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So what's the big deal about using more than 65% of the barrel capacity? You will have problems with unmelt and melt uniformity that will result in parts that do not look right or function properly. As the screw turns and backs up to make the shot, the feed section of the screw actually becomes shorter, from the point of view of the pellets (see Fig. 2). With large shot sizes, a good portion of the feed section winds up behind the feed throat, and there are only a few flights at most to prepare the granules for the transition or compres- sion zone. Why is this a problem? The feed section is only meant to feed, not melt the granules. But it also provides some heat transfer, and by effectively shortening the feed section you are not adequately preparing the granules for the transition zone. You may get by if you're molding amorphous resins, as they melt like butter. They can soften before getting up to their melt temperature, so they are easier for the transition zone to handle. But you are still asking for unmelt. If you are processing a semi-crystalline resin, you have an entirely different situation. Semi-crystalline resins melt differently, like ice. That is, they stay hard until they reach their melting temperature. In addition, you can bring semi-crystalline resins up to their melting temperature, but they'll stay solid, just like ice; you need to pump extra energy into them to cause melting. This is called the latent heat of crystallization. Basically it is an energy barrier, and it can cause severe problems in melting and screw wear. Most semi-crystalline resins not only do not soften but require nearly twice the BTUs (energy) to melt. For example, amor- phous ABS takes about 150 BTU/lb to bring it from room temperature to melting. The same amount of polyethylene will take about 350 BTU/lb. Also, factor in the short time your cycle leaves to melt this plastic. Now you'll understand why some processors see flights eroded or even completely missing when they pull screws. In addition, you can now also explain why you have unmelt in your parts. Some will argue that the metering zone will finish melting. Not so—it is merely a pump. FIG 2 Feed Section Shortens as Screw Develops Shot As the screw turns and backs up to make the shot, the feed section of the screw actually becomes shorter from the point of view of the pellets. As a result, the material is not being adequately prepared for the transition section, which is where it melts. 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. Contact john@scientificmolding.com; scientificmolding.com. Semi-crystalline resins melt differently, like ice. They stay hard until they reach their melting temperature. Make Perfect Parts and Save Money! • Preload cores for perfect parts. • Save $$$$ with a smaller cylinder. • Simplify mold operation and design. • Smaller mold base = low cost parts. • Fully Preloads at only 1,500 psi. • Stays locked without hydraulics. • 400˚ F systems available. • Rush 0-2 day shipping available. pfa-inc.com N118 W18251 Bunsen Drive Germantown, WI 53022 (262) 250-4410 Fax (262) 250-4409 KOR-LOK ® , Hydra-Latch ™ and Hydra-Jaws ™ are trademarks of PFA, Inc. ©2016 PFA, Inc. All Rights Reserved. Mold OPEN Core Out Mold CLOSED Core OUT Mold CLOSED Core IN Core PRE-LOADED Against FULL Injection Pressure CAN'T DO IT! CAN'T DO IT! KOR-LOK ® TRADITIONAL SIDE-ACTION VS. SIDE-ACTION SYSTEMS SYSTEMS YES! YES! YES! YES! YES! YES! SIDE-ACTION SYSTEMS Designed & Made in USA HYDRA-JAWS ™ Every Mold, Every Machine, Every Time! Adjusts to Fit Every Mold Quick and Secure Consistent Clamping Force 22 JULY 2017 Plastics Technology PTonline.com I N J E C T I O N M O L D I N G K now How