Exam Style Questions: Polymer Processes

Question 1

Explain why extrusion is suitable for mass or continuous production (2 marks)

Answer 1

Award 1 mark per relevant point up to a maximum of 2 marks.

Process is fast / highly automated / profiles can be used continuously / less skilled – labour required / produces continuous lengths that can be cut to length in line.

Question 2

Extrusion is used to create many polymer profiles.

Give three reasons why extrusion is used to uPVC window frames (3 marks)

Answer 2

Award 1 mark per relevant point up to a maximum of 3 marks.

• Profiles need to be produced in long lengths (1) and cut to the appropriate
  length (1).
• As extrusion uses metal dies to shape the uPVC into profiles, highly detailed profiles can be produced (1).
• Cooling the uPVC in a water bath as it is extruded means the material will hold
  its profile and remain stable (1), giving it even strength (1).

Question 3

Give one mechanical method of producing polymer sheet and explain three stages of the process (4 marks)

Answer 3

Award 1 mark for the method and 1 mark for each stage up to a
maximum of 4 marks

Calendaring
- Polymer pellets are heated and softened
- The softened polymer feedstock is extruded through a series
of heated rollers
- This process flattens and stretches the polymer
- The final rollers cool the sheet and it is either rolled or cut to length

Question 4

Evaluate the advantages and disadvantages of using jigs, patterns, and moulds to
batch produce polymer products. Comment on both machine-made and hand-made production aids in your answer (6 marks)

Answer 4

Advantages:
- Both machine-made and handmade production aids increase repeatability,
accuracy and reduce time spent manufacturing
- Jigs can help make accurate angles, for example when line bending
- A jig might be used to help fold a polymer sheet and to make sure that folds
run perpendicular to the edges of the sheet
- Jigs may help with positioning and dimensioning, for example when drilling
a hole
- Patterns enable moulds and thus copies to be made
- Machine made tool steel moulds can be used repeatedly as they are very
hard and resistant to wear
- Moulds and patterns can be made from a range of materials depending on
the product and the finish required

Disadvantages:
- Handmade jigs are subject to inaccuracies
- It may be difficult to achieve the desired quality of finish on a handmade
pattern
- Machine made patterns and moulds are expensive to produce and would
have to be reused many times to recuperate the cost
- If production aids are broken or lost production may be delayed
- Patterns and mould must be used with a release agent or materials could
stick to the inside
- Finishes rely on the surface finish of a mould and so any faults could be
costly

Question 5

Give three reasons why vacuum forming is suitable process to manufacture the
HIPS seed pot (4 marks)

Answer 5

• The pot has a thin wall thickness
• It is a tapered shape with no undercuts
• HIPS is an appropriate material for vacuum forming
• A number of seed pots can be formed in one go
• Although any excess flashing can be trimmed, it doesn’t matter if the edges are not perfectly smooth
• Mould can be reused again and again
• It is a cost-effective way of producing a low-cost product

Question 6

List at least four things that must be considered when creating a suitable mould (4 marks)

Answer 6

• The mould must have corners and edges rounded with a small radius
• The mould must have a positive draft angle > 3° to ensure easy removal
• The mould must have vent holes drilled where there are dips in the profile
• The mould must not have too deep a profile
• The mould must have a smooth finish
• Undercuts must be avoided

Question 7

Give one reason why blow moulding is best suited to continuous or mass production (2 marks)

Answer 7

• High set-up costs of extrusion equipment, dies and moulds means that it would not be a cost-effective solution to use for batch or mass production
• Air pressure needed to press the parison into the walls of the mould is high, and expensive specialist equipment would be required, as it could not be done
  using compressed air from a pneumatic workshop system or air tools

Question 8

Which one of the following is a method of creating hollow forms? (1 mark)

• Thermoforming
• Rotational moulding
• Injection moulding
• Compression moulding

Answer 8

Rotational moulding

Question 9

Give two suitable forming processes that would have been used in the manufacture of the plastic jerry cans (2 marks)

Answer 9

Award 1 mark per relevant point up to a maximum of 2 marks.

• Blow moulding (jerry can)
• Injection moulding (lid)

Question 10

List five stages of the rotational moulding process (5 marks)

Answer 10

Award 1 mark per relevant point up to a maximum of 5 marks.

• The cold mould is loaded with polymer power/granules and if applicable.
• Powdered pigment.
• The mould is then placed in a rig and put into a huge oven.
• It is heated slowly
• Whilst in the oven it is rotated through on axes
• The molten polymer coats the inside of the mould
• Once the desired wall thickness is reached the mould is cooled
• Slight shrinkage allows the moulded form to be removed

Question 11

Layup or laminating is a technique used to form thermosetting polymers to create rigid forms. Evaluate the advantages and disadvantages of this forming process (6 marks)

Answer 11

Advantages:
- Can be used to create very large structural rigid forms
- Good strength to weight ratio
- Mouldings are comparatively lightweight for their size
- A smooth coloured finish can be achieved using a pigmented gel coat
- Can be used in conjunction with various different fibres including glass
fibre, carbon fibre and Kevlar which creates a composite
- It can be used to produce products like speed boats, rowing boats,
canoes, wind turbine blades and caravan panels
- It creates a waterproof, largely chemical resistant shell
- It does not corrode
- It is relatively low maintenance
- Additives can be used to increase UV resistance

Disadvantages:

- Can crush/shatter on impact
- It can be hard to repair neatly 
- Polyester resin has a shelf life
- It is not recyclable