Economics of Forming
Before discussing the economics of forming it should be emphasised that there may be other limiting factors apart from purely economic ones. For example, injection moulding cannot, in general, be used to mould objects having re-entry curves or reverse tapers. It is, however, possible to mould slight undercuts in certain materials, though not in others. Undercuts are often moulded in polypropylene screw caps, for instance, into which may be inserted imitation jewels in, say, polystyrene. Polypropylene is a sufficiently resilient material for this purpose but a similar attempt using polystyrene for the caps would lead to fracture when the cap was removed from the mould (see Fig. 1). Similar restrictions on reverse tapers and reentry curves apply to articles thermoformed from sheet but here there is a little less restriction since the combination of a fairly thin sheet and a resilient material will allow some degree of freedom.
Unable to display preview. Download preview PDF.