Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 47 of 67

The first installment of this two-part series (see May issue) dis- cussed the concepts of process capability and the pressure- volume-temperature (PVT) diagram. Optimum packing of the part will lead to lower variation and therefore higher process capability (Cp) and process capability index (Cpk). The goal at every stage should therefore be to try and achieve the optimum part weight. Achieving this optimum weight is related to the fill and pack-and- hold phases and therefore everything related must be considered. The temperature of the part must also be considered when ejected, since it is related to the dimensional stability. There are five pillars that contribute to the quality of an injec- tion molded part (Fig. 1). Each must be considered and optimized before production of the part. Cp and Cpk are measures of the capability of the molding process to produce good parts. The higher the numbers, the better the chances of molding accept- able parts—or the lower the chances of having rejects. Since process development and establishing a molding process consti- tute the last of the five pillars, it often becomes the burden of the process engineer to improve the Cpk. Achieving the required Cpk should be considered at every stage of each of the five pillars. In the fill phase the cavity is filled with molten plastic. In the pack phase, additional plastic is packed in to compensate for shrinkage. The challenge is to add the plastic before the cooling melt reaches its no-flow temperature. If the part is underpacked and the plastic temperature inside the cavity is below the no-flow temperature, no amount of pressure will help in packing the additional plastic. Above the no-flow temperature, as the packing time and pressure increase, the part weight increases. The higher the pressures and times, the higher the part weight. (Parts can and will get overpacked, a topic that will be discussed in a later article.) At the higher part weights, the number of molecules inside the cavity starts to stabilize and the variation between shot to shot therefore reduces. This reduction in the variation there- fore helps to increase the process capability. The goal should therefore be to achieve maximum packing without overpacking the cavity. The proof is in the data (Fig 2). A two-cavity mold was used for this experiment. Thirty parts were molded with 2000 psi and 8000 psi of plastic Each contributes to molded-part quality, and each must be optimized before production begins. Improving Molding Process Capability: The Role of the Five Essential Pillars By Suhas Kulkarni, Fimmtech QUESTIONS ABOUT INJECTION MOLDING? Visit the Injection Molding Zone. Five pillars contribute to the quality of an injection molded part, and each must be considered and optimized before production of the part. FIG 1 The Five Pillars for Achieving Part Quality and Consistency PART 2 46 AUGUST 2018 Plastics Technology QUALITY Part Design Material Molding Process Injection Molding Machine Mold Design and Moldmaking Tips and Techniques

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