GB2239836A - Injection moulding process - Google Patents

Abstract

A process for producing injection moulded plastics mouldings, the process including the steps of - (a) producing separable mating resilient mould inserts which abut along opposed abutment faces to define therebetween a mould cavity; (b) inserting the resilient mould inserts into opposed movable mould parts of a plastics injection moulding apparatus, the mould parts being movable into abutment for moving the mould inserts into abutment to define said mould cavity, Figs. 3,6 (not shown), (c) injecting molten plastics into said cavity to create a plastics moulding; and (d) moving the mould parts apart to separate the mould inserts to permit removal of the plastics moulding, (e) the separable mating mould inserts being produced by: (i) placing an article 30 of which subsequent plastics mouldings are to be produced between opposed bodies 20 of a mouldable material which is curable to form a resilient material, Fig. 2, (ii) pressing the opposed bodies of mouldable material together so as to press the article into both bodies of mouldable material so as to create an impression of the article in said mouldable material; (iii) curing said mouldable material so as to retain said impression and form separable bodies 20 of resilient material; (iv) separating the bodies of resilient material and removing said article to thereby create said mould cavity. <IMAGE>

Description

INJECTION MOULDING PROCESS The present invention relates to an injection moulding process, and articles mouldedeby the-process.

The present specification has been divided out of UK Patent Specification No. 2210319 which describes and claims an injection mould.

Injection moulding of thermoplastics to create a product requires the creation of a separable mould. The mould normally comprises two mould parts and these mould parts may incorporate movable core inserts for the moulding of intricate shapes having undercuts.

Generally, acute undercuts in a moulding are not possible without a complicated mould construction which results in expensive tooling costs. In addition such moulds can take many months to produce. Even if a mould is produced for providing undercuts, the number of undercuts provided will be restricted such that, in the main, only straightforward geometrical shapes are produced using these techniques.

These drawbacks become more apparent when articles of non-geometric random shape are to be moulded. Such articles may for instance be models of animal shapes.

Heretofore, injection moulded models of animal shapes made from plastics have lacked realistic detail in shape due to the inability of conventional injection moulding techniques to produce many randomly shaped undercuts and fine detail. In addition since the mould cavity in the injection mould parts is defined by metal it has to be shaped using erosion techniques such as spark erosion or pantography and consequently it is not possible to accurately recreate the fine detail of an original, artistically, created sculpture.

In addition it will be appreciated that because of the high cost of creating injection moulds it is not economically practical to produce a relatively small number of mouldings of one particular model and is similarly not economically practical to produce a large number of moulds for creating a wide variety of such models.

It is a general aim of the present invention to provide an injection moulding process which ls capable of producing articles of either a geometric or a nongeometric random shape, in particular replicas of sculptures such as miniatures or models of character figures.

According to the present invention, there is provided a process for producing injection moulded plastics mouldings, the process including the steps of (a) producing separable mating resilient mould inserts which abut along opposed abutment faces to define therebetween a mould cavity; (b) inserting the resilient mould inserts into opposed movable mould parts of a plastics injection moulding apparatus, the mould parts being movable into abutment for moving the mould inserts into abutment to define said mould cavity; (c) injecting molten plastics into said cavity to create a plastics moulding; and (d) moving the mould parts apart to separate the mould inserts to permit removal of the plastics moulding, (e) the separable mating mould inserts -being produced by: (i) placing an article of which subsequent plastics mouldings are to be produced between opposed bodies of a mouldable material which is curable to form a resilient material; (ii) pressing the opposed bodies of mouldable material together so as to press the article into both bodies of mouldable material so as to create an impression of the article in said mouldable material; (iii) curing said mouldable material so as to retain said impression and form separable bodies of resilient material; (iv) separating the bodies of resilient material and removing said article to thereby create said mould cavity.

Attachment means, such as a rigid backing plate or bolts, for subsequently attaching to the mould insert into the injection mould may be located within the cast mould parts so as to be attached to the resilient material during the curing process. It is envisaged that such attachment means may be secured to the resilient material after removal from the cast mould. It is also envisaged that the cast mould itself could be used as attachment means in which case the resilient material is retained within the cast mould part after the curing process.

Thereafter the resilient mould inserts can be fitted to an injection moulding machine and used to produce replicas of said article. Preferably said mouldable material is a composition based upon a synthetic rubber or a natural rubber and which on the application of heat is cured to define said resilient material. Preferably said mouldable material is reclaimed rubber originating from tyres. Curing of the rubber is achieved by maintaining the cast mould parts, whilst closed, at an elevated temperature for a sufficient length of time to enable vulcanisation of the rubber to occur.

In view of the resiliency of said resilient material and the resiliency of the plastics material used for creating the moulding, the moulded article may have acute undercuts which do not prevent the article being removed from the mould cavity.

In addition fine detail of the original article is recreated by the mould since the mould cavity is formed as an impression of the original article.

In view of the simplicity of the process according to the present invention it is possible to create a high quality of reproduction for geometric or non-geometric shapes very economically. In particular the present process thereby makes it economically practical to produce high definition moulds for producing small quantities of replicas of an original model and also to produce a wide variety of such moulds. Large volumes of such replicas can also be produced simply by replacing worn out moulds.

An example of a process of producing a mould according to the present invention is hereafter described with reference to the accompanying drawings, in which: Figure 1 is shown as a schematic side section view of a pair of mould parts shown in an open position; Figure 2 is a similar view to Figure 1 showing the mould parts closed so as to mate; Figure 3 is a schematic end view of a mould platen in an injection moulding machine; Figure 4 is a schematic perspective view of a pair of separated mould inserts according to one embodiment of the invention; Figure 5 is a schematic perspective view similar to Figure 4 showing a second embodiment of the invention; and Figure 6 is a schematic part cross-section through opposed mould platens illustrating mould inserts in situ In Figures 1 and 2 there is shown a pair of cast mould parts 12, 14 each of which comprises a relatively rigid body 16. Each part 12, 14 includes an abutment face 17 which are in mutual contact when the mould parts made and each abutment face 17 is provided with a recess 18 which is filled with a mouldable material 20 which is curable to form a resilient heat stable material. A rubber composition based upon rubber reclaimed from tyres has been found suitable for moulding thermoplastics such as polypropylene.

The outer surface of the mouldable material 20 is arranged to be substantially contiguous with the abutment face 17 and prior to creating an impression is preferably coated with a release agent such as talcum powder.

An original sculpture or article 30 from which replicas are to be made is positioned between the mould parts 12, 14 and these are then closed by, for instance, a press (not shown). Closing of the cast mould parts causes the article 30 to be pressed into the opposed faces of the mouldable materials and causes an impression to be made therein. Due to the flow characteristics of the mouldable material a natural separation line around the periphery of the article is achieved between the opposed surfaces of the mouldable materials.

The closed cast mould parts are now heated to cause the rubber compound to vulcanise and thereby form a heat stable resilient material. The mould . parts are now separated and the article is removed to leave a mould casting 50 (see Figures 3 and 4).

The bodies of resilient material 28 may now be removed from the cast mould parts 12,14, thereby defining a pair of mould inserts 100, in block form, formed from the resilient material. The pair of mould inserts 100 can be directly mounted into opposed mould platens 40 (only one of which is shown in Figure 3) of an injection moulding machine. Attachment means, for example bolts 101 Figure 4) or a rigid backing plate 110 (Figure 5) may be received to the resilient material prior to insertion of the mould insert into the mould platens 40. The backing plate 110 may include one or more bolts 111.

Advantageously the attachment means are located with the cast mould parts so that the attachment means become secured to the resilient material during the curing process.

It is envisaged that the backing plate 110 may be attached to the resilient material after casting, for instance by bonding.

Preferably, as schematically illustrated in Figure 6, the resilient mould inserts 100 when located within the opposed platens 40 are positioned such that the separation surfaces 120 of the mould inserts 100 are located slightly above the abutment faces 41 of the platens 40. Accordingly when the mould platens 40 close, the opposed surfaces 120 of the mould inserts initially contact one another and then are placed under a resilient loading caused by compression of the resilient material as the platens reach their fully closed position. The resilient loading on the opposed surfaces 120 resists flashing occuring during the injection moulding process.

As illustrated in Figure 2, it is envisaged that location formations, such as pegs 25, may be spaced about the mould cavity such that when the mould insert 100 are closed the location formations co-operate to resist relative slidable movement of the opposed surfaces 120.

As illustrated in Figure 3, it is envisaged that the cast mould parts can be used as attachment means. In this case the opposed surfaces 120 of the resilient material are substantially contiguous with the abutment faces 17 of the mould parts.

The body 16 of each mould part has an outer shape complementary to that of a location recess formed in a platen 40 of an injection moulding machine. Accordingly the body 16 can be accurately located in the platen 40 whilst being easily removable to facilitate replacement of the mould inserts.

The platen 40 shown in Figure 3 includes radially extending channels 41 along which plastics is injected.

Each body 16 has a channel 42 formed therein for communication with a respective channel 41. After curing and separation of the mould parts 12, 14 a channel (not shown) is cut into the resilient material for feeding plastics material from the channel 42 of the body to the mould cavity portion formed therein.

When casting the resilient inserts the size of each recess 18 of the cast mould parts and the amount of mouldable material contained therein is chosen to provide sufficient mouldable material to provide an adequate impression. Preferably in order to assist heat dissipation during the moulding process the amount of mouldable material is chosen to be a minimum to obtain the above criteria. In addition it is envisaged that the bodies 16 may be provided with ducts for coolant fluid which communicate with coolant ducts formed in the supporting platen 40.

In addition, the resilient material at least in the vicinity of the mould cavity is preferably arranged to be of a minimum thickness so as to resist distortion of the mould cavity arising from fluid pressure of the moulten plastics during the injection moulding process.

In order to assist heat dissipation the mouldable material may be adapted to improve its heat conductivity.

For instance the chemical composition of the mouldable material may be adjusted to maximise its heat conductivity and/or the mouldable material may include particles of a good heat conductor such as metallic particles dispersed therein. In addition, or as an alternative, coolant conduits, such as metallic pipes, carrying a coolant may extend into the recess 18 so as to be partly or wholly surrounded by the mouldable material.

Furthermore, blocks of suitable metals may be located within the mouldable material to act as heat sinks.

These blocks may be in direct contact with the body 16 to thereby provide a good path of heat conduction from the mouldable material and into the body 16.

Claims (7)

1. A process for producing injection moulded plastics mouldings, the process including the steps of (a) producing separable mating resilient mould inserts which abut along opposed abutment faces to define therebetween a mould cavity; (b) inserting the resilient mould inserts into opposed movable mould parts of a plastics injection moulding apparatus, the mould parts being movable into abutment for moving the mould inserts into abutment to define said mould cavity; (c) injecting molten plastics into said cavity to create a plastics moulding; and (d) moving the mould parts apart to separate the mould inserts to permit removal of the plastics moulding, (e) the separable mating mould inserts being produced by: (i) placing an article of which subsequent plastics mouldings are to be produced between opposed bodies of a mouldable material which is curable to form a resilient material; (ii) pressing the opposed bodies of mouldable material together so as to press the article into both bodies of mouldable material so as to create an impression of the article in said mouldable material; (iii) curing said mouldable material so as to retain said impression and form separable bodies of resilient material; (iv) separating the bodies of resilient material and removing said article to thereby create said mould cavity.

2. A process according to Claim 1, wherein each of the bodies of mouldable material is contained within a casting mould part, each of the casting mould parts comprising a relatively rigid body having an abutment face containing a recess, the process including filling the recess with said mouldable material until the outer surface of the mouldable material is substantially contiguous with the abutment face.

3. A process according to Claim 2, wherein after removal of the article said resilient material is removed from each of the casting mould parts to define said separable mould inserts.

4. A process according to Claim 3, wherein attachment means are attached to said resilient material for attaching the resilient material to said mould parts.

5. A process according to Claim 1, wherein the mould inserts are placed in said movable mould parts so that the mould inserts abut to form said cavity prior to the mould parts abutting one another.

6. A process according to Claim 1, wherein prior to placing the article between said bodies of mouldable material the surface of the mouldable material is coated with a release agent.

7. A process for producing injection moulded plastics mouldings substantially as herein described with reference to the accompanying drawings.