Manufacturing with Polymers

Question 1

What are the advantages of injection moulding?

Answer 1

Advantages of Injection Moulding
• High production rates.
• High tolerances are repeatable.
• Wide range of materials can be used.
• Low labour costs.
• Minimal scrap losses.
• Little need to finish parts after moulding.
• Both thermoplastics and thermosets can be used.

Question 2

What are the disadvantages of injection moulding?

Answer 2

Disadvantages of Injection Moulding
• Expensive equipment investment.
• Running costs may be high.
• Lead times for production of die sets may be long.
• Modifications to product, and therefore mould, can be costly.
• Difficult to accurately meter shot size.

Question 3

What are the advantages of rotational moulding?

Answer 3

• Low pressure therefore moulds are relatively simple and inexpensive (e.g. aluminium/wood) and also, therefore, lead times can be short.
• Stress-free mouldings.
• Can have uniform wall thickness.
• Can contain resin reinforcement and cast-in features such
as thread inserts.
• Can produce very large and thick walled articles (12mm).
• Intricate shapes and undercuts are possible.
• Very little waste polymer.
• Most thermoplastics and some thermosets can be used.

Question 4

What are the disadvantages of rotational moulding?

Answer 4

Disadvantages

• A relatively slow process.
• A limited choice of plastics types as feedstock.
• Material costs relatively higher.
• Loading and unloading is labour intensive. Bosses and ribs are difficult.
• Can have non-uniform wall thickness.

Question 5

What are the two types of Blow Moulding?

Answer 5

Extrusion blow moulding

Injection blow moulding

Question 6

How does Injection Blow Moulding work?

Answer 6

Injection cycle.
The polymer melt is pre-formed into a ‘parison’ in the injection mould section.

Moulding cycle
The product is formed by inflating the heated parison against the walls of the mould.

Question 7

How does Injection Extrusion Blow Moulding work?

Answer 7

Injection cycle.
The polymer melt is extruded into a ‘parison’.

Moulding cycle
The product is formed by inflating the heated parison against the walls of the mould.

Question 8

What is compression moulding?

Answer 8

The process of moulding a material in a confined shape by applying pressure and usually heat.
Used almost exclusively for
thermosetting materials.
Used to produce mainly electrical products.
Relatively low mould cost. Parts have low residual stress.

Question 9

What is Transfer Moulding?

Answer 9

A process of forming articles by fusing a plastic thermoset material in a chamber then forcing the whole mass into a hot mould to solidify.

Question 10

What are the advantages of Transfer Moulding?

Answer 10

• Since the mould has a runner system, multiple cavities
are easier.
• There are lower moulding pressures in this process
than compression moulding.
• Transfer moulded parts inherently have less flash (excess material that runs along the parting line of the mould) than their compression moulded counterparts because the mould remains closed when the plastic enters the mould cavity. However, the sprue must be removed.

Question 11

What are the disadvantages of Transfer Moulding?

Answer 11

• The mould is more complex than a compression mould.
• Flow induced and orientation problems are more pronounced (similar to injection moulding) because the material is essentially injected into the cavity at high speeds.
• Transfer moulding still produces more waste material than compression moulding because of the sprue, the air holes and the overflow grooves that are often needed to allow air to escape and material to overflow.

Question 12

What is Thermoforming

Answer 12

This process consists of heating thermoplastic sheet to a formable plastic state and then applying air and/or mechanical assists to shape it to the contours of a mould.

Question 13

What is vacuum forming

Answer 13

Vacuum forming is a simplified version of thermoforming.
A sheet of a thermoplastic is heated to a forming temperature, stretched onto, or into, a single-surface mould, and held against the mould by applying a vacuum between the mould surface and the sheet.
The vacuum forming process can be used to make product
packaging, car dashboards, refrigerator linings, etc.

Question 14

What are the advantages of vacuum forming?

Answer 14

• Low pressures mean low-cost tooling –Wood, plaster.
• Also means a short manufacturing time for tooling.
• Medium to large parts can be made economically in small quantities

Question 15

What are the disadvantages of vacuum forming?

Answer 15

• Relatively slow process.
• Re-entrant
• Plastic sheet feedstock must be prepared.
• There is a certain amount of waste from brim offcut (can sometimes be reground and reused)

Question 16

What is pre-stretch (bubble) vacuum forming?

Answer 16

Once the plastic has reached it’s forming temperature or ‘plastic’ state it can be pre- stretched to ensure even wall thickness when the vacuum is applied.

Pre-stretch is an invaluable feature when forming deep draw parts with minimum draft angles and high mould surface detail.

Vacuum, air pressure and optional aids such as a plug assist are then used to assist in moulding the heated, stretched plastic.

Question 17

What are the four steps to injection moulding?

Answer 17

1) Melting
2) Injection
3) Cooling
4) Ejection

Question 18

How does the injection system work in Injection Moulding?

Answer 18

• Extruder consists of a barrel with heater bands outisde and a rotating screw inside
• As it turns, the screw can move axially back and fourth
• This allows the melt to accumulate in the front of the barrel as the screw retracts
• When the shot is delivered, the screw moves forward, forcing the shot into the mould cavity
• The screw is constantly turning to keep resin melted.

Question 19

What is a mould core used for?

Answer 19

It is used when there are internal cavities that are not accessible from the normal halves of the mould