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Materials & manufacture · National 5

Commercial processes

7 cards · 6 quiz · 2 exam

Design & Manufacture · National 5 · mrkellydt.co.uk

What you're learning in this topic

Learning intention WALT

We are learning about commercial processes, specifically to understand:

  • the uses and identifying features of injection moulding, die casting, rotational moulding, vacuum forming and sand casting
  • the features of a good vacuum-forming and sand-casting pattern

Success criteria WILF

  • I can describe the uses and identifying features of injection moulding, die casting, rotational moulding, vacuum forming and sand casting
  • I can describe the features of a good vacuum-forming and sand-casting pattern

Moulding and casting

ProcessUsesIdentifying features
Injection moulding (thermoplastic)Complex, accurate parts in high volumes: casings, toys, bottle crates, sunglasses framesEjector pin marks, injection point, flashing, draft angles, high-quality finish, strengthening ribs
Die casting (metal, e.g. aluminium alloy)Accurate complex metal parts in high volumes: toy cars, pencil sharpeners, door handlesSame as injection moulding: ejector marks, flash, draft angles, high finish
Rotational moulding (thermoplastic)Large hollow forms: kayaks, bins, traffic barriers, play equipmentHollow, seamless, uniform wall thickness (thicker in corners), good finish, slight draft
Sand casting (metal)Low-volume solid or hollow metal parts: engine blocks, manhole covers, one-offsRough "orange peel" texture, draft angles, rounded corners and fillets; often needs machining
Vacuum forming (thermoplastic sheet)Simple shallow forms: packaging trays, masks, bath tubs, chocolate insertsThin walls (thinner at corners and bottom), open tray shape, generous draft angles, rounded corners

How the processes work

Four of the processes in the table above, step by step.

hopper heaters round the barrel screw mould split line ejector pins injection point molten plastic pushed forward
Plastic granules fall from the hopper into the heated barrel. The turning screw melts and mixes them, then pushes the molten plastic through the nozzle into the mould at the injection point. The plastic cools against the cold mould, the two halves open at the split line, and ejector pins push the part out. Draft angles on the sides let it release.
molten metal forced in steel die (two halves) split line: flash may form here ejector pins
Molten metal, such as aluminium or zinc alloy, is forced under pressure into a two-part steel die. It cools quickly against the die, which then opens at the split line. Ejector pins push the casting out, leaving small circular ejector marks. Any thin flash squeezed out at the split line is trimmed off. The die is reused, so every casting is identical.
heated oven plastic powder rotates on two axes mould hollow Finished part uniform wall thickness
Plastic powder is put into a closed mould, which is heated in an oven while it rotates on two axes. The powder melts and coats the inside of the mould evenly. The mould is cooled, still rotating, then opened. The product is hollow and seamless, with a uniform wall thickness (a little thicker in the corners).
copedrag split line sprueriser molten metal fills the cavity sand packed round the pattern, which is removed before pouring
The pattern is packed in sand in a two-part moulding box: the cope (top) and the drag (bottom). The halves are separated at the split line so the pattern can be removed, leaving a cavity. Molten metal is poured down the sprue; the riser lets air escape and shows the mould is full. When cool, the sand is broken away and the casting is fettled: the runners are cut off and the edges filed.

Patterns for vacuum forming and sand casting

  • Draft (taper) on vertical faces so the pattern can be removed.
  • Radiused corners and edges: no sharp corners; material flows and the pattern releases.
  • Smooth surface: any marks transfer to the product.
  • Vacuum forming: pattern must be flat-backed, have no undercuts, and have vent holes in hollows so air is drawn out.
  • Sand casting: pattern is often split in two halves; made from a stable material like MDF or wood.
platen: air sucked out through holes clamp draft angle radiused corner vent hole sheet pulled tight flat back
The heated sheet is clamped in the frame, the platen raises the pattern into it, and the air is sucked out so the sheet is pulled tight onto the pattern. A good pattern has draft angles and radiused corners so it releases, a flat back, and vent holes so air is drawn out of hollows. The sheet stretches and thins most at corners and on deep sides.
  • Sand casting: the pattern is slightly larger than the product to allow for shrinkage as the metal cools; a sprue and riser are added so metal can be poured in and air escape; cast parts are fettled (runners cut off and edges filed) afterwards.

Watch the processes

Key terms

Injection mouldingHigh-volume complex thermoplastic parts. Ejector pin marks, injection point, flash.
Die castingHigh-volume complex metal parts in aluminium/zinc. Same features as injection moulding.
Rotational mouldingLarge hollow seamless plastic forms: kayaks, bins.
Sand castingLow-volume metal parts with rough sandy texture.
Vacuum formingThermoplastic sheet pulled over a pattern: thin walls, draft angles.
Vent holesAllow trapped air out of hollows in a vacuum-forming pattern.
Draft angleA taper so the product or pattern releases from the mould.

Quick check

1. A plastic kayak is hollow, seamless and has an even wall thickness. Process?

  1. Injection moulding
  2. Rotational moulding
  3. Vacuum forming
  4. Die casting
Show answer
B: Rotational moulding. Rotational moulding makes large hollow forms.

2. Small circular marks on the inside of a plastic casing are evidence of…

  1. Flashing
  2. Sand grains
  3. Ejector pins
  4. Layer lines
Show answer
C: Ejector pins. Ejector pins push the part out of the mould.

3. Why must a vacuum-forming pattern have draft angles?

  1. To make the formed sheet stronger and stiffer
  2. To give the product a smoother surface finish
  3. To make the walls of the product thicker
  4. So it can be removed from the formed sheet
Show answer
D: So it can be removed from the formed sheet. A taper lets the pattern release.

4. A cast-iron manhole cover with a rough texture was most likely made by…

  1. Sand casting
  2. Die casting
  3. Injection moulding
  4. Vacuum forming
Show answer
A: Sand casting. Sand leaves a rough surface.

5. Which process suits a toy car body in zinc alloy made in large numbers?

  1. Sand casting
  2. Die casting
  3. Vacuum forming
  4. Rotational moulding
Show answer
B: Die casting. High volume, accurate metal parts.

6. Why are there vent holes in vacuum-forming patterns?

  1. To reduce the amount of plastic sheet used
  2. To stop the pattern overheating in the machine
  3. So air trapped in hollows can be drawn out
  4. To make the pattern lighter to lift out
Show answer
C: So air trapped in hollows can be drawn out. Otherwise the plastic won't pull into hollows.

Exam-style questions

1. Name an appropriate process to manufacture the plastic casing of a games controller and explain why it is suitable. (3 marks)

Show marking guidance
  • Injection moulding (1)
  • Produces complex forms with strengthening ribs accurately (1)
  • High-quality surface finish, no further finishing (1)
  • Economical for high volumes despite expensive moulds (1)

2. Describe two features of a vacuum-forming pattern and explain why each is needed. (4 marks)

Show marking guidance
  • Draft angles (1) so the pattern releases from the formed sheet (1)
  • Radiused corners (1) to prevent thinning/tearing and help release (1)
  • Vent holes (1) to let trapped air out of hollows (1)

Printable worksheet

An I do, we do, you do worksheet for this topic, with a separate answer sheet.

Worksheet (PDF)Answers (PDF)

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