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In the last two columns (June and August), we have been discussing the general rules used to calculate cycle times for injection molded parts, with a particular emphasis on how we determine the temperature of the polymer that will allow for ejection of the part without deformation during demolding or subsequent distortion of the part as it cools to room temperature. Many tools reference either the deflection tem- perature under load (DTUL) or some per- centage of that value. A wider survey of literature on the topic even uncovered a reference to the Vicat softening temperature when molding amor- phous polymers, a value that is somewhat higher than the DTUL. To provide for a useful treatment of this topic, the wall thick- ness of the part must be considered, since this will influence factors such as the rate of heat transfer through the part from the center to exterior surface and from the exterior surface to the mold wall. This, in turn, will influence the actual temperature of the polymer that is supposed to be captured in the term for the ejection temperature (TE). The focus of this article is to obtain an understanding of the how the modulus of a polymer increases as it cools in the mold. As we illustrated in last month's column, amorphous polymers and semi-crystalline polymers exhibit differences in temperature-dependent behavior. The big event in the thermal history of an amorphous polymer is the glass transition. If the temperature of an amorphous polymer is increased from room temperature, eventually enough energy is imparted to the material to create independent motion along significant lengths of the indi- Cycle Time: Science vs. Rules of Thumb vidual polymer chains that make up the material. When this occurs, the elastic modulus of the polymer declines rapidly. The plot for PC provided in Fig. 1 shows this event occurring between approximately 140 and 160 C (284 and 320 F). Over this relatively narrow temperature range, the modulus of the material declines by approximately 99%. As the polymer cools in the mold, the reverse process takes place. The molten material entering the mold begins to build modulus rapidly as it cools through this temperature interval. The layers of flowing material that are in direct contact with the mold will undergo this change at the most rapid rate. Interior layers will cool more slowly, due to their distance from the cavity and core surfaces and the fact that polymers are relatively poor conductors of heat. The temperature of the mold surface will influence the rate of heat transfer, and this will be most noticeable on the part surface. But it is a given that the temper- ature of the mold must be set below the glass-transition temperature of an amorphous polymer to ensure that the material "sets up." The benchmark temperatures that we have been talking about— the Vicat softening temperature and the HDT (or DTUL)—at the two most commonly used stresses of 66 and 264 psi are shown on the modulus curve for the PC. It should be obvious that at the Vicat softening temperature there is little chance that the material will achieve an ejectable modulus. But by the time we reach the HDT values the modulus of the material is approximately 75-80% of what A general lack of understanding of the relationship between modulus and temperature leads to some poor practices when process conditions are established. To properly calculate cycle time, you need to understand how the modulus of a polymer increases as it cools in the mold. Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS By Mike Sepe PART 3 Amorphous polymers and semi-crystalline polymers exhibit differences in temperature-dependent behavior Here, the elastic modulus of PC, an amorphous material, declines between approximately 140 and 160 C. Modulus vs. Temperature Behavior for Amorphous and Semi-Crystalline Polymers Elastic Modulus, MPa Temperature, C 0 50 100 150 200 250 3000 2500 2000 1500 1000 500 0 Nylon 6 Polycarbonate Tg Tg Vicat Softening Temperature HDT @ 264 psi HDT @ 66 psi FIG 1 26 SEPTEMBER 2017 Plastics Technology PTonline.com K now How MATERIALS