Manufacture

A major pitfall, when moving from metal to composite design, is neglecting manufacturing considerations at the outset. Metals are largely isotropic and their properties are generally easier to predict, therefore it is less important to think about manufacturing considerations whilst in the material selection and design stages. However, with composites this can lead to major problems because material selection, design and manufacture all go hand in hand. Choosing one material over another could have a massive impact in terms of possible design geometries and manufacturing options.

The basic requirement of all composite manufacturing processes is to maintain the desired pressure and temperature throughout the component over the required period of time [23]. If the temperature and pressure are controlled effectively, and the matrix and fibres are held in position until the composite becomes cool enough (or has set sufficiently in the case of thermosets) to be ejected, then manufacture will be successful. Manufacturing processes can be split into two categories; open mould and closed mould. In the open mould category there are wet lay-up processes, bag moulding and curing processes, and autoclave moulding processes. In the closed mould category there is transfer moulding, compression moulding and injection moulding. Another manufacturing process, which does not fit into either category, is filament winding. Several of these processes are compared in figure 12.

composite manufacturing comparison

Figure 12: Comparing the performance of composite manufacturing processes. Reproduced from [4].

Open mould processes

Wet lay-up processes involve either hand lay-up or spray-up; in either case impregnation occurs during the moulding process. Hand lay-up is a skilled process and therefore labour is expensive, however the cost of the mould and other capital investments are usually fairly low. This makes it suited to short-run components, where a fast cycle time is not imperative, such as boat hulls. Spray-up cannot achieve high a volume fraction - therefore it is not suitable for high performance lightweight applications. Both spray-up and hand lay-up are most suited to large sheet-like components such as boat hulls and furniture. Unlike other processes they require little or no pressure.

Bag moulding is essentially an extension to the lay-up process; the lay-up mixture is covered in a flexible airtight bag and either an external pressure is applied or a vacuum is created inside the bag-mould enclosure. This improves the properties of the resultant composite by driving out volatile substances and increasing the volume fraction of reinforcement.

Autoclave moulding uses a combination of a vacuum bag and a pressure chamber to produce components with high geometrical complexity and very good mechanical properties. Almost invariably it is prepregs that are used in autoclave moulding (although dry filament windings can also be cured in an autoclave), the moulding cycle can take many hours.

A universal disadvantage of open mould processes for the production of a carabiner is that the component is only in contact with one side of the mould. This is fine for asymmetric sheet-like objects such as panels, hulls, and covers as they generally only require a good surface finish on one side. However, a carabiner is symmetric about the longitudinal plane (figure 13) - this means that you could effectively only produce 'half' a carabiner using open mould techniques.

carabiner cross section

Figure 13: Open mould processes only allow the component to contact the mould on one side - which means the other side will be flat and have a relatively poor surface finish.

In all processes that use laminates there are a number of options to consider in terms of the form of the reinforcement fibres. There are both unidirectional and woven fabric options. Unidirectional options include: tape, single tows, strips and unidirectional fabrics (defined as having greater than 80% warp). Woven fabrics include: balanced fabrics and multiaxials. Figure 14 describes the effects of using different fibre forms.

comparing fibre reinforcements chart

Figure 14: Comparison chart for laminate fibre reinforcement forms [4].

Closed mould processes

In transfer moulding a closed mould contains pre-placed fibre layers and is filled with liquid resin at low pressure. Transfer moulding makes use of thermosets only, the low viscosity of the uncured thermoset resin means that pressure requirements are low; consequently tooling costs can be kept low. Transfer moulding is generally used for high strength and high stiffness components with production in the tens of thousands - as such it is a definite candidate for carabiner manufacture. A three dimensional fibre preform, with fibres oriented optimally for the tensile and bending loads expected in a carabiner, could be infused with a suitably tough resin.

Another liquid moulding method is reaction injection moulding (RIM), in this process the resin is injected at high pressure into a mould. The mould may contain a fibre preform, or the resin may contain short fibres/filler already. RIM can be used for more complicated 3D components and has a high initial capital investment; however cycle times are generally very short so it is economical for large production series. Alternatively, injection moulding can make use of thermoplastics; this further reduces the cycle time as the product does not have to cure in the mould. However, as thermoplastic melts are very viscous, this process requires very high pressures and thus has considerable tooling costs. Injection moulding makes use of short fibre reinforcement only and can be highly automated.

If it is possible to formulate a short fibre reinforced polymer that has the necessary mechanical properties, then injection moulding (or RIM) would be an excellent candidate for carabiner manufacture. Although the initial capital investment is high - injection moulding would enable the complex geometry of a carabiner body to be made in a single process. In addition, once manufacture begins there will be minimal labour costs and production capacity will be very high thanks to fast cycle times. The mechanical properties of injection moulded composites rarely compete with laminated composites because their fibres are relatively short and their alignment is hard to control as it depends on the flow path during injection. However, the strength of most high performance laminates considerably exceeds the requirements - therefore an injection moulded polymer does not need to match laminate strength, it must only be of comparable strength to the aluminium alloy currently in use.

injection moulding diagram

Figure 15: A schematic diagram of the thermoset injection moulding process [8].

Compression moulding involves using a compression press to apply heat and pressure to a sheet or bulk moulding compound. The heated moulding compound flow into the shape of the mould and usually produces sheet-like components. Cycle times are generally low and the process is often used in the automotive industry to produce covers and housings. Prepregs and laminates can also be used in a compression moulding process - generally giving the resulting product better mechanical properties. Rubber-block and hydro forming techniques enables more complex geometries and reduce the risk of wrinkles in the component. Compression moulding is certainly a possible candidate for carabiner production. A roughly carabiner shaped charge pattern would be cut from a sheet moulding compound such that it would cover near to 90% of the mould surface. A charge would normally cover somewhere between 20 and 90% of the surface - covering a larger portion of the surface is beneficial for structural applications because it keeps flow induced orientation to a minimum.

Filament winding is the process of wrapping a tow or band of fibres around a rotating mandrel. Either prepreg fibres are used or fibres are dipped in wet resin as they are wound. The winding angle and pattern can be tailored to the loading requirements and a high fibre volume fraction is possible which means parts generally have good mechanical properties. However, shapes with reverse curves cannot be wound, in addition, winding is effectively limited to parts with rotational symmetry. This makes it difficult to use in carabiner manufacture as the carabiner shape precludes the use of normal filament winding apparatus. A radical redesign of the carabiner shape may enable the use of filament winding.

Another composite manufacturing process that may be worth considering it that used for tennis racket production. A carbon fibre prepreg is wrapped around an inflatable tube; the tube and prepreg are then inserted into a closed mould. The tube is pressurised and the mould is heated until the racket consolidates [31]. This produces strong and lightweight rackets that can not only withstand the impact of a tennis ball travelling at 140 mph - but can also send the ball hurtling in the opposite direction at a similar speed. This process could be adapted for carabiner manufacture. It would require the carabiner to be hollow which could potentially increase the cross section beyond the maximum acceptable value - this depends on what the minimum possible size is for an inflatable tube capable of applying the required pressure.

The variety of composite manufacturing processes reflects the variety and complexity of composite materials. The composite field is continuously changing - new combinations of materials are frequently created whilst new variations and modifications to existing manufacturing methods are introduced.

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