I received this question the other day…
When I am setting up a process with a hot runner system, I always have to adjust the hot runner nozzles to avoid unbalanced parts. Do you know of any techniques to help eliminate this problem?
Although adjusting the temperature of your hot runner drops can help balance your tool, there are a few alternative ways molders use to help balance their tool.
1) Adjustable drops – In many hot runner tools, the height of the hot runner drop can often be adjusted to increase or decrease the thickness of the specific drops
2) Use true balanced runner systems – Many hot runner manifolds are not built with balance and symmetry in mind. Additionally, most hot runner systems do not use features such as Beaumont’s Melt Flipper to balance the shear within the hot runner system.
3) Balance the clamping – All platens deflect, and many older platens will be somewhat concave. Check and measure these conditions to ensure spacing or additional bolster plates are not needed. Additionally, a review of parallelism during clamping can be very helpful.
4) Balance the cooling – In many injection molds, the cooling supply each individual cavity may not be properly balanced. This is very common when the part geometries are not symmetrical… resulting in variations the effectiveness of the cooling from part to part. You may want to measure and compare the coolant temperature and flow going to each region of the tool.
5) Balance of venting – This can be an often overlooked cause of cavity imbalance. The effectiveness of the melt entering the mold cavity is based much on the air’s ability to get out the plastic’s way. I have seem many molds with virtually no venting to interior cavities… or even caes where the inner cavities actually vent to the out cavities. causing all sorts of complications in gas removal.
When specifying new hot runner systems or new tooling, try to incorporate systems which can be easily adjusted. Many molders see great benefits to the additional control and flexibility brought forth through the use of valve and thermal gate systems.