A method of closing the ends of longitudinally extending through-holes formed in the wall of a resin pipe

Abstract: A method for closing the opening of each of the through-holes formed in the pipe-wall of a resin pipe and extending parallel to the axis of said pipe, comprises pressing one end portion of said pipe in the direction parallel to said axis in a mold for closing said opening, said mold being connected to a high frequency current source, and heating said end portion by applying a high frequency current to said mold so as to melt the hole-partition walls for partitioning said through-holes, and the inner surface of the through-holes, thereby closing said opening.

Description: This invention relates to a method for closing the opening of each of the through-holes formed in the pipe-wall of a resin pipe and extending parallel to the axis of said pipe. In order to reduce the weight of a pipe and the amount of materials used in a pipe, and to provide a pipe with an improved strength against flattening load so as to minimize deformation due to stress caused by earth load, etc., attempts have been made by forming a multiplicity of axially extending through-holes 13 in spaced parallel relation in the pipe-wall of a resin pipe, for example, as shown in Figure 1. Such a resin pipe 1 is used for buried sewage piping, etc., since it has considerably high flattening rigidity characteristics. The usual method for connecting such resin pipes 1 is to insert a diametrically enlarged end (socket side) of a pipe into a diametrically non-enlarged end (spigot side) of another pipe, and then to join the pipes together by applying an adhesive or by attaching a seal ring. However, such method has a disadvantage in that the through-holes 13 formed in the pipe-wall of the pipe communicate with various parts within and/or outside of the piping, whereby fluid leakage from the piping or inflow of underground water into the piping may occur. Accordingly, such pipes cannot be used as they are. It is necessary to close the through-holes 13 on the socket side or spigot side after extrusion moulding or secondary processing of the pipe. In JP-A-60 115 429 there is disclosed a method of closing axial through-holes by softening the tube and inserting it into the mould until the tube end reaches an inversely tapered section of a tube end moulding section. A vacuum suction is then performed, and an external mould for crushing the tube end rim is moved to press the outer surface against the inner surface and therefore close the through-holes. In Japanese Published Examined Patent Application No. 57-44111, as Figures 1 and 7 illustrate, it was proposed to place a heated pipe end 10 in a mould 103, then press the pipe end 10 against the mould 103 while rotating the mould 103, whereby the hole-partition walls 130 are twisted. By twisting the pipe end 10 in such a way, the inner and outer surfaces 11, 12 thereof are pressed together, the through-holes 13 being thus closed. However, the portion subjected to such treatment exhibits contractions formed on the inner surface 11 and/or the outer surface 12, which may lead to the development of some structural defects and thus to a breakage of the pipe. Further, the through-holes cannot be completely closed by the proposed method, because the closing is performed only by contact bonding which is caused by deforming or crushing the pipe-wall. In addition, the operation required for rotating the mould 103 is very complicated. In Japanese Published Unexamined Patent Application No. 54-103480 there is disclosed a method in which, as Figures 1 and 8(a) and (b) illustrate, hole-partition walls 130 of the pipe end 10 are broken by a cutting tool 104 while the pipe end is heated. Then the inner and outer surfaces 11, 12 are pressed by a pressing mould 105 to be contact-bonded together. In this method, the adhesion of the crushed portions is not satisfactory. This method also involves a number of stages and is complicated. As afore-mentioned, in the two methods, the through-holes 13 are merely crushed by pressing. This involves a high possibility that the once-closed portion may be reopened when the pipe is subjected to secondary processing such as diametrical enlargement, after closing of the through-holes, or when any shock such as dropping, or any property change with time is encountered. If resin pipes made of polyvinyl chloride, which has a melting temperature and a decomposition temperature in close proximity to each other are to be connected with each other, the heating temperature and heating time must be strictly controlled, and, therefore, the operation is very complicated. Besides aforesaid methods, there is known a method in which a caulking material such as a silicone resin is injected into the through-holes 13 to plug or close them. However, the injection of such caulking material into individual through-holes is quite troublesome. Such a method of closing the holes naturally involves variations in sealing performance among the through-holes. Furthermore, the silicone resin and polyvinyl chloride are incompatible with each other. Therefore, this third method does not assure a satisfactory closing effect. It has been also proposed to attach a blind plate made of a resilient material to the wall-end of the resin pipe 1. However, such blind plate may be easily detached from the end only by applying a small force. Therefore, this method is not a satisfactory one. Most molecules in an electrically low conductive material (i.e., a dielectric material) have self -contained positive and negative charges of equivalent amounts, and each individual molecule thus forms an electric dipole. Therefore, if a dielectric material is introduced into a high-frequency field, an individual molecule in the dielectric material begins to rotationally oscillate under the high-frequency field, and dielectric loss is caused through the friction between adjacent molecules to generate heat. This exothermic phenomenon occurs in every portion of the dielectric material, with a result of a temperature rise in the material. In this conjunction, the energy loss developed in the dielectric material is given by the consumed electric energy P as the following equation (1) indicates: P = kfE² ε tan δ (1) where, f represents the frequency (Hz), E represents the field strength (V/cm), ε represents the dielectric constant (F/cm), tan δ represents the dielectric dissipation factor, and k represents the constant of proportion. Between the dielectric dissipation factor tan δ and the power loss factor (dielectric power factor) cos  consumed as thermal energy when an AC voltage is applied, a relationship exists which can be expressed by the following equation (2): tan δ = cos √ 1 - cos²  (2) In the case of a plastic material, δ is generally small (i.e.,  is about 90°), hence, in other words, the dielectric dissipation factor tan δ is approximately equal to the dielectric power factor cos . Therefore, if the frequency f and the field strength E are fixed, the electric energy P (value of the generated heat) depends on the product (dissipation factor) of the dielectric constant ε and the dielectric power factor cos  (or dielectric dissipation factor tan δ). The greater the value of the dissipation factor, the greater the value of the generated heat becomes. The high-frequency dielectric heating is a technique utilizing such exothermic phenomenon of a dielectric material in a high-frequency field. Unlike other heating methods, it is of an internal heating method, and has the following characteristic features: (1) uniform heating of the object to be heated including the interior thereof is possible; (2) heating it a high rate is possible, with ease of temperature control; (3) any particular portion of the object can be selectively heated; and (4) high heating efficiency can be achieved. According to the present invention, there is provided a method of closing the ends of axially extending through-holes at one end portion of a resin pipe having formed in the wall thereof a plurality of the axially extending through-holes, the method comprising the steps of: preheating and softening at least the end portion of the resin pipe; inserting said preheated end portion into a first mould comprising an outer mould capable of surrounding part of the outer surface of the end portion of the pipe and an inner mould adapted to be received by the inner surface of said end portion of the pipe in such a manner that a tip of the end portion extends a given length from the first mould; pressing a second mould comprising an inner mould and an outer mould to the end portion in an axial direction so that the tip of the end portion is pressed into a cavity formed between the inner mould and the outer mould, the cavity having a depth less than that of the tip, wherein the inner surface and the outer surface of the tip are pressed toward each other to close the through holes; and applying a high frequency current to the second mould so as to melt the tip by dielectric heating. Preferably the pipe is diametrically enlarged after the preheating step by diametrically enlarging a section of the inner mould. The inner mould and outer mould may have radially sloping surfaces adapted to allow the end portion of the pipe to be diametrically enlarged or reduced. A resin pipe to which the present invention is applied has a plurality of through-holes formed in the wall thereof and extending parallel to the axis of the pipe, as illustrated in Figure 1. As the material of such a resin pipe 1, a thermoplastic resin having a large dissipation factor can be advantageously used. Examples of such resin include rigid or non-rigid polyvinyl chloride, polyamide, and methacrylate. Among them, non-rigid polyvinyl chloride is preferred because its dielectric power factor rapidly increases with a rise in temperature, and thereby it melts or softens at relatively low temperatures. Therefore, when the resin pipe is made of non-rigid polyvinyl chloride, through-holes 13 of a pipe end portion 10 can be readily closed without subjecting the pipe end portion 10 to any particular preheating or presoftening. On the other hand, a resin pipe 1 made of rigid polyvinyl chloride, which has a lower dielectric power factor than non-rigid polyvinyl chloride, does not melt or soften until the temperature becomes close to its melting temperature. Therefore, if the end portion of the resin pipe 1 is directly subjected to high-frequency dielectric heating, thermal stress may be concentrated on a portion having a smaller sectional area such as a hole-partition wall 130, and such portion is thus liable to scorching and/or decomposition. If the resin pipe 1 is made of rigid polyvinyl chloride, it is desirable that, as Figures 2(a) and 2(b) illustrate by way of example, the end portion 10 or a portion adjacent thereto of the resin pipe 1 is preheated uniformly by a hot plate 21 or by a heating oven 22 having a heater 20. Through such preheating it is possible to provide an improved dielectric power factor (i.e., greater dissipation factor) and thus to perform high-frequency dielectric heating effectively. The preheating temperature depends on factors such as the material and configuration of the resin pipe 1. For example, with a resin pipe 1 made of rigid polyvinyl chloride, the preheating temperature may be suitably set within the range of 100 to 150°C. Thus, the invention described herein makes possible the objects: (1) to provide a method for closing the opening of each of the through-holes formed in the pipe-wall of a resin pipe and extending parallel to the axis of said pipe which can completely close the opening of the through-hole, uniformly and instantaneously without decomposing the resin; (2) to provide a method for closing the opening of through-holes of a resin pipe which can assure the closing performance of the closed portion for a long period of time regardless of secondary processing, such as diametrical enlargement, or stress such as impact or repeated flattening compressive loads; (3) to provide a method for closing the opening of through-holes of a resin pipe which can be very easily and economically performed; (4) to provide a method for closing the opening of through-holes of a resin pipe which can diametrically enlarge an end portion of the pipe concurrently with closing the opening, thereby improving the economical efficiency; (5) to provide a method for closing the opening of through-holes of a resin pipe in which the high-frequency dielectric heating is employed to heat internally the resin pipe, thereby preventing the resin from adhering to the surface of a mold; (6) to provide a method for closing the opening of through-holes of a resin pipe in which the closing can be completed in a short period of time to avoid the decomposition of the resin. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a fragmentary perspective view showing an example of a resin pipe having through-holes to be closed in accordance with the method of this invention; Figures 2(a) and 2(b) are front views in section showing, by way of example, a heating plate and a heating oven respectively for preheating a resin-pipe; Figure 3 is a sectional view in front elevation showing one example of an apparatus for practising the method of the invention; Figure 4 is a front sectional view showing another example of such apparatus; Figures 5(a) and 5(b) are explanatory views respectively showing one step of the method in which the apparatus of Figure 4 is used; Figures 6(a) and 6(b) are explanatory views respectively illustrating one step of the method in which another apparatus is used; and Figures 7, 8(a) and 8(b) are front sectional views illustrating prior art methods. In the prior art methods for closing the opening of through-holes, the step of heating-melting and the step of pressing are carried out separately. Therefore, the once-melted resin becomes cooled before the step of pressing is begun. As a result, satisfactory fusion bond cannot be achieved by the conventional methods. In contrast, the method of this invention is such that heating-melting and pressing, and fusion bonding are carried out substantially concurrently; therefore, even when resin material which has a melting temperature and a decomposition temperature in close proximity of each other is used, the through-holes can be closed completely. An apparatus for closing the opening of through-holes employable in practising the method according to the invention is described below. An apparatus for conducting the method of this invention, as illustrated in Figure 3, comprises an inner mould 316 for forming a socket, disposed in front of the inner mould 311 of the first mould 31, and a split-type outer mould 317 for forming a socket, disposed in front of the outer mould 312 of the first mould 31. Said inner mould 316 has a split mould 319 disposed centrally in the axial direction thereof, with a cone-shaped space 320 provided in the middle of the split mould 319 and extending in the axial direction. A conical rod 318 which is reciprocable in the axial direction of the mould 316 is in engagement with the space 320, so that the diameter of the split mould 319 can be increased or reduced by changing the position of the rod 318. Since such construction is known in the art, the detail thereof is not described here. On the basis of the same principle as that of the preceding example the process is conducted as follows: a resin pipe 1, one end portion of which is preheated, is inserted into the inner mold 316; the split mold 319 is diametrically enlarged by moving the rod 318; the outer periphery of the socket portion is peripherally clamped by the split-type outer mold 317; then a high-frequency current is applied to the inner and outer molds 311, 312 while the mold 31 is forced into contact with the front end portion of the resin pipe 1, whereby a socket having an annular channel 321 for housing a seal member such as rubber packing, is formed simultaneously with the closing of the through-holes 13 of the resin pipe 1. The split-type outer mold 317 has the function of preventing the resin pipe 1 from buckling when the pipe 1 is forcedly inserted while it is melted by the high-frequency dielectric heating. Another embodiment of an apparatus for closing the opening of through-holes, as Figure 4 illustrates, is of such arrangement that the through-holes are closed while one end portion of the resin pipe 1 is diametrically enlarged. This apparatus includes a mold 42 for holding the pipe-wall portion of the resin pipe 1 , between inner and outer molds 421, 422 of the mold 42, a pressing device 32 for pressing the end portion 10 of the resin pipe 1 in the axial direction of the pipe, and a high-frequency oscillator 33 connected to mold 42. Each of the inner mold 421 and an outer mold 422 has a radially sloped surface adapted to allow the end of the resin pipe 1 to be diametrically enlarged. The inner mold 421 is fixed to a stationary table 5 by a support member 51. As is the case with Figure 3, the inner and outer molds 421, 422, at least with respect to their sloped surfaces 24 and 25 and adjacent portions, are made of a material having high electric conductivity, such as brass or copper. The molds 421, 422 are connected to the high-frequency oscillator 33. The mold 42 has a pressing ring 423 which is connected by an arm 331 to the pressing device 32, presses the pipe end portion 10 positioned in the housing space 414 defined by the inner and outer molds 421, 422, and closes the through-holes 13 formed in the pipe-wall. The method in accordance with this invention using the above described apparatus will be explained below. First, as illustrated in Figure 5(a), a resin pipe 1 made of rigid polyvinyl-chloride with its one end portion preheated and softened in a hot air oven is placed in the inner mold 421 and is diametrically enlarged. In this conjunction, the pipe end 10 is positioned in the space 414 for housing the pipe end portion. Then, as Figure 5(b) shows, the outer mold 422 is pressed against the inner mold 421 so that the resin pipe 1 is held between them. Subsequently, a high frequency current is applied to electrodes 24, 25 while the pressing ring 423 is pressed by the pressing device 32 against the end portion of the resin pipe 1. The frequency and field strength of and application time for the high frequency current may be suitably determined according to the material, configuration and size of the resin pipe 1, and the distance between the electrodes. In the case that the resin pipe is made of rigid polyvinyl chloride and has a wall thickness of about 10 mm, a high frequency current of 40 MHz may be applied at a power output of 3 kW for 5 to 30 sec. The pressing force given by the pressing ring 423 is more than 2 kg/cm², or preferably within the range of 2 to 6 kg/cm². The pipe end portion 10 held between the inner and outer molds 421, 422 and diametrically enlarged is then pressed by a pressing ring 423 in such a way that the inner surface 11 is bent toward the outer surface 12, and simultaneously the end portion is heated and melted by applying high frequency current, thereby the end portions of the inner and outer surfaces 11, 12 are bonded together. Since the surfaces 11, 12 of the pipe 1 are in contact with the inner and outer molds 421, 422, respectively, the surfaces are not subject to melting. Only the inner surfaces of the through-holes and the hole-partition walls at the pipe end portion are melted and bonded, with the result that the through-holes are closed. In this conjunction, the molten resin fills in the through-holes as the molten resin is pushed under the pressing force; thus, the area of bondage is increased to enhance the closure strength of the through-holes. In this embodiment, electrodes 24, 25 are positioned in the inner and outer molds 421, 422 respectively. The electrode 24 may be disposed in the pressing ring 423. Instead of the inner mold which is adapted to enlarge the diameter of the pipe end portion, an inner mold having a sloped and diameter-reducing portion may be employed. In this case, the resin pipe is forcedly inserted into the inner mold having such a diameter-reducing portion and pressed with the outer mold, whereby the end portion of the pipe can be diametrically reduced. Another method for closing the opening of through-holes will be explained below. One end portion of the resin pipe 1 (preferably a portion extending from the pipe end over a distance equivalent or similar to the thickness of the pipe) is heated and softened. The resin pipe 1, so heated, is inserted into a heating mold 61 in the same manner as illustrated in Figure 6(a). The mold 61 comprises inner and outer molds 611, 612 which have radially sloped surfaces. The resin pipe 1 is pressed by a pressing device 32 against the innermost end of the mold and along the axis of the pipe, whereby the inner and outer pipe surfaces 11, 12 at the pipe end are brought closer to each other. The heating mold 61 may be heated so that the end portion of the resin pipe is maintained to be heated and softened. The heating temperature is selected according to the material and wall thickness of the resin pipe, and usually it is set at about 200°C. Then, as Figure 6(b) illustrates, the resin pipe 1 is forcedly inserted into the mold 62 by the pressing device 32 in the axial direction of the pipe. As in Figure 3, a high-frequency current is applied to the mold 62 by the high frequency oscillator 33. The pipe end which has been already heated and softened, as stated in the above explanation, is inserted into the mold 62 as it is pressed against the wall 623 of the mold 62, and is heated through high-frequency dielectric heating. Therefore, the inner and outer surfaces 11, 12 of the pipe end are bent inwardly or in the reverse direction to the pressing direction and melted. The inner mold 621 of the mold 62 is of a columnar shape and the outer mold 622 is of a cylindrical shape. Therefore, the innermost end space of the mold 62 is generally coextensive with the pipe end, and thus the pipe end is fused and bonded while retaining its wall thickness as it is. Thus, there is no likelihood that the once-closed through-holes will regain their open state upon such secondary processing as a diametrical expansion or upon an application of any impact. Thus, the closed pipe end has excellent strength characteristics. In this example, the closing of the opening of through-holes, the diametrical enlargement of the pipe end, and the formation of a socket having an annular channel for receiving a seal member such as a rubber packing can be conducted concurrently, as in the examples shown in Figure 3.

Claims: A method of closing the ends of axially extending through-holes (13) at one end portion (10) of a resin pipe (1) having formed in the wall thereof a plurality of the axially extending through-holes (13), the method comprising the steps of: preheating and softening at least the end portion (10) of the resin pipe (1); inserting said preheated end portion into a first mould comprising an outer mould (317) capable of surrounding part of the outer surface (12) of the end portion of the pipe and an inner mould (316) adapted to be received by the inner surface (11) of said end portion of the pipe in such a manner that a tip of the end portion extends a given length from the first mould; pressing a second mould (31) comprising an inner mould (311) and an outer mould (312) to the end portion in an axial direction so that the tip of the end portion is pressed into a cavity formed between the inner mould (311) and the outer mould (312), the cavity having a depth less than that of the tip, wherein the inner surface (11) and the outer surface (12) of the tip are pressed toward each other to close the through holes (13); and applying a high frequency current to the second mould (31) so as to melt the tip by dielectric heating. A method according to claim 1, wherein after the preheating step, the pipe is diametrically enlarged by diametrically enlarging a section of the inner mould (316). A method according to claim 1 or 2, wherein the inner mould (316) and outer mould (317) have radially sloping surfaces adapted to allow the end portion of the pipe (1) to be diametrically enlarged or reduced.

Inventors: MURATA MASAHIRO, TAKAHASHI KAORU, MURATA, MASAHIRO, TAKAHASHI, KAORU
Applicants: SEIKISUI CHEMICAL CO LTD, SEKISUI KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: B29C57
IPC codes: B29C57, B29L31