Blow Molding
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Severe Sagging of Polypropylene '2079851475616104457 Parison
Problem Phenomenon
The extruded parison sags and becomes longer due to its own weight before the mold closes, becoming thinner at the top and thicker at the bottom.
Parison sagging causes uneven wall thickness distribution of the product, and in severe cases the parison breaks or the mold cannot close normally.
Cause Analysis
(1) The melt temperature is too high, and the melt strength of polypropylene is low
(2) The parison extrusion speed is slow, and the parison hanging time is long
(3) The parison is too long or the wall thickness is too thin
(4) The MFR of the raw material is high, and the melt viscosity is low
(5) The high environmental temperature aggravates sagging
Solution
(1) Appropriately reduce the extrusion temperature to the lower value within the range of 200-280°C to increase melt strength
(2) Increase the extrusion speed and shorten the parison hanging time
(3) Shorten the parison length or increase the parison wall thickness
(4) Select a grade with a lower MFR to increase melt strength
(5) Control the workshop environmental temperature and avoid high temperature
Key Takeaway
Parison sagging is caused by low melt strength or slow extrusion, and needs to be solved by lowering the temperature, increasing the speed, and material selection.
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