Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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so that the plastic resides inside long enough for the plastic to melt, but not so long as to degrade. Therefore, the percentage use of the barrel should not be too low or too high. This percentage is usually considered to be 15 to 20 on the low side and to 70 to 80 on the high side. The lower the number, the higher the chances of degradation, especially for heat-sensitive resins and/or longer-cycle parts. The shear from the screw rotation must also be minimized. The higher the shear, the more you will experience issues with melt homogeneity. All these issues will result in melt consistency and therefore lead to lower process capability. The choice of the barrel size is therefore important. The process must also not be pressure limited. Molding Process: As has been the underlying theme, optimal packing of the part will give the best process capability. The above four factors help in achieving this goal. The process engineer must realize not to compro- mise part dimensions for process capability. In other words, an underpacked part may achieve the required dimensions but may not have a good Cp value. LINKING THE PILLARS Apart from the above reasons linked to the five pillars, there are several other factors that are common among them: Molding Process Window: This is related mainly to part design, mold design, and the mold build. The larger the molding window, the more robust and more capable the process. The ABOUT THE AUTHOR: Suhas Kulkarni is the founder and president of Fimmtech, San Diego, an injection molding service-oriented firm focusing on Scientific Molding. Fimmtech has developed several custom tools that help molders develop robust processes, and its seminars have trained hundreds of individuals. Kulkarni is an author of the book, Robust Process Development and Scientific Molding, published by Hanser Publications. Contact: (760) 525–9053;; Materials with high shrink value will have a higher varia- tion than those with lower shrinkage. leading reason for smaller process windows is parts beginning to flash at pack pressures slightly higher than those used to just pack them out. This indicates the mold shutoffs may be inadequate. It could also be because a section in the part is too thick, requiring the molder to use excessive pressures. If the machine tonnage is insufficient, parts will flash before they are packed out to an optimal level. Processors will therefore use lower packing pressure, leading to inconsistency. Pressure-Limited Process: This is related to material selection, part design, mold design, or machine selection. A pressure-limited process will not allow sufficient pressure to fill and pack the part, leading to inconsistencies. A material with a low L/T limit can make a process pressure limited. Molders usually suggest using lower viscosity materials to help them fill the part easier at lower pressures. Thin sections in the part design can also increase the required pressure, making the process pressure limited. The location of the gate must be in an area that will let plastic flow easier into the ends of fill, making it less prone to pressure limitation. Sometimes in thin-wall parts, the required pressures can exceed 30,000 psi plastic pressure and therefore a machine with a higher pressure capacity must be selected. The above are some of the factors to be considered. There are several other factors not mentioned here. It is important that each project is looked at with a holistic approach, avoiding the over-the-wall engineering approach. This will ensure the success of the project. During this study there was another interesting observation that was made between cavity-to-cavity process capabilities. This will be discussed in a later article. 48 AUGUST 2018 Plastics Technology T ips & Technique s

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