Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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We have spent a lot of time on the subject of crystallinity and there is a lot more that we could say. The emphasis of this discussion has been on cooling rate, since it is within the control of the processor. But there are other factors that can influence crystal- linity. The process-related items on the list play a relatively minor role compared with cooling rate, but they should be under- stood to appreciate the big picture. The other factors are related to aspects of mold design and to the material itself. One process parameter that has a minor but measurable influence on crystallization is the melt temperature of the polymer as it enters the mold. The higher the temperature of the material, the longer it will take to cool to the point where crystallization stops. It is important to under- stand that we are referring to actual melt temperature, not the barrel settings. Anything that can alter the energy content in the polymer, such as screw rotation speed or backpressure, can also affect the rate of cooling and therefore the degree of crystallinity. The other process condition that has an unexpected effect on crystallization is the pack and hold pressure profile. Higher pressures compress the material to a greater degree, forcing the polymer chains closer together and restricting their mobility. Since crystallization depends upon mobility, higher degrees of packing discourage crystallization. This effect can be observed in a pressure-volume-temperature (PVT) diagram as shown in the accompanying graph. This shows the change in the specific volume of a PP as it cools while under different pressures. Larger changes in volume correspond in part to the achievement of a greater degree of crystallinity. It is A Processor's Most Important Job apparent that this change in volume becomes smaller as the applied pressure increases. Both the melt temperature and the pack and hold pressure profile have a relatively small effect on crystallinity compared with cooling rate, but they are measurable, and they have been confirmed in laboratory tests. Another interesting influence is orientation. This is a phenomenon that occurs whenever a polymer flows. The act of moving the polymer produces an uncoiling of the entangled chains; and the straighter chains, aligned closely together, can produce zones of higher crystal- linity. Thermoformers that produce parts in PET are quite familiar with this phenomenon and have learned that PET in a sidewall of a deep- draw container will exhibit a higher degree of crystallinity than the bottom of the container where the sheet did not stretch. This can affect the properties of the container. Too much crystallinity will reduce the toughness of the material and can result in brittle failures. In injection molding, this type of crystallization is most often observed as a function of gate location and part geometry and is especially important in materials that can crystallize over an extended period of time, such as PE and PP. The higher the temperature of the material, the longer it will take to cool to the point where crystallization stops. There are several process-related issues that influence crystallinity besides cooling rate. Let's examine a few. By Mike Sepe PART 7 Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS This pressure-volume-temperature (PVT) diagram shows the change in the specific volume of a PP as it cools while under different pressures. Larger changes in volume correspond in part to the achievement of a greater degree of crystallinity. PVT Diagram for a Filled PP Temperature, °C 10 68 126 184 242 300 Specific Volume, cc/g Pressure, MPa 0 50 100 150 200 0.949 0.917 0.885 0.853 0.820 0.788 0.756 0.724 18 AUGUST 2018 Plastics Technology PTonline.com K now How MATERIALS

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