Polymer Processing

What are the main categories of polymer processing methods?

Forming

casting

moulding

machining

joining

additive manufacturing

What are thermoplastics and how do they behave in processing?

Thermoplastics soften and flow when heated and solidify when cooled, allowing them to be reshaped multiple times.

What are thermosets and how do they behave in processing?

Thermosets cure irreversibly during processing, forming cross-links that prevent reshaping when reheated.

What is extrusion in polymer processing?

A continuous process where molten polymer is forced through a shaped die to form long shapes with a constant cross-section.

What are the key components of an extruder?

Hopper, heated barrel with rotating screw, die, and cooling system.

What is the main function of the extruder screw?

To convey, melt, and mix the polymer before forcing it through the die.

What is injection moulding?

A high-volume process where molten polymer is injected into a closed mould to form complex shapes.

What is the cycle of injection moulding?

Melt → Inject → Cool → Eject

What is a key advantage of injection moulding?

Produces high-precision, complex parts with excellent surface finish.

What is blow moulding used for?

Forming hollow objects like bottles by inflating heated plastic into a mould.

What are the types of blow moulding?

Extrusion blow moulding and injection blow moulding.

What is compression moulding?

A method where heated polymer is placed in a heated mould and compressed to form the final shape.

What materials are typically used in compression moulding?

Thermosets like phenolics and reinforced plastics.

What is transfer moulding?

A process where a charge is preheated in a chamber and then forced into a mould cavity.

What is rotational moulding?

A process for forming hollow parts by rotating a mould in two axes while heating.

What are the steps in thermoforming?

Heat a plastic sheet, form it over a mould using vacuum or pressure, and cool it.

What is calendaring?

A method for producing thin sheets or films by passing molten polymer through heated rollers.

What is foaming in polymer processing?

Creating a cellular structure in the polymer, usually using a blowing agent.

What is the difference between physical and chemical foaming?

Physical foaming uses gases; chemical foaming uses compounds that release gases when decomposed.

What is the goal of additive manufacturing for polymers?

To build up parts layer-by-layer from CAD models, often using thermoplastics.

What are common methods of polymer additive manufacturing?

Fused filament fabrication (FFF), stereolithography (SLA), selective laser sintering (SLS).

What is welding in polymer joining?

Fusing materials using heat and pressure, often via ultrasonic, hot gas, or friction methods.

What are adhesives used for in polymer joining?

To bond surfaces using chemical curing agents, ideal for dissimilar or thin materials.

What are design considerations in polymer processing?

Shrinkage, warpage, residual stress, and orientation due to flow patterns.

What is a gate in mould design?

The entry point for molten polymer into the mould cavity.

Why is cooling rate important in polymer processing?

It affects crystallinity, mechanical properties, and internal stresses.

What are some common polymer processing defects?

Short shot, flash, sink marks, voids, weld lines, and burn marks.

What is the effect of molecular orientation in processed polymers?

Improves strength in flow direction but may cause anisotropic properties.

What is the importance of rheology in polymer processing?

Describes flow behavior; viscosity influences pressure, shear rate, and processing conditions.

What are fillers and how do they affect processing?

Added to improve stiffness, reduce cost, but may increase viscosity and affect flow.

What are plasticisers?

Additives that increase polymer flexibility and processability by lowering Tg.

What is polymer extrusion?

A continuous process where molten polymer is forced through a shaped die to form long shapes with a constant cross-section.

What is injection moulding used for?

To produce high-precision, complex shapes in high volumes by injecting molten polymer into a mould.

What is compression moulding?

A method where heated polymer is placed in a heated mould and compressed to form the final shape.

What is thermoforming?

A process where a plastic sheet is heated, formed over a mould, and cooled to create shaped products.

What is polymer casting?

A low-pressure process where liquid polymer is poured into a mould and cured.

What are thermoplastics?

Polymers that soften when heated and can be reshaped multiple times without chemical change.

What are thermosets?

Polymers that irreversibly cure and form cross-linked structures that cannot be remelted.

What are the main types of polymer moulding?

Injection, compression, transfer, reaction injection, and rotational moulding.

Why is cooling important in polymer processing?

It solidifies the polymer and affects final properties like crystallinity and internal stress.

How does crystallinity affect polymer properties?

Higher crystallinity improves stiffness and chemical resistance but can reduce transparency.

How do additives affect processing?

They can alter viscosity, curing time, strength, flexibility, and surface finish.

What is cycle time in injection moulding?

The total time to complete one full moulding cycle: injection, cooling, and ejection.

What is annealing in polymer processing?

A heat treatment used to relieve internal stresses and improve dimensional stability.

What is a weld line in moulded parts?

A line formed where two flow fronts meet, potentially weakening the part.

Why is venting important in moulds?

To allow trapped air and gases to escape, preventing voids and burns.

What is warpage in polymer parts?

Distortion due to uneven cooling, internal stresses, or part design issues.

What is shrinkage in moulded parts?

The reduction in part dimensions as the polymer cools and solidifies.

What is the gate in a mould?

The location where molten polymer enters the mould cavity from the runner system.

What are common defects in injection moulding?

Short shots, flash, sink marks, voids, weld lines, and burn marks.

What parameters influence polymer processing?

Temperature, pressure, cooling rate, cycle time, and polymer viscosity.

What are common tooling components in injection moulding?

Heated barrel, screw, hopper, nozzle, and a mould with cavities and cooling channels.

How does pressure affect polymer moulding?

It helps fill the mould cavity completely and can reduce porosity and defects.