Monday, April 24, 2017

Optimization (Improvisation?)

Last time we posted, we had just completed our molds. Since then, we have actually had the opportunity to test, optimize, and do a final production run on the gears molds. Here we will walk you through the journey that started as optimistic young machinists and lead to (somewhat) experienced injection molders.

Our first day of injection molding optimization we showed up ready to run a successful first round. We set fairly arbitrary initial parameters and put the machine on semi-automatic. During the first run we eagerly anticipated the first tiny gears to come out and....nothing!

All the gears got stuck in the cavity! We spent a few long minutes prying each individual gear out of the cavity and generously sprayed some release agent into the mold. Take 2:

This time our little gear poped out, there is still hope! We set to work filing away at the gates so that there would be more resistance as they pull apart and once again spray some release agent. Take 3:

Medium and small gear! Improvement and hope! We then started looking at the parameters and increased cooling time. Take 4:

After take 4 we completed our gears successfully and all popped out! We then took the gears out to our base piece to test the press fit. Not even close! Our gears floated around the pegs. This meant that we were back to the mill machine. 

Since the holes were too small all it took was a single extra pass around the core and our mold was ready to test again. 

Second Optimization Test:
We tried our mold with smaller gear holes. This time they press fit perfectly! 

Snapped onto the new base piece, they looked great and there was enough room for the cover and ring to fit around them! Looking closer at the gears we noticed a few problems that we set out to fix in the following tests. Firstly, the gears were off center. While this did not change the machine parameters, it definitely did not fit into the aesthetic appeal of our yo-yo. This turned out to be an easy fix. We simply put brass pins that had a smaller diameter in to replace the regular pins. As we attached the core, we would push it all the way down and to the right before attaching it. Now we had centered gears. On to more optimization. 

Flash: The unfortunate case when extra plastic escapes the mold and creates a flare of plastic around the parts. To solve this issue we reduced our shot size so that it would correctly follow our pressure profile.

Short shot: We actually reduced the shot size to the point that there was not enough plastic injected to fully fill the mold. We proceeded to change the shot size until we came to a happy medium at 5.2 inches.

Dimples and weld lines: Some of the final problems that we encountered were dimples and weld lines in our part. We realized that overall, it was inevitable that we would have some dimples and weld lines since our part has different thicknesses and the plastic entered from only one side. However, in the spirit of optimization we set out to minimize the visual effects of each defect.

Overall we changed a number of parameters:
  • Clamping force: Placed at max to stop flash
  • Shot size: decreases and increased to 5.2 inches
  • Gate walls: Files down so that more plastic could pass
  • Brass Pins: Added to center the gears
  • Cooling time: Decreased to 15 seconds so that the gears did not get stuck
  • Release agent: Generously applied

After three optimization runs and our final production run we ended up with a whole stack of three different colors of gears. 

Here is an image of the final parameters used in our production run:

Stay tune for our first assembled yo-yo!

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