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Thread: Material cushion variation

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  1. #1
    Join Date
    Dec 2012
    Location
    Serbia
    Posts
    107

    Material cushion variation

    Hi,

    On one of products i have problem that not in each shot is same material cushion. Anyhow i have like 100 shots OK, than few with less or more cushion than should be. On this parts i have shout thru material (is overmold), or not fulfilled part. And that again all is OK for some time.
    Now i need to fix this problem, but i cant relay find solution.
    I check injection pressure, switch over volume, switch over pressure, cycle, dosage and injection time, and all values are within tolerances.
    First what i looked was screw coupling but since i dont have problems on other products (different material) i excluded that as problem, also we use only virgin material.
    Wet material test didn't give us wanted result, so im wondering what else i could try to prevent this from happening.

    Best regards,
    Nemanja.

  2. #2
    Join Date
    Dec 2012
    Location
    Serbia
    Posts
    107

    Re: Material cushion variation

    Forgot to mention that material is PA 6.6 GF30.

  3. #3
    Join Date
    Aug 2011
    Posts
    541

    Re: Material cushion variation

    Nylon should be consistently dried.

    KOM

    brent

  4. #4
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    199

    Re: Material cushion variation

    How big of cushion difference are we talking about? Electric or hydraulic?
    If those data points are clearly 'outliers', you could have a sticky valve somewhere on your injection, or maybe during dosage.

  5. #5
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    199

    Re: Material cushion variation

    May be worth while to clean the linear transducer or replace it if you're getting odd readings

  6. #6
    Join Date
    Aug 2014
    Location
    Michigan
    Posts
    71

    Re: Material cushion variation

    It would be helpful to know what % of the barrel you are using for each shot and the amount of cushion variation you are seeing. When you say "Wet material test didn't give us wanted result"; what was the result? With regards to the moisture, you are using PA 66 GF30, which neat (unfilled) PA has a max moisture of 0.20%. However, you have a glass filled PA and glass absorbs zero water so the max moisture for PA66 GF30 should be 0.14% (approximately). I have seen data sheets from the material manufacturer for PA66 GF50 that states the max moisture is 0.20% which is absolutely wrong. PA66 GF50 should have a max moisture of 0.10% because 50% of it is glass and glass absorbs no water.

    You also mentioned that it is an over-molded process. Is the mold loaded by automation or a human? If it is loaded by a human, is their cycle time consistent every shot? when the cushion goes out of tolerance did they have a long cycle before that? If your operator runs a long cycle time then your residence time will increase causing the material to drool out of the barrel and fluctuations in your process. You can combat this by adding time to your cooling time and extruder delay (if you raise your cooling time by 5 seconds then raise your extruder delay 5 seconds), this will buy you more time without adding residence time. Be careful with this as it might change dimensions on your part so if you go this route, make sure you have your QA department do a dimensional check on the parts after. Its best just to get an operator that can keep up with the machine if possible.

    Chris has an excellent suggestion, before you go messing with your process, check your linear transducer to make sure it is not a mechanical/electrical issue.

    Hope this helps,

    Husky

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