Bozilla Corporation March 2010 Newsletter

When a mold is designed to utilize valve gates to fill the cavity, the sequencing of the valve gates can be used to minimize clamp force. This statement may seem familiar, but the method being referenced may not seem so familiar. We will discuss how to sequence the valve gates in order that the flow front reaches the end of the flow path.

If a mold is designed with several valve gates, the valve gates should be placed in a position so the cavity fills in a balanced fashion. More times than not, this is easier said than done. Unfortunately, the slightest imbalance in a cavity can cause a pressure spike resulting in a clamp force spike.

I'll explain:

If there are several valve gates being used to fill a mold and the polymer flow front from one valve gate reaches the end of the cavity before the others, the pressure will begin to spike at that valve gate forcing the polymer to move to the unfilled regions. The problem with this scenario is that the unfilled regions should be filled by the other valve gates. When it's not, a pressure spike is created which creates a clamp force spike. At the end of a filling stage if there are multiple valve gates creating a pressure spike, the clamp force will increase very rapidly resulting in the need for a larger Injection Molding Machine (IMM). Unfortunately, in the world of injection molding, it's not always as perfect as we want it to be due to many extraneous circumstances.

So how do we solve this pressure spike situation due to imbalances in the tool?

To solve this potential clamp force spike issue, we run a mold filling simulation analysis utilizing the exact valve gate nozzle locations in the tool. The analysis will reveal any imbalance during filling. We would then adjust the timing of the valve gates in order to reduce the pressure spike in the cavity thus decreasing the clamp force. This practice has been studied and proven many times to be extremely effective.

We would like to emphasize that this technique is only used when the cavity is known to be filling in an imbalanced fashion. It is always preferred to have the nozzles located so the cavity fills in a balanced fashion. It is not recommended to close and open valve gates during the filling and packing cycles as it will have an impact on the stress within the polymer and can create the potential for part warpage.

If you wish to understand more about this process, please contact us.

Bozilla Corporation is now a Gold Certified consulting partner with Autodesk Inc.

(http://www.moldflow.com/stp/english/services/consulting_partners.htm)

Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs. We invite you to visit our website at www.BozillaCorporation.com.