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Thread: Voids in polypropylene parts - raw material problem?

  1. #1
    Join Date
    Apr 2021

    Voids in polypropylene parts - raw material problem?

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    Hi all,

    We having been having problems with intermittent voids over the past year while making a polypropylene part for our customer. These voids will appear over the course of a few weeks in the thick wall section of the part, then will disappear completely for another few weeks, only to reappear again. There seems to be some correlation between the raw material we use and when the defects occur. We buy the specific grade of raw material from one manufacturer, but it appears that they have two different manufacturing sites for this specific grade of PP material. When we use material from site 1 we seem to have problems, but when we use material from site 2 the problem seems to go away. We have tried to process the problem out of the part but as it is a validated process there is only so much that we can do at this current moment.
    Unfortunately it doesn't seem as it's going to be as easy as asking for material from site 2 only, the material manufacturer is very resistant to the idea that there could be differences between the two sites, and will take some convincing otherwise.
    See the two pictures attached and let me know what you think. One is raw material granules from site 1 (pellets with one big void/bubble inside), the other from site 2 (pellets with smaller multiple bubbles inside). I would have expected them to look more similar than they actually are.

    Thanks in advance,

  2. #2
    Join Date
    Feb 2022

    Re: Voids in polypropylene parts - raw material problem?

    Looks like they might be pushing their material too fast, so you are not getting a smooth flow at sight 1. Since you are probably not going to get much cooperation from them as long as their material certs are within tolerance, are you allowed to adjust your back pressure with your validated process? I would try to play with your screw speed, back pressure, or temps to increase your consistency.
    I understand the pain of validated cycles all too well LOL.

  3. #3
    Join Date
    Apr 2021

    Re: Voids in polypropylene parts - raw material problem?

    Yeah, you're right about the cooperation, or rather lack of it. Certs say everything is fine so there couldn't possibly be anything wrong we're being told. Radio silence from the material supplier when asked about their manufacturing methods, the differences in the pellets, including images of the differences between the pellets, and images of cut parts of pallets from site 1 showing their hollow centre. But it's no big surprise there's no response forthcoming as of yet.
    We know what will fix the problem, but it will require a time consuming revalidation in order to make finished parts from the material supplied from site 1. And of course someone will have to foot the bill for that, and the usual arguments that go with those situations.
    At the moment we can't make any parts for our customer. Raw material supplier is saying you'll have to take whatever material we can get for you given the global supply chain problems of the past two years coupled with what's been happening in Ukraine recently. Oh, and also a substantial increase in unit cost per ton for the material too, thank you very much...!
    We'll have to see how it pans out over the next few weeks...

  4. #4
    Join Date
    Oct 2014

    Re: Voids in polypropylene parts - raw material problem?

    I wonder if it is a "wide spec" material, where the MFI could have variation and still be "in spec". Can you correlate the defect to specific melt flows?
    I had a similar problem where material had to be at low end of spec to produce acceptable parts.
    Just a thought.

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