GB2328395A

GB2328395A - Injection-compression moulding

Info

Publication number: GB2328395A
Application number: GB9815250A
Authority: GB
Inventors: John Cameron
Original assignee: Kurt Mueller
Current assignee: Kurt Mueller
Priority date: 1998-07-15
Filing date: 1998-07-15
Publication date: 1999-02-24
Anticipated expiration: 2018-07-15
Also published as: GB9815250D0; GB2328395B

Abstract

A method of injection-compression moulding an elastomer containing a blowing agent in a two-part mould 34,35, comprises partially closing the mould 34,35 to leave a gap between the faces of the two mould halves 34,35, damming the periphery of the mould, by means of a retractable dam 32, such as to close the gap and injecting the elastomer into the gap; allowing the fluid elastomer to spread and expand within the gap; fully closing the mould (34,35) to compress the elastomer against the mould faces and the dammed periphery of the mould whereby the dam is retracted within a recess and allowing the compressed elastomer to solidify. The method is particularly useful for moulding a speaker cone piston or for overmoulding an annular elastomeric surround on to the periphery of a speaker cone piston.

Description

INJECTION'COMPRESSION MOULDING METHOD AND MACHINE This invention relates to a method of injection-compression moulding an elastomer containing a blowing agent in a two-part mould; a method of moulding a speaker cone piston in a two-part mould; a method of overmoulding an annular elastomeric surround on to the periphery of a speaker cone system; and an injection-compression moulding machine. It is particularly, although not exclusively, useful in the manufacture of speaker cones.

Loud speaker cones are described for example in US-A-3961378, typically using a cone of paper flexibly mounted at its periphery for axial movement, and with a voice coil at the centre operating in an annular magnetic air gap. Research and development of speaker cones over the years has aimed at improving the fidelity of sound reproduction, reducing the complexity and cost of manufacture, and reducing the mass, this latter aim being particularly important for automotive use. Broadly speaking the more rigid the cone piston, which is the paper or plastics diaphragm, the less total harmonic distortion; and the greater the excursion, i.e. the range of axial movement, then the louder the sound produced by the speaker. More sound can be produced for less effort when lighter weight speaker cones are used, and this enables lighter weight magnets to be used, leading overall to lighter loud speakers. A variety of plastics moulding techniques have been used, one example being described in US-A-3961378, and another, involving split line moulding, being disclosed in GB-A-2228391 and more recently in EP-A-552040. These publications illustrate a particular difficulty in the moulding of speaker cones, namely the poor flow characteristics of molten elastomers when moulding components of thin crosssection, particularly the flexible surround for the speaker cone piston. We have also recognised that existing elastomeric speaker cones and cone surrounds suffer from in-built stresses caused during the moulding process; such stresses give rise to structural irregularities and reduced acoustic performance and reliability.

Accordingly, the purpose of the present invention is to overcome or mitigate the disadvantages of previous techniques for manufacturing speaker cones.

The invention provides a method of injection-compression moulding an elastomer containing a blowing agent in a two-part mould, comprising: partially closing the mould leaving a gap between the faces of the two mould halves, damming the periphery of the mould such as to close the gap and injecting the elastomer into the gap; allowing the fluid elastomer to spread and expand within the gap; fully closing the mould to compress the elastomer against the mould faces and the dammed periphery of the mould and allowing the compressed elastomer to solidify.

The invention also provides a method of moulding a speaker cone piston in a two-part mould, comprising: partially closing the mould leaving a gap between the faces of the two mould halves, damming the periphery of the mould such as to close the gap and injecting an elastomer into the gap; allowing the fluid elastomer to spread within the gap; and fully closing the mould to compress the elastomer against the mould faces and the dammed periphery of the mould and allowing the compressed elastomer to solidify.

Further, the invention provides a method of overmoulding an annular elastomeric surround on to the periphery of a speaker cone piston, comprising: positioning the cone piston in a mould cavity defining the intended shape of the surround, partially closing the mould leaving a gap between the faces of the two mould halves, damming the gap on both the inside and the outside of the intended annular shape of the overmouid, allowing the fluid elastomer to spread within the gap; fully closing the mould to compress the elastomer against the mould faces and the dammed periphery of the mould and allowing the compressed elastomer to solidify and to bond to the cone piston periphery.

Further still, the invention provides an injection-compression moulding machine comprising: two opposed mould parts; means for driving the two mould parts together along an axis so that their opposed faces meet upon full closure of the mould and define the final shape of an injection moulding; an axially-retractable peripheral dam on at least one of the mould parts facing the other mould part and projecting axially from the periphery of the mould, the dam being resiliently biased to project outwardly by a predetermined extension; and control means for causing the driving means to drive the two moulds together to a partially-closed position at which the dam of one mould part engages the periphery of the other mould part to enclose the gap between their opposed faces, whereby to shut off fluid elastomer when it is injected into the gap; and for then causing the drive means fully to close the mould and to cause the dam to retract against its resilient bias, so as to compress any elastomer in the mould and to mould it to its final shape.

With the benefit of the present invention, the gap allows the molten elastomeric material to be injected at much lower pressures than with existing injection-moulding techniques. The elastomer spreads easily in the relatively wide gap. After the subsequent compression, we have found that the solidified moulding has relatively minor irregularities, leading to exceptionally good acoustic performance and reliability. We have also found that it is possible to reduce substantially the thickness of the final moulding, easily down to 0.3 mm and, in principle, down to substantially lower thicknesses. Thus it is now possible to make products of lighter weight and reduced thickness, with advantages especially in their flexibility and in their acoustic properties. In the case of annular elastomeric surrounds, for example, where part is very thin and part is relatively thick, the thicker part can retain a cellular structure, and thus be of lighter weight, where a blowing agent is used with the elastomer.

In order that the invention may be better understood, preferred examples will now be described, by way of example only, with reference to the accompanying drawings, in which Figure 1 is an elevation of a two-part mould for a speaker cone; Figures 2 and 3 are cross-sections taken respectively along the lines A to A and B to B of Figure 1, through the two-part mould in the closed configuration, and further inciuding a cone piston held in the mould cavity; Figures 4 and 5 are respectively a perspective view and a side elevation of a circular speaker cone manufactured using a mould different from that shown in Figures 1 to 3; Figure 6 is a cross-section on the axis of a two-part mould in its open configuration for moulding a speaker cone piston, in accordance with a first embodiment of the invention; and Figure 7 is a cross-section, to an enlarged scale, of part of an injectioncompression moulding machine containing a speaker cone and arranged to overmould an annular elastomeric surround, in accordance with a second embodiment of the invention.

Before the invention itself is illustrated, two-part injection moulds for overmoulding an elastomeric surround will be described with reference to Figures 1 to 5, by way of background.

The two-part injection mould shown in Figures 1 to 3 is of generally conventional construction, and is mounted for automatic reciprocation along the mould axis to and from the closed position illustrated. Apparatus, not shown, is provided for inserting a cone piston 10 into the open mould cavity before an injection process, and for releasing the completed speaker cone following the injection process. The main novel feature of interest is the shape of the mould cavity which defines the annular surround for the cone piston; and the hot tip injection points 22A to 22D shown in Figure 1.

In this example, the cone piston 10 is a paper diaphragm with a central aperture. An annular elastomeric surround is injected over the peripheral edge of the cone piston using the hot tip injection process. As shown most clearly in Figure 2 and Figure 3, the annular surround consists of two flat ring portions joined by a thin, flexible semitoroidal portion 14. The inner flat ring portion of flange 12 overlaps the periphery of the cone piston 10, and is defined between an inner edge 11 and an outer edge 13. The inner ring 12 is formed integrally with the semitoroidal joint portion 14 and has a similar thickness to it. A relatively thick flat outer ring portion or flange 16 is formed integrally with the outer edge 15 of the semitoroidal joint 14.

During moulding, the mould faces define a shut-off region 17 which defines the outer edge of the outer flat ring portion 16, preventing the molten elastomeric material from flowing radially beyond this edge.

The molten elastomeric material is injected through four spaced hot tip injection points 22A to 22D shown in Figure 1. (In other examples, two or three hot tips could be used.) One such injection point is shown as 22 in Figure 3. As shown in Figure 3, the hot tip injection point 22 communicates with a hot manifold 24 within block 23 of the right hand mould part 20. The mould is designed so that the elastomeric material spreads evenly around the portion of the mould cavity which defines the intended future shape of the elastomeric surround 11 to 16. Thus the mould faces of the mould parts 20 and 21 are machined precisely to give the thickness required, allowing for the thickness of the cone piston 10.

During injection moulding, the elastomeric material spreads throughout the mould cavity within the shut-off area 17, and the thin inner flat ring portion 12 bonds with a fusion-bond joint to the face of the cone piston 10, in the example where the cone piston is made of paper. Whether the cone piston is of paper or of thermoplastics or other plastics materials, the bond is formed during the solidification of the elastomeric material and is assisted by the pressure between the faces of the mould parts 20, 21: it is formed as a lap joint.

With this injection method, no sprue is formed anywhere except in the form of the circular ends of the hot tips 22. These take the form of small cylindrical projections on the surface of the elastomeric surround, and have no deleterious effect on the loud speaker.

Due to the hot tip method of feeding the elastomer molten material to the mould cavity, no runner system is required, so no degating is required, and no waste material is generated requiring regrinding; also, there is no need for any post-moulding operation, such as the removal of sprue.

A suitably adapted mould. corresponding to that of Figures 1 to 3, can be used to make a three part speaker cone as shown in Figures 4 and 5. In this example, the cone is of circular section, but oval or dther sections would of course be possible. In this example, a thin, relatively flexible elastomeric joint 44 is formed by injection moulding between preformed relatively rigid inner and outer mouldings 43 and 41. Preferably, the preformed rigid mouldings are of a hard material, usually thermoplastics such as polypropylene. They need not be formed of the same material, but this is advantageous in reducing the cost of manufacture. The flexible joint 42 has the same general shape as that joint 12 to 16 shown in Figures 1 to 3, in that it has a substantially semitoroidal thin joint portion 42 formed integrally with an inner flat ring portion 44 which forms a lap joint with the cone piston 43 whose aperture 46 is at the centre. The outer periphery of the flexible elastomeric joint 42 is also bonded as a lap joint to the outer, relatively rigid surround or flange 41.

Whilst a variety of injection moulding techniques could be used, including split line moulding, the hot tip injection moulding process described with reference to Figures 1 to 3 is preferred.

We have found that a satisfactory lap joint can be formed in such a process, between the elastomeric flexible material and the relatively hard thermoplastics material of the cone piston 43 and the outer surround 41. It is easier to optimise the strength of this bond when the same material is chosen for the outer surround 41 as for the cone piston 43.

A first embodiment of the invention will now be described with reference to Figure 6. This is a cross-sectional view of a two-part injection-compression moulding machine for moulding a speaker cone piston. The moving mould half 34 carries a mould with a generally conical surface 302. The fixed mould half 35 carries a mould with a correspondingly-conical recess 301, the two mould surfaces 301 and 302 being formed precisely to define the final shape of the speaker cone, typically 0.3 mm thick, when the peripheral flat faces of the mould halves engage upon full closure of the mould. Relative axial movement of the mould halves is guided by guide pillars 37, of which one is shown in Figure 6, projecting axially from the fixed half, received in guide bushes 36 on the moving half 34.

Molten elastomeric material is injected in the direction 308 along a material feed path 306 in a sprue bush 307. A conventional sprue puller 305, with a reverse taper, is arranged on the end of an ejector pin 304 and is axially movable within a punch 303 arranged to project through a central circular aperture in the moulded speaker cone. Once the cone has been moulded and allowed to solidify, sprue fills the sprue puller 305 and material feed path 306 and also constitutes a disc 309. In order to remove the mushroom-shaped sprue 305, 306, 309, the punch 303 is driven towards the right in Figure 6, breaking the sprue away from the speaker cone. The sprue puller 304 is pulled towards the left in Figure 6, retracting the sprue. The sprue puller 304 is then caused to release the sprue which drops free of the mould halves; the speaker cone is then removed ready for the next cycle.

In accordance with the invention, an annular dam 32 is formed in the fixed, half 35 of the mould, around the periphery of the mould face 301. The dam is a cylindrical ring, mounted for a short axial movement relative to the mould, guided by several bolts 33 disposed equi-anguiarly around the periphery. The dam 32 slides axially in a correspondingly ring-shaped recess in the mould, with a close fit. As shown in Figure 6, the dam is fully extended, and in this example it projects 3 mm towards the surface 302. The dam 32 is resiliently biased outwardly, by springs (not shown) mounted over the bolts 33 inside the ring-shaped recess.

The dam 32 forms a peripheral shut-off for the mould, and allows moulding to be initiated with the mould in a partially-closed position, in this example 3 mm wider than the fully-closed position. As the mould moves axially from its partially closed to its fully-closed position, the dam 32 is gradually retracted against the spring force, maintaining the shut-off throughout the process. With the mould in its fully-closed position, the dam is fully retracted into the fixed half of the mould.

The machine of Figure 6 is operated under automatic electronic control, which allows the speed and position to be programmed for each phase. Such injection moulding machines are conventional and need not be described here.

Elastomer is injected along the material feed path 306 while the mould is partially closed, with the mould faces 3 mm apart and the dam 32 engaging the opposite mould face. The fluid elastomer easily spreads from the injection point radially outwardly, at low pressures. The quantity of material injected is sufficient to partially fill the mould, typically extended for half the radius. The mould halves 35 and 34 are then moved to the fully closed position, in a compression stroke, as the fluid elastomer spreads further radially outwardly to the dam 32 and is then compressed axially to assume its final configuration and then to solidify. In fact, the compression can be triggered during any phase of the injection stroke, either during or upon, completion of the injection stroke: the back-pressure is sufficient to stop the injection proceeding.

In this example, the cone piston is compressed from its original thickness of 3 mm down to 0.3 mm, but even thinner cones could be formed using this technique, for example down to 0.1 mm. Also, the 3 mm gap could be any suitable gap, for example in the range 1 to 6 mm.

The pressures and gaps will be dictated in part by the materials used. It is not essential use a blowing agent, but we have found that this takes full advantage of the invention, and enables lightweight products to be made. Thus the elastomeric material is mixed with a blowing agent before injection. It then expands within the gap in the mould, as it spreads radially. As it expands, it forms a cellular structure with gas bubbles. We believe that during the subsequent compression stroke, the bubbles are squashed and elongated relatively uniformly, and to a degree which depends upon the ultimate thickness of each different section of the mould. It is this which reduces substantially the in-built stresses which were characteristic of previous high pressure injection moulding techniques for such products.

It will be appreciated that whilst in this example the fluid is injected on the axis, it could instead be injected at any point or at multiple points.

In one example, standard polypropylene materials are used, with a percentage of mineral, for example talc, mica or glass, as a filler. Glass fibre is a useful form of filler, and a major portion of the fibre lengths can be retained during moulding. Polycarbonate can be used in place of glass fibre. As an altemative to polypropylenes, other thermoplastics materials, such as Delrin , an acetyl resin made by DuPont.

The invention can also be used to produce an overmould with the shape as shown in Figure 2, for example using the hot tip injection method described with reference to Figures 2 and 3. The shut-off necessary for the overmoulding is provided by two transversely-spaced dams 30, 30A as shown in Figure 7 to an enlarged scale. Each dam is formed similarly to the dam 32 of Figure 6. In the example of a circular periphery, the dams would be circular rings formed concentrically in the mould. Other shapes such as ovals or irregular shapes would of course be possible alternatives. The configuration of the overmould and of the periphery 10 of the speaker cone piston, are described above, using the same reference numerals, with reference to Figure 2. As shown in Figure 7 the dams 30, 30A are mounted on axial bolts 31, 31A in the fixed half of the mould.

Claims

1. A method of injection-compression moulding an elastomer containing a blowing agent in a two-part mould, comprising: partially closing the mould leaving a gap between the faces of the two mould halves, damming the periphery of the mould such as to close the gap and injecting the elastomer into the gap; allowing the fluid elastomer to spread and expand within the gap; fully closing the mould to compress the elastomer against the mould faces and the dammed periphery of the mould and allowing the compressed elastomer to solidify.

2. A method of moulding a speaker cone piston in a two-part mould.

comprising: partially closing the mould leaving a gap between the faces of the two mould halves, damming the periphery of the mould such as to close the gap and injecting an elastomer into the gap; allowing the fluid elastomer to spread within the gap; and fully closing the mould to compress the elastomer against the mould faces and.

the dammed periphery of the mould and allowing the compressed elastomer to solidify.

3. A method of overmoulding an annular elastomeric surround on to the periphery of a speaker cone piston, comprising: positioning the cone piston in a mould cavity defining the intended shape of the surround, partially closing the mould leaving a gap between the faces of the two mould halves, damming the gap on both the inside and the outside of the intended annular shape of the overmould, allowing the fluid elastomer to spread within the gap; fully closing the mould to compress the elastomer against the mould faces and the dammed periphery of the mould and allowing the compressed elastomer to solidify and to bond to the cone piston periphery.

4. A method according to Claim 2 or Claim 3, in which the elastomer includes a blowing agent and the fluid elastomer expands as it spreads within the gap to form a cellular structure which is then compressed upon full closure of the mould.

5. A method according to any preceding claim, in which the elastomer is of a thermoplastics material.

6. A method according to any preceding claim, in which the said axial gap, when the moulds are partially closed, is from 1 to 6 mm.

7. A method according to Claim 6, in which the said axial gap is about 3 mm.

8. A method according to any preceding claim1 in which the axial thickness of the final mould or of portions of the final mould is less than 0.5 mm.

9. A method according to Claim 8, in which the said axial thickness is less than or equal to about 0.3 mm.

10. A method according to Claim 4, or any of Claims 5 to 9 as appendant to, Claim 4, in which the quantity and timing of injection of the elastomer are such that the blown fluid continues its expansion transversely within the gap during the closure of the mould from the partially closed to the fully closed position, and reaches the dammed periphery before it has solidified.

11. A method according to any preceding claim, in which the elastomer is injection at only one port into the mould.

12. A method according to any preceding claim, in which the elastomer is injected using a hot manifold and hot tip system.

13. An injection-compression moulding machine comprising: two opposed mould parts; means for driving the two mould parts together along an axis so that their opposed faces meet upon full closure of the mould and define the final shape of an injection moulding; an axially-retractable peripheral dam on at least one of the mould parts facing the other mould part and projecting axially from the periphery of the mould, the dam being resiliently biased to project outwardly by a predetermined extension; and control means for causing the driving means to drive the two moulds together to a partially-closed position at which the dam of one mould part engages the periphery of the other mould part to enclose the gap between their opposed faces, whereby to shut off fluid elastomer when it is injected into the gap; and for then causing the drive means fully to close the mould and to cause the dam to retract against its resilient bias, so as to compress any elastomer in the mould and to mould it to its final shape.

14. An injection-compression moulding machine according to Claim 13, adapted for overmoulding an annular surround on to a preformed substrate, one of the mould parts being shaped to accommodate the substrate during overmoulding; in which there is one further dam, on the same or the opposite mould part, spaced from the said dam in a direction transverse to the axis and arranged to co-operate with the same dam to shut off an annular gap between the mould faces overlying, in use, the portion of the substrate on which the overmould is to be made.

15. A speaker cone piston formed using the method of Claim 2 with a blowing agent in the fluid elastomer.

16. A speaker cone having a cone piston and an annular elastomeric surround joined flexibly to the periphery of the cone piston, made using the method of Claims 3 and 4 or of any claims dependent to Claims 3 and 4.

17. A method of injection-compression moulding, substantially as described herein with reference to the accompanying drawings.

18. A method of manufacture of a speaker cone piston, substantially as described herein with reference to Figure 6 of the accompanying drawings.

19. A method of injection-compression moulding an annular elastomeric surround joined flexibly to the periphery of a speaker cone piston, substantially as described herein with reference to Figures 1 to 7 of the accompanying drawings.