Plastic Caps cracking for unknown reason 1

[gileswood]

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

 

[boora]

Have you contacted your supplier, and is the resin of the specifications that you requested?
There seems to be a problem of possibly a contaminant of a noncompatable resin present.
What are the temperatures for your process?

 

[Demon3]

First I would suggest not using a blend of polymers, that is likely the cause of the failures. Hardly any polymers mix, even different types of PE. One polymer will be in there as particles thus ruining the strength of your part.

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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[patprimmer]

I agree with Demon3

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.

Regards

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[dwightdixon]

I presume you have molded this part from this same material and in this mold for years. Right? I presume you have looked to see if the cracks are not in the same location and not predominately in the same cavities. Right? I presume you have not changed anything in your manufacturing process or testing procedures. Right? I presume you only run one color and it does not occur when you run natural. Right? I presume you do not run any regrind or changed your regind proceedures. Right? I presume the caps will not fail, if you do not assemble them to the bottles and store them. Right? I presume you have preventative maintenance proceefure to be sure all vents are open and the failures do not occur at the knit lines that would be a problem if you did not maintain the vents. Right? I presume you use the blended resins because you are adjusting the tensil modulus of the material to maintain high retention force of the cap on the bottle without cracking the cap. Right? The first thing I would do is look at non-pigmented caps using Polarized light to see that the failures all occur at high stress points or knit lines. The next thing I would do would be to adjust the formula down to a lower modulus and rest a bunch of caps for resistance to cracks and acceptablity of cap retention force. I woud also look at possible ESCR contamenents which might be getting on the caps before you assemble them to the bottles. I would also develope a destructive sample test that might involve use of an ESCR agent to make sure you do not get into this problem again. These are the things I have done successfully before when we got into these problems and they have alway worked to prevent future problems.