Monday, November 17, 2014

Process Optimization: Retaining Ring



Since our last post of the Retainer Ring we have made a few modifications during the optimization process.

Changes to Mold

First Injection 

During the first injection (with the mold from the previous post) the ring had trouble ejecting from the mold. The small, plastic runners to the ejector pins would fold over with the force of the pins rather than push out the ring from the mold. 

Re-Machining 

To give better structural support to the ejector pin runners we added a thick plastic ring around our part and over each ejector pin. This gives better support because the ejector pins would have to bend all six runners in order to cause the same problem. This allowed for all the ejector pin force to used to eject the part 



Second Injection

During the second injection run the parts were ejecting well. No further changes to the mold are needed. 


Optimization Parameters 

Below are our injection molding parameters for this part. 


The cooling time was chosen because higher cooling times caused the part to shrink around the mold and get stuck to the core mold so well that we had to pry the part off with pliers. So the trade-offs for cooling time with our part is increased dimensional accuracy on the inner diameter versus forced need to get the part off the mold. 

The injection speed was set fairly high at first (the max injection speed on the 2.008 IM machine is 6.3) to make sure the entire ring was filled before freezing (cooling time is proportional to the feature's thickness squared thus our thin part could freeze before filling it all if we do not inject the plastic fast enough). 

The pressure profile is optimize to be the max pressure we can have with out flashing. 

Production Run

Last week we ran production on our retainer ring (and our yoyo body and thermoform cover!). Production ran smoothly for the most part (we had a bit of problem with flash at first but increased the clamping forces and lower the injection speeds reduced that significantly). 




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