Trouble Shooting of Blow Moulding Process

Pitted surface appearance

Possible Reasons:

• Air entrapment inside the mould
• Condensation
• Low Blow Rate
• Low Blow Pressure

Possible Solutions:

• Vent the mould to reduce the air entrapment.
• Check mould finishing, polished mould causes air entrapment. Very coarse mould surface results in rough part surface. Sand blasted mould surfaces are usually desirable.
• Check mould for condensation coolant temperature
• Increase the size of air orifice to allow faster blow rate
• Increase blow pressure.

Vertical groves and lines in the part

Possible Reasons:

• Dirty and / or damaged die

Possible Solutions:

• Clean die surface.
• Scratches in die or core, to be removed
• Check for contamination.
• Set the parison straight to eliminate hold up in die.

Irregular markings on the part

Possible Reasons:

• Water condensate or water leakage in the mould

Possible Solutions:

• Run faster cycle.
• Run warmer mould.
• Stop leakage in the mould.

Rough part surfaces and / or poor gloss.

Possible Reasons:

• Rough parison

Possible Solutions:

• Increase die temperature
• Increase mould temperature
• Increase melt temperature
• Increase extruder speed
• Use low molecular weight resin
• Clean die core

Rippling on parison / melt fracture

Possible Reasons:

• Unsuitable die shape
• Extrusion rate too high

Possible Solutions:

• Increase die land
• Increase die diameter
• Reduce extruder speed

Thin areas or holes on the part surface

Possible Reasons:

• Airflow in head is faster
• Parison is not straight
• Moisture in resin
• Contaminated resin in head or die

Possible Solutions:

• Reduce airflow through head
• Set parison
• Pre heat the resin
• Clean the head and die core

Poor wall thickness distribution – longitudinal

Possible Reasons:

• Parison sagging
• Parison control not working

Possible Solutions:

• Change extrusion rate
• Decrease melt temperature.
• Use low M.F.I. resin.
• Check parison control timings.
• Check parison control pressure.
• Check flow control valve setting.

Poor wall thickness distribution- circumferential

Possible Reasons:

• Non symmetrical parison
• Non symmetrical article

Possible Solutions:

• Set parison straight
• Use larger die
• Optimise article weight
• Use larger die

Excessive dimensional shrinkage of article

Possible Reasons:

• Low blow pressure
• Insufficient cooling
• Melt temperature is too high

Possible Solutions:

• Increase blow pressure
• Decrease mould temperature
• Increase cooling time
• Decrease melt temperature.

Warpage

Possible Reasons:

• Insufficient cooling
• Non uniform cooling

Possible Solutions:

• Reduce mould temperature
• Check for efficiency of water flow in mould channels
• Increase cooling time.
• Check efficiency of mould venting.
• Provide more cooling, especially in pinch of area of the mould.

Parison doughnut formation

Possible Reasons:

• Core temperature is low
• Core gone inside the die

Possible Solutions:

• Allow core to reach temperature equilibrium with rest of the system
• Lower the core slightly (to get the weight of the article, reduce the core diameter)

Thinning or stretching at parting line

Possible Reasons:

• Low blow pressure
• Pinch off is too sharp

Possible Solutions:

• Increase blow pressure
• Check efficiency of airline
• Widen pinch of land
• Reduce head and die temperature

Parison tail sticking to the article

Possible Reasons:

• Parison too long

Possible Solutions:

• Shorten Parison length.
• Increase pinch-of land to compression cool the tail

Black specks in parison

Possible Reasons:

• Degraded resin
• Hold ups in the die head

Possible Solutions:

• Thoroughly clean and purge extruder. Avoid prolonged shutdowns with resin held at high temperature
• Improve streamlining of the melt inside

Contamination

Possible Reasons:

• Poor house-keeping

Possible Solutions:

• Keep production area clean