Polymer processing and moulding processes

Question 1

What type of atom structure do thermoset polymers have?

Answer 1

Amorphous structure, with crosslinked chains.

Question 2

Out’ve thermoplastic and thermoset polymers, which can be reused?

Answer 2

Thermoplastics.

Question 3

What type of polymer is processed through extrusion?

Answer 3

Thermoplatics.

Question 4

What products are made using extrusion?

Answer 4

Pipes, guttering, sheets or film.

Question 5

Describe the extrusion process briefly.

Answer 5

Polymer pellets added to hopper, heated and forced by barrel through die. constant cross section.

Question 6

What types of extrusion are there?

Answer 6

Extrusion blow moulding - Used for pipes. Air blown through centre of pipe as its being extruded to stop it collapsing.
Extruded wire: Polymer coating extruded around wire.

Question 7

What types of moulding are used for thermosets?

Answer 7

Compression moulding
Transfer
Reaction Injection Moulding (RIM)

Question 8

What type of moulding is used for thermoplastics?

Answer 8

Rotational moulding

Injection moulding

Question 9

Describe Compression moulding, and its advantages and disadvantages.

Answer 9

Hydraulic press used to compress to halves of a heated mould.
Advantages:
-Simple low cost process
-Low die wear and low scrap rate
-High volume production rate
-Metal threads and inserts can be moulded in.
Disadvantages:
-Complex shapes are difficult
-Thick and heavy parts take a while to cure.

Question 10

Describe Transfer moulding, and its advantages and disadvantages.

Answer 10

Thermoset is preheated in a transfer pot before being forced into the mould at high pressure using a plunger.
Advantages:
-Less pressure is required as polymer soft due to preheating.
-Multiple cavities are used so high yield rate.
-High accuracy
-Short production cycles
Disadvantages:
-Expensive (tooling)
-Lots of waste product.
-Slow production rate compared to injection moulding

Question 11

Describe Reaction Injection moulding, and its advantages and disadvantages.

Answer 11

2 monomers mixed and injected into mould.
Advantages:
-Large design flexibility 
-No moulded in part stresses
-Good for large thin walled parts
Disadvantages:
-High capital investment
-Slow cycle time compared to standard injection moulding
-Expensive raw materials.

Question 12

Describe Rotational moulding, and its advantages and disadvantages.

Answer 12

Polymer added to mould. Mould is spun about x and y axis to cover inside walls of the mould. The process takes place inside of an oven.
Advantages:
-Cheap mould because it doesn’t need to withstand high pressure.
-Versatile technology

Disadvantages:

• Energy intensive and not environmentally friendly.
• High labour cost.
• high cost of raw materials.
• Long cycle times.

Question 13

Describe the Injection moulding process.

Answer 13

Extruder used to inject molten material into mould and cooled.

Question 14

Describe the construction of an injection mould.

Answer 14

Sprue: Entry point from extruder into mould.
Gate: Controls flow rate into the part.
Runners: Carry plastic to individual part cavities.

Question 15

What is the equation for clamping force for injection moulding?

Answer 15

Clamping force = Pressure inside of mould * Projected are of mould cavity

Question 16

What are the 4 injection moulding defects and how to fix them?

Answer 16

Short shots: Partial filling of part due to solidification before part has filled completely. Fix Increase temp/ increase pressure.
Flashing: Material escapes between mould sections. Fix: increase clamping force of mould or decrease injection speed.
Sink marks: Localised depression in part. Fix: Redesign part to have uniform thickness, or increase packing pressure after injection.
Weld lines: Boundary lines on part. Fix: Higher melt temp, higher injection pressure.

Question 17

What are 4 injection moulding variations?

Answer 17

Multi component: Mould multiple types of plastic into the same mould. e.g rear car light cluster.
Insert moulding: Moulding around a part in the mould.
Over moulding: Moulding a product preassembled
Ice moulding: Moulding 2 parts of the same polymer separated by a cooled insert.

Question 18

What is the equation for extrusion blow moulding blow up ratio?

Answer 18

Blow up ratio, BUR = Bubble diameter, Db / Die diameter, Dd

Question 19

What are the main variables that define moulding processes?

Answer 19

How the heat and pressure is applied.

How the polymer is delivered to the mould.