Patent Description:
Injection molds and injection-molded workpieces are known in the art. Usually, a thermoplastic material is used as a first material which is injected into a mold in order to form a workpiece. It is sometimes necessary to provide a workpiece made from a first material with an insert that is made from a second material which differs from the first material. An insert may, for example, be a magnet or a bushing. Usually, the insert is placed in the injection mold, whereby the position of the insert is secured by holding members. The holding members are provided with contact surfaces that abut the insert and thereby fixate it in the mold. The mold is then filled with the first material, thereby overmolding the insert at least in parts.

Injections molds for overmolding inserts, the molds having holding members with round or trapezoidal cross-sections, are known from <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

An injection mold is often intended to produce large quantities of workpieces one after the other. The holding members in the injection mold must therefore be adapted to securely fixate an insert even if the size of different inserts varies from insert to insert. In other words, the tolerances for the inserts have to be taken into account. It is therefore a typical solution to adapt the holding members to be able to secure the largest insert which is still in accordance with the tolerances for the insert. In the case that an insert is used which is in accordance with the tolerances, but smaller than the largest possible insert, the insert may not abut all holding members in the injection mold. In other words, a gap may be formed at least in parts between one or more holding members and the insert. When the mold is then filled with the first material, said material will at least partially flow into gaps between the holding members and the insert. This material will form burrs or flashes of material which may cover the insert at least in part.

Afterwards, when the workpiece is used in harsh environments and exposed to stresses, for example vibrations, a burr may become detached from the remaining workpiece. Such a situation may be critical, for example in cases where the workpiece is used inside a gear box or in another device with moving parts. The burr may get stuck between moving parts and damage the device. Furthermore, when loose burrs get inside valves or filters, said valves or filters may become clogged.

It is an object of the invention to provide an injection mold, an injection-molded workpiece and a method as described above, which reduce or even eliminate the risk of burrs detaching from a workpiece.

For the method as mentioned in the beginning, the object is achieved by a method defined in claim <NUM>. For the injection-molded workpiece as mentioned above, the object is achieved by a workpiece as defined in claim <NUM>. Preferred embodiments of the method and the injection-molded workpiece are defined in the dependent claims.

The invention solves the aforementioned object in that a burr may still be formed. However, said burr will strongly adhere to the remaining workpiece. Thereby, the risk of the burr becoming detached is reduced or even eliminated. Due to the convex cross section of the contact surface of the holding projection, it may always be possible for the first material to flow in a region between the holding projection and the insert. The contact surface of at least one holding projection does not abut the insert and a continuous material film is created which is strongly connected to the remaining workpiece. In a case where the insert fully abuts the contact surface or in parts, the first material will flow into the region between the contact surface and the insert until a further flow is blocked by the insert. Thereby, the material will follow the convex shape of the contact surface, wherein it is ensured that a thickness of the material increases from a contact point of the holding member with the insert towards a direction away from the insert. Thereby, this burr may be securely connected to the remaining workpiece.

The contact surface is not defined as only the area of an exact contact point which may be in direct contact with an insert, but also as the surface in a vicinity of said contact point. For example a surface that is arranged within a radius around said contact point, said radius being about <NUM>% to <NUM>% of a thickness of the first material in the vicinity of a recess formed by the holding member.

To summarize, a workpiece according to the invention may still be provided with a burr. However, this burr is securely attached to the remaining workpiece such that the risk of detaching the burr during operation may be decreased.

In the following, further improvements of the invention are described. The additional improvements may be combined independently of each other, depending on whether a particular advantage of a particular improvement is needed in a specific application.

According to a first advantageous embodiment, the at least one contact surface may have an overall convex cross section in at least one or more intersecting planes. As a consequence, the holding projection may have an overall pin shape. This may lead to a securely fixated burr. A holding projection having an overall pin shape preferably has a rotational symmetry along an axis of rotation that is coaxial with the direction of projection of the holding projection.

In order to facilitate the flow of the first material between the holding projection and the insert, the at least one holding projection may taper from the base towards the at least one contact surface. This may also facilitate the removal of the workpiece after injection molding.

The at least one holding projection preferably has an overall wedge-shaped cross section. In other words, at least two opposing sides or flanks of the holding projection may taper towards the at least one contact surface. The convex contact surface may form a top of the wedge.

According to a preferred form of the holding projection, the holding projection may have at least one flank that is in inclined with respect to a surface normal on the contact surface at an angle between <NUM>° and <NUM>°, more preferably between <NUM>° and <NUM>°. The surface normal on the contact surface is preferably parallel with or identical to a longitudinal axis of the holding projection along which the holding projection projects from the base.

In order to securely fixate an insert during injection molding, the at least one holding projection and/or the complete at least one holding member is preferably formed as a rib, wherein the top of the rib has the convex cross section. The cross section may thereby be a cut perpendicular to a longitudinal direction of the rib. Said longitudinal direction of the rib is preferably perpendicular to a direction of projection along which the holding projection projects.

The overall shape and the proportions of the holding projection may influence the flow of the first material. According to a particularly advantageous shape, the at least one holding projection may have, in a cross section, a length and a width wherein the width at the half length is between <NUM>% and <NUM>% of the length. The cross section is thereby a cross section that cuts through the contact surface, in particular a cross section that extends parallel with the direction of projection along which the holding projection projects. The length may be measured between the base of the holding member and the contact surface, in particular, along the direction along which the holding projection projects. The width is preferably measured perpendicular to said length.

In order to form a convex shape that allows the first material to flow between the holding member and the insert, the at least one contact surface preferably has a radius of curvature that lies in a range between <NUM>% and <NUM>% of the material thickness of the first material in the vicinity of the recess formed by the holding member. Said material thickness generally corresponds to the length of the holding projection and/or the depth of the recess. More preferably, the radius of curvature lies in a range between <NUM>% and <NUM>% of the material thickness.

The radius of curvature may, in the alternative, be related to a width of the holding projection. In particular, the radius of curvature is preferably between <NUM>° and <NUM>°, more preferably between <NUM>° and <NUM>°, of the width of the holding projection. Said width is thereby preferably a width which is measured at the half maximum of a length of the holding projection.

The holding member comprising the at least one holding projection may be interchangeable such that different holding members may be used with the injection mold. The injection mold is provided with a plurality of holding members.

The injection-molded workpiece may further be improved in that a material film consisting of the first material covers a bottom of the recess at least in parts, wherein the material film is connected to at least one flank of the recess. The at least one flank of the recess may in particular be a side wall. At least one material film is connected with both flanks of the recess. The bottom of the recess is preferably the region where the concave shape is located or, in other words, in the vicinity of the insert.

The at least one recess preferably tapers towards the insert.

A material thickness of the first material preferably decreases, in particular continuously, from at least one flank of the recess towards a bottom of the recess. In other words, the material thickness increases from the bottom towards the flanks. Thereby, a burr formed at the bottom of the recess may be securely fixated to the remaining material.

The at least one insert is preferably made from metal. More preferably, the at least one insert is a magnet or a bushing. An insert may also be covered with or embedded in a plastic material. Just by way of example, a plastic bonded magnet may also be used as an insert.

The at least one injection-molded workpiece is preferably a workpiece that is adapted for being used inside a gear box. Additionally or alternatively, the at least one workpiece may be adapted for being submerged in oil or other substances. In both cases, the workpiece may benefit from the advantages of the invention. In particular in gear boxes, loose burrs may provide a risk for damaging the device when they get into moving parts. Furthermore, when loose burrs get inside valves or filters, said valves or filters may become clogged.

Preferably, the workpiece is produced by an injection mold as described herein, into which at least one insert made from the second material is inserted and which is at least partially filled with the first material. Consequently, the at least one workpiece according to the invention is preferably, at least in parts, formed complimentary to the injection mold as described herein.

In the following, the invention and its improvements are described in greater details using an exemplary embodiment and with reference to the drawings. As described above, the various features shown in the embodiment may be used independently of each other in specific applications.

In the following figures, elements having the same function and/or the same structure will be referenced by the same reference signs.

<FIG> shows an injection mold <NUM> according to an example. The injection mold <NUM> is shown above an injection molded workpiece <NUM>. <FIG> may depict a situation in which the workpiece <NUM> is removed from the mold <NUM>, after the injection molding process is finished.

For a better understanding, the workpiece <NUM> is also shown in a perspective view in <FIG>.

The injection mold <NUM> is at least partially formed complementary to the workpiece <NUM>. The injection mold <NUM> is provided with a cavity <NUM> into which a first material <NUM> may be injected. The first material <NUM> is preferably a thermoplastic material.

The injection mold <NUM> is provided with a holding member <NUM> for securing an insert <NUM> using overmolding said insert <NUM>. The holding member <NUM> has a base <NUM> from which a holding projection <NUM> projects into the cavity <NUM>. The holding projection <NUM> is provided with a contact surface <NUM> for fixating the insert <NUM>. In other words, the contact surface <NUM> may abut the insert <NUM> at least in parts. Generally, the mold <NUM> is provided with a plurality of holding members <NUM>, wherein at least one of the holding members <NUM> abuts the insert <NUM> with its contact surface <NUM> during overmolding of the insert <NUM>.

The contact surface <NUM> has an overall convex cross section <NUM>. Said convex cross section <NUM> is seen along an intersecting plane which extends parallel with a contact direction <NUM> along which the holding member <NUM> and the insert <NUM> are brought into contact with each other. It should be noted that the holding projection <NUM> preferably projects along a direction of projection <NUM> which may be coaxial with said contact direction <NUM>. The contact direction <NUM> may therefore be seen as a projection direction or longitudinal direction of the holding projection <NUM>. To summarize, the holding projection <NUM> has a rounded shape at least in an area in which it abuts the insert <NUM> during overmolding. The intersecting plane is identical with the drawing plane of <FIG>.

The contact surface <NUM> may have an overall convex cross section <NUM> in at least one more intersecting plane (not shown here). Such a shape may result in an overall pin shaped holding projection <NUM>. The aforementioned one more intersecting plane is preferably arranged perpendicular to the first intersecting plane mentioned above.

The at least one holding projection <NUM> preferably tapers from the base <NUM> towards the contact surface <NUM>. Since both sides or flanks <NUM> of the holding projection <NUM> are inclined with respect to a surface normal <NUM> of the contact surface <NUM>, the holding projection has an overall wedge shape. The surface normal <NUM> is parallel with the contact direction <NUM>. Preferably, the surface normal <NUM> is coaxial with the contact direction <NUM>. The arrow showing the contact direction <NUM> in <FIG> and the dashed line showing the surface normal <NUM> are drawn separated from each other for the sake of visibility.

Both flanks <NUM> are preferably inclined with respect to the surface normal <NUM> at an angle <NUM>, which is preferably between <NUM>° and <NUM>°, more preferably between <NUM>° and <NUM>°.

The holding projection <NUM> has a length <NUM> that is measured along the direction of projection <NUM>. The length <NUM> extends between a height of the base <NUM> and the contact surface <NUM>. A width <NUM> of the holding projection is preferably measured at half of the length <NUM> perpendicular to the contact direction <NUM> in the intersecting plane. The width <NUM> is preferably between <NUM>% and <NUM>% of the length <NUM>, more preferably between <NUM>% and <NUM>%.

The contact surface <NUM> has, as mentioned above, an overall convex shape that results in the convex cross section <NUM>. The convex shape may be described by a radius of curvature <NUM> which is indicated by a dashed half circle in <FIG>. The radius <NUM> is preferably between <NUM>% and <NUM>%, more preferably between <NUM>% and <NUM>% of the width <NUM>.

Perpendicular to the intersecting plane, the holding member <NUM> preferably has an overall shape of a rib. In other words, the convex shape of the contact surface <NUM> extends in a direction perpendicular to the widths <NUM> and length <NUM>. In the alternative, the holding member <NUM> may have a rotational symmetry around an axis of symmetry that is coaxial with the direction of projection <NUM>. In this case, the holding member <NUM> may have an overall pin-shape.

In the following, the workpiece <NUM> that can be produced with the injection mold <NUM> is described with respect to <FIG>.

The first material <NUM> has an outer surface <NUM> which is in contact with the injection mold <NUM> during the molding process. The shape of the surface <NUM> is defined by the shape of the injection mold <NUM>. The first material <NUM> at least partially covers the insert <NUM>. The insert <NUM> is preferably made from a second material <NUM> that differs from the first material <NUM>. Preferably, the second material <NUM> is a metal. The insert <NUM> may, just by way of example, be a magnet or a bushing.

During the molding process, the first material <NUM> flows between the injection mold <NUM> and the insert <NUM>. As a consequence of the injection mold <NUM> being provided with the at least one holding member <NUM>, the holding member <NUM> creates a recess <NUM> in the first material <NUM> of the workpiece <NUM>. The recess <NUM> is basically formed complementary to the holding member <NUM>. In other words, a depth <NUM> of the recess <NUM> is basically identical to the length <NUM> of the holding projection <NUM> and a width <NUM> of the recess <NUM> is basically identical to the width <NUM> of the holding projection <NUM>. The length <NUM> and the depth <NUM> are preferably basically identical with a material thickness <NUM> of the first material <NUM> in the vicinity of the recess, whereby deviations of about <NUM>% may be possible.

In the vicinity of the insert <NUM>, or, in other words, at the bottom <NUM> of the recess <NUM>, the recess <NUM> has an overall concave shape. This concave shape is formed complementary to the convex shape of the holding member <NUM>. Consequently, a radius <NUM> of the curved shape is basically identical to the radius <NUM> of the convex shape of the holding member <NUM>.

Due to the convex shape of the contact surface <NUM>, the first material <NUM> may easily flow between the holding projection <NUM> and the insert <NUM> during the molding process.

The recess <NUM> tapers towards the insert <NUM> as a result of the wedge shape of the holding projection <NUM>.

In the region where the contact surface <NUM> is not in abutment with the insert <NUM>, a material film <NUM> may be formed between the contact surface <NUM> and the insert <NUM>. This material film <NUM> may cover the bottom <NUM> of the recess <NUM>. The material film <NUM> is connected with at least one of the sides or flanks <NUM> of the recess <NUM>. In a region where a gap exists between the contact surface <NUM> and the insert <NUM>, the material film <NUM> is connected with both opposed flanks <NUM>, forming a continuous material film at the bottom <NUM> of the recess <NUM>.

The material film <NUM> is, due to the curved shape of the material <NUM> that borders the recess <NUM>, securely fixated with the workpiece <NUM>. A material thickness <NUM> of the material film <NUM> increases from the bottom <NUM> towards the flanks <NUM>.

The risk of the material film <NUM> getting detached is thereby decreased. In other words, the material film <NUM> may form a burr or a flash. However, this burr may remain on the workpiece <NUM>. Expensive removal of the material film <NUM> can be omitted. The workpiece <NUM> may be used in harsh conditions in which burrs that are not formed like the material film <NUM> according to the invention getting detached from the workpiece and risk a failure of a device in which the workpiece is used.

As already mentioned above, the injection mold <NUM> is preferably provided with a plurality of holding members <NUM> that have overall rib-shapes. These rib-shaped holding members <NUM> produce groove shaped recesses in the workpiece as it is depicted in <FIG>.

In the following, further advantageous shapes for a holding member <NUM> and a holding projection <NUM> are briefly described with respect to <FIG>, wherein each Figure shows a holding member being in contact with an insert.

For the sake of brevity, only the differences in contrast to the embodiment described with respect to <FIG> are described in detail. It should be noted that each of the following figures shows a cross-section through the holding member <NUM>. The overall shape of the cross section may either be an overall rib-like shape, wherein the cross section shows a cut perpendicular to a longitudinal direction of a rib.

In the alternative, the holding members <NUM> may have overall pin-shapes, wherein the holding members <NUM> are basically provided with a rotational symmetry around a rotational axis that is coaxial with the direction of projection <NUM>.

In the following figures, <FIG> relate to extreme angles of the flanks <NUM> which can still be used in order to take advantage from the holding member <NUM> according to the invention. In <FIG>, the upper and lower limit for the angle <NUM> of the flanks <NUM> according to a preferred range of said angle <NUM> are shown. Finally, <FIG> relates to a holding member <NUM> with parallel flanks <NUM> and a convex contact surface <NUM>.

In <FIG>, a holding member <NUM> and a holding projection <NUM> are shown wherein the angle <NUM> between the flanks <NUM> and the surface normal <NUM> is about <NUM>°. The radius <NUM> of the curvature of the contact surface <NUM> is about <NUM>% of a thickness <NUM> of the first material <NUM> adjacent to the recess <NUM>. The thickness <NUM> of the first material <NUM> is generally equal to the depth <NUM> of the recess <NUM> and/or to the length <NUM> of the holding projection <NUM>. As mentioned before, these dimensions relate to an example for a holding projection <NUM> having a large angle <NUM>. Even with this large angle <NUM>, the workpiece <NUM> may still benefit from the advantage of the invention. In other words, a material film <NUM> will form at the bottom <NUM> of the recess <NUM>.

In <FIG>, an angle <NUM> for inclined flanks <NUM> that is small compared to the angle <NUM> of the embodiment described with respect to <FIG>, is shown. This angle <NUM> is about <NUM>°. The radius of curvature <NUM> of the contact surface <NUM> is about <NUM>% of the material thickness <NUM>. As mentioned before, the thickness <NUM> corresponds to the depth <NUM> of the recess <NUM> and/or the length <NUM> of the projection <NUM>. Also this embodiment may benefit from the curved contact surface according to the invention in that a material film <NUM> will form at the bottom <NUM> of the recess <NUM>.

In <FIG>, two examples are shown, which show the limits of a preferred region of angles <NUM> for inclined flanks <NUM> of the holding projection <NUM>. Thereby, the angle <NUM> shown in <FIG> represents a lower limit of the preferred range and is about <NUM>°. The angle <NUM> shown in <FIG> is about <NUM>° and represents the upper limit of the preferred range. To conclude, the angle <NUM> for inclined flanks <NUM> of the holding projection <NUM> is preferable within in the range between <NUM>° and <NUM>°. However, as mentioned above, also angles <NUM> which are out of this preferred range, can still be applied and the workpiece <NUM> may still benefit from the advantages of the invention. The radii of curvature <NUM> for the contact surface <NUM> are, in both embodiments, about <NUM>% of the material thickness <NUM>.

As can be seen above, the radii of curvature <NUM> for all embodiments described so far, lie within the preferred range of radii <NUM> between <NUM>% and <NUM>% of the material thickness <NUM>. Said material thickness <NUM> corresponds to the depth <NUM> of the recess <NUM> and the length <NUM> of the holding projection <NUM>. Furthermore, all embodiments so far have radii of curvature <NUM> that lie within the more preferred range of <NUM>% to <NUM>% of the material thickness <NUM>.

In <FIG>, another holding projection <NUM> is shown, wherein said holding projection <NUM> is also provided with a contact surface <NUM> having a radius of curvature <NUM> which is about <NUM>% of the material thickness <NUM>. However, in contrast to the aforementioned embodiments, the flanks <NUM> are not inclined with respect to the surface normal <NUM>, but are parallel with each other. Such an embodiment may also be combined with the embodiments as shown before. In particular, parallel flanks <NUM> may be combined with a contact surface <NUM> that has a radius of curvature <NUM> that differs from <NUM>% of the material thickness <NUM>.

Claim 1:
Method of producing a workpiece (<NUM>) by injection molding in an injection mold (<NUM>), wherein the injection mold (<NUM>) comprises a plurality of holding members (<NUM>) for securing an insert (<NUM>) during overmolding, the holding members (<NUM>) having a base (<NUM>) and a holding projection (<NUM>) projecting from said base (<NUM>), wherein the holding projection (<NUM>) is provided with at least one contact surface (<NUM>) for fixating the insert (<NUM>), wherein the at least one contact surface (<NUM>) has an overall convex cross section (<NUM>) in at least one intersecting plane, characterized in that the contact surface (<NUM>) of at least one holding member (<NUM>) does not abut the insert (<NUM>) and a continuous material film (<NUM>) is created between the holding projection (<NUM>) and the insert (<NUM>).