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Thread: Calculating Percentage of Machine Shot Volume

  1. #1
    Join Date
    Apr 2012
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    Shenzhen, China
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    Calculating Percentage of Machine Shot Volume

    Just found out that the company I am working for uses a totally diferent method for calculating the percent of shot volume used.

    There mothod is based on stroke: (srew stoke for part - cushion)/max machine stroke.

    I am used to correcting max shot volume using melt density of the material used then dividing the shot weight by the corected max machine shot volume.

    The method they are using gives a totally different result. I do not agree with their method. Looking for others opinion.

  2. #2
    Join Date
    Aug 2014
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    Saint Paul, MN
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    135

    Re: Calculating Percentage of Machine Shot Volume

    When you say “corrected max shot volume”, do you mean you are calculating a max shot weight based on the resin you are using? Is your corrected quantity in grams (not cubic cm)? If so the two methods should be equivalent, although yours might be a bit more accurate. On the other hand, I would trust a calculation based on observed max shot more than one based on a number (max shot weight or volume) pulled out of the manual. You could try your calculation slightly differently: Use melt density to convert your shot weight to a shot volume and divide that by max shot volume.

  3. #3
    Join Date
    Feb 2016
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    Re: Calculating Percentage of Machine Shot Volume

    Linear calculation is better in my opinion- based on the assumption the process is already optimized and shot size/transfer positions are already established.
    Only time I would use density of material is to calculate an ESTIMATED shot size going into a new process.

    If you calculate shot volume using density you'll need to use the MELTED plastic density in the equation (not the COOLED plastic density). The density on the data sheet is generally given in pellet form so this would be incorrect to use in such equation. Even then you'd need to convert that volume to volume in Polystyrene because that's how the machine manufacture will dictate shot size.. (If I'm off here, please someone correct me!)

    Also don't forget to subtract decompression (along with cushion) to get an accurate shot size.

  4. #4
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    Apr 2012
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    Re: Calculating Percentage of Machine Shot Volume

    I use a calculator that I downloaded from Reiloy USA. I made a modification to it so it is all in grams. I am working on another that is in ccm.

  5. #5
    Join Date
    Feb 2016
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    Re: Calculating Percentage of Machine Shot Volume

    Quote Originally Posted by tbramel View Post
    I use a calculator that I downloaded from Reiloy USA. I made a modification to it so it is all in grams. I am working on another that is in ccm.
    Personally I would still go off what the actual machine outputs are over the calculated shot size..

    How could you be sure the melt densities are accurate?

    The volume of plastic changes as temperature is increased/decreased as well as its density (mass of a material per its given volume) See attached
    Click image for larger version. 

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    Unless you had an exact volume/temperature graph of your specific material grade (leaving pressure out of the equation) it would still really only be an estimated shot size, whereas machine outputs are actual values.\

    Just my 0.02

  6. #6
    Join Date
    Aug 2011
    Posts
    560

    Re: Calculating Percentage of Machine Shot Volume

    Good discussion folks!
    Technically, both should get you the same result. As discussed above, I would calculate the actual melt density at the given temperature and also don't forget the set Back Pressure and then use the formula. Remember the back pressure will also change the amount of plastic you have in the barrel.
    -Suhas

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