Polymers - 2

Question 1

Moulding processes for thermosets

Answer 1

Compression, transfer, reaction injection

Question 2

Moulding processes for thermoplastics

Answer 2

Rotational, injection

Question 3

Compression moulding

Answer 3

Charge is punched by a heated mould

Question 4

Typical compression parameters

Answer 4

200°C+, 10-150 MPa, cycle time 0.5 to 5 mins

Question 5

Why won’t compression moulding be used on thermo plastics?

Answer 5

The mould would have to be cooled to set plastic

Question 6

Types of compression mould

Answer 6

Flash, positive, semi-positive

Question 7

Transfer moulding

Answer 7

Charged into pot to be heated prior (liquid so equalised pressure), intricate with no flash

Question 8

Major use of transfer moulding

Answer 8

Integrated circuit packaging

Question 9

Reaction injection moulding

Answer 9

Polymer synthesised in situ in mixing head, pressure until set, large small production run

Question 10

Common situ reaction mould polymer

Answer 10

Polyurethane

Question 11

Rotational moulding

Answer 11

Polymer granules are spun in 2 axis, melting and coating the inside mould
(V large pieces, low pressure so cheap)

Question 12

Injection moulding

Answer 12

Two types; plunger and reciprocating screw (similar to extruder additional motion)
High capital cost but highly automated

Question 13

Advantages of reciprocating screw injector

Answer 13

Better melt homogenity, pressure capability, control of shot volume

Question 14

Process cycle for moulds

Answer 14

Injections, hold time, withdrawal, freeze time, opening, extraction, closing

Question 15

Process parameters temperatures

Answer 15

3 heating zones specified, final one about metal point +30°C

Moulds - 40-90°C cooled, uniform to prevent distortion

Question 16

Process parameters pressure

Answer 16

Injection - high P (70-200MPa) due to viscosity, finer parts needs more P
Hold - to maintain good features, removed when gate has frozen

Question 17

Why are thermosets heated to 200°C in moulds

Answer 17

To promote cueing

Question 18

Why do thermoplastics require dying before processing?

Answer 18

They are hydroscopic

Question 19

Ratings are done for injection moulding machines by

Answer 19

Max shot size and clamp force

Question 20

How does clamp force limit max area of moulding?

Answer 20

Clamping force = P x A

Question 21

Main parts of a moulded assembly

Answer 21

Core +cavity, sprue, runners, gate, ejector pins

Question 22

Types of runner system

Answer 22

Cold, 2 plate - common, attached
Cold 3 plate - auto separation
Hot runner - always molten, saves scrap, faster but more expensive

Question 23

Preventing defects in mould designs

Answer 23

Constant thickness, cooling channels, insulation, avoiding premature freezing and weld lines

Question 24

Injection mould benefits

Answer 24

High rate production, dimensional control, versatile, moulds can be v expensive

Question 25

The main area of reducing cycle time

Answer 25

Freeze time

Question 26

Injection moulding variations

Answer 26

Multi-component, insert moulding, over-moulding, ice moulding

Question 27

Injection blow moulding

Answer 27

Injection moulded -> blow moulded -> stripped

Bi-axial strengthening (withstand pressure eg soda)