion <NUM> can be prevented. Furthermore, the interval between holding pins 50A to 52A of the second die <NUM> and the interval between the holding pins 50A to 52A of the second die <NUM> in the length direction of the core base portion <NUM> are shortened, thereby holding the core base portion satisfactorily stably.

(<NUM>) Like the cleaning portion recesses 14F and core base portion recesses 14Fa of a cleaning portion 2F shown in <FIG>, the cleaning portion recess 14F and core base portion recess 14Fa of the first side portion and the cleaning portion recess 14F and core base portion recess 14Fa of the second side portion can also be arranged so as to face each other with a core base portion 12F interposed therebetween, and to allow the depth direction to be inclined at an angle θ2 in the circumferential direction with respect to the mold opening and closing direction of the second dies <NUM> and <NUM> (see <FIG>). However, the cleaning portion recesses 14F and <NUM> on the first side and the second side portion can also be formed so as to be shifted in the axial direction similarly to the cleaning portion recesses 14E.

In the cleaning portion 2F shown in <FIG>, only a second set of the cleaning portion recesses <NUM> from the tip end of the cleaning portion <NUM> shown in <FIG> is replaced by the cleaning portion recesses 14F, and first and third sets from the tip end of the cleaning portion 2F are constituted by the cleaning portion recesses <NUM> arranged so that the depth direction coincides with the mold opening and closing direction of the second dies <NUM> and <NUM>. Here, one or plural sets of the cleaning portion recesses <NUM> at an arbitrary position in the cleaning portion <NUM> can also be replaced by the cleaning portion recesses 14F.

When the angle θ2 exceeds <NUM>°, holding pins <NUM> (see <FIG>) provided in adjacent cleaning soft portion molding portions <NUM> sometimes interfere with each other, so that the angle θ2 is set preferably to <NUM>° or less, desirably <NUM>° or less. Note that plural types of cleaning portion recesses having different angles θ2 with respect to the cleaning portion can also be provided.

As described above, when the cleaning portion recesses 14F are provided at positions where the angle θ2 is provided with respect to the mold opening and closing direction, in the case of forming the plurality of cleaning protrusions 21b, which protrude outward, in the cleaning soft portion <NUM>, a degree of freedom in arrangement layout of the cleaning protrusions 21b can be improved. In other words, the cleaning portion recesses 14F are formed by holding pins which hold the core base portion <NUM> in the central part of the second molding space <NUM>. Since the positions of the holding pins <NUM> can be adjusted in the length direction and the circumferential direction with respect to the second molding space <NUM> without interfering with the forming positions of the cleaning protrusions 21b, the degree of freedom in the arrangement layout of the cleaning protrusions 21b can be improved. Further, since the handle base portion <NUM> is formed flat, when the space between the molars is cleaned with the handle base portion <NUM> being gripped with the fingers, the cleaning portion <NUM> is curved as shown by a virtual line in <FIG> within a plane substantially perpendicular to the plane including the handle base portion <NUM> (that is, in the mold opening and closing direction). Since the cleaning portion recesses 14B are disposed so that the depth direction thereof forms an angle θ2 with respect to the mold opening and closing direction, the core base portion 12F can be effectively prevented from being broken from the cleaning portion recess 14F as a starting point.

Next, a method for manufacturing the interdental cleaning tool will be described.

As shown in <FIG>, the method for manufacturing the interdental cleaning tool includes: a base portion molding step of filling a synthetic resin material into first molding spaces <NUM> of the first dies <NUM> and <NUM> to mold the base portions <NUM>; and a soft portion molding step of placing, in the second molding spaces <NUM> of the second dies <NUM> and <NUM>, the base portions <NUM> molded in the first dies <NUM> and <NUM> and then filling an elastomeric material into the second molding spaces <NUM> to form the soft portions <NUM>. Note that the first dies <NUM> and <NUM> correspond to a first mold and second mold of a first die, respectively, and the second dies <NUM> and <NUM> correspond to a first mold and second mold of a second die, respectively.

At the base portion molding step, as shown in <FIG>, a synthetic resin material is filled into the first molding spaces <NUM> of the first dies <NUM> and <NUM> to mold the base portions <NUM>. More specifically, the first dies <NUM> and <NUM> used have: a plurality of the first molding spaces <NUM> that are arranged in parallel, and each have a core base portion molding portion 32a and a handle base portion molding portion 32b; ; a pair of connecting portion molding portions <NUM> each formed between the adjacent handle base portion molding portions 32b to communicate therewith; a runner <NUM> formed on the base end side of the plurality of first molding spaces <NUM>; and gates <NUM> through which the first molding spaces <NUM> communicate with the runner <NUM>. When supplied to the runner <NUM>, a synthetic resin material is allowed to pass through the gates <NUM> and filled into the first molding spaces <NUM> to allow a plurality of the base portions <NUM> to be simultaneously molded. Thus, a primary molded product 10A is molded, which includes the plurality of base portions <NUM>, a runner portion <NUM>, gate portions <NUM>, and the connecting portions <NUM>. The base portions <NUM> may be molded one by one. However, the simultaneous molding of a plurality of base portions <NUM> makes it possible to improve the productivity and to transfer the base portions <NUM> at the same time by holding the molded runner portion <NUM>, which can improve the workability and thus is preferred. Each gate <NUM> may be formed at any position on the base end portion side opposite to the core base portion molding portion 32a of the first molding space <NUM>, more preferably on the base end portion side opposite to the core base portion molding portion 32a of the first molding space <NUM> with respect to the connecting portion molding portion <NUM>. However, a side gate should preferably be formed as the gate <NUM> at the base end portion of the first molding space <NUM>, so that a possibility of causing the gate portions <NUM> of the primary molded product 10A to be pinched between the second dies <NUM> and <NUM> can be reduced in the process of loading the primary molded product 10A into the second dies <NUM> and <NUM>. Alternatively, a hot runner may be provided instead of the runner <NUM> including a cold runner in the first dies <NUM> and <NUM>. However, the use of the hot runner can make the first dies <NUM> and <NUM> large and increase manufacturing cost. Therefore, the runner <NUM> including the cold runner is preferably provided. In addition, the plurality of base portions <NUM> can be stably connected to one another by the runner portion <NUM>, which can improve the handling properties of the primary molded product 10A in the process of transferring the primary molded product 10A into the second dies <NUM> and <NUM>, and thus is preferred. In addition, a pin gate having, for example, a cylindrical or spindle shape with a diameter of <NUM> to <NUM> can be preferably adopted as each gate <NUM> to allow use of a cold runner, and the gates <NUM> can be arranged at narrow intervals, which can make the molded product small.

In the soft portion molding step, as shown in <FIG> and <FIG>, the primary molded product 10A molded in the first dies <NUM> and <NUM> is placed in each of the second molding spaces <NUM> of the second dies <NUM> and <NUM>. Then, the second molding space <NUM> is filled with an elastomeric material to mold the soft portion <NUM>, whereby the interdental cleaning tool connected body <NUM> in which a plurality of the interdental cleaning tools <NUM> are continuously connected in parallel to one another is obtained.

First, the second dies <NUM> and <NUM> used in the soft portion molding step will be described. The second dies <NUM> and <NUM> have a plurality of the second molding spaces <NUM>, which are formed at positions corresponding to a plurality of the base portions <NUM> of the primary molded product 10A molded in the first dies <NUM> and <NUM>. In addition, the second dies <NUM> and <NUM> also have fitting spaces <NUM>, <NUM> and <NUM> formed to accommodate the runner portion <NUM>, a plurality of the gate portions <NUM>, and the connecting portions <NUM> in the primary molded product 10A. Between the second dies <NUM> and <NUM> and the base portion <NUM>, the cleaning soft portion molding portion <NUM> surrounding the core base portion <NUM> is formed as the second molding space <NUM>. Gates <NUM> which open to the tip end portions of the cleaning soft portion molding portions <NUM> are formed on the mating surfaces 40a and 41a of the second dies <NUM> and <NUM> on the tip end side of the cleaning soft portion molding portions <NUM>. The plurality of gates <NUM> communicate with a common runner <NUM> formed in the second dies <NUM> and <NUM>. The elastomeric material is supplied from the common runner <NUM> to the plurality of second molding spaces <NUM> via the plurality of gates <NUM>. The gates <NUM> are preferably set to be a diameter of <NUM> or more and <NUM> or less.

In the second dies <NUM> and <NUM>, a pair of tip end side holding pins <NUM>, a pair of intermediate portion holding pins <NUM> and a pair of base end side holding pins <NUM> are provided so as to respectively correspond to the tip end side portion, intermediate portion and base end side portion of the cleaning soft portion molding portion <NUM>, and so that the holding pins of each pair face each other. These three sets of holding pins <NUM> to <NUM> are provided so as to be freely movable in a direction substantially perpendicular to the mating surfaces 40a and 41a of the second dies <NUM> and <NUM>, in other words, in the mold opening and closing directions of the second dies <NUM> and <NUM>. As shown in <FIG>, the core base portion <NUM> of the base portion <NUM> allows the tip end portions of these three sets of holding pins <NUM> to <NUM> to protrude into the cleaning soft portion molding portion <NUM>, and sandwiches the core base portion <NUM> between the tip end portions of the holding pins <NUM> to <NUM> of the respective sets, whereby the core base portion <NUM> is accurately positioned and held in the central part of the cleaning soft portion molding portion <NUM>.

When the cleaning soft portion molding portion <NUM> is filled with the elastomeric material, the core base portion <NUM> is softened by being exposed to the high temperature elastomer material, and the core base portion recesses 14a are formed on the outer peripheral surface of the core base portion <NUM> by the tip end portions of the holding pins <NUM> to <NUM>. A concentration of a large stress occurs at the forming position of the core base portion recess 14a when bending force is applied to the cleaning portion <NUM> at the time of inserting the cleaning portion <NUM> into the interdental space or during interdental cleaning by the cleaning portion <NUM>, and this concentration may cause the breakage of the core base portion <NUM>. Therefore, in order to prevent the breakage, a maximum depth d of each of the core base portion recesses 14a is set to <NUM> or less, preferably <NUM> or more and <NUM> or less.

Specifically, with regard to each of the holding pins <NUM> to <NUM>, the cross-sectional shape thereof is formed in a shape elongated in the length direction of the second molding space, in this embodiment, a Japanese straw ricebag shaped (rectangular shape with curved short side portions, rectangular shape with rounded corners), the maximum length dimension thereof along the length direction is set to <NUM> or more and <NUM> or less, more preferably <NUM> to <NUM> or less, still more preferably <NUM> or more and <NUM> or less, most preferably <NUM> to <NUM> or less, and the cross sectional area thereof (sectional area of the cross section perpendicular to the axial direction of the tip end portion) is set to <NUM><NUM> or more and <NUM><NUM> or less, more preferably <NUM><NUM> or more and <NUM><NUM> or less, still more preferably <NUM><NUM> or more and <NUM><NUM> or less. The pins are formed into a shape elongated in the length direction, whereby the influence of the Karman vortex during the molding can be suppressed. In addition to the bale shape (rectangular shape with curved short side portions, rectangular shape with rounded corners), various shapes including long shapes in the spiral direction of the cleaning portion shaft, such as an elliptical shape, a rectangular shape, a teardrop shape and a parallelogram shape, are possible.

The cross-sectional area of the tip end portion of the holding pins <NUM> located on the most tip end side is set to be substantially identical in size to one another or smaller as compared with the cross-sectional areas of the tip end portions of the holding pins located at other positions. In addition, the cross-sectional areas of the intermediate portion holding pins <NUM> and the base end side holding pins <NUM> are set to be substantially identical in size to one another, or the cross-sectional area of the intermediate portion holding pins <NUM> is set larger than the cross-sectional area of the base end side holding pins <NUM>. That is, a passage area at the tip end side portion of the cleaning soft portion molding portion <NUM> is small. Accordingly, the cross-sectional area of the tip end side holding pins <NUM> is made as small as possible, whereby a flow resistance of the elastomeric material is set as small as possible. In addition, the influence of the Karman vortex generated during molding is suppressed, whereby poor filling of the elastomeric material with respect to the cleaning soft portion molding portion <NUM> can be prevented, and excessive melting of the core base portion <NUM> can be prevented. Accordingly, such a small cross-sectional area of the tip end side holding pins <NUM> is preferable. However, it is also possible to provide a plurality of sets of the intermediate portion holding pins <NUM> at intervals in the axial direction. It should be noted that the cross-sectional area of the tip end portion of the holding pin has very little change in area due to shaking or expansion/shrinkage during molding. Accordingly, even if these factors are taken into consideration, it can be presumed that the cross-sectional area becomes substantially the same as an area of the opening portion of the cleaning portion recess to be formed by the holding pin.

Each of the tip end side holding pins <NUM> is provided within a range of <NUM> from a tip end portion 46a of the cleaning soft portion molding portion <NUM> toward the base end side, each of the intermediate portion holding pins <NUM> is provided within a range of ±<NUM>% of the length of the cleaning portion <NUM> in the axial direction around an intermediate point between the tip end side holding pin <NUM> and the base end side holding pin <NUM>, and each of the base end side holding pins <NUM> is provided within a range of <NUM> from the base end portion of the cleaning soft portion molding portion <NUM> toward the tip end side. The respective holding pins with the above-described cross-sectional areas are arranged at these positions, whereby it can be expected not only to ensure the fixation of the core base portion <NUM> during molding but also to prevent the influence on the molded product due to the Karman vortex generated during molding.

In the soft portion molding step, in a state in which the primary molded product 10A is placed in the second molding space <NUM> and the mold is closed, as shown in <FIG>, a pair of the tip end side holding pins <NUM>, a pair of the intermediate portion holding pins <NUM>, and a pair of the base end side holding pins <NUM> are protruded in the cleaning soft portion molding portion <NUM>, as shown in <FIG>. Then, the core base portion <NUM> is held by these three sets of the holding pins <NUM> to <NUM>, the elastomeric material is injected and supplied to the plurality of gates <NUM> through the common runner <NUM>, and the elastomeric material is filled into the cleaning soft portion molding portion <NUM>. At this time, the core base portion <NUM> is softened by the heat of the elastomeric material, and the core base portion recesses 14a are formed by the tip end portions of the holding pins <NUM> to <NUM>. Then, the maximum depth d of each of the core base portion recesses 14a is set to <NUM> or less, preferably <NUM> or more and <NUM> or less. Therefore, the core base portion <NUM> can be prevented from being broken due to the occurrence of the concentration of the large stress at the forming position of the core base portion recess 14a when the bending force is applied to the cleaning portion <NUM> at the time of inserting the cleaning portion <NUM> into the interdental space or during interdental cleaning by the cleaning portion <NUM>. Further, the cross-sectional area of the holding pin <NUM> is set smaller than the cross-sectional area of the holding pins <NUM> and <NUM>. Accordingly, the filling of the elastomeric material from the tip end portion of the cleaning soft portion molding portion <NUM> toward the base end side thereof is not inhibited by the holding pins <NUM> to <NUM> as much as possible. Even if an injection pressure somewhat varies, the core base portion <NUM> can be prevented from being curved, and the cleaning soft portion <NUM> constituted by the elastomeric material can be molded with high accuracy.

In this way, the base portion <NUM> is covered with the soft portion <NUM>, and then the runner portion <NUM> and the gate portions <NUM> which are made of the synthetic resin are removed. In addition, a runner portion <NUM> and gate portions <NUM> which are made of the elastomer molded with the runner <NUM> and the gates <NUM>, are removed. In this way, the interdental cleaning tools <NUM> are obtained.

Tip end surfaces of the holding pins <NUM> to <NUM>, which abut against the core base portion <NUM>, are constituted by flat faces perpendicular to the axial direction of the holding pins <NUM> to <NUM> as shown in <FIG>. However, the tip end surfaces may be constituted by circular arc surfaces along the outer peripheral surface of the core base portion <NUM> as in holding pins <NUM> shown in <FIG>, or may be each constituted by a pair of inclined planes connected in an isosceles triangle as in holding pins <NUM> shown in <FIG>. With this configuration, a contact area between the core base portion <NUM> and each of the holding pins <NUM> and <NUM> can be set large, and a depth of the core base portion recesses 14a can be set shallow. In addition, the core base portion <NUM> can be accurately held in the central part of the second molding space <NUM>, and moreover, the holding properties for the core base portion <NUM> can be improved. Accordingly, this configuration is preferable. Moreover, holding pins having tip end surfaces with different shapes may be arbitrarily combined and used.

Here, when a bottom surface shape of the core base portion recesses 14Ca is formed into an inverted V-shaped mount shape in which a center is raised, as shown in <FIG>, a tip end supporting surface <NUM> abutting against the core base portion <NUM> of each of the holding pins <NUM> is formed such that a central part <NUM> is recessed as compared with both side parts <NUM> and <NUM> when viewed from the axial direction of the cleaning soft portion molding portion <NUM>, as shown in <FIG>. In addition, the tip end supporting surface <NUM> is formed into a shape having regions which abut against at least both side parts <NUM> from the beginning when the holding pin <NUM> contacts the outer surface of the core base portion <NUM>. As a result, unlike the conventional holding pin with a flat tip end or a raised center, the core base portion <NUM> is held in a stable posture by force dispersed from the beginning of the contact of the holding pin by the at least both side parts <NUM>. Therefore, the holding pin <NUM> can hold the core base portion <NUM> without deeply biting into the core base portion <NUM>, and the depth of the core base portion recess 14a to be formed can be made shallow.

More specifically, as shown in <FIG>, both side parts <NUM> are first brought into contact with the outer surface of the core base portion <NUM>, and hold the core base portion <NUM> in a stable posture at at least two positions as compared with the conventional flat pin. the holding pin <NUM> usually bites into the core base portion <NUM>, due to the core base portion <NUM> that is not completely cooled down and is in a soft state, or due to own weight thereof or the like. However, the holding pin <NUM> supports the core base portion <NUM> at at least two positions from the beginning, and therefore, force thereof is dispersed. Instead of biting deeply from the beginning like the conventional flat pin, the holding pin <NUM> receives the force on the entire tip end supporting surface <NUM>, and such a biting depth is suppressed.

Then, particularly while the elastomer is being filled and molded, the core base portion <NUM> is thermally expanded or softened by being exposed to a high temperature elastomer, and furthermore, the core base portion <NUM> receives force and vibrates during the filling with the elastomer, and the holding pin <NUM> attempts to further bite into the core base portion <NUM>. However, in the present invention, the core base portion <NUM> is held in a stable posture at two positions from the beginning, and the above-described vibrations can also be effectively suppressed. Therefore, final biting is also suppressed to be shallow.

In this embodiment, the shape of the tip end supporting surface <NUM> of the holding pin <NUM> is formed into a shape recessed substantially in a V shape when viewed from the axial direction of the cleaning soft portion molding portion <NUM>. However, the present invention is not limited to this. As long as the shape of the tip end supporting surface <NUM> is such a shape having regions which abut against both side parts <NUM> from the beginning of the holding pin <NUM> contacting the outer surface of the core base portion <NUM>, it is preferable that the shape of the tip end supporting surface <NUM> may be, one having a gentle shape that is substantially U shape when viewed from the axial direction of the cleaning soft portion molding portion <NUM> as shown in <FIG>, and may be a shape in which protrusions <NUM> are provided on both side portions when viewed from the axial direction of the cleaning soft portion molding portion <NUM> as shown in FIG. <NUM>(b), for example.

Both side parts <NUM>, which abut against the core base portion <NUM> from the beginning, do not necessarily have to include the exemplified side end edges of the tip end supporting surface <NUM>, but may be intermediate regions between the central part <NUM> and the side end edges. In addition, in each of the above-described examples of the tip end supporting surface <NUM>, such a shape is adopted, in which the central part <NUM> initially maintains a gap with the outer surface of the core base portion <NUM> without abutting against the outer surface. However, the present invention is not limited to this. The tip end supporting surface <NUM> just needs to have a shape in which at least both side parts <NUM> sandwiching the central part <NUM> abut against the outer surface from the beginning. Such a shape in which the central part <NUM> also abuts simultaneously is also included in the present invention. Specifically, a shape shown in <FIG>, which is substantially U-shaped as in <FIG> and has the entire surface of the tip end supporting surface <NUM> abut against the core base portion <NUM> from the beginning, is also included in the present invention.

The arrangement positions of the holding pins <NUM> to <NUM> with respect to the second dies <NUM> and <NUM> can also be configured as follows.

For example, only the second set of the holding pins from the tip end of the cleaning soft portion molding portion <NUM> can be replaced by the holding pins <NUM>. In addition, the first and third sets from the tip end of the cleaning soft portion molding portion <NUM> can be constituted by the holding pins arranged so that the axial direction thereof coincides with the mold opening and closing direction of the second dies <NUM> and <NUM>. However, one or plural sets of the holding pins at arbitrary positions in the cleaning soft portion molding portion <NUM> can be replaced by the holding pins <NUM>.

When the angle Θ2 exceeds <NUM>°, the holding pins <NUM> provided in the adjacent cleaning soft portion molding portions <NUM> sometimes interfere with each other, so that the angle Θ2 is set preferably to <NUM>° or less, desirably <NUM>° or less. Plural types of the holding pins <NUM> having different angles Θ2 can also be provided in the second dies <NUM> and <NUM>.

As described above, the holding pins <NUM> may be provided at the positions where the angle Θ2 is provided with respect to the mold opening and closing direction. Under such a condition, when the plurality of cleaning protrusions 21b, which protrude outward, are formed in the cleaning soft portion <NUM>, the degree of freedom in arrangement layout of the cleaning protrusions 21b can be improved. In other words, the holding pins <NUM> are molded by the holding pins <NUM> which hold the core base portion <NUM> in the central part of the cleaning soft portion molding portion <NUM>. Since the positions of the holding pins <NUM> can be adjusted in the length direction and the circumferential direction with respect to the cleaning soft portion molding portion <NUM> so that the holding pins <NUM> do not interfere with the forming positions of the cleaning protrusions 21b, the degree of freedom in the arrangement layout of the cleaning protrusions 21b can be improved. Further, the handle base portion <NUM> is formed flat. When the space between the molars is cleaned with the handle base portion <NUM> being gripped with the fingers, the cleaning portion <NUM> is curved within the plane substantially perpendicular to the plane including the handle base portion <NUM> (that is, in the mold opening and closing direction). Since the holding pins <NUM> are disposed so as to form the angle Θ2 with respect to the mold opening and closing direction, the core base portion <NUM> can be effectively prevented from being broken from the core base portion recess 14Fa, which is formed by each of the holding pins <NUM>, as a starting point.

Next, a description will be given of analysis results of finite element analysis, in which the influence of the shape, arrangement positions, depth and cross-sectional area of the core base portion recesses 14a with respect to the bending force of the core base portion <NUM> was examined.

First, a description will be given of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM>, in each of which the finite element analysis was performed. As a common configuration of the core base portion <NUM>, a rod-shaped core base portion having a diameter of <NUM> and a length of <NUM> was used, in which core base portion recesses with a predetermined depth were formed in a first side portion and a second side portion. In the examples, tests were carried out while placing the first side portion at a lower position. Accordingly, a "lower surface" coincides with the first side portion, and an "upper surface" coincides with the second side portion. Further, the "planar shape of the recess" means a shape obtained when the core base portion recess is observed from above. The "diameter or width of the recess" means a diameter when a planar shape of the core base portion recess is circular, and when the planar shape is other than being circular, means a maximum length among lengths between intersections of planes (VS in <FIG>) perpendicular to a central axis (CL in <FIG>) of the core base portion and the end portions of the core base portion recess. The "length of the recess" means a maximum length among lengths (UL in <FIG>) between intersections (T, B in <FIG>) of the plane (BS in <FIG>) including the central axis (CL in <FIG>) of the core base portion and the end portions of the core base portion recess. The "depth of recess" means a "maximum depth of the core base portion recess".

In Examples <NUM> to <NUM>, as a common configuration, the recesses centered on the positions of <NUM> and <NUM> from the tip end portions on the first side portion and second side portion of the core base portion were provided (totally <NUM> spots). That is, at the positions of <NUM> and <NUM> from the tip ends, two sets of the recesses, each set of which makes a pair formed at opposite positions shifted from each other by <NUM> degrees in the circumferential direction, were provided.

In Example <NUM>, a bale shape (rectangular shape with curved short side portions, rectangular shape with rounded corners) was adopted for each of the recesses, in which a width along the circumferential direction of the core base portion was set to <NUM>, a maximum length in the axial direction of the core base portion was set to <NUM>, an opening area was set to <NUM><NUM>, and a maximum depth was set to <NUM>.

In Example <NUM>, a rectangular shape was adopted for each of the recesses, in which a width along the circumferential direction of the core base portion was set to <NUM>, a length of the core base portion in the axial direction was set to <NUM>, an opening area was set to <NUM><NUM>, and a maximum depth was set to <NUM>.

In Example <NUM>, a rectangular shape was adopted for each of the recesses, in which a width along the circumferential direction of the core base portion was set to <NUM>, a maximum length of the core base portion in the axial direction was set to <NUM>, an opening area was set to <NUM><NUM>, and a maximum depth was set to <NUM>.

In Example <NUM>, on the first side portion of the core base portion, there was formed a bale shaped (rectangular shape with rounded corners) core base portion recess which had a depth of <NUM> was centered on the position of <NUM> from the tip end portion of the core base portion, had both end portions formed into a semicircular shape with a diameter of <NUM>, and had a length set to <NUM> in the axial direction of the core base portion. In addition, on the second side portion of the core base portion, there was formed a bale shaped (rectangular shape with rounded corners) core base portion recess which had a depth of <NUM> was centered on the position of <NUM> from the tip end portion of the core base portion, had both end portions formed into a semicircular shape with a diameter of <NUM>, and had a length set to <NUM> with respect to the axial direction of the core base portion. Then, an interval (gap) between the core base portion recesses on the first side portion side and the second side portion side in the axial direction of the core base portion was set to twice the maximum axial length of the core base portion recesses. That is, an interval between the core base portion recesses of the first side portion and the second side portion in the axial direction (UL direction in <FIG>) of the core base portion was set to twice the maximum axial length of the core base portion recess. For the test, the first side portion was placed on the "lower surface". The "interval between the core base portion recesses of the first side portion and the second side portion in the axial direction of the core base portion" is obtained by the following procedure. First, a plane (BS in <FIG>) including UL and a center line (CL in <FIG>) of the core base portion is set. Next, perpendicular lines are drawn to CL from the intersections (B, T in <FIG>) of the respective core base portion recesses of the first side portion and the second side portion and the BS, and intersections of the perpendicular lines and CL are obtained. There is obtained a minimum length between the intersection of the perpendicular line and CL, which is obtained from the first side portion, and the intersection of the perpendicular line and CL, which is obtained from the second side portion. The obtained minimum length is defined as the "interval between the core base portion recesses of the first side portion and the second side portion in the axial direction of the core base portion".

In Example <NUM>, there were formed bale shaped (rectangular shape with rounded corners) recesses which had both end portions formed into a semicircular shape with a diameter of <NUM>, and had a length of the recess set to <NUM>, as the core base portion recesses. In addition, an interval between the core base portion recesses of the first side portion and the second side portion in the axial direction of the core base portion was set to three times the maximum axial length of the core base portion recesses.

In Example <NUM>, the core base portion <NUM> was configured similarly to that of Example <NUM> except that a rectangular recess in which a width of the recess was set to <NUM> and a length of the recess was set to <NUM> was formed, as each of the core base portion recesses.

In Example <NUM>, the core base portion <NUM> was configured similarly to that of Example <NUM> except that the interval between the core base portion recesses of the first side portion and the second side portion in the axial direction of the core base portion was set to <NUM> time the maximum axial length of the core base portion recess.

In Example <NUM>, the core base portion <NUM> was configured similarly to that of Example <NUM> except that the interval between the core base portion recesses of the first side portion and the second side portion in the axial direction of the core base portion was set to one fourth times the maximum axial length of the core base portion recess.

In Example <NUM>, the core base portion <NUM> was configured similarly to that of Example <NUM> except that the depth of the recess was set to <NUM>.

In Example <NUM>, as the core base portion recess, a rectangular recess was formed in which a width of the recess was set to <NUM>, a length of the recess was set to <NUM>, and a depth of the recess was set to <NUM>. In addition, the interval between the core base portion recesses of the first side portion and the second side portion in the axial direction of the core base portion was set to one fourth times the maximum axial length of the core base portion recess.

In Example <NUM>, the core base portion <NUM> was configured similarly to that of Example <NUM> except that the depth of the recess of the core base portion recess was set to <NUM>.

In Comparative Examples <NUM> and <NUM>, similarly to Examples <NUM> to <NUM> the recesses centered on the positions of <NUM> and <NUM> from the tip end portions on the first side portion and second side portion of the core base portion were provided (totally <NUM> spots) as a common configuration. That is, two sets of the recesses, each set of which makes a pair formed at opposite positions shifted from each other by <NUM> degrees in the circumferential direction, were provided at the positions of <NUM> and <NUM> from the tip ends. In Comparative Example <NUM>, each of the recesses was formed into a circular shape with a diameter of <NUM>, in which an opening area was set to <NUM><NUM>, and a maximum depth was set to <NUM>.

In Comparative Example <NUM>, each of the recesses was formed into a circular shape with a diameter of <NUM>, in which an opening area was set to <NUM><NUM>, and a maximum depth was set to <NUM>. Since the planar shape of the recesses of Comparative Examples <NUM> and <NUM> is circular, the "diameter of recess" is defined as the "width of recess".

Then, with regard to each of the core base portions of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM>, one end portion thereof was fixed, and the core base portion was horizontally supported in a cantilever manner so that the core base portion recesses were disposed on the upper and lower surfaces of the core base portion. In this state, a force of <NUM> N was applied downward to the other end portion of the core base portion. At that time, a stress distribution in the periphery of each of the core base portion recesses was analyzed by finite element analysis. In addition, a maximum value of the stress in the periphery of the core base portion recess was obtained. The analysis results and the maximum stress values are shown in Tables <NUM> to <NUM>.

Table <NUM> shows results of the analyses on the recesses in the second side portion at the positions of <NUM> from the tip ends of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM>. Table <NUM> shows results of the analyses on the recesses in the first side portion at the positions of <NUM> from the tip ends of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM>. In the analysis results of Table <NUM>, only the stress distributions of the core base portions in the peripheries of the core base portion recesses on the upper surface (second side portion) side are shown. In the analysis results of Tables <NUM> to <NUM>, only the stress distributions of the core base portions in the peripheries of the core base portion recesses on the lower surface (first side portion) side are shown. Further, in each of the analysis results shown in Tables <NUM> and <NUM>, a mesh is described in a background of the core base portion so that the outer shape of the core base portion becomes clear. In addition, a maximum point stress value at an edge portion of the recess was set as each of the stress values.

Table <NUM> and Table <NUM> show the following. As in Examples <NUM> to <NUM>, if the opening has a shape elongated in the axial direction of the core base portion, the stress values are reduced at both of the position of <NUM> from the tip end and the position of <NUM> from the tip end, and the core base portion hardly breaks even when the core base portion is bent greatly near each of the positions at the time of insertion into the space between the front teeth and insertion between the molars.

In addition, Tables <NUM> to <NUM> show that the core base portion recesses of the first side portion and the second side portion are arranged to be shifted in the axial direction so as not to overlap each other in the circumferential direction of the cleaning portion as in Examples <NUM> to <NUM> regardless of the depths of the core base portion recesses being the same or different. In such a situation, the stress value becomes small, so that the core base portion becomes less breakable even if the core base portion is bent at the time of the insertion into the space between the molars, and so on. From Examples <NUM> to <NUM>, it is understood that the stress value tends to decrease as such a separation distance between the core base portion recesses of the upper and lower surfaces in the axial direction becomes larger. Further, it is understood that the stress value becomes smaller when the shape of the core base portion recesses is the bale shape (rectangular shape with curved short side portions, rectangular shape with rounded corners) or rectangular shape rather than a circular shape.

Claim 1:
An interdental cleaning tool (<NUM>) comprising:
a base portion (<NUM>) made of a synthetic resin; and
a soft portion (<NUM>) made of an elastomer that covers at least a part of the base portion (<NUM>),
the base portion (<NUM>) including a handle base portion (<NUM>), and a core base portion (<NUM>) that has an elongated shaft-shape and connects continuously to a tip end portion of the handle base portion (<NUM>),
the soft portion having at least a cleaning soft portion (<NUM>) that covers the core base portion (<NUM>),
the handle base portion (<NUM>) constituting a handle portion serving as a grip, and
the core base portion (<NUM>) and the cleaning soft portion (<NUM>) which constitute a cleaning portion (<NUM>) for interdental cleaning,
wherein a plurality of cleaning portion recesses (<NUM>) which penetrate the cleaning soft portion (<NUM>) and form recesses in the core base portion (<NUM>) are formed on each of a first side portion and a second side portion of the cleaning portion (<NUM>) at an interval in an axial direction of the cleaning soft portion (<NUM>), and at least one of the plurality of cleaning portion recesses (<NUM>) is formed so that an opening of the core base portion recess (14a) formed in the core base portion (<NUM>) has a shape elongated in the axial direction of the cleaning portion (<NUM>),
wherein
at least one set of two cleaning portion recesses (<NUM>) among plural sets of the cleaning portion recesses (<NUM>) paired between the first side portion and the second side portion is formed at an interval in the axial direction of the cleaning portion (<NUM>) so as to be avoided from overlapping each other in a circumferential direction of the cleaning portion (<NUM>),
the at least one set of the cleaning portion recesses (<NUM>) formed at the interval in the axial direction of the cleaning portion (<NUM>) has, therebetween, an interval in the axial direction of the cleaning portion (<NUM>), the interval being set to one fourth or more of a maximum axial length of the cleaning portion recesses (<NUM>).