What do your calculator and a garden chair have in common?

Have you ever wondered how the electronic devices you’re surrounded with were made? We’ve already published the 3D models and the source code of our calculator, but we thought it would be great to also share our whole manufacturing process.

Mass production of such a device requires a huge number of steps. Let’s start from the top!

Injection molding

Most if not all plastic products are injection molded, from disposable knifes to garden chairs. This is a widespread process because it allows complex parts to be manufactured at a high rate and for a reasonable unit price. Our calculator is no exception.

Injection molding is a simple concept : filling a cavity — the mold — with hot molten plastic. Once the plastic cools down and solidifies, we can retrieve the plastic part. Sounds easy, doesn’t it? In practice, it’s a bit more complicated.

A simple example

Let’s take the bottom part of our calculator as an example since it’s the simplest part of the device.

Bottom part of the NumWorks calculator

Let’s suppose the mold is in a single block. We could fill its cavity with liquid plastic from an opening. But how would we retrieve the part once it would have solidified?

Sure, we could break the mold apart, but that would mean manufacturing a whole new mold for every single part. That would be incredibly expensive because molds need to be machined out of extremely strong alloy to withstand molten plastic. And anyway, it would be way too tedious to machine the mold cavity from a tiny opening.

The solution is actually very simple: build a mold in two parts. Just like a waffle mold!

Ejecting the part

Once the part has cooled down, we open the mold to retrieve it. But plastic always shrinks when it cools down, so the part always sticks to the innermost mold half. To detach the part from the mold, we drill some holes in the mold and slide metal rods to poke the part out: those are called ejector pins.

To optimize the production speed, the part is ejected from the mold before it has fully cooled down. It’s still a bit soft, and when the ejector pins jab the part out, they leave a mark on it.

Those ejector pin marks are visible on many plastic products like disposable knives or garden chairs. On products made out of several parts like our calculator, those marks are hidden on the inside faces of the parts. This way, they’re invisible on the assembled product!

Once the part has been ejected from the mold, an operator retrieves it and checks it for defects. It’s then ready for the rest of the assembly process!