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Thread: Overmold Possibility

  1. #1
    Join Date
    Jan 2020
    Posts
    1

    Overmold Possibility

    I'm looking to overmold some metal strips in plastic. Imagine a plastic panel with dimensions 15"x6"x1/8" and with metal strips that are .01875" in thickness. The only thing is I don't want any of the metal showing. Effectively these would be floating in the mold before being surrounded by the plastic. What's the best method to achieve this and is it even possible?

  2. #2
    Join Date
    Nov 2012
    Posts
    106

    Re: Overmold Possibility

    Hi, first of all, I do not have any experience with "invisible" metal inserts. Only with visible :-) Do not také my advice too seriously.
    1) you have to persuade your customer, that small area of visible metal strip is cool and he will be happy with this.
    2) you can put the strip into the tool and some kind of praying can help.
    3) The fixation in the tool is necessary and you have to cover the "spot" with next proces step - something like 2K moulding

    :-)

  3. #3
    Join Date
    Oct 2016
    Location
    McMinnville, OR
    Posts
    168

    Re: Overmold Possibility

    Unless you did it as a two shot process or secondary process to cover it you will have to have some metal showing. Something needs to hold the metal plate in place in the mold if not it would float in the mold and give you odd parts. So you would have some small areas of uncovered metal.

  4. #4
    Join Date
    Dec 2012
    Location
    Serbia
    Posts
    84

    Re: Overmold Possibility

    Hi,

    Is surface important? We do some sort of "holders" that we put on metal plates. They put metal plates in wanted position, but on other hand they are visible on surface. Holders are molded from same plastic as final overmold.
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  5. #5
    Join Date
    Aug 2011
    Location
    Upstate of South Carolina
    Posts
    654

    Re: Overmold Possibility

    First thing make sure you have enough part thickness to allow the inserts to become totally encapsulated within the part. You will need to support the inserts in a way that holds them in place as the polymer fills the cavity to keep from getting pushed around. That would require a core function and the means to start the retraction in the sequencer so you can find the perfect timing. The strips may require Pre-heating due to different cooling rates between polymer and metal strips or to keep the melt front from freezing prematurely.
    Lastly since we don’t know what you’re trying to mold; I’d offer that this could possibly be achieved using metal fibers added the polymer; possibly a wire screen inserts, or carbon fiber filled resin. If you want some sort of shielding for a computer, you could have the parts plated in say copper.
    BR,
    Rick

  6. #6
    Join Date
    Aug 2014
    Location
    Saint Paul, MN
    Posts
    159

    Re: Overmold Possibility

    I like the idea of using plastic blocks to locate the part, but here are a couple of other issues to consider:

    The overmolded plastic shrinks, but the metal doesn't. You have thicker sections of plastic between metal strips and thinner sections over the strips. The metal constrains how the plastic can shrink, so you may end up with more warpage than you would with the plain plastic panel. Also the plastic over the strips is severely constrained in how it can shrink, so depending on the plastic and the size of the metal strips it could crack or split.

  7. #7
    Join Date
    Jun 2014
    Location
    EU
    Posts
    92

    Re: Overmold Possibility

    Hello,

    interesting topic.
    I would go the way of holding or fixing the strips into the cavity too.
    Inserts like Nemanja has, can be one way. The other is tool design with core positioning like Rick introduce. Or You can also combinate this two things together.

    You can use spring/preassure loaded core unless you know (or can predict) the cavity preassure vs preassure needed for core pushing (it can be changed as You need by spring change).
    Or You can time the core into pull position dependig on the time or cyclus state (core pull - starting holding preassure etc.) and than fill the gap by holding preassure.

    In the overmolding its critical the gate position.
    What is bad news - the simulation of this is not well evolved. Only Autodesk moldflow has some function for overmolding.

    Also very important is to choose right grade of polymer for Your appliaciton.

    J

  8. #8
    Join Date
    Jan 2020
    Posts
    7

    Re: Overmold Possibility

    I would say most of this is right in overmoulding I have always had a section exposed normally that is also used for sleeves or bolts. But maybe (and doubtful it will work) use two core pins near the gates that are magnetic pins fill and as you start low speed pack pull the cores flush to face of the mold and finish filling out the holes from the pins with pack and hold but like everyone else has stated polymer selection is critical

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