What are some key properties of Polymers?
(1) low density relative to metals and ceramics
(2) good strength-to-weight ratios for certain (but not all) polymers
(3) high corrosion
resistance
(4) low electrical and thermal conductivity.
Why are polymers used instead of metals?
Ease of shaping, Lower energy, Less handling and product finishing.
What are limitations of polymers?
Lower Strength, Lower Stiffness, limited service temperatures, degradation from radiation and having viscoelastic properties
What are the differences between the three types of Polymers
Thermoplastics are solid at room temperature, become viscous liquids when heated, and don’t degrade with repeated heating–cooling cycles.
Thermosets soften and flow when first heated, then harden irreversibly. Unlike thermoplastics, reheating causes them to degrade and char rather than soften.
Elastomers are rubbers with extreme elasticity, able to stretch up to 10 times and fully recover.
What is a Polymer Melt?
A thermoplastic heated to a viscous liquid
What is pseudoplasticity (shear thinning)?
Viscosity decreases with shear rate — the fluid thins as it flows faster.
What does the viscosity of a polymer melt depend on?
Shear rate and Temperature
What is viscoelasticity?
Material exhibiting both viscous and elastic properties, like how a polymer melt can flow through a die but expands once through it.
What is polymer extrusion used for?
Produces long plastic parts with uniform cross sections
What is the general process for polymer extrusion?
A rotating screw forces plastic through a heating chamber
and then through a heated die
What are the sections that the barrel is divided into, and their functions in a polymer extrusion process?
Feed Zone: feedstock is moved from the hopper and is preheated
Compression Zone: root diameter increases, decreasing flight depth, softening and compressing the polymer to eliminate air pockets
Metering Zone: polymer melt is homogenised and is pressure-controlled to push a specific amount through the die.
How does the melt flow during polymer extrusion?
The screw rotates inside the barrel, forcing the melt towards the die. This mechanism is called drag flow.
What happens to the melt in the polymer extrusion process just before the melt reaches the die?
The melt passes through a screen pack which filters out contaminants and hard lumps, helps build pressure, straightens the flow and removes the elastic memory from the screw motion.
What is die swelling in polymer extrusion?
When the plastic expands when exiting the die opening
When would blow film extrusion be used, and what is the general process?
Used to produce a tube made from a thin film. Begins with the extrusion of a tube that is immediately drawn upwards (while still hot) and inflated by blown air, then guide it into a pinch roller that squeezes the tube together after it has cooled.
What is the process of polymer injection moulding?
Polymer is heated to a highly plastic state and forced to flow under high pressure into a mould cavity where it solidifies and the moulding is then removed from cavity.
When should injection moulding be used?
In high volume production processes as the cycle time is very short.
What are the two parts in injection moulding?
The injection unit, which melts and delivers polymer melt (very similar to an extrusion process, and the clamping unit, which opens and closes the mould each injection cycle
What are the features of a two-plate mould used in polymer injection moulding?
Cavity: Geometry of part but oversized for shrinkage
Sprue: Leads from nozzle to mould
Runners: Leads from sprue into cavity
Gate: Constricts the flow of plastic into cavity and
Ejector pins: To eject part at the end of the cycle.
What are the clamping designs used in polymer injection moulding?
Toggle clamps: high speed and high force are most suited to
relatively low tonnage machines
Hydraulic clamps: higher-tonnage machines
Hydromechanical: large tonnages (150-1000 ton)
What are the phases of cavity pressure during injection moulding?
Filling Phase, Packing phase, Holding phase, (gate sealend), Residual cavity pressure.
What is changed in the injection moulding process when the polymer is a thermoset rather than a thermoplastic
The resin is brought up to its melt point, but doesn’t want to go past it, so the screw is straight instead of tapered, and the mould is heated to crosslink the thermoset so that it retains its shape.
What is the general process of blow moulding?
A solid-bottom hollow tube is placed between two mold
halves and heated. The heated tube is then expanded into
the sides of the mould with compressed air
What is blow moulding used for?
Making one-piece hollow plastics parts with thin walls such as bottles in typically high quantities
