Residence Time Calculations .....

[Suhas]

Hello All,
'I went with the flow' and for about couple years now (after my book was published), I realized that I have been doing and telling the wrong calculation for residence time. The calculation of (Max shot/Mold Shot Weight) times the cycle time is an estimate but not accurate. If have a 20 gm mold shot and a 100 gm barrel then we do not build up all the 100 gms at one time and inject 20 at a time. The calculations are a little more complicated and also based on screw design, etc.
Another part that complicates this is that the plastic does not reach the processing temperature till it reaches a certain part of the screw. So where is the point where we start the calculations?
I don't have the answers yet, but may be some day we can come with a 'close calculation'.
Food for thought

Regards,
Suhas

[brentb]

Suhas,
I meant to tell you, but the student shouldn't correct the teacher.
I used to run the press, and when the resin was just covering the screw in the feed throat, add a different pellet or two or three (depends on shot size), then stopwatch the time until the color appeared (hot runner can be tricky, beware).
KOM

brent

[Suhas]

Hi Brent,
We are all students and we are all teachers. I bet you in the next 25 years there will be new and cool stuff that we will be using in molding. If you read some older texts you will see some of those techniques are obsolete and make no sense.
PLEASE do point out my misconceptions. I still have a lot to learn.
Thanks buddy!
Suhas

[brentb]

I was kidding, I didn't catch the "error".

Someday we must have a thread about being " over-anal" in molding!

KOM

brent

[rickbatey]

Brent,
That would be anal-retentive. I think that's a big part of what drives us toward perfection in this business!
Rick

[mp8020]

Suhas-

What is the correct method you have identified. I have been using the method on pg 92 of you book, but have been doing some research and found other methods that produce different results by a factor of 1.4x or 2.0x.

I have found this general method that does not account for screw design completely; however it does give a better under understanding of usable volumes in the barrel.

Machine shot size, "M," in grams
Maximum injection screw stroke, "L," in diameters
Shot size being molded converted to the same melt density as polystyrene, "S," in grams
Cycle time of molding, "C," in seconds

Multiply "M" by two if "L" is less than four. Multiply "M" by 1.4 if "L" is greater than or equal to four. For example, if "M" is 200 grams and "L" is four, the result is 280.
Divide the result by "S." For example, 280 divided by 100 grams is 2.8.
Multiply the result by "C." For example, 2.8 times 15 seconds yields 42. Under these conditions, the barrel residence time is 42 seconds.


It seems this would also effect %shot which is what i am more interested in as I have been on a bit of a crusade for my company to become more focused on the 20/80 rule in an effort to reduce part to part and run to run variation.

Do you have and formulas you can share that are corrected or does your search continue?

thanks
Matt

[Suhas]

Hi Matt,
Thanks for the above info. Here is the issue with most calculations: If the max machine shot size is 100 gms, and the shot weight (Parts + Runner) are 40 gms, we build 40 gms shot and inject it. We do not build a 100 gms, shoot 40 gms, and then another 40 gms and then shoot 20, build 20 and then shoot the rest. - Can't happen anyways. If that was the case, most formulas would be close. But then, how do you take into consideration the volume of the molten plastic on the screw? A larger metering section will keep the melt for a longer time. So the search continues. But .....As I was writing this, I just thought of something. But let me put this in the right way and get back to you all.
We all get stuck in a rut with tunnel vision, I think I have an out of the box practical, non-engineering calculation solution for this ... I think ... I am in the bath tub now, when I shout Eureka, you will know :)
Suhas