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Here’s another trick of the trade that will make your life easier. The subject is viscosity and the fact that plastic changes viscosity in processing, and if you change injection speed, viscosity changes big-time. Many of you have seen or worked with this in the form of a viscosity curve (see illustration).
To control your process you have to control injection velocity or fill time, and the tolerance is especially tight. Fill time must be kept to ±0.04 sec, because polymer viscosity is so sensitive to shear rate. Sorry, no arguments here: You must control fill time to keep your parts consistent shot to shot, run to run, machine to machine. Setting up a process so that the machine (rather than you) controls fill time will make your life on the shop floor easier. You might just get to eat lunch and go home on time, not to mention making higher-quality parts with less scrap.
If the strategy is to control fill time to ±0.04 sec, how do we do it correctly and safely? To control speed in a car you need to have more power available than what you are currently using. If you are going up a steep hill on cruise control, you need enough power from the engine to maintain the 70 mph you want. The same concept applies to controlling first-stage filling on an injection machine, but with one difference: If you apply too much power (pressure), you can damage a slide or other delicate part of a mold. In the case of a multi-cavity tool, there is the chance that a cavity will accidentally block and you could be trying to push four cavities worth of plastic into three. Remember that the first stage (filling), with the hold pressure set near zero, should only make a short shot.
Whether your machine is electric or hydraulic, closed or open loop, it doesn’t matter—to keep fill time constant while running requires that extra pressure be available. This is the “Delta P,” the pressure difference between what was used and what pressure was available.
As viscosity varies due to different resin lots, colors, regrind percentages, or moisture content, you want the machine to adjust for viscosity changes so you do not have to. If the machine has enough extra pressure and the new batch of resin is stiffer, it will use more pressure, like a car climbing a hill. If the material is of lower viscosity, the machine will use lower pressure. In fact, each shot should use a little different pressure because of the hundreds of variables that exist in processing.
The question now becomes how much extra pressure is needed for your machine to control fill time and adjust for viscosity changes? Some folks recommend 10% to 20% extra. At times that might be enough, but it would be better to know for sure, not guess or assume. For an easy, 30-minute test to find the exact Delta P required for your machine, visit this page of my website: scientificmolding.com/articles/delta_p_procedure.pdf. Remember to do the experiment at three different speeds, once a year. As a rule for hydraulic machines, Delta P is 200 to 400 psi. Electrics usually require 1500 to 4000 psi.
John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS and other specialties. E-mail firstname.lastname@example.org or visit scientificmolding.com.