Gardner Business Media

Hot Runner Technology OCT 2017

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October 2017 7 CONTRIBUTOR: David Hoffman is director and instructor for plastics edu- cation and training for Beaumont's American Injection Molding Institute (AIM). The AIM Institute would also like to thank John Blundy for his contribution to this article. For More Information: American Injection Molding Institute / 866-344-9694 / Beaumont Technologies / 814-899-6390 / Again, this refers to the portion of the equa- tion involving the radius (r), which is raised to the fourth power. Steps in the flow channel can create dead spots, which do not flush well when purging and cause color-change issues and materi- al-degradation problems. Also, the steps make it possible for material to sit in the manifold for extended periods, which increases the likeli- hood for thermal degradation of the polymer. The polymer's natural flow path around the channel intersections also causes these dead areas. This happens even if the channel inter- sections are perfectly aligned (see Figure 3). 6. System cost. The cost of the system is often one of the most important things to con- sider, but relying solely on quotes from various suppliers is not the best way to approach the issue. Familiarity with the various options each hot runner supplier offers is helpful. For exam- ple, some suppliers may provide the ability to separate the manifold from the mold while the mold is still in the molding machine, reducing maintenance and downtime. Another thing to think about is how the channels are manufac- tured. The manufacturing method may affect color changes and require additional clean-out procedures that expand on traditional purging approaches. (See the sidebar for a list of com- mon direct and indirect costs when evaluating the overall system.) It is often said that the devil is in the details, and this certainly applies to hot runner system evaluation and performance. Issues arise, so it is important to be aware of some common challenges and potential sources of variation, including high pressures at assembly points, co-existence with a cold mold and platens, gating issues (cooling, clogging, drooling, string- ing), processor skill level, shot size, residence time, leaking (see Figure 4), valve pin seating, shear flow imbalances, power consumption and thermocouple placement and contact. Making a more informed decision requires a thorough understanding of the hot run- ner system's design, manufacturing process and available options. It also requires a solid understanding of troubleshooting and mold qualification standards. Staying on top of the items described in this article will reduce downtime, improve part quality and decrease overall costs. / HR FIGURE 4: This leaking manifold caused plastic to encase the hot runner wiring.

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