INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT

(51) International Patent Classification 4 (11) International Publication Number: WO 85/ 0 B29C 67/22, 45/32, 43/18 // B29K 105:04, 105:06 Al (43) International Publication Date: B29L 9.00 26 September 1985 (26.0

(21) International Application Number: PCT/GB85/00107 (74) Agent: T. K. CONNOR & CO.; 2 Chichester Chancery Lane, London WC2A 1EG (GB).

(22) International Filing Date: 19 March 1985 (19.03.85)

(81) Designated States: AT (European patent), AU, BE

(31) Priority Application Number : 8407309 ropean patent), CH (European patent), DE ( pean patent), FR (European patent), GB (Euro

(32) Priority Date: 21 March 1984 (21.03.84) patent), JP, LU (European patent), NL (Europea tent), SE (European patent), SU, US.

(33) Priority Country: GB

Published

(71) Applicant (for all designated States except US): H. R. With international search report.

SMITH (TECHNICAL DEVELOPMENTS) LIMIT¬ ED [GB/GB]; New Mill, Crawley Road, Witney, Ox¬ fordshire OX8 5TF (GB).

(72) Inventors; and

(75) Inventors/Applicants (for US only) : SINGH, Devendra, Pal [IN/GB]; ASHTON, John, William [GB/GB]; Street Court, Kingsland, Leominster, Herefordshire HR6 9QA (GB).

(54) Title: METHODS AND APPARATUS FOR MAKING ARTICLES FROM FOAMED THERMOPLASTICS TERIALS

(57) Abstract

Description is made of different methods of moulding foamable thermo¬ plastics materials. One method de¬ scribed provides that thermoplastics materials are foamed in a mould having a 'substitute' core which may comprise a number of members, interposed be¬ tween two parts of a mould. Thermo¬ plastics material is placed between the mould parts and the (or each) of the in¬ terposed substitute core member(s). In this way a plurality of foamed moulded articles are produced in a single mould¬ ing operation. The substitute core may comprise a thermoplastics material pre¬ form whereby a composite sandwich construction can be moulded. Another described process provides for the pro¬ duction of thermoplastics material arti¬ cles having an evenly distorted cell structure achieved by providing that the material to be foamed is heated whilst held der pressure. Upon release of the pressure rapid expansion o ^' f the foaming material takes place which is constrained by shape of the mould. As a result a cell structure is achieved in the foamed article in which the dimensions of the cell in or more directions differ from the dimensions in other directions. Two methods of achieving this are described. Desc tion is made of the moulding of thermoplastics materials making use of diaphragm pressure unit having an elastomeric aphragm sealingly stretched across a surface of a punch part of a mould. Making use of this unit to hold the preform m rial under pressure until the foaming temperature is reached - and then suddenly releasing the pressure on the diaphra enables a foamed material to be produced with the characteristics described.

FOR THE PURPOSES OFTNFORMAΗON ONLY

Codes used to identify States party to the PCT on the frontpages of amphlets publishing international appli¬ cations under the PCT.

AT Austria GA Gabon MR Mauritania

AU Australia GB United Kingdom MW Malawi

BB Barbados HU Hungary NL Netherlands

BE Belgium rr Italy NO Norway

BG Bulgaria P Japan RO Romania

BR Brazil KP Democratic People's Republic SD Sudan

CF Central African Republic ofKorea SE Sweden

CG Congo KR Republic o Korea SN Senegal

CH Switzerland LI Liechtenstein SU Soviet Union

CM Cameroon LK Sri Lanka TD Chad

DE Germany, Federal Republic of LU Luxembourg TG Togo

DK Denmark MC Monaco US United States of America

IT Finland MG Madagascar

FR France ML Mali

TITLE: METHODS AND APPARATUS FOR MAKING ARTICLES FROM FOAMED THERMOPLASTICS MATERIALS

DESCRIPTION

The invention relates to methods and apparatus for making articles from foamed thermoplastics materials,

We have disclosed elsewhere proposals for the production of foamed thermoplastics materials (for example polysulphone, polyethersulphone, polycarbonates and the like) in which the polymer material is steeped in a solvent or partial solvent therefor (e.g. acetone, ^' butanone or the like) to form a precursor which (after the possible addition of foaming ageiits, fillers and the like) is placed in a mould and heated to form a foamed thermoplastics article. Optionally the precursor material may be compressed in a mould (whilst possibly being heated) to form a pre-for . The pre-form may then be heated (e.g. in an air circulating oven or fluidised bed) in a mould to form the finished article. A number of different ways of making sandwich composite articles are known. In general tlese comprise the production of a foamed -core' material to which are fixed, for example by bonding, other thermoplastics layers - e.g. fibre resin reinforced sheets. The usual methods of producing such sandwich composite structures provide that the central core, and any layers to be atta ^' ched to it, are separately moulded and that all the items are brought together in a further mould in which they are joined together. An object of the invention is to provide a method of producing a moulded sandwich composite structure which reduces both the time and the cost of its formation compared with the known methods and which makes use of substantially fever moulding tools than has hitherto been possible.

Again, in one prior proposal (see U.K. Patent Specification 2 083 04.5) we describe a method of making

a foamed thermoplastics material the internal structure of which comprises an array of substantially regular, evenly sized cells. In certain circumstances it is desirable that the internal cell structure of a finished thermoplastics article, whilst being regular, comprise an arry of non-spherical cells - that is to say it is sometimes desirable to provide that the cells within a foamed thermoplastics article have one dimension greater than the other two dimensions; or two dimensions each longer than the third.

A further object of the invention is to provide a method of achieving this desired result.

One aspect of the invention .provides a moulding process for producing foamed thermoplastics materials in a mould having a first and a second part, including the steps of interposing a third part between the first and second parts, placing material to be foamed between the first and third parts, and between the third and second parts, and then moulding the material to produce a least two separate moulded articles.

Said third part may comprise a plurality of members and material to be moulded may be placed between adjacent ones of them whereby more than two articles are produced in a single moulding operation. The process may further include the steps of first producing two fibre reinforced thermoplastics layers, second removing said third part and using said mould without said third part to make a thermoplastics preform and third placing the thermoplastics -preform in the mould in place of said third part between said two fibre reinforced thermoplastics articles _r and initially moulding to produce a composite, sandwich, article.

A second aspect of the invention provides a moulding process for producing a foamed thermoplastics material article in which a precursor material or preform is placed in a mould and held under pressure whilst the mould is heated, the pressure being substantially

instantaneously released whereby a moulded foamed thermoplastics material article is produced the structure of the cells in which are distorted.

Another aspect of the invention provides a mould for use in the above processes," and having a first part being a cavity block, a second part being a core or punch part and a third part for interposition between said first and second parts, said third part being a substitute core part. The third part may comprise a plurality of members and may be configured to produce foamed articles of the same or different shapes.

A further aspect of the invention provides a pressure diaphragm unit for use in a moulding process and incorporating a body part having on one surface thereof an elastormeric material diaphragm; the body part including channels therethrough to a surface behind said diaphragm through which hydrostatic and/or pneumatic pressure may be applied to the diaphragm. The diaphragm may be held on said face of the body by a clamping plate configured to cooperate with a mating surface of a spacer or ejector ring forming part of a cavity block; the diaphragm unit and cavity block together comprising a mould. _: - r

We now propose a method for the production of shaped articles which not only overcomes the problems found with known processes but which also gives control of the orientation and structures of cells in the finished article.

In the processes we propose a precursor material is heated to a predetermined temperature for a predetermined time; during this time volatiles/gases liberated by heating (or by reaction) are produced and other reactions may take place (e.g. cross-linking, polymerisation or chain lengthening processes). These factors mainly determine the selected temperature to which, and the length of time for which the precursor material is heated. Other facters could include the vapour pressure of the liberated volatiles/gases and the glass transition temperature of the materials used. The precursor material may be subjected to direct pressure (the closing pressure) or left, within a predetermined space to be filled as the precursor material expands.

It is possible to add to the precursor material composite laminates or coating layers (skins) on either side thereof which may be coated with adhesive or made sticky with a solution or mixture of a solvent and/or polymeric material. Alternatively the skins may be separately manufactured and later applied as second stage process.

The temperatures applied to the sides of the precursor are significant. The temperatures applied may be the same on opposite sides or different.

In the latter case predetermined temperature differential is applied to the precursor so as to give desired properties to the finished product. Pressure may be applied instantaneously, slowly, or cyclically (i.e.

such that the pressure is applied, released, applied and so on for a predetermined number of cycles) until the final pressure is applied and maintained. Generally pressure is uniform but different pressure zones could be forged to give desired changes in properties-.

Pressure release is at a predetermined rate which results in the expansion of the product. The boundary of the confined space is opened to a predetermin size thus giving the precise shape and thickness required. The general technique is applicable in expansion processes involving unidirectional (or essentially unidirectional) or bidirectional cell dimension control. For the sake of convenience a method involving, 'essentially unidirectional' cell expansion will be briefly described.

In this method two main walls of the cavity to be foamed into (as described above) are formed from a shaped structure (e.g. a metal fabricated mould). One or both of the. ^' walls could be made of an elastomeric material, which again could be of the required shape or ^' of a shape which upon pressurization adopts the required shape. The precursoB material is placed in the mould and compression applied (e.g. by pneumatic or hydraulic means). The material is heated (e.g. by hot air, by the diaphragm itself including a conductive electrically heatable element, of, if hydraulic pressure is used, by heating the hydraulic fluid. If an elastomeri membrane is used it is important that it should be extensible to the required strain and have a very low "permanent set", it should also be hard enough to smooth out any uneveness in the precursor material.

These techniques may be extended to an extrusion method in which a die and extended die section are so arranged that the material is permitted to expand only in a preferred direction.

These techniques have applicability in the production of shaped foamed articles e.g. air ducting

with complicated shapes, radomes etc.

Embodiments of the invention will be describedr with reference to the accompanying drawings in which:- Figure 1 is a highly schematic sectional view of a first mould embodying the invention,

Figure 2 illustrates sections of elements moulded with the mould of Figure 1,

Figure 3 illustrates other elements which may be made with moulds similar to the mould of Figure 1. Figure 4- illustrates the use of the mould of

Figure 1 to make a composite sandwich structure,

Figure 5 illustrates a mould embodying the invention specifically for forming foamed cores having cells with predetermined structures, Figure 6 illustrates schematically a pressure diaphragm unit embodying the invention,

Figure 7 illustrates an alternative moulding technique using the unit of Figure 6,

Figure 8 illustrates a sectional view of a component embodying the invention,

Figure 9 illustrates an injection moulding tool for forming the component of Figure 8,

Figure 10 illustrates a blow moulding tool for forming a component such as shown in Figure 8, Figure 11 illustrates a unit such as shown in Figure 6, and

Figure 12 illustrates the moulding technique for use with the unit of Figure 11.

The known moulding techniques can be considered to be categorised in three main groups determined by their components, i.e. moulds, for .producing:-

(i) Flat laminates of constant thickness, (ii) Shaped, solid laminates of constant or varying thickness. (iii) Cored laminates i-r±th constant or varying wall thickness. The first of these uses moulds which are the

simplest form of compression mould and are inexpensive to make and maintain. Generally the moulds comprise two stainless steel facing sheets and a chase or keep ring. In use the sheets or other materials to be moulded are laid-up on one facing sheet, within the chase ring, and the other facing sheet then placed on top of it. The completed mould is placed within the platens of a press and moulding takes place. The chase ring is thinner than the required final laminate thickness and any excess moulding material 'flashes out' via the gap between the chase ring and the facing sheet. This very simple form of moulding relies on the press platens being accurately flat and parallel. The second category uses semi positive or "Positive Flash" moulds. In these moulds the material is placed in a mould cavity in one half of the mould. A punch forming the other mould half is pushed into the mould cavity to effect moulding. The opposing faces • of the mould cavity may be similarly (to produce constant thickness articles) or differently shaped (to produce articles, the sections of which are of varying thickness). The mould cavity may take a considerable bulk of moulding material, prior to compression, and is therefore suitable for the high bulk-factor of "Film Stacking" techniques in which a plurality of films (layers) of thermoplastics material are laid one on another (or between layers of fibre impregnated with the polymer material) in the mould. The mould is closed, leaving a flash gap between the mould halves arid any excess moulding material flashes via this gap into a "pinch-flash" land area. Small moulds are heated by heated press platens and cooling is achieved naturally or by a cooling medium (e.g. cooling channels in the press platens). Larger moulds may have in-built heating and cooling systems.

The third category includes cored semi positive or "cored-positive flash" moulds. These moulds apart from the extra tooling required for the core, are

similar to the semi-positive moulds described above.

Various types of press may be used to compress the moulds e.g. downstroke, upstroke, angle and transfer presses. The presses may be hydraulically or penumatically actuated - such presses are generally well known and will not be further described.

The following description is of methods and processes making use of moulds not known from the prior art, as described above.

Figure 1 illustrates a mould embodying the ^" invention which will hereinafter be referred to as a Substitute Core Mould. This type of mould, could also be termed a

"Multi- Plate" compression mould (but should not be confused with a multi-impression mould, a term used in injection mould design) .

- The mould shown in Figure 1 comprises a mould cavity block or first mould part 10 the surface of which is configured to shape the outer surface of a first component 12 to be moulded; a second mould part or core 14 the surface of which is configured to shape the inner surface of a second component 16 to be moulded, and a substitute core 18 which lies between the elements 12 and 16 and is configured to shape the inner surface of

the outer element 12 and the outer surface of the inner element 16 respectively. Mould part 10 is mounted on a cavity block backplate 20 located on a fixed press plate 22. The core" block 14. supports a top sheet 24. which in turn supports a core backplate bolster 26 on which the movable press platen 28 bears.

The edges of the cavity block 10, substitute core 18 and core block 14 are shaped as shown to provde •'Core Puller Hooks" 30 and 32 for the elements 12 and 16 respectively. A packing ring 34 is provided as shown. Other parts of the mould (ejector pins and striker plates, cooling and heating means and locks etc.) which are of known form are not shown' for sake-of clarity. To form the two elements 12 and U - in a single moulding process - the material forming the element 12 is laid on the surface of the cavity block. 10. The substitute core 18 is placed (and perhaps locked) in position. The material for forming the element 16 is laid up on the exposed surface of the substitute core 18 to the desired thickness. The core block 14 is then placed (and perhaps locked) in position. The mould i's completed by positioning the top sheet 24 ^anα ^ the moving platen brought down to bear on the mould. The mould is heated (by the internal heaters - not shown) and pressure maintained by the press. After sufficient time the heaters are turned off and the mould is cooled (or allowed to cool) whilst the preesure is maintained. After cooling the platen 28 is lifted away, and the mould is opened. The elements 12 and 16 are then removed (with the substitute core 18) e.g. with the use of the ejector pins (not shown) .

The core block 14 may be permanently clamped to the top movable press platen if desired. The cavity block and ejection system may be permanently clamped to the fixed table of the press, or positioned against stops and clamped if it is necessary to withdraw the

cavity block (e.g. for ease of layin -up) . — ^•

Figure 2 illustrates the elements made in the _^ mould of Fig ^' ure 1. • ^"

It will be appreciated that the mould shown in Figure 1 has enabled the simultaneous production of two elements (elements 12 and 16) with a substantial reduction in process time and tooling compared with known moulding Techniques. It will be appreciated that by variation of the configured surfaces of the cavity block 1 , the core block 14 and the substitute core

18 other shapes of elements may be made e.g. as shown at Figures 3A and 3B. It will also be appreciated that it is possible to provide a plurality of substitute cores and thereby produce more than txro elements at a time such as are illustrated at Figures 3C and 3D.

The elements made by a mould similar to that of Figure 1 may be combined with a preform to produce a foamed core 'A' sandwich composite structure.

In such a technique the preform for the core * ^* n y be made in the mould shown in Figure 1, (or in a mould such as described below if particular characteristics . for the final product are required ,

Thereafter the same mould be used to form the final sandwich composite structure. To use the mould of Figure 1 to produce the preform certain modifications need to be made to it, viz: the ejector system is altered. The process is then as follows:-

A crepe preform is placed in the cavity block 10, and the core block 14. is located. The temperature is allowed to rise. The movable platen is released and allowed to retract by an amount necessary to produce an evenly thick foam core. Tlereafter the mould temperature is further raised to the foaming temperature at which it is retained for a period sufficient for foaming to be completed.

Then the mould is allowed to cool (or cooled) after which the press platen 28 is lifted and the mould opened to allow the foamed core to be 'extracted.

Figure '4 illustrates the use of the mould shown in Figure 1 to produce a composite sandwich construction making use of a core produced in the above way. It will be noted that, the substitute core 18 has been replaced by the foamed core 40. The method of formation of the composite is substantially as already described with reference to Figure 1. The outer shin moulding 12 is positioned. The foamed core 40 is placed in the mould. The inner shin moulding 16 is positioned and the core block 14 positioned. The moulding process then continues as already described to produce the composite structure.

An alternative method of forming the foamed core will be described with reference to Figure 5 which enables the production of foams have desired cell characteristics. This type of mould is for the specific production of foam when the dimensional increase in length, breadth and thickness from the crepe pre-form to the completed foam is appropriate.

The mould shown in Figure 5 includes a chase 60 located between facing sheets 62 and 64. The facing sheet 62 is attached to a fixed platen 66 of a press and the sheet 64 bears, in use, the moving platen 68 of the press. Loose pressure plates 70 are provided as shown. A lock plate 72 couples the facing sheet 64 to an extension 74 of the chase as shown. Ejector pins 76 and spring stops 78 are provided as shown.. Ejector plates 80 and gear 82 are operable to lift the loose plate 70 and formed core out of the mould when the moulding process is finished. n operation a crepe preform is placed in the chase 60 between facing sheets 62 and 64 and the press closed. The platens 66 and 68 are heated and pressure is

applied for a minimum of five minutes. The press is rapidly opened and the crepe rapidly expands beyond the chase ring 60. The inner wall of ring 60 is machined at an angle of (e.g. 25°) to minimise surface resistance against the expanding foam.

The moulding technique restrains the expanded foam (to obtain dimensional stability and consistency) whilst offering minimal surface resistance to the foams movement during its expansion. An alternative method of forming a composite article, using a single semi-positive compression mould, makes use of a method we term isostatic moulding.

The method makes use of a pneumatic/hydrostatic press an as ancillary pressure diaphragm unit. With this method the skin 12 and 16 are produced as noted above. The core block 14 of the mould of Figure 1 is then replaced with a unit such as shown schematically at Figure 6. The unit of Figure 6 comprises a_body 100 through which extend conduits 102 feeding hydraulic fluid to a volume 104 behind a diaphragm 106. A second, sealing, • elastomeric diaphragm 108 is provided as shown. The conduits 102 are coupled to a reservoir 110 of hydraulic fluid by a network including a pump 112, pressure guages 114, accumulator 116, shut off valve 118, non return valve 120, pressure regulator 122 and pressure relief valve 124 as shown.

The skin elements 12 and 16 are repositioned in the cavity block 10 and the pressure diaphragm unit lowered into a loading position (see Figure 7). Then syntactic powder is poured through loading ports 138 in a loading ring 140, until the cavity 130 is full. Slight pressure

is applied by operation of pump 112, to extend the diaphragm and give a semi-rigid core. Pressure covers 142 are fitted and the press closed to the mould position. Full pressure is applied to the "pressure diaphragm" to compress the syntactic powder to form a sintering wthin the outer skin 12.

The pressure diaphragm unit is then depressurised and the press opened. To make a composite structure in this way the inner and outer skins of the component are produced as described with reference to Figure 1 and then treated as follows:-

The inner surface of skin element 12 and the outer surface of skin element 16 are coated with a concentrated solution of acetone and polymer material. This gives a polymer enriched contact surfaces for good adhesion between skins and foam core. The moulded core puller hooks 30 and 32 are removed from the elements 12 and 16.

The elements 12 and 16 are replaced in the mould with the substitute core 18. The mould is closed and heated to approximately 15.0°C to dry off the acetone from the coatings. When the skins have dried the mould is ^" opened and the elements removed from it.

The pressure diaphragm unit is replaced by the core block 14. ^" The loading ring 140 is removed and the skin elements 12 and 16 are positioned in the mould.

The mould is partially closed and heated to 325°C - 350°C. Thereafter the mould is fully closed and cooled.

It will be appreciated that the techniques described above permit the manufacture of complex struct¬ ures e.g. components such as shown in Figure 8 that have the properties of a specific thermoplastics material

their outer surface and the mechanical advantages of a laminated or foamed central core.

The component shown in Figure 8 has a core l60 of expanded foam or compression moulded laminate formed in accordance with any of the methods noted above, and a outer skin 162 formed by injection moulding.

The production of this component is achieved as follows, first the core is produced by the techniques noted above. The core 160 is then located as an insert ⁰ in an injection mould and the component completed by an injection moulding process.

Figure 9 shows the core 160 within the parts 164, 165 and 166 of an injection mould to which the injected polymer material is introduced via an injector 5 168.

Figure 10 shows how a complex shape may be made by blow moulding the core 160 from a foam extruder 170 into an extruded outer skin 172 within an extrusion moulding tool 174- _* Figure 10 shows the tool both 0 before (A) and after (B) the foam core 160 has been blown.

This particular technique combines the use of a "Blow Moulding machine" and a "Foam Extrusion machine". First the outer surface 172 of the component is extruded into the open 'halves' of the mould. The 'halves' of 5 the mould are then closed, (pinching the extruded skin) onto the nozzle of the F.oa extruder 170 which is then operated. The expanding foam from extruder 170 forces the skin 172 against the mould surfaces, creating the desired foam filled component. ^* *- ¹ The research we have carried out has indicated to us that for the most effective moulding of thermoplasti material foamed articles it is necessary for the initial crepe to be compressed evenly during its heating/foaming. For this reason .we consider that the isostatic compression moulding we now propose, making use of the pneumatic/hydrostatic diaphragm unit described above, is to be preferred, system making use of this

technique will be described with reference to_ Figures 11 and 12.

Figure 11 illustrates, diagrammatically, a hydrostatic compression mould system including a cavity block 200 the surface of which is configured to the give the desired shape of the article to be made. The cavity block receives a diaphragm pressure unit 202 to which hydraulic fluid is fed via a line 204. The unit 202 incorporates an hydraulic oil heating chamber 206 through which electrical heating- elements 208 extend. Chamber 206 is connected to the rear of the diaphragm 210 by a line 212. The diaphragm 210 is clamped onto the body of the unit 202 by an annular plate 214. A spacer/ejector ring 216 is provided ■ on the cavity block 200, as shown, on which the plate ' . 214 rests in use. The ring 216 incorporates a vent tap 218. Channels 220 for passage of heating and/or cooling fluid are provided for passage of heating and/or cooling fluid are provided in both the block 201 and the body of the unit 202.

Line 204 is coupled via a non-return valve 222 and pump 224 to a reservoir of hydraulic fluid 226. A branch line 228 leads, via a shut off valve 230 and a dump/bleed valve 232 to a cooling reservoir 234 in which heated hydraulic fluid is allowed to cool before being passed (via a shut off valve 236) to the reservoir 222. A relief valve 238 . bypasses, the dump/blee valve 232 as shown. Pressure guages 240 are provided as shown. We propose that relatively small moulds be free standing (having suitable locking mechanisms) and that larger moulds be located in a suitable press for moulding to take place. ■

Moulding using the system of Figures 11 and 12 is ^■ carried out as follows:-

The unit 202 is heated to the full processing temperature (e.g. 200°C) for sufficient time to ensure

that the diaphragm 210 and hydraulic fluid are at the processing temperature. The cavity block 200 is heated to approximately 130°C. A crepe preform (made in any suitable way) is placed in the cavity block and the mould is closed and clamped.

Hydraulic pressure at 200 psi (1.38MN/ _m ² and the complete mould heated to 200°C with the vent tap 218 open.

When the mould has reached the processing temperature the vent tap 218 is closed and the hydraulic pressure is released. The foam expands pushing the diaphragm 210 back against the body of the unit 202 as shown in Figure 12. Then the mould is cooled (using the cooling channels 220) to approximately 100°C after which the mould is opened allowed the foamed component 242 " to be ejected and removed from the ejector ring.

It will be _. seen that this particular form . of mould lends itself to the techniques described above for obtaining foams having regularly distorted cell structures.

Although described as hydraulically operated - it will be appreciated that the diaphragm unit could be penumatically operated.

It will be noted that the hydraulic/pneumatic pressure diaphragm unit makes it po ^" ssible for 'normal ¹ moulding ^' techniques to be used. Further it will be appreciated that it is possible, by heating the unit to one temperature and the cavity block to another to vary the characteristics of the foamed materials by moulding them with a temperature differential across their sections.

In producing good quality moulded products it is necessary for the production moulds to be made of good quality, e.g. mould grade tool steels which give dimensional stability and reliability at the processing temperatures required. The mould surfaces are desirably smooth, e.g. chrome plated. The mould cavities need ^' to

be machined to the correct dimensions to achieve good quality moulding.

Before any mouldings are made a suitable (e.g. P.T.F.E.) release agent is desirably baked onto the surfaces of the mould tool to create a permanent release film. To achieve this the release agent is sprayed onto all mould surfaces and the mould tool heated appropriatel at this temperature for three hours.

During the moulding operations the wiping action of the materials in the mould ^' may remove a small percentage of the release film and to maintain a good release properties a fresh coating of release agent may be applied to the mould surfaces between moulding operations.