Moulding 1 - Polymer processing Flashcards
Polymer processing:
* What is polymer matrix behaviour during moulding ?
* How do moulding parameters, in particular temperature, affect properties
of finished component ?
Different approaches for converting
* dry fibres and thermoset resins,
* thermoset prepregs
into composite components
What is the Kamal-Sourour model?
When thermosets are moulded, a curing reaction occurs. When energy is applied, the hardener generates reactive species which react with monomers or oligomers to initiate curing. This process is exothermic. It can be described by the Kamal-Sourour model:
dα/dt = (k1+k2.α^m)(1-α)^n
α = degree of cure
t = time
m,n = order of reaction
where k is described by the arrhenius equation.
What are the resin processing stages?
- Storage - uncured resin system is in a glassy, brittle state when stored at low temperature
- Moulding - When temperature is increased beyond the glass transition temperature the resin becomes liquid
- Curing - Resin crosslinks to form a single molecule (gelation). after gelation, resin is no longer a liquid and not processable. The reaction continues after gelation.
- Post-curing - Optional heat treatment to obtain a higher degree of crosslinking and better mechanical properties.
- Cooling - When crosslinked resin is cooled below glass transition temperature, it changes from rubbery to hard. This is the service temperature range.
These can be classified:
* A-stage - Early stage of chemical reaction where resin has just been mixed and is still a liquid at room temperature. Used “out of the bucket” in most processes.
* B-stage - Intermediate stage of reaction where resin is partially cured. It is tacky at room temperature, and to prevent further progression it is stored at low temperatures. When heated it becomes liquid and the curing reaction resumes. Prepregs contain B-stage resin.
* C-stage - The finished component, resin is solidified.
look at graphs for thermoset vt diagrams
Define isothermal and non-isothermal curing.
When moulding, the resin temperature is raised to curing temperature. If the curing process is non-isothermal, then the temperature is not controlled and continues increasing slightly due to the exothermic reaction. If it is isothermal, the temperature is controlled and kept at the curing temperature.
During cooling, volume shrinkage is greater for non-isothermal cure than for isothermal cure resulting in higher residual stresses.
How is temperature related to modulus of cured thermoset resin?
In cured resin, high glass transition temperatures Tg are related to high degree of cure i.e. density of crosslinks and modulus.
Below Tg, resin is glassy and modulus is high, and the modulus decreases with increasing temperature.
Above Tg, lightly-crosslinked resin is more rubbery with significantly reduced modulus. Highly crosslinked resin is less affected. If the temperature is increased any more, the resin will degrade.
What are the structure of thermoplastics and how are they processed for moulding?
Thermoplastics are polymers in their final state and there is no chemical reaction during moulding. Amorphous thermoplastics consist of entangled chain molecules and semi-crystalline means the molecules are partially structured in lamellae forming larger structures known as spherulites.
They can be stored at room temperature unlike thermosets and are amorphous thermoplastics are formable when their temperature is increased beyond their glass transition temperature. Semi-crystalline thermoplastics become liquid at temperatures above their melt temperature.
When moulded thermoplastics are cooled below their glass transition temperature they change to a hard, brittle state (service temperature range).
look at graphs for vt diagram for thermoplastics
How is temperature related to modulus for thermoplastics?
Amorphous thermoplastics are hard and brittle below the glass transition temperature Tg. when they exceed Tg they transition to a rubbery state and have a significant reduction in modulus, similar to non-crosslinked thermosets. At the melt temperature they become liquid.
Semi-crystalline thermoplastics have a high modulus below Tg, with a relatively small reduction in modulus at Tg (similar to highly-crosslinked thermosets), with a significant drop at Tm.
Give two examples of an open mould process.
- Wet lay-up: a dry reinforcement mat or fabric is impregnated manually with thermoset resin, and left to cure at room temperature. An optional gel coat is added first, then fabric, wet-out, and consolidate with roller. This is repeated to build up layers. This has low investment costs and is very flexible, but is time consuming with low reproducibility.
- Spray-up: This is where a mixture of thermoset resin and chopped fibres are sprayed onto a mould surface. The composite is then left to dry at room temperature. Typically glass fibres and polyester resins. It uses rovings which is a low cost material format, has high deposition rates and not as labour intensive (can be automated). However it has low reproducibility and poor dimensional control like wet lay-up.
What is autoclave processing?
An autoclave is a heated pressure vessel used for processing lay-ups from thermoset prepregs where the B-stage resin and fibrous reinforcement are already combined. The prepreg is laid on tooling surface, and lay-up is enclosed in a vacuum bag. This tooling with lay-up is moved into autoclave where it is cured at high temperature to induce cure, and an applied pressure and vacuum for lay-up consolidation.
Ideally the tooling is made from the same material as the laminate, but it is frequently made from metal (Al, steel, Invar) which are more durable.
High part quality can be achieved with low void content, but with high costs due to time consuming and labour intensive set up, investment and operating costs, as well as cost of prepreg. Therefore used in applications where quality is paramount such as aerospace or motorsports.
What are the autoclave cycle steps?
- Increase temperature to reduce resin viscosity
- Dwell: resin can flow at reduced viscosity and the trapped air can escape as the vacuum is applied
- Ramp up to cure temperature and increase pressure to compress residual voids
- Hold pressure and temperature until curing reaction is complete
- Cool down to demoulding temperature, vent to ambient pressure
What is liquid composite moulding?
Liquid composite moulding (LCM) involves placing a dry fibrous reinforcement in tooling that applies compaction pressure, which is then impregnated with liquid resin using a flow driving pressure gradient. It is then left to cure. Types include Resin Transfer Moulding (RTM) and Vacuum Infusion (VI).
Permeability of the reinforcement and viscosity of the resin flow determine if all parts of the reinforcement are completely impregnated.
What is Resin Transfer Moulding?
RTM is a type of LCM where a preform is placed between two stiff matched tools. Resin and hardener are mixed and injected into a cavity, and the mould is heated to accelerate curing. The cured part is demoulded.
This allows geometrically complex components to be produced with good surface quality and fibre volume fractions around 50%.
High pressure RTM also exists with cycle time reduced from hours to <10 minutes.
What is Vacuum Infusion?
VI is a type of LCM where a preform is placed between stiff tooling and a vacuum bag. The resin flow is driven by the vacuum only (pressure difference of 1 bar), components are typically cured at room temperature.
This gives good surface quality, and has lower tooling costs than RTM. Suitable for large components. However, it is slow, has poor dimensional control and uses lots of consumables.
