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about twice as much energy to melt as an ABS (about 716 BTU/kg vs. 342), yet their processing temperatures are similar. Twice the energy requirement for the nylon means your processing ducks had better be in a row. So now we know what we must do to melt nylon vs. ABS from an energy standpoint. But there is another issue to make your processing life harder. Semi-crystalline resins stay hard until they reach their melting temperature. It's like melting ice. Ice does not change in hardness significantly as it warms up from -10 C (14 F) to -0.5 C (31 F). Fall on ice at either temperature and I doubt you will feel any differ- ence in hardness. Like ice, semi-crys- talline resins do not soften until they reach their melting temperature. They stay hard until they meet two criteria: First, you put in enough energy to bring them to their melting point; and second, you have to put in another dose of energy to overcome the heat of fusion (melting)—that is, break up their ordered chain pattern. It is like an energy barrier to melting. How does the screw and barrel handle this difficult situation? The semi-crystalline pellets flow from the hopper into the feed throat and fall between the deep flights of the feed section of the screw. The feed section augers the pellets forward and compacts them, forcing air and some volatiles out the hopper (it is a vent). The feed section may warm the pellets, but it does not and should not do any melting. The material then reaches the transition or melting zone, where the root diameter of the screw tapers thicker to provide compression of the pellets against the barrel wall. This compression, along with friction of the pellet against the barrel wall, drives energy into the pellets to raise their temperature. The problem is that not all granules get to the barrel-wall/flight interface— some get the energy needed to melt and some do not. Solid-bed breakup occurs, which leads to some partially unmelted pellets getting through the transition and metering zones. These partially unmelted pellets can wind up in the part and wear the screw. For the sake of discussion, let's say you are also running liquid color and using a general-purpose screw design. Liquid color often uses an oil carrier, which lowers the friction of the pellet against the barrel, which in turn reduces the energy transfer for melting. Is this really something you want to happen? Turn off the liquid color; does the screw rotation time decrease? A general- purpose screw with a 20: 1 L/D ratio has only five flights in the transition zone. This is a significant problem with shot sizes larger than about 40% of the shot capacity. It's often recommended that barrier screws be used instead, but they often cause degradation, producing excessive black specks. You're better off with a screw designed to provide melt uniformity. Amorphous resins melt differently. They require substantially less energy and are easier to melt. They melt like frozen butter. An amorphous pellet at room temperature is hard, but as it warms up it begins to soften. With more energy, amorphous materials continue to soften until they are suitable for molding. They do not stay hard until they reach the melting point, and they do not have to overcome the heat of fusion. A partially unmelted pellet may be like taffy or a marshmallow. If it gets wedged between a screw flight and the barrel wall, it will squash without severe damage or wear to the screw or barrel. Amorphous resins thus are more forgiving during the melting process. A general- purpose screw may provide acceptable processing, but again, that's not my recommendation. In a nutshell, semi-crystalline pellets are more difficult to melt uniformly than amorphous pellets. A general-purpose screw might process amorphous pellets, but with shot sizes at the small end (below 20% of barrel capacity), and at the high end (above 40%), most molders will have problems with semi-crystalline pellets. I don't recommend barrier designs. Instead, specify a screw design that provides melt uniformity with a minimum L/D ratio of 20:1, though 24:1 is preferred. The solid break-up that can occur when processing semi-crystalline materials leads to some partially unmelted pellets getting through the transition and metering zones of the screw. These partially unmelted pellets can wind up in the part (left) and wear the screw (above), even this one running only PE. Amorphous resins melt like frozen butter. Like ice, semi- crystalline resins do not soften until they reach their melting temperature. PTonline.com 42 FEBRUARY 2018 Plastics Technology I N J E C T I O N M O L D I N G K now How 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.