Thermoforming and Casting Flashcards
(13 cards)
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.
