Page 327 - Plastics Engineering

310 Processing of Plastics

t Air

Air
+ I

Heated sheets placed Inflation Ejection
between open mould halves

Fig. 4.54 Dual sheet forming

Example 4.7 A rectangular box 150 mm long, 100 mm wide and 60 mm
deep is to be thermoformed from a flat sheet 150 mm x 100 mm x 2 mm.
Estimate the average thickness of the walls of the final product if (a) conven-
tional vacuum forming is used and (b) plug assisted moulding is used (the plug
being 140 mm x 90 mm).
Solution
(a) The initial volume of the sheet is given by

Aihj = 150 x 100 x 2 = 3 x lo4 mm3
The surface area of the final product is

Aj = (150 x 100) + 2(100 x 60) + 2(150 x 60)
= 4.5 io4 mm2
Therefore, from equation (4.28)

3 x 104
hf = 4.5 x 104 = 0.67 mm
(b) If plug assist is used then it could be assumed that over the area
140 mm x 90 mm, the wall thickness will remain at 2 mm. The volume of
this part of the moulding will be
Val= 140 x 90 x 2 = 2.52 x lo4 mm3

This would leave a volume of (3 x 104-2.52 x lo4) to form the walls. The
area of the walls is

A,=(2~100~60)+(2~150~60)=3~ lO4~*