GB2126149A

GB2126149A - Moulds for injection moulding

Info

Publication number: GB2126149A
Application number: GB08308238A
Authority: GB
Inventors: Stephen John Pentlow; Robert Paul Ziff
Original assignee: STYLO MATCHMAKERS INT
Current assignee: STYLO MATCHMAKERS INT
Priority date: 1982-08-25
Filing date: 1983-03-25
Publication date: 1984-03-21
Also published as: GB8308238D0

Abstract

A method for forming an injection moulded article such as a shoe comprises forming, by coating, a mould around a master of the article using a settable moulding compound. The master mould thus formed is secured in a mould casing of injection equipment and a moulding material, such as PVC, is injected into the mould to reproduce the article. The master mould may be formed directly in the mould casing to which it may be bonded by an adhesive, or may be formed in a mould box and subsequently transferred to the mould casing to be secured therein.

Description

SPECIFICATION Injection moulding apparatus and a method of forming an article in said apparatus This invention relates to a method of forming an article by means of an injection moulding process and also relates to injection moulding apparatus for carrying out the method.

It is already known to form articles, such as shoes, from plastics materials by means of an injection moulding process in which the plastics material, typically PVC or polyurethane, is injected into a mould formed by a metal die. The shape of the article is, of course, defined by the internal contours of the die. It is sometimes desired to give the surface of the article a particular appearance or to have particular patterns impressed therein and this is achieved by providing an appropriate surface finish to the die by cutting the pattern into the surface of the die. Typically, this is carried out by electroetching the surface of the die or by machine or hand-working the die. However, the known methods, which all involve the "working" of the metal of the die do not enable a very high definition for a pattern on the finished article to be achieved.As a result, attempts to reproduce surfaces having fine detail, such as crocodile skin, have not been successful, the resultant finishes being relatively crude and unattractive.

The present invention seeks to provide a new method of forming a mould which overcomes the limitations of the known methods.

According to one aspect of the invention there is provided a method of forming an article by means of an injection moulding process, including the steps of placing a master of said article in a mould casing with a mould space between said article and the casing, filling the mould space with a settable moulding compound to form a mould of said master, which is supported by the casing, removing the master, closing the casing and injecting a moulding material into the mould to reproduce said article.

According to another aspect of the present invention there is provided a method of forming an article by means of an injection moulding process, including the steps of, placing a master of said article in a mould box, filling the mould with a settable moulding compound to form a mould of said master, inserting the mould of said master in a mould casing which is such as to support the mould, closing the casing and injecting a moulding material into the mould to reproduce said article.

The moulding compound is preferably a pourable high definition moulding compound such as silicon rubber. Alternatively, it is possible to use as the moulding compound a polyurethane synthetic plastics material which may be injected into the mould box.

The present invention also provides injection moulding apparatus for carrying out the method including a mould casing, a mould of a master formed of a settable moulding compound and adapted to be positioned in the casing for support by the casing, means to close the casing, and means to inject moulding material into the mould to reproduce the master.

Preferably, location means are provided to locate and secure the mould in the mould casing.

In one form, the location means may comprise a plurality of recesses in the mould casing in which projections on the mould are located. The projections in the mould may have embedded therein screw-threaded nuts which are engageable by screws or bolts in the casing. In an alternative form, the location means is provided by a bonding agent applied to the interior of the mould casing whereby said mould of the master is bonded to the mould casing.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying informal drawing in which:~ Figure 1 shows a sectional end view of a mould casing and, Figure 2 shows a sectional plan view of Figure 1,and Figure 3 shows a sectional plan view of a further embodiment of the invention.

Figure 1 shows a schematic end view of a mould for an injection moulding process having a rigid mould casing 1 which may be formed of metal, epoxy resin or other suitable material and which is split into two parts 2 and 3 along a central plane 4. Within the mould casing there is located a mould 5 which is formed, by a method to be described hereinafter, of a setting moulding compound such as silicon rubber. After being formed, the mould 5 is cut along a dividing line so that when it is placed in the mould casing the dividing line is aligned with the central plane 4.

The interior of the mould 5 defines the shape of the article to be moulded. The mould 5 is formed to be a snug fit in the casing so that its exterior surface is supported throughout by the mould casing 1.

to ensure that the mould 5 is accurately located in the casing 1 and secured in position in the embodiment shown in Figures 1 and 2 it is provided on its exterior surface with projections 6 which locate in associated recesses 7 in the casing 1. A screw threaded nut (not shown) is embedded in some or all of the projections during the formation of the mould 5 and the mould casing 1 is provided with screw threaded studs or bolts 8 which engage the nuts to secure the mould 5 rigidly and immovably to the casing 1.

As shown more clearly in Figure 2, this embodiment is intended particularly for the formation of an injection moulded shoe and therefore the interior of the mould 5 is adapted to receive a last 9 so that the shoe is defined by the space between the last 9 and mould 5. The last 9 is secured in known fashion to the casing 1.

After the mould 5 has been secured in the two casing parts 2 and 3 and the last 9 has been secured in position, the mould casing 1 is closed by an end plate 10 which, in conjunction with the last 9, defines and closes the interior of the mould 5, the outline of the sole of the shoe being formed in the end plate 10. The sole of the shoe can be defined by an additional part of the mould 5 which would be secured to the end plate 10, but in practice, the high definition provided by the mould 5 is not usually required for a sole. The shoe is then formed in a conventional way by means of an injection moulding process in which a moulding material such as PVC or polyurethane is injected by means of ram injection or extruder injection into the mould through a passage (not shown) in the end plate 10. The injection moulding process follows conventional techniques.

The mould 5 is formed by placing a master of the article to be formed, a shoe in the present example, in a mould box, the interior profile of which corresponds to the interior profile of the metal casing 1. The present invention is particularly applicable to the formation of shoes having a fine pattern or ornamentation such as is found on crocodile skin or lizard skin. The shoe of the natural article which is to be reproduced is supported on a last in the mould box which is then filled with a high definition settable moulding compound to form a master mould of the shoe.

The moulding compound is preferably silicon rubber, which is pourable into the mould box to provide, in effect, a gravity moulding. When the moulding compound is set, it can be removed from the mould box and thus forms a mould of the master, for insertion in the mould casing for reproduction of the shoe. The use of silicon rubber is advantageous because the mould then has a degree of flexibility which assists in its removal from the mould box and the shoe master, but it is not essential that it be flexible.

Figure 3 shows a modified embodiment in a view corresponding to that of the embodiment illustrated in Figure 2. In this embodiment the mould 5 is formed directly in the mould casing 1.

The master of the shoe is supported in the casing 1 on a suitable last and the silicon rubber moulding compound is poured into the space between the mould casing and the shoe where it sets to form the mould 5. The shoe and last are then removed. Since the mould is formed of a resilient material it has a tendency to be compressed when subjected to pressure. The thickness of the mould 5 is therefore controlled within predetermined limits throughout its profile to reduce the tendency to compress when the final moulding compound is injected into the mould.

The interior surface of the mould 1 is suitably prepared to receive the moulding compound and this may be by treating the surface so that the mould bonds directly thereto or by coating it with a bonding agent prior to pouring the moulding compound.

It will be appreciated that the mould would be formed in a separate casing for subsequent bonding to the mould casing.

After being removed from the mould box, the mould is cut, with a very fine cutting edge which may be a blade or a laser along its centre line so that one half can be secured in the casing part 1 of the mould casing and the other part in the other part 3 of the mould casing 1. Because of the resilience of the moulding compound the two parts of the mould 5 mate accurately in the mould casing so that the normal flash line which is obtained in the moulding process is virtually eliminated.

In an alternative form, the moulding compound is polyurethane but this material, although it may be advantageous in certain instances, requires a high pressure moulding technique and it is therefore inherently more expensive than the use of a pourable moulding compound such as silicon rubber.

It has been found that the use of a mould "insert" formed of such a high definition moulding compound enables the finest detail on the master to be moulded into the reproduced article so that it is possible to reproduce by an injection moulding process a realistic reproduction of such finishes as crocodile skin.

Although described with reference to the injection moulding of shoes, it will be appreciated that the present invention is readily applicable to the injection moulding of other articles where a high definition is required for a pattern or profile on the finished article. It will also, be understood that many variations may be made within the general inventive concept of providing a mould "insert" formed from a "master" by means of a moulding compound.

Claims

1. A method of forming an article by means of an injection moulding process, including the steps of placing a master of said article in a mould casing with a mould space between said article and the casing, filling the mould space with a settable moulding compound to form a mould of said master, which is supported by the casing, removing the master, closing the casing and injecting a moulding material into the mould to reproduce said article.

2. A method of forming an article by means of an injection moulding process, including the steps of placing a master of said article in a mould box, putting a settable moulding compound in the mould box to form a mould of said master, inserting the mould of said master in a mould casing which is such as to support the mould, closing the casing and injecting a moulding material into the mould to reproduce said article.

3. A method as claimed in Claim 2, wherein said settable moulding compound is a pourable high definition moulding compound.

4. A method as claimed in Claim 3, wherein the compound is a silicon rubber.

5. A method as claimed in Claim 2, wherein said mould of the master has screw-threaded means moulded therein by which the mould can be secured to the mould casing.

6. A method as claimed in Claim 1, 2 or 3, wherein a bonding agent is applied to the interior of said mould casing prior to said moulding compound being poured.

7. Apparatus for carrying out the method as claimed in Claim 1 including a mould casing, a mould of a master, the mould being formed of a settable moulding compound and being adapted to be positioned in the casing for support by the casing means to close the casing and means to inject moulding material into the mould to reproduce the article.

8. Apparatus as claimed in Claim 6, wherein a bonding agent is applied to the interior of said casing whereby said mould of the master is bonded to said mould casing.

9. Apparatus as claimed in Claim 5, wherein the mould casing has a plurality of recesses adapted to receive projections on the mould to locate the mould in the casing.

10. Apparatus as claimed in Claim 6, wherein the projections have screw-threaded nuts embedded therein which are engageable by screws or bolts passing through the casing.

1 1. A method of forming an article by means of an injection moulding process, substantially as described herein with reference to and as illustrated in any one or more of the accompanying drawings.

12. Apparatus for forming an article having a high definition surface finish by means of an injection moulding process.