Page 326 - Plastics Engineering

Processing of Plastics 309

(c) Matched Die Forming
A variation of thermoforming which does not involve gas pressure or vacuum is
matched die forming. The concept is very simple and is illustrated in Fig. 4.53.
The plastic sheet is heated as described previously and is then sandwiched
between two halves of a mould. Very precise detail can be reproduced using
this thermoforming method but the moulds need to be more robust than for the
more conventional process involving gas pressure or vacuum.

Sheet heating Forming Ejection

Fig. 4.53 Thermofonning between matched dies

(d) Dual-Sheet Thennoforming
This technique, also known as Twin-Sheet Forming, is a recent development. It
is essentially a hybrid of blow moulding and thermofonning. Two heated sheets
are placed between two mould halves and clamped as shown in Fig. 4.54. An
inflation tube at the parting line then injects gas under pressure so that the sheets
are forced out against the mould. Alternatively, a vacuum can be drawn between
the plastic sheet and the mould in each half of the system. This technique has
interesting possibilities for further development and will compete with blow
moulding, injection moulding and rotational moulding in a number of market
sectors. It can be noted that the two mould halves can be of different shapes
and the two plastic sheets could be of different materials, provided a good weld
can be obtained at the parting line.
4.4.1 Analysis of Thennoforming
If a thermoplastic sheet is softened by heat and then pressure is applied to
one of the sides so as to generate a freely blown surface, it will be found
that the shape so formed has a uniform thickness. If this was the case during
thermoforming, then a simple volume balance between the original sheet and
the final shape could provide the wall thickness of the end product.

Aihi = Afhf (4.28)

where A = surface area, and h = wall thickness (‘i’ and ‘f’ refer to initial and
final conditions).