Composites- Manufacturing 1 Flashcards
Generic strategy for manufacturing composites
The fibre and matrix must be maintained at the correct shape, temperature and pressure throughout both the process and part
How does shape of fibres dictate manufacturing route?
Short fibres can effectively be processed like plastic additives whereas long fibres need dedicated processing techniques
How does choice of matrix dictate manufacturing route?
Thermoplastics have a higher viscosity in molten state than most thermoset polymers making them harder to combine with reinforcements
How does viscosity influence what is required to generate flow?
High viscosity matrices require high forces making it harder to combine with fibres.
Low viscosity matrices require low forces making it easier to combine with fibres.
Standard approaches for processing high viscosity composites
Like short fibres in thermoplastic melts. Use high forces. Primary examples are injection moulding, extrusion and hot pressing. Alternative is to use short flow distance. Primary examples are powder processing, co-mingling and hot pressing.
Advantage of using thermoplastics in terms of storage
Despite being harder to combine with reinforcements, once combination has been achieved, thermoplastic composites systems are stable and can be stored for many months or years prior to part manufacture
Standard approaches for processing low viscosity composites
Like thermosetting resins. More easily combined with fibres so low force processing routes preferred. Primary examples are wet lay-up, pultrusion, filament winding, RTM, pre-preg lay-up, compression moulding. It is possible to bleed resin out of the fibres if too high a force is used. Higher forces can be applied after the resin becomes solidified
Storage disadvantage of low viscosity resins
Combining them with fibres is relatively easy but it’s also easy for the two phases to separate so storage options are limited and resin is generally cured immediately
Powder processing
Used for thermoplastic and thermoset. Some matrices available as powders. E.g UHMWPE thermoplastic or phenolics thermoset. These can be processed by mixing the powder with the reinforcement by a low energy mixing method. Hot pressing then melts or reacts the matrix to form the solid composite. Minimal resin flow is required
Hot pressing
Prepared mixtures of reinforcement and matrix can be hot pressed. Apply pressure and temperature over time. Can use thermoplastic sheets like GMT or thermosetting moulding compounds. Applications are large, flat or moderately curved parts such as car body panels, dashboard panels and component cases. Basically press down on the prepared mixture to force it into shape between plates. Flash can be released out sides
Injection moulding
Used to make particle or short fibre reinforced thermoplastic or thermosetting polymer composites. Much easier with thermoplastics. Feed in the resin into a barrel and force it to other end using a rotating screw. Screw presses it against barrel more as goes along and it melts (thermoplastic) until it is injected into a mould. Barrel must be cooled with thermosets. Applications often small, intricate parts like high T resistant electrical parts with a phenolic matrix
The 5 generic steps common to thermoset composite manufacturing processes
- Preparing the fibres and matrix
- Combining the fibres and matrix
- Void removal
- Consolidation
- Resin curing
Preparing the fibres
Unidirectional cloth, woven fabrics and randomly aligned mats all need cutting to shape and size. Process can be automated using computer controlled ply-cutters. Required plies are given in a ply book
Preparing the matrix
Weigh resin first as is often the most viscous and harder to measure accurately. Mix in curing agents at the required ratio. Often use an equivalent weight which allows the correct molar ratio to be used. E.g epoxy equivalent weight (EEW). Note the pot life which starts when curing agents are mixed in
Preparing the fibres (preforms)
Preforms are dry fibres arranged into correct shape, size, thickness and fibre orientation prior to resin injection. Often used in RTM and made by specialist preform manufacturers. Most traditional textile/fabric processes can be used to produce preforms. Like winding, weaving and stitching, braiding and knitting.
