Thermoforming and Casting Flashcards
What is thermoforming? Give an example.
Production of plastic products from sheet and film. Food packaging.
Briefly describe the thermoforming process.
TP heated then pressed against a mould.
What temperature does an amorphous TP need to be heated to for thermoforming?
Above the glass transition temperature.
What temperature does a semi crystalline TP need to be heated to form thermoforming?
Just below Its melting temperature.
Describe 4 thermoforming methods.
Vacuum forming: Air sucked out of the plastic to pull it tight against the mould.
Pressure forming: External pressure applied to the outside of plastic to force it against mould.
Mechanical forming: Same method as metal stamping.
Free blowing: Pressure used without a mould.
What are the pros of thermoforming?
- Inexpensive tooling
- Scrap can be reused
- Medium production rates.
What are the cons of thermoforming?
- High part cost(compared to injection moulded parts)
- Parts with under cuts cannot be formed.
Describe casting.
Filling mould with liquid polymer. Simple, slow and inexpensive. Examples: Gears, bearings and wheels.
What is monomer casting?
Monomers mixed together and pored into mould together.
In a casting scenario, what is potting and encapsulation?
Component covered with a resin guard to protect it from environment. Used for electrical components.
In a cast metal microstructure what are the different regions?
Chill zone: Small irregular grains on the edge.
Columnar zone: Long thin grains in the direction of the centre.
Equiaxed zone: Ordered grain structure in centre of casting.
What is the equation for solidification time?
T (solidification time) = Cm (mould constant) * (Volume of casting/Surface area of casting) ^ n
n = 2
What are 3 ways metal shrink when casting?
Thermal contraction in the liquid phase.
Solidification shrinkage.
Thermal contraction in the solid phase.
