Complications From Changing Screw Diameter

I just received this interesting question yesterday from a medical molder…

We are planning to change screw size because we are using less than 20% of the shot size capacity of the press, can you give us a little information how this problem affect our process control and how big is the impact on it if we change the screw size to have it in the advised range please? Also if we change the screw and barrel size do we need to make a lot of modifications to our electric molding machine?
My Response
First, I just wanted to re-iterate the comments I made in a previously blog entitled ‘The 80-20 Rule For Available Shot Size‘.
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!
As a medical molder, you can create some potential liability since the residence time will be so high relative to the process. This also creates variability in the melt quality due to longer residence time distributions and very small cushions relative to size of the machine.
When you upgrade to the smaller screw… the biggest changes will be in the melt temperature and shear rate during screw recovery. I strongly recommend taking a melt temperature measurement during the current configuration so that you can better match it after the change. To match the shear during screw recovery, you may want to determine the circumferential speed used during screw recovery so that it can be matched on the new barrel. To calculate this, multiply the RPM times the screw diameter times pi… (RPM)x(d)x(π).
A little more about this topic is discussed on my post entitled ‘The Affect Of Screw Diameter On Shear Rate‘.
As with most processes, you should do your best to match the process outputs such as fill time and melt temperature.
Since the machine is electric, you will have to work with the manufacturer to adjust the settings to ensure the proper plastic pressure is reported on the controller.
Additional Thoughts
You are very likely to encounter a more solid and reliable process with the new configuration… plus you will significantly reduce risks associated with higher residence times and residence time distributions.

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