Revised Link Project Proposal with UMIST and Lucas Industries plc.
S F Bush
Section 2: The Project
The central objective of the project is to find out how to design the optimum manufacturing route to a given product. A key concept in realising this objective is the integration of product design, design of manufacturing process and choice of raw materials. A short-hand expression for this is Integrated Design and Manufacture or IDM.
The project will confine itself to polymer composites as a class of material and to aerospace and automotive products as end-points, but otherwise there is no theoretical restriction. Of particular concern is the question of manufacturing rate and flexibility. While automotive parts are typically made in hundreds of thousands if not millions, aerospace parts are numbered in thousands. A major goal for aerospace manufacture must be therefore to see how far the economics of large-scale production can be obtained by high variety techniques. The high-variety concept is also attractive for automotive products as fashion-tailoring is increasingly superimposed on basic styles.
Section 2.1: Polymer Composites
It is not the intention of the project to develop or investigate the development of fundamentally new types of composites, but to explore individual types or combinations of existing types in a new or optimised manufacturing process. It is properties realised in made artefacts which matter, not those obtainable in simple test shapes – though often these provide useful upper bounds to performance.
Any polymer combined with another distinguishable material may be regarded as a composite, but it is materials in fibre form which generally provide the most striking improvements to a polymer’s mechanical and thermal properties. The project will therefore take fibres as its starting point for reinforcement, while not ruling out other forms such as platelets.
Fibres themselves are available in two basic forms: discrete lengths and continuous. The discrete fibres may be as short as 0.3 mm (as is typical of glass reinforced thermoplastics such as nylon) or as long as 25 mm as in polyester styrene sheet moulding compounfs.
The main fibre types are:
GLASS (E-glass and S-glass)
CARBON (high modulus HM and ultrahigh modulus UHM)
ARAMID (typically Kevlar 29 and Kevlar 49)
In round therms these types have all broadly the same breaking strengths, but differ in their stiffness (and cost). Glass is far and away the most used overall, but carbon fibre is favoured for aerospace applications. Aramid has only slight use as a reinforcement fibre, though often found on its own (e.g. in ropes).
Man-made polymers are now available commercially in thousands of grades though there are only around twenty generic types in regular use. Of these twenty types around twelve are thermoplastic. For thermoplastics, moulding takes place at above 200 oC with injection pressures of 100 bar or more, so substantial moulds and machines are always involved. Eight types may be classed as thermo-setting resins. The uncured resins will generally enter the mould at temperatures well below 100 oC and at modest pressures of around 5 bar, so moulds and machines are less substantial and therefore cheaper than for thermoplastics. The curing process (chemical cross-linking) for thermosets in the mould is exothermic and temperatures will often rise well above 100 oC. Control of this exotherm is often a rate limiting step.
Instead of entering a mould as a fluid, both classes of polymer may be shaped by a die pressing on a flexible sheet made up of the polymer resin and any sold filler or reinforcement used. Heat is applied through the die, but its function is somewhat different for the two classes. For thermoplastics, the heat is required to take the polymer above its melting point so that it can flow to the die shape; for thermosets closure of the die is usually sufficient to cause the (uncured) resin to flow to the die shape: heating is requred to initiate curing of the resin.
In the formed state, unreinforced, the room temperature strengths of most polymers lie in the range 30 to 100 MPa. The key factors distinguishing so-called advanced materials are not so much strength per se but (a) the relatively elevated temperatures at which significant strength is maintained, in possibly a steam environment, and (b) the capacity of these materials to be efficiently reinforced. A requirement of increasing importance is the captacity to withstand actual fire which is not quite the same property as temperature resistance.
Broadly speaking then for thermoplastics, raw material costs and processing costs in the fluid sate increase with temperature resistance in the solid state. As with thermosets, environmental resistance (water, uv, etc) is a function of the particular chemical structure of the polymer. Among the thermosets, epoxide resins have been found to have a good combination of basic strength, reinforcement capability and environmental resistance for many structural applications, and are unlikely to be displaced by resins within the thermoset class. The main competition to epoxide resins is thus likely to come from the newer temperature resistant thermoplastics and from different types and ways of introducing reinforcement into the manufactured artefact.