Plastic Caps cracking for unknown reason
Plastic Caps cracking for unknown reason
(OP)
I am having a problem with the cap we manufacture for our sports bottle. It develops serious cracks between 1 and 7 days after being moulded: the mix is a 60% LLDPE (Sasol Lm3160) and 40% LDPE (Sasol LT019) we have tried using pigment and masterbatch at the regular 2-3% but we are still getting these cracks. can anyone explain this and offer some guidance??
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Giles Wood
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Giles Wood






RE: Plastic Caps cracking for unknown reason
There seems to be a problem of possibly a contaminant of a noncompatable resin present.
What are the temperatures for your process?
RE: Plastic Caps cracking for unknown reason
Second, it could be additives giving ESCR so make sure you know what's in the pigment masterbatch. It could be full of stearates, masterbatchers put those in there because they are cheap and so thay make more money. Make sure the pigment masterbatch is the same kind of PE you choose for the part so it mixes.
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RE: Plastic Caps cracking for unknown reason
It is most likely ESC or incompatibility of the blend.
Another posibility is neucleation from the pigments causing stress induced by excessive shrinkage.
Make sure your mould and melt are correct temperature and not to cold.
Make sure your pack pressure is correct.
Make sure your shrinkage is correct.
Do some mouldings in the blend without masterbatch, and if they crack, do some in each resin straight.
If they are cracking a few days after moulding, while in storage, it is most likely ESC with the stress being introduced through post moulding shrinkage.
The short answers are decrease the post moulding shrinkage or increase the elongation at break. How to do these is another much longer story.
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RE: Plastic Caps cracking for unknown reason