The 80-20 Rule For Available Shot Size

I received this common question via e-mail the other day…

I heard that it’s good to have shot size from 20 to 80 percent of the machine’s available shot size. Has this conclusion was tested, or has somebody published a white paper study?
My Response
Although I am not familiar with studies to validate this specific conclusion, but you can find studies on many of the aspects which contribute to this general rule of thumb. I can give a good argument to support both the 80% and 20% limits. Even more important… is the fact that you can easily generate your own data to validate/test any of these arguments with your specific processes.
80% – This rule of thumb is provided to give a buffer to allow for process variation for a couple compounding reasons… (1) A good cushion should be between 5-10% of the overall shot size. (2) Many machines require 2-5% of the shot size to decompress the screw after recovery. (3) The check ring will typically vary 2-5% during fill resulting in a similar variation in cushion size. When you add these variations up, you need a 10-20% buffer to help ensure you can properly fill the part.
20% – The typical general purpose screw contains approximately 1-2 shots of material within the flights of the screw. This means that a process running at 50% capacity will have an estimated barrel residence time between (2) and (4) * (cycle time). Likewise, a machine running at 20% capacity has an approximate residence time between (5) and (10) * (cycle time). If you bring this to the extreme, a process running at 5% capacity could have a barrel residence between 2000% and 4000% of the cycle time!
These are rules of thumb, and therefor there are always exceptions. With the use of Accurate process controls and short travel check rings, you may be able to violate the 80% rule. I never recommend violating the 20% rule as it is likely to affect the part quality, process stability, and it will waste a large amount of energy.
Additional Thoughts
Unlike most manufacturing processes, the compressibility and shear thinning characteristics of plastics cause inherent variability in the process.
  1. MJ left a comment on 2009/09/29 at 3:52 pm

    Hello Andy.
    Shot size meaning: transfer position+amount of material+decompression?

  2. Andy Routsis left a comment on 2009/09/29 at 4:30 pm

    Hello MJ,
    Shot size refers to the amount of material from the end of screw recovery to the final cushion. This typically does not include screw decompression. As the material fluctuates, you will need room to adjust this value to maintain an adequate cushion.
    For example if the shot capacity was 100mm, screw recovery ended at 80mm, and the cushion was 7mm, the shot size is 80-7=73mm. This would use 73% of the machine’s capacity. The decompression does not factor in the equation.

  3. MJ left a comment on 2009/09/30 at 11:19 am

    Hello Andy.
    We running products, where I have shot size less than 20% of machine availability. Every set-up I measured different shot weight (without Hold pressure). Is it possible, that so small amount of material can cause of my problem?

  4. Andy Routsis left a comment on 2009/09/30 at 11:55 am

    I agree,
    The long residence time can cause potential degradation and breakdown of the polymer chains. This may make acceptable parts but can make the melt inconsistent.
    In this case, the travel required to seat the check ring is becoming an increasingly larger percentage of the overall shot size.
    Additionally, if you are running a hydraulic molding machine, the amount of distance necessary to stabilize the injection unit will consume a significant percentage of your overall shot size.

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