Understanding Hot Tip Design

Alternative designs are available that will either eliminate or dramatically reduce skewing the flow of plastic.

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While sampling a hot-to-cold-runner mold, a particular shop was fighting variations that appeared to be related to steel imbalances. It was difficult to show any variations in the actual steel that related to the data collected, but during a color change, operators noticed a few colored streaks in the runner following the hot tip. These streaks were very specific in location and consistency.  

Around the same time, another shop was experiencing variations in filling and part quality. After evaluating its data, this shop also looked at the mold steel for variations, but again no glaring evidence pointing to the culprits was found. However, similar streaking was noticed in the hot sprue.  

In contemplating the influence of the hot tip design, the second shop turned the hot sprue to three different orientations and made short shots of the runner system at each orientation. The results of this test, shown in Figure 1, confirmed the shop’s suspicion that the hot tip was skewing the plastic flow and causing variations in part quality.

Fortunately, alternative hot tips are available that will either eliminate or dramatically reduce skewing the flow of plastic. The hot tip for the mold in Figure 1 was retrofitted with one of those options, and the filling variations and associated problems were immediately eliminated.

The solutions are not always that simple, however. Some alternate hot tip designs come with their own set of challenges. For example, some are more expensive than conventional tips, while others may create other challenges in processing certain materials. Understanding hot tip design and its role in processing quality parts is essential to selecting the most effective solution for your application.

Let’s take the variation shown in Figure 1 and increase it by the number of drops in a multi-drop hot runner system. There is a very high likelihood that each hot tip in that system is in a different orientation relative to the part and/or cold runner sections. If the hot runner system is feeding a cold runner system, each cold runner section could be filling differently, causing the parts also to fill and pack differently, which ultimately leads to increased process and part quality issues. 

If the hot runner system is direct-feeding parts, then skewed filling patterns inside the cavities may cause the parts to be dimensionally different from each other or to warp differently. The filling pattern has a direct influence on orientation, packing profiles and shrinkage within each cavity, so if their filling patterns are different, the molded parts are likely to be different.

Lastly, consider the case in which you take a mold that has been running well out of the machine for routine maintenance and cleaning. The hot sprue or hot drops are removed, cleaned and then reinstalled in the hot runner system, but most likely in a different orientation than they were originally. After the mold is put back into operation, you find that the parts are not filling the same and therefore quality control personnel are unhappy. Everyone says nothing changed: “All we did was clean the mold!” Sound familiar? Understanding hot tip design will help identify a solution.

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