<p>Crystallinity plays a significant role in determining the quality of molded polymeric products. It affects the mechanical, thermal, physical, and biodegradation properties of final products. Higher crystallinity yields higher impact strength, while lower crystallinity yields higher water absorption and faster degradation. Therefore, the crystallinity level within a product should be tuned based on the product's desired outcome. The crystallinity degree is determined by the crystallization rate, which is influenced by many processing factors such as cooling rate and mold temperature in injection molding. Moreover, there is not much research conducted to numerically investigate crystal size that develops locally in injection molded products and its relation to cooling rate and mold temperature. In this regard, the influence of the cooling rate and mold temperature in injection molding is investigated numerically to understand their local effect on crystallization rate and crystal size. The molded geometry was designed by using SolidWorks to perform the simulation. The dog-bone specification is based on the American Society for Testing and Materials ASTM, an international standards organization. Autodesk Moldflow is a software simulation that produces high-end plastic injection molding CAE. The suggested melting temperature by the software for Polypropylene was 220 °C, and the suggested mold surface temperature ranged between 20 °C to 80 °C. The lowest cycle times that could be achieved without sacrificing properties were determined first. Then, the simulations were conducted at the lowest cycle times, which were 30 and 50 seconds, and at the two previously mentioned mold temperatures. Each mold temperature cycle time of 30 and 50 seconds was simulated, resulting in different cooling rates. The results showed that cooling rate and mold temperatures were found to play a significant role in the crystallization kinetics and crystals size. Moreover, crystallinity was found to vary within one product based on the design and the distance between the investigated section and the gate. The least crystallinity point was found to be near the gate, and the largest crystal size was found to be at the furthest point away from the gate. Moreover, there was a positive correlation between crystallization rate and cooling rate. In contrast, the relationship between cooling rate and crystal size was found to be in which when cooling rate decreases, crystal size increases. It is important to consider these factors when molding as not only processing parameters were found to influence properties but also the design of the product and gates.</p>

The Influence of Cooling Rate and Mold Temperature on Polymers Crystallization Kinetics in Injection Molding

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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