MYP Design Moulding and Casting Notes

Moulding Makes the Whole Shape in One Go

1. Moulding and casting pour or force a soft or molten material into a hollow cavity and let it harden into the finished shape.
2. Cutting throws away offcuts and forming works bend by bend, while a mould produces every surface of the part in a single operation.
3. The cavity copies everything, so a scratch left by a milling cutter in the tool shows up on all 40,000 parts that come out of it.
4. Every mould needs draft angles on its vertical faces and a planned way of getting the part out, or the cavity becomes a trap.
5. Materials shrink as they cool, often 1 to 2 percent for a thermoplastic, so the cavity is cut slightly oversize to land on the right finished size.

Injection Moulding: Melt, Inject, Cool, Eject

1. Thermoplastic granules drop from a hopper onto a turning Archimedean screw running through a heated barrel, which melts and mixes them as it carries them forward.
2. The screw then shoves forward like a ram and pushes the melt through the sprue and runners into a closed steel mould at pressures over 100 MPa.
3. Water flows through cooling channels drilled inside the tool, and once the part is solid, ejector pins push it out and the mould closes again.
   1. A full cycle for a small part takes roughly 10 to 60 seconds, which is where the huge output per day comes from.
4. The solidified sprue and runners snap off and are granulated and fed back into the hopper, so waste is low.
5. Turn over any plastic lid and you can find the small round witness mark where the melt entered and the faint parting line where the two halves of the tool met.
6. It suits complicated thermoplastic parts in very large numbers: bottle caps, interlocking bricks, wheelie bin lids and kettle bodies.

Blow and Rotational Moulding Make Hollow Things

1. Blow moulding starts with a hot tube of softened polymer, the parison, clamped between the two halves of a cold mould.
2. Compressed air blown into the tube pushes it outwards until it touches the mould walls and freezes there as a thin hollow shell.
3. Only the outer surface is shaped by the tool, so a PET drinks bottle or an HDPE milk bottle has a controlled outside and a wall around 0.3 mm thick.
4. Rotational moulding puts a weighed charge of polymer powder into a hollow mould, then heats it while turning it slowly about two axes at once.
5. The powder melts and tumbles into an even coating over the whole inside surface, giving a thick walled seamless part with no joins to leak.
6. Water tanks, wheelie bins, traffic bollards and kayaks are made this way, and the cycle takes minutes rather than seconds because nothing is under pressure.

Extrusion: One Cross Section, Any Length

1. Extrusion pushes softened material through a shaped hole in a steel plate called a die, the way toothpaste takes the shape of the tube opening.
2. The cross section stays the same for the whole length, and the cooled product is simply cut off at whatever length is needed.
3. Plastic drinking straws, guttering, curtain track, garden hose and uPVC window sections are all extruded.
4. Aluminium is extruded too, hot rather than molten, which is where greenhouse frames and heat sink sections come from.
5. A die is far cheaper than an injection mould because it is one shaped plate with no moving halves and no ejector pins.
6. The limit is obvious once you see it: extrusion cannot produce a shape that changes along its length, so a bottle is out of reach.

Compression Moulding Is How Thermosets Are Shaped

1. A weighed slug of thermosetting powder or preform is dropped into the open, heated lower half of the mould.
2. The upper half presses down, and the combination of heat and pressure squeezes the material into every corner of the cavity.
3. While it is held there the polymer cures, forming permanent cross links between the chains so it can never be melted and reshaped again.
4. That curing takes time, so the cycle is longer than injection moulding, where the part only has to cool.
5. Melamine picnic plates, electrical socket faceplates and saucepan handles are compression moulded, because a thermoset stays rigid when it is hot.

Casting Metal: Sand for One Offs, Dies for Millions

1. Sand casting rams damp moulding sand around a wooden pattern held in a two part box, then the pattern is lifted out and the halves are closed again.
2. Molten aluminium at around 700 degrees Celsius is poured down the runner, while risers let the air escape and top the casting up as the metal shrinks.
3. The sand mould is broken apart to release the casting, so a fresh mould has to be rammed up for every single part.
4. That makes sand casting cheap for one offs and small runs, but the surface comes out grainy and usually needs machining where it matters.
5. Die casting forces molten zinc or aluminium alloy into a reusable steel die under pressure, which gives thin walls, crisp detail and a surface good enough to plate.
6. A die costs a great deal and then survives hundreds of thousands of shots, which is why model cars, door handles and engine housings are die cast.

The Mould Is the Expensive Part, So Volume Decides

1. An injection mould is machined from hardened tool steel, polished by hand and can cost tens of thousands of pounds and take weeks to make.
2. That is a fixed cost paid once, then divided between every part the tool ever produces.
3. A 20,000 pound tool spread over 500 parts adds 40 pounds to each one, and the same tool spread over 500,000 parts adds four pence.
4. So the number you plan to make picks the process: vacuum forming or 3D printing for tens, rotational moulding for hundreds, injection moulding for tens of thousands and up.
5. Changing the shape after the tool is cut means cutting a new tool, which is why companies 3D print and test the part before anyone touches the steel.
6. The same trade off between setup cost and unit cost runs through every scale of production, and it gets its own article in this unit.