what products can plastics be shaped into
- sheets
- insulation for wires
- moulded parts
why are plastic shaping processes important
- less energy required than for metals
- less product handling
- unlimited variety of part geometries
What are the two types of plastics
Thermosets and Thermoplastics
what is the process called when heating a thermoplastic until it softens till the consistency of liquid
Polymer melt
does viscosity increase with temperature or decrease
Decreases as it get more liquid as temperature increases
what are the two main components of an extruder
The Barrel and the Screw
what is a screen pack
Before reaching the die, the screen pack is a series of wire meshes supported by a stiff plate containing small axial holes. The part below the Breaker Plate
What’s the purpose of a screen pack
- Filters out contaminants and hard lumps that hasnt melted
- builds pressure in metering section
- straightens flow of polymer melt and removes it’s “memory” of circular motion from screw
what’s the difference between a sheet and a film
- Film has thickness below 0.5 mm where as sheet is bigger
- A film is mostly used in photographic films where as sheets are used for window glazing
What are some of the thermoplastic polymers?
- Polyethylene
- polypropylene
- polyvinylchloride
- cellophane
Injection moulding cycle
- mould is closed
- melt is injected into cavity
- screw is retracted
- mould is opened and part is ejected
plastic shrinkage factors
- fillers in plastic tend to reduce shrinkage
- high pressure forces more material into cavity to reduce shrinkage
- longer time allowing more material to go in at a slower pace
- higher temperature = lower polymer viscosity allowing more material to be packed into mould to reduce shrinkage
What is reaction injection moulding
where two highly reactive liquids are mixed and injected into mould cavity create a hard outer surface and a soft inner surface for example bumpers
What is blow Moulding
Where the air pressure is used to inflate the soft plastic inside the mould cavity
What is Rotational Moulding
The process which uses gravity to rotate the inside mould to allow for a hollow component
What is Thermoforming
Where a sheet is heated and is deformed by pressure created from a vacuum
difference between positive and negative thermo moulds
positive = convex shape Negative = concave shape
what is a net shape
where the finished component needs no finishing and is ready to be used
important properties of polymer melts include?
- Viscosity
- Viscoelasticity
what does extrusion mean
a long version of the finish component and will also have a constant cross section
