Parison reasons

Great care is taken that we design blow molds to avoid thin or weak regions that could result in premature failure. Molds are designed to avoid excessive draw into corners that would result in locally thin areas. For this reason, blow molded products invariably have rounded corners. Another potential source of weakness is the pinch-off line. To compensate for this fact, it is common to program the parison to produce a thickened base. 

Parison cooling significantly impacts the cycle time only when the final parison thickness is large. In thin blown articles the mold is opened when the pinched-off parts have solidified so that they can be easily stripped off thus they are the rate-controlling element in the cooling process. For fast blow molding of even very thin articles, the crystallization rate must be fast. For this reason, HDPF, which crystallizes rapidly, is ideally suited for blow molding, as are amorphous polymers that do not crystallize at all. 

In extrusion blow moulding a parison may sag under its own weight after leaving the die, with unwanted and perhaps random thinning as a result. In part this behaviour may be attributable to an elastic effect (to uncoiling of the chains) but also to viscous flow as the molecules move in relation to each other. It is reasonable to believe that the elastic element will become more important with ... 

The location of the nozzle should also be carefully planed because the different flow velocity in various areas within the cavity can appear as flow lines. The extrusion blow molding process with the accumulator unit can also be the reason for flow lines. Usually the incoming melt is separated by the ram, which causes a flow line in the back. This can sometimes be moved to the parting line of the tool, where it becomes almost invisible. In cases where this is not feasible, it may be possible to create a spiral action (turbulence) when filling the accumulator system. The resulting flow patterns are then mixed as they exit, creating the parison. 

If B 1, there is marked temperature gradient, but if B 1, there is not. For a sheet of polymer of thickness >1 mm cooled by water or by contact with a steel mould, B 3> 1. When B > 10, it is a good approximation to say that the polymer surface temperature is immediately reduced to that of the cooling medium. For a blow-moulded parison (Section 5.5.2) of thickness L = 0.002 mm, cooled in still air on the outside, B = 2. If the lower end of the parison cools for 150 s before the mould closes, it is a reasonable approximation to ignore the temperature gradient through the wall, and calculate the average temperature drop at the end of the parison. 

The parison extrudes down vertically and the process relies on the hot strength of the plastic to hold the parison weight in shape. For this reason blow moulding uses far more viscous materials than would normally be employed for the injection moulding process. A low viscosity material would simply pour out of the die onto the floor or split off before the parison had formed. 

The process of inflating the parison is primarily one of planar extensional flow especially away from the ends of the parsion. Since the ends of the parsion are constrained as the parison expands, the thickness of the wall decreases as the diameter expands, leading to primarily planar extensional deformation. Eor this reason the blow molded part contains primarily orientation along the circumferential or hoop direction and hence exhibits mechanical anisotropy. 

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