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Tech Tip February 99

Hot Runner Tool manifolds

In hot runner tooling the use of a manifold is necessary to transport the material to the drops and than through the gates to the cavity or runner system. The consideration of such a system is that it eliminates the cold runner system and the wasted material associated with it. The down side is the cost of such system and control for the system. This than should explain why most systems are in high productivity tools with high volume parts. There are lots of questions to ask and points to consider when getting into such systems.

The first question would be what steps and plans are there for the hot runner system and tool in general. What are your steps to designing a hot runner system? Do you design the system yourself? Do you buy it from one of the hot runner suppliers, giving them all the information and they do the design work? Do you buy some components off the shelf and than make and design some yourself? Have you selected the material to be molded and the grade? I find it amazing that people build system and have not narrowed down the material to be used. That should be decided before you start, so that the pressure drop through the manifold can be calculated correctly. This would include the specific grade within that family of material.

Have you considered the pressure drop through the system and how much you are going to use to just push the material through the system and than the gates? The pressure within the manifold needs to be calculated and designed for, thus you must know the machine and pressure that will be available. From a safety standpoint a nameplate on the side of the tool should state maximum allowable pressure for that manifold system.

What type of manifold system, is it annular or cylindrical for the flow channels? There is a difference in the pressure drop just between these two systems for the flow channel in the manifold. Knowledge of this can be beneficial to the outcome of the overall mold, process and quality of parts produced. Please remember we are talking plastic pressure here. I have run into systems that use from 5000 psi to more than 18000 psi to just push material through the manifold. If we have only 20000 psi to start, we used 25% to 90% of the available pressure just to get through the manifold so we only have 75% to 10% of the available pressure left to fill and pack the parts. Has anyone seen a hot, runner tool that you couldn't quite fill and pack the parts, but if a few of the drops were shut down it filled okay?

Another approach is to do a mold flow on the part to find what pressure is necessary with a safety factor. Than subtract this from the available pressure of the machine that will be used, thus giving the manifold designer the maximum pressure loses he can have within the system. This approach is sometimes done from the beginning of the design of the tool. It just depends on the technology available or necessary for the job. Understand that the manifold is the distribution and that in most systems would include the gates in the calculation but these numbers may be separated out.

Thanks for the time and more next month.


Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Chemical