Hot Runner Considerations, Part 3

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The Tech Tip about Tips

For this tech tip the focus will be on the actual hot tip itself which will include those tips used in full direct gated hot runner systems and hot-to-cold runner systems (one or more hot drops feeding a cold runner system).

Remembering back at least a decade ago while sampling a hot-to-cold runner mold in Illinois, a client was fighting what we refer to as “steel variations”. It was difficult to show any variations in the actual steel that made sense to the data that was collected. But during a color change, we noticed a few colored streaks in the runner following the hot tip. The streaks were very specific in location and consistency.

The streaking always stuck in the back of my head as something interesting. Shortly afterward, another client was sampling a mold for which we helped design the runner system. He called and said they still had variations in the filling and part quality. After evaluating the data, we encouraged them to evaluate the mold steel for variations, but yet again nothing was glaringly obvious. Then we noticed something about the hot sprue, and we started putting two and two together. We evaluated the design of the hot tip and asked if they could turn the hot sprue to three different orientations and provide short shots of the runner system at each orientation.

They performed the test and provided the pictures shown in Figure 1. The results confirmed the suspicion that the hot tip was in fact skewing the plastic flow and causing the variations in part quality.

Figure 1

Figure 1

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

But it is not always that simple. Some of the alternative hot tip designs do not come without their own set of challenges. Some are more expensive than conventional tips while others may create various processing challenges with some materials. So care must be taken in understanding the design of the hot tips and which may provide the best solution for your application.

Now, let’s take that variation and increase it by the number of drops in your 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 system is feeding a cold runner system (Figure 2), each cold runner section could be filling differently, thus causing parts to fill and to pack differently, which ultimately leads to increased process and part quality issues. If the system is direct feeding parts (Figure 3), then perhaps the parts may be dimensionally different or have variations in warp due to the skewed filling patterns. The filling pattern has a directly influence on orientation, packing profiles, and shrinkage within each cavity. So if the filling patterns are different, the molded parts may be different.

Lastly, consider the case where you may take a good running mold out of the machine for routine maintenance and cleaning (yeah, I know…who does such a thing?). The hot sprue is cleaned out and the hot tip is put back into the hot sprue but most likely in a different orientation. The mold is started back up and you find out that the parts are not filling the same and quality control is not happy. Everyone says nothing changed…all we did was clean the mold!

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