Injection Molding Optimization

Since we last updated, Team Yolette has been hard at work injection molding and reworking our molds in order to finally manufacture plastic parts for our yo-yo! In this blog entry, we highlight the challenges we have faced and overcome in manufacturing the red and black insert part discussed in the last blog entry.

On our first run of injection molding the “color ring,” we found the correct lengths of ejector pins & shim by finding the difference between the size of the mold and the given dimension of the whole ejector pin & mold assembly, and then loaded our molds into the injection molding machine. Using a set of starting process parameters similar to previous parts, we injection molded our first parts, hopeful, but not expecting that it would work on the first attempt. After adjusting the screw feed stroke such that plastic was filling over the whole profile and the mold was filled as much as possible, we found that our part was not being filled entirely. 

First try at color ring

In order to remedy this, we decided to machine additional runners that looped around to the edge of the part opposite the sprue so that it would fill better.

We milled out two additional runners on the cavity side, and tried again. However, this time we found that because of the large size of the new runners, the part was getting stuck in the cavity after molding, and was therefore not ejecting (we should probably have put these runners on the core). In order to remedy this, we went back to the mill and added ejector pins on the runners, which we then threaded in order to help the core side ‘grip’ the part, keeping it on the correct side (core) until ejection. This new mold worked well, and the only remaining defects we observed were that there was some dishing in the rectangular pieces, which we found to be acceptable since it does not have any mating and is nearly invisible, and that the mold stuck a tiny bit in the core side, but this was not a problem as it still ejected consistently. We used a very short cooling time in order to allow the part to shrink more, as the shrinkage was initially smaller than we predicted (2%). We were therefore able to use a short screw feed delay time, allowing for rapid production.

Color ring cavity

Color ring core

Final process parameters

Next time on our blog: completed yo-yos!!