Page 328 - Plastics Engineering

Processing of Plastics 31 1

This ignores a small area in the base of the box, outside the edges of the plug.
Hence, the thickness of the walls in this case would be
(3 x IO") - (2.52 x IO")
h, = = 0.16 mn~
3 x 104
These calculations can give a useful first approximation of the dimensions
of a thermoformed part. However, they will not be strictly accurate because in
a real situation, when the plastic sheet is being stretched down into the cold
mould it will freeze off at whatever thickness it has reached when it touches
the mould.
Consider the thermoforming of a plastic sheet of thickness, h, into a conical
mould as shown in Fig. 4.55(a). At this moment in time, t, the plastic is in
contact with the mould for a distance, S, and the remainder of the sheet is in
the form of a spherical dome of radius, R, and thickness, h. From the geometry
of the mould the radius is given by
H - Ssina
R= (4.29)
sinatana

D

Fig. 4.55 Analysis of thermo forming

Also the surface area, A, of the spherical bubble is given by

A = 2zR2(1 - COSCY) (4.30)
At a subsequent time, (t + dt), the sheet will be formed to the shape shown in
Fig. 4.55(b). The change in thickness of the sheet in this period of time may