TL;DR
Parison wall variation is usually an uneven die gap, worn die pin, or incorrect parison programming. Check die concentricity before adjusting parison programming.
What you might see
- uneven wall thickness in finished bottles
- pinch-off flash heavier on one side
- parison sags to one side during extrusion
- part weight varies part to part
Likely causes
Die pin off-center causing uneven annular gap
Non-uniform melt temperature in the die head causing viscosity variation
Parison programming (PWDS) set incorrectly for the part profile
Worn die pin or die bushing allowing eccentric melt flow
Required tools
- Ultrasonic wall thickness gauge
- Digital scale for part weight
- Heat-resistant gloves
- Die adjustment wrench set
Safety first
- Die heads operate above 200 C. Wear heat-resistant gloves and avoid skin contact with any metal surface near the die.
- Hot molten plastic can cause severe burns on contact. Never place hands under the parison while extruding.
- Ensure the mold clamping system is locked out before making any die adjustments with molds open.
Procedure
- 1
Stop production and allow the die head to reach thermal equilibrium at the set temperatures.
Warning: Die head and die pin surfaces reach temperatures exceeding 200 C during operation. Wear heat-resistant gloves when inspecting or adjusting the die area. - 2
Extrude a length of parison onto a flat surface and measure the wall thickness at four quadrants (top, bottom, left, right) using a wall thickness gauge.[1]
- 3
If one quadrant is consistently thinner, loosen the die adjustment bolts and move the die pin toward that thin area to open the gap on that side.
- 4
Re-extrude after each adjustment and re-measure until the four-quadrant variation is within specification.[1]
- 5
Verify that all barrel and die head temperature zones are at setpoint using the machine temperature controller readout.
- 6
Review the parison wall distribution program (PWDS) to ensure the profile matches the part geometry requirements.
- 7
Run 20 production cycles and weigh the parts to confirm part weight variation is within the acceptance range.
Sources
Bekum Bekum H-Series Blow Molding Machine general technical documentation, Bekum
Bekum H-Series blow molding machine operator manual, die head adjustment and parison thickness control (general)
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Clamping faults are usually a hydraulic pressure shortfall, worn toggle pin, or position sensor misalignment. Verify hydraulic pressure at the clamp cylinder before adjusting the mechanical system.
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