Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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example, if you set 850 psi (59 bar), you will get 850 psi hold pressure for the time you set on the second-stage or hold timer. However, if the screw reaches the zero position or "bottoms out" at any time during the hold stage, there is a good chance you'll make a bad part. Without that "cushion" of material in front the screw, the hydraulic pressure is not developing any plastic pressure in the nozzle, and most likely the part won't be properly packed. On an electric machine, if the screw reaches the zero posi- tion, the results will be the same. However, the controller is often programed to reduce the hold pressure to some minimum value to avoid unnecessary load on the expensive electric servo motor. The controller knows the screw cannot move forward any further and subsequently reduces the forward pressure. While I prefer units of volume for shot size and cutoff position, cushion is best stated as a distance. Picking a particular volume as a rule of thumb would be a very small distance on larger screw diameters. My target is about 6 mm, or 0.25 in. This assumes that the numbers on the controller screen are accurate. If the position sensor is not calibrated correctly, you could be reading a 3-mm cushion on the controller when in fact the screw is at the bottom or end of the stroke. Safety tip: it is critical that the screw tip never touch the barrel end cap, be- cause it has the power (pressure) to blow the end cap off. There is always a minimum clearance; my understanding it is about 1.5 mm (0.060 in.). The hydraulic cylinder has stops in it to prevent going beyond this, providing the screw is the correct length. (I am not sure how electric machines handle this.) Finally, how much shot-to-shot variability is acceptable? In an ideal world the answer is zero. Unfortunately, variability is a fact of life due to check-ring design, wear factors, contamina- tion, lack of maintenance, and the fact that most nonreturn valves have a propensity to leak differently from shot to shot. If you would like to get an idea of how much they leak, do a simple test on any given part. Note the actual screw start posi- tion before injection and the cushion using definition number 1 above. You can now calculate the volume of the stroke using the equation for the volume of a cylinder: Volume=π × r² × stroke length. Now catch all the plastic parts and runners, etc. that drop out of the mold. Weigh them all. Find the melt density for the plastic you are using, and calculate the volume of molten plastic needed to make the parts you weighed. Let me know if the volume calculations match. I'll bet that the calculated shot vol- ume is 1.3 to 2 times the volume calculated from the plastic weight. Because of these issues with non-return valves, my cush- ion window is ±1.0 mm. (It should be less for a plunger.) ABOUT THE AUTHOR 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 or visit Visit the link belowfor the full line of IMS Molding & Extrusion Supplies: Trusted source for Molding& Extrusion Supplies, Mold Components & Supplies, Equipment &Engineering Visit us on theweb or call toll-free 001.888.304.1307 (Mexico) © 2012, IMS Company. All rights reserved. • Cutting and Degating – Gate Cutters, Flash Trimmers and Deburring Tools • MoldMaintenance – Mold Releases, Mold Cleaners and Rust Preventives •Temperature Control – Heater Bands, Thermocouples and Temperature Controllers •Part Production – Screws, Barrels andNozzles • Process Cooling – Manifolds, Fittings and Hose ... andMUCH MORE! Since 1949 PLASTICS TECHNOLOGY MAY 2012 29

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