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Minimizing Injection Molding Flash - Zero Rework Project

Assuming that your injection mold is of a quality and robust standard, the reason for flash (burrs) around the parting line results from the mold opening up slightly during the fill pressure and ultimate packing pressure being greater than the Clamp Tonnage of the press. In larger plastic parts that have significant “Projected” surface area the calculation for Press Tonnage is sometimes underestimated and the actual shot will spread open the injection mold ever so slightly----enough that Flash (Burrs) will form at the parting line. Two basic corrective actions that are typically looked at to resolve this issue are; 1) The addition of “Parting Line Locks” which are designed to add extra insurance in keeping the parting line closed during injection molding cycle. Or 2) Look at the Injection Pressure Profile and see if the pressures can be reduced by adjusting the Temperatures for both the injection mold as well as the plastic material. Or 3) Verify that you have ample Venting at the parting line, as well as in “Interior” areas of your plastic part.

It has been my experience that high injection pressures are commonly used to overcome pockets of trapped gas/Air that are not easily vented and cause a number of problems. Over the years I have become an advocate of “Vacuum Evacuation” for use in injection molds that had complex flow patterns and numerous Air/Gas traps. A side note; “Always perform a “Short Shot” series of partial fills, perhaps 40, 60, 80 and 90% which will allow you to have a visual capture of problem areas that require venting. If you eliminate these trap spots you will find that Injection and Hold Pressures can be reduced and that Flash (Burrs) are minimized if not eliminated.

If you are gathering inputs for a "Zero Rework" program, the Flash (Burr) issue is one of many injection molding problems to be considered. I have been involved with a number of Quality Initiatives that were specific to injection molded plastic parts, from Clear Lenses to larger Cosmetic components and to attempt to explain all of the "Lessons Learned" and "Best Practices" which are employed by "Best In Class Suppliers" can fill volumes. I have long looked at the Automotive Industry as being on the Leading Edge of Plastic Injection Molding Technology----Both with Resin (Polymer Science) as well as the latest processing capabilities for Injection Molding Machines and Material Delivery Systems. Since the "Best In Class" molders are typically rated on specific products that may not relate to yours, I would strongly suggest that you partner with your Material Suppliers to better understand the latest "Best Practices" being used today----and possibly for the Near Term Research Technologies on the horizon. During the 1980's & 90's I partnered with GE and Bayer and this proved to be a Win-Win for all involved.

Your "Zero Rework" project should also include partnering with experts from the Welding equipment that is used. I suppose that you use Ultrasonic Welding on your PA parts. PA typically requires high energy and a robust shear joint design for proper welds. Once the melt is initiated during the first stage of the weld the PA material is common to have a "Splash" of material and if not contained by the weld joint design will usually splash to the outside of the mated parts being welded----Flash (Burr)? If this is one of your quality issues that require rework it would be a good idea to consult with Welding Experts, I was partnered with Herrmann, Branson and Dukane which were positioned for global technical support, although other companies offered quality products. I should also add that the ability and cost for purchasing quality (Well Tuned) Horns for you custom welding applications is a key factor for process capability----thus quality. I have encountered custom horns that were not tuned properly and there was no way they could produce a good weld. So make sure that you have Technical Assistance during your part/weld design and the initial production startup to verify that everything performs as expected. Todays U/S Welders are also capable of Parameter controllers with QC limits that will record every weld and alert when the set limits are violated. This is a great option that will stop production when something goes wrong with the setup----rather than producing bad plastic parts.

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