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There are hundreds of variables to contend with in developing a 24/7 injection molding process, and it can be difficult to decide on which to focus. Some are more important than others, but no matter where you rank a given variable, the fact is the process will not run unless each is controlled to what- ever the resin and part need. My pick this month is melt unifor- mity, and it should be in your top 10 list of variables. By uniformity I mean not just melt temperature, but melt consistency, meaning no swirls, streaks, or partially unmelted pellets. If you want consistent dimensions and performance 24/7, you must have melt uniformity. Our focus here is on the difficulty in achieving melt uniformity between amorphous and semi-crystalline resins. These resins melt differently, and a processor needs to understand how each of these polymers melts to obtain melt uniformity. Normally, the differences relate to the different arrangement of the polymer chains in a part. With amorphous polymers, the chains are random—meaning they have no specific order or alignment, something like tangled pieces of string. Semi- crystalline polymers have a structure or ordered pattern of chain alignment. The prefix "semi" is used to note that not all the polymer chains in a semi-crystalline part are crystal- lized. There are areas of crystallinity and areas of random (amor- phous) chain orientation within a given part. Colors, additives, and cooling rate influence the degree of crystallinity, which changes part size and properties. Melting Amorphous vs. Semi-Crystalline Plastics Regardless of whether the polymer you are processing is semi-crystalline or amorphous, melt uniformity is required to obtain consistent part dimensions and performance. Understanding how each melts allows the processor better control in processing. Whether semi-crystalline or amorphous, each relies on the same sources of energy for melting: the barrel, screw, and heater bands. Most of the energy, about 80%, comes from friction of the pellets against the barrel wall and compression in the transition zone of the screw. The remainder comes from the heater bands around the barrel. The energy-transfer mechanism is the same for both amorphous and semi-crystalline resins. However, this is where the similarity in melting these plastics ends. An important factor that differentiates them is the amount of energy required to melt each. A kilogram of a semi-crystalline resin, such as nylon 6, requires significantly more energy than 1 kg of an amorphous resin such as ABS. In fact, nylon takes If you want consistent dimensions and performance 24/7, you must have melt uniformity. Understanding the differences in how each melts is crucial to obtaining melt uniformity. Get more insights on Injection Molding from our expert authors: short.ptonline.com/moldingKH Learn more at PTonline.com KNOW HOW INJECTION MOLDING Structure Difference Between Semi-Crystalline and Amorphous Polymer Chains Semi-Crystalline Semi-crystalline polymers have a structure or ordered pattern of chain alignment. With amorphous polymers, chains are random—meaning they have no specific order or alignment, something like tangled pieces of string. 2D View Molten Solid Amorphous 40 FEBRUARY 2018 Plastics Technology PTonline.com K now How By John Bozzelli INJECTION MOLDING