Troubleshooting problems with random copo PP

[Wiseco]

Hi,

I've made some test using random copo PP on an injection-blow machine. All the test results in some defect in bottles. It's like small cracks or micro-fractures in the interior of the bottle. Someone knows what could be the cause? It's the first time that I work with polyolefin platics so I'm a bit lost with these new problems.

I can post some photos if it's needed.

Thanks!

[Will Call]

It sounds like you've got moisture in you air.

[Wiseco]

Does PP are more sensitive to that compare to other plastics? Because we have never faced this problem before.

[Indy]

Cracks would make me believe a separation of the blend occurred or chilling cycle to cold. Just guessing
indy

[Wiseco]

Maybe I'm blowing them too quickly? I'm at 80psi on the air and the melt temp were at 400-425F.

[Indy]

It looks like low blowing air and possible moisture in the molds being the problem. Could be the cycle timing too.
Best Indy

[Will Call]

I agree with Indy and would add that the preform may be too cold. Those don't look like cracks to me but more like bubbles and flow lines

[Indy]

I guess you could add vacuum and venting problems to my expanding list here. tehe

Best indy

[Wiseco]

I must say that I thought it was from the injection stage like air entrapped between core rods and preforms but it doesn't seem it's that.

I agree with you Will, I think it's the preform that is too cold or core rods that are so they cool preforms but curiously, I've retarded the injection to the max so the plastic would stay less time on the core rods before being transfer and blow but either way, these bubbles stay. In the startup, bubbles are near the tip of core rods but as the mold setup get warm, bubble form upper in the bottle. I don't see some logical in the fact that can be moisture as the opening for air are at top most section of the blown section of the bottle.

I'm making those tests on a styrene mold setup

[Wiseco]

I guess you could add vacuum and venting problems to my expanding list here. tehe

Best indy

Yeah I saw that PP is sensitive to venting compare to styrene.

Another thing, I try to get my injection mold hotter possible to gain clearness but as you see in the first photo, it makes a big bubble in the top most section due to the plastic that stick a bit to the cavity so the plastic get pull away from the core rods. Maybe small cracks or bubbles are causes by that...but I don't think as at startup, bubbles are at the bottom most section of the bottle.

[Wiseco]

PP doesn't need to be dryed before processing it, right? I mean like PET?

[Indy]

I don’t think drying is necessary for pp unless your blend has something else in it. You don’t say if it’s happening in every mold but you could try throwing a piece of masking tape in the mold but not in the parting line to see if it is a fill or mold problem.

Best indy

[Dr. Dick]

I don’t think drying is necessary for pp unless your blend has something else in it. You don’t say if it’s happening in every mold but you could try throwing a piece of masking tape in the mold but not in the parting line to see if it is a fill or mold problem.

Best indy

Unless as reported in another p dot com post the resin supplier is adding water to the resin . I would do a quick check for moisture in your raw material.

[Wiseco]

Just to give some updates, I've talked and read a lot to find a solution to my problem. One honorable guy in the blow molding trade told me that my temp settings were probably too cold and probably core rods should have a release coating. Some of my core rods are out for coating, should come back this week so I will run some tests. Also, I've received 3 grades to play with.

I was suspecting a release problem at the first state of development and this honorable guy just confirmed that. I'm a bit proud to have suspect this in the first state but not very proud for not still stick to my idea...

I'll post results soon.

[Indy]

Since you brought this back up I was wondering if you have done anything with the needles. Could they be to long or to short, just a thought?

Best Indy