Page 128 - Module DMV20173

6.0 POLYMER PROCESSING

13. The nip rolls, one rubber coated and the other polished steel, (stainless steel or chrome

plated) are pressed tight enough together not to allow air to escape from the inside of
the blown bubble.
14. The nip rolls are driven at a faster speed than the extrusion, producing biaxial strain on

the blown film.
15. This causes the film produced to be stronger in all directions.
16. The flat tube is then brought down over the nip rolls to be wound onto a drum.
17. Blow molding is a very economical process used to produce hollow plastic products
(Figure 6.12).

18. A number of different methods of extrusion blow molding have been developed.
19. Factor such as the size of the part, the number to be made and the type of part will
influence the decision about which technique will be employed.

Figure 6.12 Close-up of the production of bottles

6.6.1 PROCESS CHARACTERISTIC

1. Inflates a softened parison tube to the con tour of a mold cavity Uses thermoplastics

2. Forms thin-walled hollow products.
3. Parting lines are present.
4. Wall thickness can be increased by increasing the parison tube wall thickness flash is
present but is minimal.

7.6.2 PROCESS SCHEMATIC

1. This setup is typical of many blow-moulding processes (Figure 6.13).
2. The parison tube is positioned in the mold cavity, and air pressure is then applied to the
parison, which forces the plastic to form to the mold cavity.

BPLK 160 DME 1083