WO1992005216A1

WO1992005216A1 - Modelling materials and processes

Info

Publication number: WO1992005216A1
Application number: PCT/GB1991/001600
Authority: WO
Inventors: Roger Mark Sloman
Original assignee: Advanced Composites Group Ltd.
Priority date: 1990-09-21
Filing date: 1991-09-18
Publication date: 1992-04-02
Also published as: AU8535591A; GB9020649D0

Abstract

A modelling material for use in the manufacture of models for pattern making, tooling and the like. The material comprises a resin matrix and filler material in the matrix, the resin matrix comprising a thermoplastic material such as polysulphate, polyethersulphure, phenoxy, polyarylether before, polyethylene, polypropylene, polystyrene, polysulphate, poletherimide, polyethosulphate, or polphenyl sulphide, or a combination thereof.

Description

Modelling Materials and Processes

This invention relates to synthetic modelling blocks, and materials for forming them.

In a wide variety of industries such as engineering, aerospace, automotive, construction master models are produced for design, development or production purposes. The model may be manufactured from metal, graphite, wood, plaster, synthetic resins, plastics and other suitable materials.

Wood is still often used due to low cost, ease of working and availability, but it suffers from inherent drawbacks, and is gradually being replaced by other, synthetic products.

Wood can swell or shrink due to temperature and humidity variations and can be too soft for processes where pressure is used such as in an autoclave. In addition, the grained nature of the material can lead to differential expansion and shrinkage, leading to warpage.

Since the primary function of a model (or pattern) is to replicate as exactly as possible the size and shape of the final component or structure required, it is obviously inherently unsatisfactory if the material used is significantly affected by normal environmental changes in such a way that distortion of the shape occurs.

In an effort to minimise these problems, various types of 'synthetic wood' modelling blocks have been developed. These would typically be based on thermosetting resins such as epoxide or polyurethane resins filled with various materials.

Whilst these materials are homogeneous with reduced thermal expansion coefficients, and higher temperature performance capability, and are as suitable as wood for being shaped and smoothed, production of the material can be problematic, and the final characteristics of the material are still not ideal for some applications. The production process involves the use of matrix resins which rely on a heat-producing chemical reaction to harden, i.e. they are thermosetting resins.

In large masses, this may produce an uncontrolled rise in temperature, commonly referred to as an exotherm, leading to cracks and voids, and in extreme cases, complete disruption of the material.

This factor also limits the size of the block that may be produced, especially in terms of the thickness of material, due to the poor heat transfer characteristics of these materials.

There is also a time constraint on production methods, as large masses must be allowed to harden slowly under temperature controlled conditions. Increasing temperature to accelerate hardening will exacerbate the exotherm problem.

The maximum temperature at which standard synthetic modelling block materials, currently available, may be utilised is about 120°C, and may have a limit of only 60 - 100°C, depending on the use conditions and requirements.

Above these temperatures, the material softens and distorts. In order to achieve this maximum operating temperature, the material must be 'post cured' , that is, subjected to elevated temperatures for a period of time. This process is time and energy consumptive, further increasing production costs.

A typical production process would be to mix the various components under vacuum and cast into metal moulds. After low temperature hardening the cast blocks would be de oulded and post cured. The blocks would then be machined to size if necessary. An alternative process is to extrude the material as a thick paste or dough, cut the extrudate into blocks, and subsequently apply heat to cure the material.

After the process for manufacturing the current standard modelling materials has been optimised, the product is still less than satisfactory in several ways.

Moisture absorption is still significant, causing the resin to swell, and potentially distort the model.

Those materials capable of sustaining high end-use temperatures of 100 - 120°C tend to be highly filled (contain a high proportion of filler), which results in their machinability being inferior to other versions, with high rates of cutter wear, and dust creation being significant problems.

In addition, the thermal expansion coefficient is higher than desired for many applications, usually being in the range 25 to 40 x 10^" per ^DC, and often rising rapidly above 60 - 100^DC as the glass transition temperatures of the resins used are exceeded.

One application for the material, which is of increasing importance, is the manufacture of models (or patterns) to be used for the production of moulds and tooling made from advanced composite materials such as carbon fibre prepregs.

The term 'advanced composite materials' (or "advanced composites") covers a wide range of materials used in many applications where high levels of structural performance are required, usually combined with high, consistent quality, such as aerospace, defence, sports, medical and industrial equipment. In most cases the desired properties are high specific strength, and/or high specific modulus, although other secondary properties such as corrosion resistance, fatigue resistance, impact energy absorption, or X-Ray transparency can lead to the selection of advanced composites. The materials consist of a combination of strong and/or stiff reinforcing fibres in a matrix of some kind, usually a thermosetting resin or thermoplastic polymer, although metal and ceramic matrices are also used.

The term 'prepreg' has different meanings in different countries and in different market sectors, and there is not yet any agreed standard definition. However, in the aerospace industry the meaning is almost universally clear, to those skilled in the art, the term referring to a combination of resin and fibres processed to give an easily handled, storeable sheet material containing all the ingredients necessary for complete cure which can then be laid into a mould and formed to a required shape, normally by a combination of heat and pressure. The term is derived from an abbreviation of the word ' preimpregnation' , this term describing the process by which the resin matrix is impregnated into the fibrous material some time before the resulting sheet material is laid into the mould and further processed to produce the required shape. In practice preimpregnation is usually carried out by specialist materials production companies, who then supply the prepreg to end users such as aircraft component producers. The long time delay between material production and use to make components usually demands that the prepreg has to be relatively stable during transport, storage and handling at ambient temperature. To have adequate stability at ambient temperature, the prepreg resin is formulated so that it does not cure significantly until relatively high temperatures, usually in the 120° - 180°C range.

Prepregs using thermosetting resin matrices are usually formulated and processed to give a resin viscosity which results in the material having a level of tack and drape which gives the most convenient handleability during layup in the mould. This is known as the B-stage. Usually this means sufficient tack and drape to allow the prepreg to form to the shape of the mould, and then stick to the mould surface. Ideally, resin should not transfer to the fingers when the prepreg is touched. For the purpose of this specification the term 'prepreg' refers primarily to the latter type, i.e. a thermosetting matrix prepreg with tack, drape and cure properties appropriate to the use of the material for the production of advanced composite moulds or tools from the synthetic modelling materials described in this specification. Prepregs made using high temperature thermoplastic polymer matrices, metal, or ceramic matrices would not be suitable for use with these materials.

U.K. Patent No. 2108038 identifies the state of the art in 1981, and the invention of a new technique using prepreg materials curing at low temperatures, rather than using the then conventional wet-layup technique or normal (above 100°C) temperature cure prepregs to produce an advanced composite mould or tool.

The advantages and disadvantages of these two then- conventional techniques are identified in U.K. patent spec. no. 2108038 and these still hold true today. In practice, both the old techniques and the new low temperature curing prepreg route are all used today, each case being treated on its merits.

Therefore, there is still a need for pattern materials which will withstand the level of elevated temperatures required for both low temperature cure, or normal temperature cure prepregs, i.e. from ambient up to 120°C, with even higher temperature capability being preferred in some cases, such as where high temperature resins, e.g. bismaleimides (BMI's), polyimides, or others are used.

As the desired cure temperature of the modelling block increases, so does the importance of its expansion coefficient, especially when accuracy is an important criterion.

Therefore a desired combination of properties for a modelling block is as follows:-

1. Sufficient elevated temperature capability to withstand the processing conditions imposed without softening or distortion.

2. Zero or low water absorption. 3. Good machinability, with low tool wear and dust, and adequate strength to resist chipping at sharp corners etc.

4. Low cost.

5. Resistant to creep or other long term distortion effects.

6. Low density, preferably well below 1.0 g/cc (for easy handleability of large blocks and patterns).

7. Produceable in thicker sections than currently standard materials, to reduce the amount of bonding necessary to build up a pattern.

8. A low thermal expansion coefficient, (not necessarily zero, as some expansion may be desirable to counterbalance other effects such as shrinkage of the materials used to make advanced composite moulds). Ideally, the thermal expansion coefficient of the modelling block is matched with the prepreg material and the processing conditions used during manufacture of an advanced composite tool or mould. 9. Capable of a more rapid production technique than conventional materials.

10. Chemically compatible with the prepreg materials used in making advanced composite moulds (some current model materials can interact with the curing agents used in low temperature curing prepregs) .

11. Low Health and Safety risks by using non-harmful ingredients as far as is possible.

12. Preferably mouldable to specific shapes using simple moulds and equipment readily available to most potential end-users. The ability to mould the material to shape reduces the time spent and technical problems involved in bonding standard blocks together to form a rough shape before machining it to final shape. Wastage is also reduced by this method.

Whilst the achievement of all these properties in one material is desirable, a number of different variants with individual properties optimised would still be a substantial advance on the current state of the art.

The invention relates to materials and methods of producing materials that can be utilised to manufacture master models, with some or all of the improved properties identified above.

The invention provides a modelling material for use in the manufacture of models for pattern making, tooling and the like, the material comprising a resin matrix and filler material in the matrix, wherein the resin matrix comprises a thermoplastic material.

The thermoplastic material may be polysulphone, polyethersulphone, phenoxy, polyaryletherketone, or another thermoplastic material, or combinations thereof. The filler may be glass spheres, hollow glass spheres, phenolic microballoons, cenospheres, perlite, or any lightweight or mineral material, either natural or synthetic, or combinations thereof.

The proportion of thermoplastic resin may be between 5 and 95 percent by weight. The proportion of filler may be between 5 and 95 percent by weight.

The method of production may be to predisperse or predissolve the themoplastic in solvent, from an intimate mix of thermoplastic and solvent, and subsequently to remove solvent by the application of heat and/or pressure and/or vacuum. Alternatively, the method of production may comprise forming an intimate mixture of solid thermoplastic and particulate filler and producing a solid block by the application of heat and/or pressure and/or vacuum. Alternatively, the method of production may comprise coating particulate filler with thermoplastic by electrostatic, solution coating or mechanical means, following which the coated material is consolidated into a block by the application of heat and/or pressure and/or vacuum. The material may be consolidated in a press or autoclave, either restrained or unrestrained.

The production of the material may be controlled to determine the porosity of the finished product.

The formulation is based on the combination of a primarily thermoplastic resin matrix and suitable solid materials. These may be combined in varying proportions to produce a number of materials ranging from impervious solid to porous block. In a modification of the basic type of formulation a mixture of thermoplastic and thermosetting resins can be used to provide different characteristics.

A porous block may be advantageous for producing tools and moulds for use in a vacuum forming process, or in the manufacture of advanced composite mould tools.

Some examples of thermoplastic resins are as follows:-

polyethylene polypropylene polystyrene polysulphone polytherimide phenoxy polyethersulphone polyaryletherketone polyphenyl sulphide

Any other thermoplastic material, may be used singly, or combinations may be used if desired. Alternatively, the matrix may be a combination of thermoplastic and thermosetting resins such as epoxies,

The solid content of the material may be, for example:-

glass spheres hollow glass spheres cenospheres ceramic spheres carbon spheres phenolic microballoons perlite alumino silicates silicas talcs clays expanded clays fibres of different types

or any other lightweight or mineral material either naturally occurring or synthetically manufactured.

Examples of methods of combining the two basic materials, resin and filler will be described by example only.

Example 1

It is possible to dissolve or disperse the thermoplastic resin in a suitable solvent system to produce a liquid. This is mixed with the filler system to form a homogeneous mix.

Heat, vacuum and/or pressure may be applied to the mix to produce a solid block. Examp l e 2

Finely ground or micronised thermoplastic in powder form is blended with the filler system to produce an intimate mixture of powders.

The mixture is subjected to heat and pressure to form a solid block.

Example 3

Particularly suitable for spherical fillers. The thermoplastic/solvent blend is coated onto the surface of the individual particles by means of a pan coater or similar. The particles are dried and consolidated by heat and/or pressure to form a block.

Example 4

The finely divided thermoplastic powder is sprayed or similarly combined with hot or electrostatically charged filler to form a coherent surface coating. The coated particles are then consolidated by heat and pressure.

Suitable processes for applying heat, vacuum and pressure are hydraulic, electric or manual presses and autoclaves. Alternatively, methods of generating heat in the material to melt the resin and cause it to flow, and thereby allow the material to be consolidated properly under the applied pressure are ultrasonic stimulation, and microwave heating.

Example 5

This example uses the following constituents, combined as described below:

Parts by Weight

Polysulphone (T.M. Udel P1700) 15

Methylene Chloride 100

Cenospheres (hollow spheres)

(T.M. Metaspheres 50) 30

Colourant 0.05

The polymer and methylene chloride are mixed until dissolved and the Metaspheres 50 added. Methylene chloride is removed by evaporation until the remaining material has a dough like consistency. The mix is placed in a suitable mould in a press, press-clave, or autoclave.

Suitable methods of removing solvent are employed, such as vacuum extraction. The solvent may be recovered for recycling.

The mixture is heated for 2 hrs at 120°C, the pressure released and the block allowed to cool.

The resultant product has a softening point of 145°C as measured by Thermomechanical Analysis and a coefficient of thermal expansion of 12 x 10^" per °C which is linear over the temperature range 20 - 140°C.

The block is easily workable by hand or machine tools and capable of being sanded smooth.

This material has been found to possess most or all of the desired properties listed above, to a sufficient degree to make the material particularly suitable for use in vacuum forming tooling, as a pattern for the manufacture of advanced composite moulds and tooling from prepreg materials, or as a mould for the manufacture of components from prepreg materials.

Different grades of the material can be made to exhibit different porosities appropriate to different end-use requirements.

Claims

Claims ;

1. A modelling material for use in the manufacture of models for pattern making, tooling and the like, the material comprising a resin matrix and filler material i the matrix, characterised in that the resin matrix comprises a thermoplastic material.

2. A material according to claim 1, characterised in that the thermoplastic material is polysulphone, polyethersulphone, phenoxy, polyaryletherketone, polyethylene, polypropylene, polystyrene, polysulphate, polyetherimide, polyethosulphate, polyphenyl sulphide, o another thermoplastic material, or a combination of any two or more of the aforesaid materials.

3. A material according to claim 1 or 2, characterise in that the filler comprises glass spheres, hollow glass spheres, phenolic microballoons, cenospheres, perlite, ceramic spheres, carbon spheres, perlite alumino silicates, silicas, talcs, clays, expanding clays, fibres, or any lightweight or mineral material, either natural or synthetic, or a combination of the aforesaid.

4. A material according to any preceding claim, characterised in that the proportion of thermoplastic resin is between 5 and 95 percent by weight.

5. A material according to any preceding claim, characterised in that the proportion of filler is between 5 and 95 percent by weight.

6. A material according to any preceding claim, characterised in that the resin matrix consists of a mixture of thermoplastic and thermosetting resins.

7. A material according to claim 6, characterised in that the thermosetting resin is an epoxy.

8. A method of producing a modelling material according to any preceding claim, characterised in that the thermoplastic material is combined with the filler material and a solvent for the thermoplastic material, and the solvent is subsequently removed to leave a block of material as aforesaid.

9. A method according to claim 8, characterised in that the solvent is removed by the application of heat and/or pressure and/or vacuum.

10. A method according to claim 8 or 9, characterised in that the thermoplastic material and a particulate filler are combined by forming an intimate mixture thereof.

11. A method according to claim 8 or 9, characterised in that a particulate filler material is coated with thermoplastic material by electrostatic or mechanical means, following which the coated material is consolidated into a block.

12. A method according to claim 11, characterised in that the coated material is consolidated in a press or autoclave.

13. A method according to any of claims 8 to 12, characterised in that the production of the material is controlled to determine the porosity of the finished product.

14. A modelling material substantially as described above with reference to Example 1, 2, 3, 4 or 5.

15. A method of producing a modelling material according to claim 14, substantially as described above with reference to Example 1, 2, 3, 4 or 5.

16. Any novel subject matter or combination including novel subject matter disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.