Monday, June 14, 2010

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

6 comments:

Haresh said...

It's really helpful
I have been applied it.
This post will be just like a sanjivani who are facing various problems in his vertical moulding machine.

faizan said...

thanks,i take help.by applying these techniques I am able to solve the problem

Anonymous said...

hello
i have an straight dark line in a forward part of the bottle, it wsnt der earlier, strtd all of a sudden, cleaned head, barrel, screw and even changed machine bt i am still facing da same problem. hope to find a solution here.
thank u

Reuben said...

What is the propotion of Preblow and Support air. I am struggling to make good 20L drums using cut and seal knife. The drum has got a tape hole on the base.

Tasnim said...

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Anonymous said...

I am having a lot of trouble with leaking bottles at the mould vents. Blowing 2Litre HDPE milk bottles on a reciprocating screw blow moulder. The vent lines are quite noticeable on the finished bottle and at times the area of the vent is thin and causes leaks
Thank you