GB2073651A - Moulding apparatus - Google Patents

Abstract

In a multi-cavity, flashless mould for moulding small rubber articles, such as seals for use in vehicle hydraulic braking systems, each mould cavity (6) is defined between a piston (10) received in a bore (12) in a first rigid platen (4) and an insert (8) firmly mounted on a second rigid platen (2), the rear ends of the pistons (10) being exposed to the pressure of a common body of fluid contained within a closed chamber (44) so that, when the mould is closed, the pistons will retract into their bores on engagement by the respective inserts until all the pistons are engaged with the inserts. The fluid body in chamber (44) then supports the full moulding pressure whilst biasing the pistons (10) against their inserts (8) with substantially equal forces. Instead of a common body of fluid the pistons may be acted upon by respective Belleville springs. To assist in preventing flash, the mould cavity (6) is separated from a surrounding flash groove (54) by very narrow land surfaces (50, Fig. 2) e.g. about 0.1 to 0.2 mm wide. <IMAGE>

Description

SPECIFICATION Moulding apparatus This invention relates to moulding apparatus, and in particular to a multi-cavity "flashless" mould for moulding small rubber articles, such as seals for use in the components of vehicle hydraulic braking systems.

A "flashless" mould is one in which the articles are moulded without any flash attached thereto as a result of the material extruding between the mould parts which come together to define the mould cavities. Rubber, when in the semi-liquid or plastic state has a relatively low viscosity, and for this reason has a greater tendency to extrude between the mould parts and form flash.

Consequently it is necessary to see that the mould parts defining all the cavities in a multi-cavity mould are closed tightly together if flash on the moulded products is to be avoided. If the articles do have any flash it has to be trimmed off in a subsequent machining operation, which is an obvious disadvantage as it increases production costs. In order that all the cavities are properly closed simultaneously, in the past the cavity defining parts had to be set very accurately upon the mould platens in which they were carried. As a result the moulds were expensive to produce and if either of the platens should become distorted, e.g. under the temperature or pressure of the moulding process, the positioning of the mould parts would be upset and the products would suffer from flash.Another disadvantage is that if one of the mould parts happened to become damaged, a new substitute part had to be set accurately in its place and the use of the whole mould was lost whilst the necessary machining was completed.

Attempts have been made to reduce these problems and in British Specification No. 958,271 there is disclosed a transfer moulding apparatus in which each of the mould cavities is defined by a plurality of inserts carried in apertures in respective plates. The plates are hinged together so that they can be folded upon one another to bring the inserts into alignment, and can be opened apart to allow removal of the moulded rubber seals. The plates and their inserts are located between an upper mould platen having sprue holes for conducting the rubber from the "bottomless" cylinder defined above it, and a flexible diaphragm carried by a lower mould platen and defining with it a fluid chamber. This chamber is filled with pressurised oil and this pressure acts on the groups of inserts defining the respective cavities through the flexible diaphragm.When the mould is closed the groups of inserts are clamped together between the upper platen and the flexible diaphragm.

Ideally the diaphragm should be sufficiently flexible that the pressure exerted on each group of inserts should be totally independent of the pressure exerted on the other groups. In reality there is no material available which has sufficient flexibility and yet is able to withstand the severe operating conditions. As a result, the pressure exerted on one insert group is never totally independent of the diaphragm deflection at adjacent insert groups. It is also necessary that the lowermost insert carrying plate be flexible so as to be deformable under the diaphragm pressure. In practice it has proved difficult to manufacture completely flashless moulds using this apparatus, and the diaphragm flexibility is only capable of compensating for small variations in the machining tolerances of the individual inserts and in their positioning in the holding plates.

This apparatus also has other drawbacks. Each of the inserts has to be individually set in its respective aperture in the carrying plate, so that the plates with inserts are expensive to make. If one of the inserts becomes damaged another has to be machined and set in its place. The diaphragm is subject to wear at the edges of the lowermost inserts, this \wear increasing the more flexible the diaphragm. The insert-carrying plates have to be removed and opened apart to remove the moulded seals, which increases the cycle time of the apparatus. Finally, the moulding apparatus with the inserts, the plates and the flexible diaphragm is rather complex in construction and operation.

A transfer moulding apparatus of simpler design is described in British Patent Specification 1409107, and it differs from that discussed above in that instead of using a flexible diaphragm and pressure chamber, a flexible upper platen is employed and the pressure in the rubber cylinder, from which the rubber is forced into the mould cavity as the mould is closed, is relied upon to press the groups of inserts together. This removes the drawbacks associated directly with the use of the flexible diaphragm, but the other disadvantages still persist. Furthermore, the upper platen takes the form of a thermally insulating plate made of a mixture of asbestos fibres and phenol resin which is appropriately cured, so it is of only low flexibility.Therefore, the pressure on each group of inserts is not totally independent of the insulating plate deflection at the adjacent groups because the plate is not flexible enough, and again totally-flashless articles are not easy to obtain. The inserts are still required to be set accurately in their carrying plates as the flexibility of the insulating plate can only compensate for small variations in the manufacturing tolerances of the separate inserts and in their mounting.

The reason for making the flexible plate thermally insulating is to avoid curing the rubber which remains in the cylinder after the cavities have been filled, so that it remains usable and does not become cured with the rubber in the cavities.

A similar form of moulding apparatus is described in British Patent Specification No. 1397908, the main difference being that one of the cavity inserts of each group is carried by the flexible insulating plate. This insert defines a constriction in the cavity facilitating separation of scrap rubber from the moulded products. This mould has the additional drawback that if the apparatus is to be used for producing different shaped seals, the insulating plate itself may need to be replaced as well as the inserts and the plates in which they are held.

It is not uncommon for the same mould press to be used for producing many different seals, possibly up to about 40 different seals- per week.

With the known moulding apparatuses described above, either two platens with fixedly mounted cavity parts, or a stack of plates with inserts therein is needed for each seal to be produced and must be stored when not in use. The space requirement may be quite large when a great number of different seals is involved and as the parts are expensive to manufacture it is of disadvantage for them to stand idle for considerable lengths of time.

The present invention aims to eliminate or alleviate the problems and drawbacks of the known moulds and accordingly provides a multicavity mould for moulding small rubber articles, comprising first and second rigid platens movable towards and away from each other, a plurality of pistons received in respective bores in the first platen and a corresponding number of inserts supported firmly on the second platen, each piston cooperating with a respective insert to define therewith a mould cavity when the platens are closed together, and the pistons being movable independently of each other in their respective bores, and means acting upon the rear ends of the pistons so that when the platens are moved towards each other, any piston engaging its associated insert before all the other pistons have engaged their associated inserts will be retracted into the bore receiving said piston until all the pistons are engaged with their respective inserts, said means being capable of supporting the full moulding pressure with which the platens are urged together during the moulding process, and said means biasing the pistons against the inserts with substantially equal forces when the full moulding pressure is supported thereby.

Such a mould can be of simple construction and comparatively inexpensive to make. Because the pistons are independent large tolerances can be compensated for and close tolerances do not need to be held during mould manufacture. The pistons and inserts do not have to be set accurately upon the platens and can be easily.

removable independently of each other to allow replacement of a damaged part. Furthermore, the same platens can be used for moulding different seals since all that is necessary is to remove the inserts and pistons and substitute others which will produce the new seal.

The moulded articles are easily removed on opening the mould,#e.g by an air jet each cavity being defined by only two parts.

In the case of injection or compressioninjection moulding ducts for supplying the moulding material to the cavities are preferably formed in the inserts, although they could be provided in the pistons.

According to one embodiment of the invention the means acting on the rear ends of the pistons comprises a body of fluid having a predetermined volume and held within a close chamber. The fluid may be a liquid, or an elastomeric solid exhibiting hydraulic properties.

In another embodiment the means acting on the pistons comprises a separate Belleville spring interposed betweenthe rear end of each piston and the inner end of the bore in which it is accommodated. A more complete understanding of the invention will be had from the following detailed description which is given by way of example with reference to the accompanying drawings, in which:~ Figure 1 is a section through a transfer mould embodying the invention; Figure 2 is a detail of the mould cavity in Fi#gure 1; Figure 3 is a plan view of a mould platen according to a modified embodiment of the invention; and Figure 4 is a section taken along the line lV-IV in Figure 3, with the piston and its retaining plate mounted in position.

Illustrated in Figure 1 is a moulding apparatus having a plurality of mould cavities, but for reasons of clarity and-simplicity only one has been shown. It should be understood that the remaining cavities are formed in exactly the same way as described below.

The apparatus comprises a rigid upper platen 2 and a rigid lower platen 4. A mould cavity 6 is defined between an insert 8 mounted in an opening in the upper platen 2, and a piston 10 slidable in a through bore 12 in the lower platen 4.

The piston is sealed to the bore by a seal 14 and is provided with a flange 1 6 which engages a shoulder defined by a counter-bore 18 to limit downward movement of the piston 10. A retainer plate 20 is attached to the upper face of platen 4 and has an opening through which the upper end of the piston 10 projects, the opening having an edge against which the flange 16 will abut to prevent the piston becoming disengaged from the bore 12 unintentionally. The insert 8 has a flange which is held against a shoulder defined in the opening in the platen 2 by an insert retaining plate 22 attached to the rear or upper surface of platen 2. The retainer plate 22 is provided with apertures 24 communicating ducts 26 formed in the insert 8 for supplying rubber to the cavity 6, and a socalled "bottomless" cylinder 28 defined by an annular cylinder plate 30. A piston 32 cooperates with the cylinder 28 and is carried by an upper support plate 34 to which the cylinder plate 30 is connected by peripherally located bolts 36 and compression springs 38. Guide posts 40 are mounted on the lower platen 4 and project from the upper surface for guiding the upper platen 2 and cylinder plate-30 into proper alignment as the mould parts are moved from their open position to the illustrated closed position.Attached to the bottom surface of the lower platen is a backing plate 42 defining with the platen 4 a fluid chamber 44 having a port 46 which is plugged-with the chamber containing a predetermined volume of hydraulic fluid. The lower end of the piston 10 is exposed to the pressure in chamber 44, as are all the pistons of the other cavities of the mould. As à result, all the pistons 10 are pushed upwardly by the same force due to the pressure of the hydraulic fluid.

In operation, with the mould parts-opened apart, a supply of uncured rubber is placed in the cylinder 28 and the mould press starts to move the parts together. Initially the upper and lower platens 2, 4 approach each other until the confronting faces of the pistons 10 and their associated inserts 8 come into abutment. Not all the pistons will engage their cooperating inserts simultaneously, and the leading pistons will engage first. As the platens continue to close together these leading pistons 10 will be pushed back into their bores 12 and the displaced fluid serves to push out the trailing pistons until they too abut their respective associated inserts 8. Only when all the pairs of inserts and pistons are engaged will the fluid pressure in chamber 28 start to build up so that further retraction of the pistons into their bores is prevented.Further closing of the press then pushes piston 32 down into the cVlinder-28 forcing the rubber-through apertures 24 and the ducts 26 in the inserts 8 into the mould cavities 6. When the press is fully closed, the full pressure of the press is supported by the fluid in chamber 44 through the pistons 10.

Because all the pistons are engaged under pressure with their inserts, rubber does not extrude between them and no flash is produced on the moulded seals. When the rubber in the cavities is cured, the press is opened, the moulded seals are removed, e.g. -by an air jet, and the mat of rubber remaining in-the cylinder 28 removed. The mould is then ready for another cycle.

From the foregoing description it will be appreciated that the mould is of simple construction, and no setting up operations are needed to position either the inserts or the pistons accurately with respect to their supporting platens because-the pistons are freely movable independently of each other to ensure that all the cavities are closed precisely each and every moulding cycle. There are no restrictions on the clearances which can be taken up due to the use of flexible plates or diaphragms as in the prior art.

In contrast the possible vertical movement of each piston may be as much as 10 mm. If one of the pistons or inserts should become damaged a replacement part, or blanking part if a replacement is not available, is easily substituted after detachment of the appropriate retainer plate 20 or 22 which may be secured in place by quick release fasteners. Furthermore, this can be carried out by the press operator since the mould does not require any tool-room machining. Consequently down-time of the apparatus on such occasions is minimised. If different seals are to be moulded the platens 2, 4 themselves need not be replaced. The inserts 8 and pistons 10 are simply removed and fresh inserts and pistons suitable for the new seal introduced in their places.

As may be seen more clearly in Figure 2, the mould cavity 6 defined between the adjoining faces of the insert 8 and piston 10 is annular and surrounded internally and externally by land surfaces 50 at which the piston and insert abut .around the inner and outer edges of the cavity 6.

According to a preferred embodiment of the invention these land surfaces 50 are made to have a very small width. In known moulding apparatus a typical width for the land surfaces is in the region of 2 mm or more, whereas-in this invention the land width is preferably of the order of 0.1 to 0.2 mm. The narrow lands are effective in helping to avoid flash at the edges of the moulded seals formed by rubber extruding between the insert and piston at their part line.

It is known to use narrow lands in compression moulds where the lands close together to provide a cutting action to sever the extruded rubber from the article being made. In injection and compression-injection (transfer) moulds no such cutting action is possible because the mould cavity is closed before the moulding material is injected into it Nevertheless narrow lands have proved helpful in precluding flash in moulds of these types. Whe#n the rubber is injected into the closed mould cavity the displaced air must be expelled between the mould parts. If it is not air bubbles will be formed in the moulded articles and they will be defective. In the mould shown in Figures 1 and 2 the insert 8 is provided with venting passages 52 for conducting away the air forced from the cavity at its inner periphery.No vents are used, of course; at the outer periphery.

With the narrow-lands 50 the rubber tends not to follow the air, but if it does extrude between the land surfaces it passes to the surrounding flash collection recesses 54 and does not remain attached to the moulded article. For ease of machining it is preferable that the land surfaces lie in planes normal to the axes of the mould parts.

These surfaces may then be produced by surface grinding operations. The insert 8 and piston 10 have comparatively wide peripheral support surfaces 58 and the land surfaces 50 are arranged to abut first when the insert and piston are brought together. The land surfaces may stand proud of the support surfaces with an effective height of about 0.01 mm. If the shape of the cavity permits, this result can be conveniently obtained grinding the support and land surfaces flat to the same level and then polishing the support surfaces only.

Turning now to Figures 3 and 4, there isshown a slightly modified apparatus. Except for the modifications described below it should be understood that the mould is essentially the same as described above with reference to Figure 1. The lower mould platen illustrated in plan view in Figure 3 has 21 bores 60 for receiving the pistons 62 of respective cavities. The bores are arranged in two coaxial circular arrays around a central bore. On the bottom surface of the platen are machined two circular grooves 64 and a linear groove 66 which also leads to a port 68. When a backing plate is fixed to the bottom of the platen the grooves 64, 66 define a passageway network interconnecting all the bores 60 with each other.

A predetermined volume of hydraulic fluid may be placed in the passageways and the inlet port 68 plugged, in which case the mould will operate as described with reference to Figure 1.

As may be seen in Figure 3, the bores 60 are plain, making them easier to machine and hence the platen less costly to produce, and the piston has a shoulder rather than a flange for engagement with the retainer plate, so it is easier to make also It is not essential for the pistons to be subject to the pressure of a common body of fluid, and instead they can be biased upwardly by separate spring means. The preferred form of spring is a stack of Belleville washers 70, as seen in Figure 4, since it is then possible to obtain a high pre-load yet a low spring rate. The pre-load of the springs is chosen so that all the springs must be compressed in order to support the full press pressure and the spring length is selected so that there is no danger of any of the pistons "bottoming". This in turn ensures that all the mould cavities will be properly closed whether or not the pistons engage the respective inserts simultaneously. With a low spring rate it is ensured that the upward forces on the pistons are substantially the same, although some springs may be compressed more than others, and the correct abutment pressures are obtained at the land surfaces surrounding the mould cavities.

Other modifications will occur to readers skilled in the art and it is intended that the scope of the invention should be limited only by the appended

Claims (14)

claims. CLAIMS

1. A multi-cavity mould for moulding small rubber articles, comprising first and second rigid platens movable towards and away from each other, a plurality of pistons received in respective Lores in the first platen and a corresponding number of inserts supported firmly on the second platen, each piston cooperating with a respective insert to define therewith a mould cavity when the platens are closed together, and the pistons being movable independently of each other in their respective bores, and means acting upon the rear ends of the pistons so that, when the pistons are moved towards each other, any piston engaging its associated insert before all the other pistons have engaged their associated inserts will be retracted into the bore receiving said piston until all the pistons are engaged with their respective inserts, said means being capable of supporting the full moulding pressure with which the platens are urged together during the moulding process, and said means biasing the pistons against the inserts with substantially equal forces when the full moulding pressure is supported thereby.

2. A mould according to claim 1, wherein each of the pistons is releasable from its respective bore and each of the inserts is releasably mounted to the-second platen, whereby the pistons and inserts are easily replaceable iridependently of each other.

3. A mould according to claim 2, wherein the inserts are secured on the second platen by a common retaining member connected releasably to the platen, and the pistons are retained in their bores by a'common retainer niember connected releasably to the first platen.

4. A mould according to claim 1, 2 or 3, wherein said means acting on the rear ends of the pistons comprises a common body of fluid.

5. A mould according to claim 4, wherein said body of fluid comprises a predetermined volume contained within a closed chamber.

6. A mould according to claim 1, 2 or 3, wherein said means acting on the rear ends of the pistons comprises a respective spring acting between each piston and the inner end of the bore accommodating the piston.

7. A mould according to claim 6, wherein each spring comprises a Belleville spring.

8. A mould according to claim 7, wherein the Belleville springs have a high pre-load and a low spring rate.

9. A mould according to any one of the preceding claims, wherein each piston and associated insert have land surfaces which abut around a periphery of the mould cavity defined therebetween when the first and second platens are closed together, air being expelled from the mould between the abutting land surfaces during each moulding operation, and said land surfaces having a very small width.

10. A mould according to claim 9, wherein the width of the land surfaces is approximately 0.1 to 0.2 mm.

11. A mould according to claim 9 or 10, wherein each piston and insert have support surfaces which abut when the platens are closed together and the land surface stand slightly proud of said support surfaces.

12. A mould according to claim 1 wherein the land surfaces stand proud of the support surfaces by about 0.01 mm.

13. A mould according to any one of claims 9 to 12, wherein each piston and insert define a flash groove extending around the mould cavity and separated therefrom by the abutting land surfaces.

14. A mould substantially as herein described with reference to the accompanying drawings.