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Thread: Material cushion variation

  1. #1

    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

    Re: Material cushion variation

    Forgot to mention that material is PA 6.6 GF30.

  3. #3
    Join Date
    Aug 2011
    Posts
    524

    Re: Material cushion variation

    Nylon should be consistently dried.

    KOM

    brent

  4. #4
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    100

    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
    100

    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
    47

    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

  7. #7
    Join Date
    Aug 2011
    Location
    Upstate of South Carolina
    Posts
    571

    Re: Material cushion variation

    What type of mold are you using for this over-mold? I’ve seen rotation over-mold where the cavity volumes are close but not the same!

  8. #8

    Re: Material cushion variation

    Hi,

    Brant, material is 100% of time dried on same conditions what manufacturer recommended.
    Chris, difference in material cushion is about 0,2-0,3cm3, i was excluding machine as problem, since i have only on this one product issue. other products that are 10x smaller dont have problem, and is almost same material. Only difference is that problematic material have some additives for self-extinguish. All linear transducers are clean regularly during weekly maintenance of machine.
    Husky, im using around 35% of barrel per shot. Wet material test give us just more often bad shots because of parameters out of tolerances.
    It is overmolded process, but cycle time is always 32,3+/-0,1s.
    Rick, we use slider table, 2x2 cavities. veight of molded parts in all cavities is 22+/-0,1g.

    Now i will check linear transducer, and i will try to reduce residence time of material, and lest see what results will be.
    Thanks for answers and tips.

  9. #9
    Join Date
    Mar 2016
    Posts
    11

    Re: Material cushion variation

    Are you running a standard heat profile on the barrel? or reverse profile? I have had instances where I wasn't getting it melted soon enough and causing blockages in the compression zone resulting in cushion variation without significant dose time changes.

  10. #10

    Red face Re: Material cushion variation

    Quote Originally Posted by Brisli View Post
    Are you running a standard heat profile on the barrel? or reverse profile? I have had instances where I wasn't getting it melted soon enough and causing blockages in the compression zone resulting in cushion variation without significant dose time changes.
    For now we running standard heat profile. But I will try reverse profile, and see what will happen.

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