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Mold Fill Analysis: Gate Location and Resin Selection

Posted by Kent Seeley on Sep 15, 2015 11:24:36 AM

The use of mold filling analysis is often a very valuable tool that can greatly benefit both plastic injection molders as well as part designers. Utilizing virtual simulation derived from 3-D models shows how plastics will fill related to product design, material, machine, mold, gating, and processing conditions. This allows for efficiencies to be achieved, projects completed on time, and customer expectations and budgets to be met by avoiding costly revisions after the mold is built.

Gate Location

Most analysis begins with the specific gate location which can have an impact on all other characteristics being analyzed – specifically the filling pattern, cooling, cosmetics, pressures and warp. The analysis makes it possible to assess the structural soundness of the design and to determine where the gate should be located. Determining the requirement for use of multiple gates relies upon the flow distance and the amount of pressure required to actually fill the mold. In turn, this can ultimately affect the clamp force during the molding process and lead to the requirement to utilize more than one gate.  Determining problematic plastic fill areas, weld lines, and if the weld lines will hold properly all contribute to this initial analysis and flow pattern.  With multi-cavity tooling, the original gate location may be impossible to achieve due to the changes in part orientation. Interrupted fill patterns can cause bubbles; either pockets of trapped gas or vacuum voids, along with cosmetic problems in that area which can inhibit part performance. Therefore, the analysis of the specific gate location(s) can ensure the importance of the part to fill evenly and uninterrupted which will avoid too much pressure on one cavity.

Resin Selection

Selecting the proper material is important for part production. The molding properties, cost, mechanical attributes, and moldability are all factors to consider related to resin selection. However, mold filling analysis can play an integral role in evaluating different resin selections with a push of a button. Once the gate location has been determined, experimenting with various resin types becomes an easy step in the analysis which allows for side-by-side comparison. An analysis of the resin can determine warp or sink potential, tendency to flash, and the ability to fill the mold. For example, a long flat part with a thicker section created by ribs will inherently be susceptible to differential shrink and warpage. Therefore, the analysis is a very useful tool in understanding and predicting which types of resins will warp more than others. A common example is the comparison between higher shrink crystalline resins to lower shrink amorphous resins. A part may warp significantly more with certain resins, so the mold filling analysis provides assistance to the OEM when considering other testing alternatives or an alternate material altogether. The same aspect pertains to the melt flow of the selected resin and whether the materials being considered will in fact fill the part. Again, an analysis can provide the experimentation of different material and a side-by-side comparison to determine whether alternative material should be utilized or, for example, whether or not an additional gate would be required. It’s important to understand that all resins react differently under various circumstances. With the use of mold filling analysis, some of the guess work can be taken out and the time to analyze many various options is greatly reduced.