Parison sagging

Therefore since the parison sag time is less than this, the above expression may be used to calculate the amount of stretching... 

Weak shoulders on bottles Parison sag Parisons too long or short Container too light Reduce melt temperature and decrease/increase extrusion rate Program increased weight... 

Shark-skin occttrs when the melt index and the coefficient n are small. This may happen when a larger amount of plasticizer is used Die swell is caused by elastic deformation in flow. It resrrlts from material expansion after it leaves the die. Increase in shear rate increases die swell. Parison sagging is caused by a decrease in pseudoplastic properties (coefficient n becoming closer to unity) and decreased friction between metal and melt. 

HOPE in this process is its melt strength, which is important to prevent parison sagging. 

Parison sag (parison parting line and blow mold parting line do not overlap)... 

Consider a scenario where a PCR tends towards melt instability at high speeds or at normal melt temperatures. Excessive parison sag and bubble or web instability are indicative of a lack of melt strength. Therefore, the addition of a low MI resin will improve the elongational viscosity of a PCR that has a tendency to excessive shear thinning and increase its melt strength. This should improve the cycle time for blow molding, improve bubble stability in blown film or increase the line speed before the onset of draw resonance in cast film. 

Blow- molding Uniform melt flow, enhanced physical properties Reduced parison sag Uniform wall thickness Bi-orientation Reduced melt fracture at high output rates Can reduce output if too high... 

The parison formation process can be considered as an annular extrudate swell and sag problems of viscoelastic fluids. The parison swell and sag have competing effects on the parison thickness parison swell results in a thicker parison whereas parison sag results in a longer and thinner parison. 

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