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). 

Process Optimization: Thermoform!

These past few weeks, we've been optimizing the thermoforming process.

Main Design Goals:
1. We wanted to reduce the webbing produced from the brass pins. With too much webbing, the thermoform piece won't fit flat underneath the retaining ring. When we cut the pieces, we wanted to have the outside of the ring be as flat as possible.
2. Reduce Thermoforming time (decrease cost)
3. Reduce Dust Specks (increase quality)

Iterations:
1. We started out with the default settings, but there was some webbing on the pins. We could also see a significant amount of dust specks on the mold.

2. We performed a few iterations to reduce the dust
- peeling the dust protector a few seconds before thermoforming
- cleaning off the plastic with compressed air
- wiping off the mold with a clean cloth. This method produced more significant results.

3. To reduce the amount of webbing, we decided to increase the heat time and increase the oven temperature. Increasing the oven temperature was also a good way to reduce thermoforming time.

4. After increasing heating time, we started to get a few problems where the piece would be deformed from being pulled off the mold. We decided that a longer cooling time was needed, so we increased the form time to allow for increased cooling before removing the plastic.




Notes from Production Run:
We made ~120 pieces in 2.5 hours. Despite optimization runs beforehand, we discovered during the production run that the machine became very warm. This resulted in a few problems, where the piece would be deformed from being pulled off the mold. As a result, we lowered the oven temperature to 640 F.

During the production run, we introduced a "glitch" in the run on purpose, in order to simulate different operating conditions. The glitch was a decrease in heating time to 200 seconds.