95 to 98% Full part - By volume or by weight

[mwohlg]

Well, here's a stupid question from a newb. How would you accurately measure the volume? Drop your parts in a graduated cylinder half filled with water?

Seems like everyone's got a scale sitting around somewhere, but graduated cylinders... are a little harder to locate.

I suppose you could also do a CT scan.

P.S. BTW, nice to meet you all. I'm Mike.

[Suhas]

Hi Mike,
Welcome to the forum. We are all learning here and so there are no stupid questions. I think I will write this in my next month's news letter but real quick when we say volume - it is simply based on looks. We need the part to be just short and that is where the pack and hold phase needs to start. There is no definite number either. By the way in actual terms - 95% weight will be equal to 95% volume correct? Weight = Volume X Density. Density does not change and so Weight and Volume are proportional to each other. More on the significance of the 95 to 98 later.
Regards and again Welcome.
Suhas

[brentb]

On a new part, how do you know the "98.55% weight? I use volume and specific gravity. I get the volume for the calculation from model or print. So I can use volume and/or weight
Trick is not to fill (and hence pack) on 1st stage injection.
I doubt the part knows if it is 90%, 95% or 98.55% full! I have yet to see data showing effects on part quality between those or other percentages in that range.

BEWARE OF DOGMA and arguing about dogma, be it in religion, philosophy, or Molding!

KOM

brent

[rickbatey]

Suhas- the density does not stay the same. The polymer expands so the volume to weight correlation shifts slightly. I follow Brent as I shoot for their 95% range based on part weight from print. But we've all too often had the argument from Product Eng that your part is too light. They too forget that those figures are calculated from theoretical numbers.
I have went back and adjusted my switchover stroke after I see where my part weight will be, but it really didn't make much difference. We all must balance part weight against part price! None of us want to throw extra dollars into the box of parts.
Rick.

[jeffieg]

i use by vol. because part size and demensions are by pack and hold, just what works for me

[Suhas]

You are right Rick on the melt density being lower that the solid density. But at a given melt temperature - volume and weight are proportional.
Suhas

[brentb]

Beware of dogma!

Many dogmatic Molders quote J. Bozzelli as if he received divine revelations from the molding gods. Though I run the risk of appearing dogmatic, let me quote from a Bozzelli article:
"On a tool that you are sure will eject the part if you make a short shot, do one trial with taking the hold time off, another leaving the time on and taking hold pressure down to near zero, and a third trial where you reduce both hold time to zero and hold pressure to near zero. You should see a different in the size of the parts. All three should be visibly short by volume (not by weight). If they are not all short it almost surely indicates that you have set the transfer point from first-stage to second-stage injection incorrectly. Further, if you are molding the same part in both an electric press and a hydraulic press, you need to check out both. You may be surprised at the difference between them."
In one of his courses he has under the Universal Setup card data: "weight and picture of 95-99% full shot. This is the weight and volume at the end of firts stage"


I say: ' Weight or volume, volume or weight, 95% 99% 90% 98.6% it depends on product and conditions like fill speed and press response, just don't hit the wall of the garage when you put that plastic car in it.

Don't be dogmatic, find out for yourself!

KOM

brent