Before I explain exactly what happened, let’s do a quick & dirty rundown of injection molding. If you want some visuals to get a more clear understanding, check out this breakdown on Aire Plastics' site: https://www.aireplastics.com/basic-injection-molding-process/ (we have no affiliation with them at all, we just thought their diagram was pretty informative).
As the name suggests, injection molding is the process of injecting molten thermo plastic into a mold to form a plastic part. In its simplest form, two halves of a mold are precisely carved, and in production are pressed together to form the cavity which is filled with plastic. Once cooled to a point that it will hold its shape but not stick to the mold or the ejector pin, the part is ejected. While there are a variety of factors that go into making a “good” part (each one could probably be a topic for a full newsletter itself...) we only had an issue with one aspect of this mold - venting.
Basic laws of physics state that two objects can’t occupy the same space at the same time. When plastic gets injected into the cavity of a mold, the air that's in that cavity has to go somewhere, so mold makers have to build in a way for air to exit from the cavity of a mold through EXTREMELY small vents. And when we say extremely small, we're talking about as small as one or a few THOUSANDTHS of an inch.
Improper venting (or in our case the total failure of a vent) can cause issues ranging from minor cosmetic defects all the way to structural failures in a part. This is a result of air - which would have otherwise been removed - remaining in the mold while the plastic solidifies. This picture from ecomolding.com illustrates this perfectly.