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Thread: Increasing gate size to 3 x wall thickness - polycarb

  1. #1
    Join Date
    Nov 2018
    Posts
    4

    Increasing gate size to 3 x wall thickness - polycarb

    I designed a part which the toolmaker struggled with for many months.

    It's a thin walled bucket, about 420mm in height.
    Wall thickness is 2.5mm but the base (where the gate is) was increased to 3mm.
    Part weight 1.3kg.

    Material is clear Polycarbonate.

    On the trials in China where tool was made, gate was originally about 3mm diameter, but then we changed it to a 6mm valve gate. The machines in China managed to produce parts like this at reasonable temps, but here in Australia we are struggling, and needing temps of around 330/340 and core temp of 90 deg C. This is on a 1200tonne machine

    This is with a PC with an MFI of 36.
    We tried CD/optical grade PC with a MFI of 70 but had issues with it cracking before ejection (likely moisture related)

    I have inspected polycarbonate containers of similar dimensions, (Cambro brand), and they have gates of approx 8-8.5mm.

    Personally I think the most obvious thing to help here would be to change valve gate to 9mm.
    This more than doubles gate surface area, and halves shear rates. I'm getting mixed opinions on whether there will be any improvement.
    From my understanding, even if the thin wall and the distance needed to travel is the issue, increasing the gate size will surely help to some degree.

    I tried modelling the flow with Autodesk moldflow however couldn't seem to change the gate from auto to manual in order to investigate.

    Any thoughts on this?

  2. #2
    Join Date
    Aug 2011
    Posts
    587

    Re: Increasing gate size to 3 x wall thickness - polycarb

    Are you pressure limited on the current machine by any chance?
    Suhas

  3. #3
    Join Date
    Dec 2016
    Posts
    131

    Re: Increasing gate size to 3 x wall thickness - polycarb

    Hi,

    If I understand this, wall thickness is 2.5-3mm and gate is 6mm, so my opinion is that increasing gate size will not help.

    Joel

  4. #4
    Join Date
    Nov 2018
    Posts
    4

    Re: Increasing gate size to 3 x wall thickness - polycarb

    So are we saying that the beautifully made polycarbonate Cambro containers have unnecessarily large gates? Surely they were sized around 8-8.5mm for a reason.

    Suhas, I presume they are running it at capacity so not sure what you're asking? No doubt a bigger machine that can achieve higher pressured would do better

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