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Complete Blown Film THE X -DIE & High Performance V -RING ALPINE AMERICAN T H E B L O W N F I L M E X P E R T S www.halpine.com T: (508) 655-1123 F: (508) 655-9337 Bottom line: Look at the time and resin costs for color and material changeovers. Would you be better off if you found a better way? Even if it is not your money, how about the time and hassle you put in? Talk to your man- ager, ask the embarrassing question: Is the above factored into the "just-in-time inventory" strategy of your plant? EDITOR'S NOTE: The author thanks Joe Cascarano, tech- nical service manager at Colors For Plastics, Elk Grove Village, Ill., for his contributions to this column. 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 john@ scientificmolding.com or visit scientificmolding.com. • p. Adjust screw rpm to provide a reason- able recovery time. • q. If all is OK, add hold pressure for the next shot. • r. As you are making parts with 100% regrind or prime material, vary backpressure from very low to high, up and down, as the screw turns on one shot. • s. On the next shot, adjust injection speed to as fast as you are comfortable with, make a shot and vary screw rpm from low to high. • t. Repeat the two previous steps two or three times, starving the screw peri- odically to allow the screw to rotate without building a shot. Vary injec- tion speed, screw rpm, and backpres- sure. Do not keep the hopper full; hand-feed if necessary. • u. Drain the hopper if you were working with 100% regrind, clean if necessary. • v. Add production-quality resin—100% virgin or appropriate ratio of virgin and regrind. • w. Set backpressure to your requirements. • x. Adjust screw rpm to take all but 2 sec of cooling or mold-closed time on normal cycles. For fast cycles, adjust this timer to ensure the cycle is deter- mined by the timers, not screw- recovery variances. Remember, you cannot set screw-recovery time. It will vary during production. • y. If you have a hot runner that is con- taminated or releasing black specks, try a purging compound specifically made for hot runners and raise the tip temperatures about 50° F (25° C), providing the resin can handle this higher temperature. • z. Say your prayers and check the parts. Ideally, our nefarious friend "Murphy" fell asleep during your purging pro- cess and you have a clean changeover. If parts show problems such as black specks, contamination, etc., you will have to make a decision to keep purging or pull the screw. No easy answer here. @plastechmag 33 Plastics Technology I N J E C T I O N M O L D I N G