Hot Runner Considerations, Part 2

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There are many suppliers of hot runner systems with each offering different features based on the performance you want to achieve. We touched on this subject in a previous Tech Tip, however, not all topics could be covered in one tip. The purpose of this Tech Tip is to highlight the importance of an aggressive and diligent quality effort and how those checks can contribute to the operational performance, or lack thereof, of the hot runner system.

You may have already compared features from one supplier to another, but have you really investigated the manufacturing and assembly process of the supplier while considering the impact on performance?

Of course all will claim excellence in manufacturing and quality, and for the most part the effort is honest. But let’s face it, plastics are not easy materials to process and small variations tend to have a significant impact on how a given material will react in the hot runner system.

We would certainly suggest that you visit the manufacturing facility. While at the facility, inquire about their standard acceptable tolerances in their manufacturing process. And remember, just because they supply a tolerance number doesn’t mean that tolerance is acceptable based on plastic flow principles, which you will see below.

Ask if they supply inspection reports stating the dimensions recorded for each critical area. Also ask if they “Bench-Test” each hot runner system and use infrared cameras to help diagnose temperature variations in the system.

To build on this, below is a short list of additional things to evaluate that will definitely impact the performance of your choice of hot runner system:

number 1  Manifold Heaters – (Figure 1) How are they installed? Most manufacturers use a press, but some do not. If the heater contact with the manifold is not uniform, there will be hot spots created in the heater element causing premature failure and non-uniform heating of the manifold itself. Process capability can also be affected especially in engineering materials.

Figure 1

Figure 1

number 2  Nozzle Heaters – Measure the ohm resistance for each nozzle heater. If the variance is greater than 4% you are not achieving a uniform process profile and the life of the less efficient heater will be reduced. This is especially true with simple temperature controllers that lack intuitive capability.

number 3  Nozzle Tip Orifice – Pressure differences in plastic injection molding are defined in the equation shown in Figure 2, where (Q) is flow rate, (l) is the runner length, (n) is material viscosity and ® the radius of the flow channel.

Figure 2

Figure 2

The purpose of pointing this out is so you can see that any slight difference in “r” value, (which includes the orifice in your hot runner nozzle, is affected to a magnitude of the 4th power. Of course, you can adjust individual tip temperature to try to overcome some of these variations, but understand that by doing this you are changing the material conditions that your parts are being molded from. Additionally the resultant variations in thermal expansion of nozzle and manifold steel could ultimately lead to other filling variations, leaking or breakage issues (Figure 3). Also understand that raising local nozzel or manifold temperatures may cause an increase in cycle time.

Figure 3

Figure 3

number 4  Stack Height – (Figure 4) How uniform is the stack height across all nozzle tips? If these dimensions vary, then the engagement with the gate will also vary, which will affect your process capability. Ask for the inspection report and what the supplier considers an acceptable tolerance…AND remember the pressure drop equation from above to see if those tolerances are acceptable for your parts and process.

Figure 4

Figure 4

 

number5Flow Channels – All gun drill manufacturers will claim variances of 0.001” per inch of travel. But do you really know if the cross drilling of flow channels has left a step at the intersection of cross drilled runners (note the equation again!)? Have the channels been scoped and honed? Pressure variations, color change and material degradation issues can occur if left unchecked.

As it is often said, whether you are using a hot runner or a cold runner, “the devil is in the details”. Checking these areas will save time in troubleshooting. In addition, if you and your supplier can manage these items you will most likely see an improvement in part quality and reduction in overall costs.

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