Peak PPSI or Transfer PPSI when doing Viscosity Study

[tbramel]

Looking for thoughts on what should be recorded when performing the Viscosity Study: Peak Pressure or Pressure at Transfer?

A customer of ours came to the factory and gave a training session for their SIM process and they use Peak Pressure.

[rickbatey]

I always use peak injection psi for anything I do. Valve gate timing can cause a much lower psi at transfer versus a massive change in peak pressure. That's what I always use when I chart my paramaters.
Rick.

[pjhall]

I agree with Rick, but check your injection graph and make sure you don't have a cold slug spike from the nozzle tip. That can really mess you up.

[Suhas]

I think this is a great question and I have been wanting to write about it for a while. In fact, I think I am going to make it my next newsletter Tech Tip!

OK, basics, basics, basics .... Why do we do a Viscosity Curve? To determine the viscosity profile so that we can find the Newtonian (flat) region. Where did this study come from? From the classic rheometric studies that is used to characterize the resin. The equipment is called as a rheometer. The only difference between the molding machine and the rheometer is that one is done inside the mold and one is done outside the mold. As the melt is being injected inside the mold, the temperature of the melt keeps changing (dropping) and therefore is called as a non-isothermal condition. Therefore an in-mold test is called as a non isothermal study and a rheometric study in the lab is an isothermal study.

The basic theory is the same for both conditions - what is the pressure required to maintain the linear speed of the piston/molding screw? Speed is calculated as Distance traveled divided by the time it took to travel the distance. Viscosity (there are several types, but keep it simple for now) is then calculated as the required pressure multiplied by the time of travel between the two points - start and the end of injection. As long as you keep the two points the same, you are injecting the same volume and so keeping everything fixed vary the injection speed - find out what pressure you need to maintain the speed and calculate the viscosity. On the molding machine, if the distance traveled is short, the required pressure will be less, the time will be less and so the viscosity will be less. Increase or decrease the distance, and the viscosity will move proportionally. Or the graph you will generate will move 'Up or Down'. But does the 'Up or Down' movement matter? No! Since you are only looking for the viscosity profile and then the flat region. We really do not care about the numerical value of the viscosity.

So to answer the question from above, if the transfer position pressure is different from the peak pressure position, it means that the the travel distances to the point of transfer and to the point of peak pressure are different. But for developing the viscosity curve? It does not matter which one you choose. Try it and post your results here.

Hope this helps,
Suhas

[joeprocess]

Why do a Viscosity Curve at all, that is my question?

[Suhas]

Hi JoeProcess,

See this link: http://fimmtech.com/index.php?id=6&subid=38

Then see this link for Exceptions: http://fimmtech.com/index.php?id=6&subid=53

Suhas

[joeprocess]

Sorry Suhas,
I know what a curve tells me I just find in unnecessary in most cases.
I used to do them for every process development. Through much data collection I found most parts (if the mold and flow paths are correctly designed) end up in the 60%-80% fill rate regardless of the material used. I'm sure there are a few odd materials that may show differently, but I've run just about everything type of material through a Viscosity Study and found not much difference.

No matter what your max. available speed on a particular press is, the curve will always show you to set your speed somewhere between 60-80%. If your press can run 200 mm/sec. the curve will say to run your speed at 140-160mm/sec. If your press can run 100 mm/sec. the curve will say to run your speed at 60-80mm/sec.

Now, I train to develop a process by injecting as fast as quality will allow and one that will run in the "least capable" press. I know that RJG also promotes a cavity balance test at varying injection speeds. I have not found this to be useful either.

[Suhas]

JoeProcess,

I agree with you. That is why I have written the exceptions also and in my seminars I put the Viscosity curve as the last one as far as priority is concerned. But it does help me to determine 'consistency potential'. I do not always stick to it. (Brent, are you there?? listening?? :) )

In regards to cavity balance, again it is important but need to know what % 'bad', If I am running forks and spoons, I do not care what the balance is as long as all the parts are good. People go through the rigorous exercise of all these studies and then block cavities off :) :) Again, read the May 2013 write up for Cavity Balance: http://fimmtech.com/index.php?id=6&subid=53

Thanks for contributing here by the way.
Regards,
Suhas

[joeprocess]

I see it now. I didn't scroll down far enough. I read the Screw Rotation title and thought it was just about that. Thanks for sharing.

[tbramel]

Suhas,

Thank you for your response. This was exactly what I was looking for.