Shaping: deforming and reforming

Candidates name injection moulding whatever the shape. A thin-walled hollow container (bottle, can) is blow moulded — injection moulding makes solid parts, not one-piece hollows. A long constant cross-section (tube, track) is extruded through a die. Injection moulding suits only small solid detailed parts. Read the form first.

Asked to bend metal, many leave out the former — the shaped block the metal is bent around — and just say "bend it in a vice." Hold the metal against a former and work it with a mallet, heating first so it does not crack; the former controls the radius. Others wrongly apply a plastic method ("line bend it") to metal.

The definition trap. Deforming shapes by force with the material solid; reforming melts it into a mould. Heat appears in both, so candidates file vacuum forming under reforming. The test is "did it change state?" not "is heat used?" Vacuum forming and bending only soften, so they deform; casting, moulding and extrusion melt, so they reform.

Choosing die casting or injection moulding for a single part is a false economy. Both need an expensive metal die or mould, and that tooling cost is the same for one part or a million. For a one-off it spreads over too few parts, so the unit cost is huge — so a one-off metal part is usually sand cast. These pay off only in high volume.

Both make thin plastic products. Blow moulding inflates a soft tube against a mould for a sealed hollow — a bottle. Vacuum forming presses a softened sheet into an open shell — a tray. A bottle cannot be vacuum formed, nor a tray blow moulded. Blow moulding melts the plastic; vacuum forming only softens.

A belief: that vacuum forming melts the plastic, making it reforming. It does not — it heats a sheet only until soft, never liquid. Atmospheric pressure then presses the softened sheet over a mould. As it stays a sheet and never changes state, vacuum forming is deforming. The draft angle (the mould's taper) lets the cooled part pull off cleanly.