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Make injection molder

  1. Oct 18, 2014 #1

    I am making an injection molding machine for fun. There is a lack of available information regarding the details of how the process works.

    There is something call the Lyman extruder that uses a wood auger bit as the injection screw. This auger is shaped much differently than the injection screws selling on alibaba.

    1. How does the screw purge air?
    2. How does the mold purge air without creating flash clogging the vent?
    3. How do I calculate the needed pressure to fill a mold? (using melt flow index, gate size, and temperature)
    4. How do I calculate the pressure a screw can generate on a fluid? (given X torque)

    Thank you for your help!

    *Disclaimer: this project is for fun, I know there's easier ways to do it like a plunger or 3d printer or resin casting. I currently do resin casting and 3d printing and such but I am obsessed with having an automated injection molder even if it can only make tiny parts and doesn't work very well.
  2. jcsd
  3. Oct 19, 2014 #2


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    I can see that you missed a question.

    1. How do I make it safe?

    An injection moulder making a part with 8" of cross sectional area will present you with the following major risks.
    Clamp force of around 30 - 50t.
    Injection pressure of up to 1,000 bar.
    Barrel temperatures of over 200°C (~400°F).
    Liquid plastic that sticks to your skin and takes a long time to cool.
    Even a plasticising pressure (back pressure) of 5 bar is sufficient to splatter the material when you purge the barrel.
    As your part gets smaller the clamp force requirement reduces. The other risks remain broadly the same.

    Please think about these risks as you proceed.

    As for you other questions
    1. I don't actually know this, but I assume back up the feed throat.
    2. Vents are sized small enough that the meniscus of the molten material will not enter it. 0.010 mm is a starting point. Most do not exceed 0.020 mm.
    3. Pack pressure is typically optimised iteratively. It can be predicted using software (e.g. Moldfow) but further optimisation is required. Figure a maximum of 1,000 bar for design purposes.
    4. You do not give sufficient information to answer accurately. However, you need to be able to exert a pressure of up to 1,000 bar.

    Please also note that the screw and check valve work like a plunger during injection. An extruder does not.
    Last edited: Oct 19, 2014
  4. Oct 26, 2014 #3
    Thank you for reply.Safety is #1. I have lexan to enclose the machine.

    I do not understand why I need 1000 bar. If I inject with 10 psi what will happen?
    Manual piston type injection molders do not use 1000 bar. The piston is usually 3/4" diameter and the operator pulls down without doing a pullup so it must be less than 140 lbs. The mechanical advantage of the lever looks 3:1. So about 741 psi.

    I cannot find information on the purpose of injecting under high pressure. I know more pressure means smaller gates, so less fiishing touches. Or more pressure means lower temps and faster cycle times. But I'm looking to do the bare minimum proof of concept. If I had a pump-assisted die cast, is that injection molding?
  5. Oct 28, 2014 #4


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    My background is industrial and not hobby. In industry, high pressures are needed to maintain part size and compliance with the cavity walls as the part shrinks. Even a slow process with large gates is likely to need 200 bar to properly and consistently pack the part out.

    You may find ~750 psi (50 bar) gives you an acceptable part. If you try it then let us know how you get on.
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