Page 287 - Plastics Engineering

270 Processing of Plastics
developments in the use of in-the-mould transfers. This technology enables
lables to be attached to bottles and containers as they are being moulded.
Fig. 4.22 illustrates three stages in the blow moulding of a complex container.

Parlscn pinched as it emerges from Mould closes to deform side cores rotate
die and then stretched between two patison into shape inwards to complete
halves d open mwid d fming
Fig. 4.22 Stages in blow moulding of complex hollow container

Analysis of Blow Moulding
As mentioned previously, when the molten plastic emerges from the die it
swells due to the recovery of elastic deformations in the melt. It will be shown
later that the following relationship applies:
BSH = B& (from Chapter 5)
where BSH = swelling of the thickness (= hl/hd)

BST = swelling of the diameter (= Dl/Dd)
2
therefore

hl = hd (BST )2 (4.17)
Now consider the situation where the parison is inflated to fill a cylindrical die
of diameter, Om. Assuming constancy of volume and neglecting draw-down
effects, then from Fig. 4.23
7tDlhl = ZDmh