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Thread: Maximum pressure inside the mold

  1. #1
    Join Date
    Apr 2018
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    Question Maximum pressure inside the mold

    Hello guys is there any general rule that shluld be considered during the inyeccion process to avoid broken the insert due the pressure applied?
    I know that it will depnd of the steel hardenesst but what will be the limit? Some times the proccess enginering apply "too much pressure" (more tha the one specified on the mold flow) in order to avoid quality issues but the results is broken inserts...what is the limit?

    thanks for your valuable help.

  2. #2
    Join Date
    Oct 2016
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    McMinnville, OR
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    103

    Re: Maximum pressure inside the mold

    I don't know that there is a simple answer to that. Depends on what "inserts" they are breaking, how big of a machine it is, how the mold was built, what it was built out of, design of the mold, and various other things. If you are talking about a pressure used on the mold flow analysis that isn't a limit. From my experience with mold flow it is just a predicted value for the speed you set. Maybe should look into why they have to use so much pressure? Just my two cents.

  3. #3
    Join Date
    Apr 2018
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    Re: Maximum pressure inside the mold

    Thanks for your reply and help.

  4. #4
    Join Date
    Aug 2014
    Location
    Saint Paul, MN
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    123

    Re: Maximum pressure inside the mold

    I have seen long, skinny core pins deflect under high injection pressure, but steel is pretty tough and I don’t think I’ve ever seen plastic pressure break a pin. I’d be more inclined to look for mechanical reasons for breakage – interference where there should be clearance and such.

    Incidentally, unless you have sensors in the cavity you aren’t going to have any idea what the cavity pressure at the breaking insert is, and even with sensors you won’t do better than a rough estimate unless the sensors are very close to the breakage site.

  5. #5
    Join Date
    Sep 2012
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    Lakewood, WI
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    Re: Maximum pressure inside the mold

    It is really easy to move steel around in a mold and the root cause is cavity pressure. Not typically during filling though, unless you are filling the cavity to 100% full on 1st stage or you are filling so slowly that the melt flowing into the cavity cools off to a point that higher fill pressures are needed to complete the filling process. Type of tool steel is a factor, but don't be fooled that a higher hardness is better. I'm not a metallurgist, but have witnessed higher percentage of core deflection causing breakage with harder materials vs. a softer material that flexes a little more. Also, keep in mind that core steel that has deflected has most certainly created an reverse draft condition on the side the steel is bent towards so the steel flexes during filling and sees pressure during ejection or mold opening depending on where the coring is.

    As mentioned before, can the overall cavity pressure be reduced? What quality issues are being seen? I'm guessing sink or dimensional? Can the tool be adjusted to reduce pressure needed? Can multiple gates be put on the part so that equal pressure can be applied from two directions? Can the core pins be telescoped into the other side of the mold?

  6. #6
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
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    131

    Re: Maximum pressure inside the mold

    Casber2311,

    The question is too broad to have a pressure limit for mold breakage. You should only use the minimum amount of pack pressure needed to pack your parts (until there's no more distortion or sink). In a perfect world the tool-steel should be adjusted to satisfy the correct dimensions when part is fully packed and process is capable.

    I would almost never rely on a mold-flow simulation to determine pack pressure.

    Try opening up your gates (increasing flow) so you can pack your parts with less machine pressure

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