I received this question the other day...
Milan
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?
My Response
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.
Additional Thoughts
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.
-Andy
Hi Andy.
What did you mean when you speak about adjusting drop in point 1?
Milan
Hello Milan,
Basically, in hot runner systems, there is typically some type of internal nozzle which distributes the polymer melt to the portion of the gate contacting the mold cavity. Often, this distance can be altered.
Best regards,
-Andy
Andy
Do you mean torpedo or aperture of nozzle?
Milan
Milan,
In some cases, yes. Basically, some runners can be adjusted, and some cannot.
The key is to determine what options you have available to yourself... and work around them.
If there is a cavity which is consistently short or over packed, you should do your best to determine what is causing this imbalance and create a strategy to improve the balance at that location.
-Andy
Andy,
Thanks for your advice.
Milan
From the above discussion between you and Milan I got the answer for what I have been looking for.
Great to hear Sunyoung, always glad to help.