Spinner part process optimization

A short-shot, or incomplete filling.

Of all the parts for our yoyo, the spinner wound up being the most difficult to optimize. The first parts we shot had two defects: short-shots, and "dieseling". Dieseling was a problem we weren't familiar with before, but happens when trapped air in the mold ignites under the modeling temperatures and pressures, leaving charred marks on the part and a very distinct smell we won't soon forget.

An example of "dieseling" on our spinner component.
Our first shots were much, much worse.

In addition to the defects, we also noticed the post we molded to hold our bearing shrunk more than we anticipated.

This gave us three problems to focus on fixing with either our mold or our injection molding parameters:

1. Fix the short-shot
2. Fix the dieseling
3. Fix the shrinkage on bearing post

Fixing the shrinkage problem

We fixed this relatively quickly by re-machining the pocket in our mold to be a bit larger and accommodate a greater shrinkage than we initially expected. No problem.

Fixing the short-shot/dieseling

These two problems actually ended up being related. What was happening in our first batch, is that air couldn't escape the mold, and the trapped air would burn our part. Our first attempt was to vary the pressure and speed profiles to try and get a more complete filling. Unfortunately, higher pressure and speed fixed the short shot, but caused another problem: flash. Flash happens when molten plastic leaks through the parting surface and causes thin little webbing on the final parts. When we backed down on pressure or speed, we got short-shot/dieseling; and when we bumped them up we got flash. So we decided to look at modifying our mold to fix the problem.

A particularly egregious example of flash in our
spinner part

The first attempt to fix the short shot was simply widening the gate. This had a minor effect, but didn't fix the problem.

The next thing we did was use a scribe to cut some small ventilation lines out of our mold for the air to escape through. This helped a little bit, but wasn't enough to fix the problem. Next, we moved on to adding some small diameter, deep pockets for the air to be pushed into. After some trial and error in number of holes, location, and depth, we eventually found a combination that worked. As a result, our parts come out with tiny whisker-like posts as the mold is filled, but these are easier to deal with than either the flash or short-shot/dieseling defects. In theory, we could lower the shot size by an amount to minimize the whisker length, but we haven't tried that and the posts break off very easily so we may focus our efforts elsewhere.

Tuned spinner coming out juuust right. Notice
the lines in the runner from the scribe marks on
the mold used for air ventilation.

Final mold after rework. Enlarged bearing post on the cavity; as well as small ventilation lines
extending radially. Also notice the blind holes drilled as a place for air to escape. These holes
fill with some plastic as well, leaving whiskers on our final part that are easy to remove by hand.

The only dimension that we wound up changing in our final part was the diameter and length of the bearing post to better accommodate the observed shrinkage from our first shots. The other modifications to our mold are to combat defects in the molding process, and don't change the dimensions of our final part. With a tuned mold and parameter sheet, we're ready to enter our production run for this component.

Final optimized parameter sheet for production run.