Plastics Technology

AUG 2016

Plastics Technology - Dedicated to improving Plastics Processing.

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• a. Make sure you have a free-flow nozzle tip. General-purpose nozzles have a dead spot in the nose and take about 50 shots to purge clean. I recommend throwing them out. If you'd rather not, take off the general-purpose tip and clean it. • b. Check that the nozzle body is as short as possible for this mold and there is no obstruction in the flow channel. • c. Purge the barrel of the existing material. I don't change shot size or temperatures until the barrel is empty. With the screw fully forward, I rely on screw rotation (not high-speed), until there is no melt stream coming out of the nozzle. • d. With the hopper and barrel empty, pull the screw back without rotation and check all the feed flights you can see. That is, pull the screw back as far as possible while looking down the feed throat (with proper eye protection). Do this in incre- ments if working alone. If there is any melted or solidified material sticking to flights in the feed zone, you probably have a leaking check valve. There should be no melted material on these flights. Clean any melted or solidified material off with a plastic rod, tube, or (second best) a wooden dowel. Please don't use metal. The screw needs to be smooth and scratch-free to work properly. Plastic should stick to the barrel and slip off the screw if things are working properly. • e. Add the purging compound of your choice, rotating the screw until it backs up to about 25% of the barrel capacity. If there is too much backpressure to allow the screw to back up, lower the back- pressure or use manual decompression to do this in increments. • f. Let it sit for a few minutes. • g. Purge the purging compound. • h. If introducing a new material, adjust temperatures as required. • i. When at the new temperatures, feed another 25% of the barrel with purging compound. • j. Purge the barrel until empty. • k. Start feeding the new material—use 100% regrind if available— to 25-50% of the barrel capacity. • l. Purge until empty. • m. Build shot size for the new material • n. Adjust backpressure to 750-1000 psi (50-70 bar) plastic pressure, providing this is appropriate for the resin you are working with. • o. Start shooting plastic into the mold with first-stage injection only (hold pressure very low…near zero). Again, use 100% regrind if possible. n s i g n E Q U I P M E N T ENSIGNEQ.COM | 616.738.9000 | SALES@ENSIGNEQ.COM • SYSTEMS INTEGRATION • CONTAINER FILLING/UNLOADING • MATERIAL CONVEYING • ADVANCED DUMPING TECHNOLOGY • WEIGH SYSTEMS • MIXING AND BLENDING Smart Solutions To Keep You Moving! BULK MATERIAL H A N D L I N G S Y S T E M S E N S I G N D E S I G N S & M A N U F A C T U R E S Changeovers take time and cost money. The chal- lenge for molders is to minimize these losses. 32 AUGUST 2016 Plastics Technology PTonline.com K now How

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