Making the Blinky diffusers: the plastic injection molding process

Injection molded diffuser samples
Injection molded diffuser samples

As an accessory for our BlinkyTile DIY Light Sculpture Kit, we are manufacturing a set of plastic diffusers. We started out by 3d printing them, however it’s not really feasible to produce 100’s of pieces in that manner, and the quality of an injection molded part is way better.

The concept behind injection molding is is pretty simple- you just create two metal shells that, when stuck together, have a cavity in between them which is the same shape as the piece that you want, and then squeeze some hot plastic in there to make the part. Of course, the devil is in the details, and you end up needing tons of pressure (literally) and precision-controlled molten plastic that can be cooled rapidly to spit the part out at the end. The result, though, is that you can produce a large volume of almost identical pieces out of precisely controlled material very quickly and economically. For comparison, it takes about 1 hour to print 4 diffuser pieces on a desktop 3d printer, but less than 30 seconds to produce two pieces using an injection molding machine- that means is over 60x faster to do an injection mold process.

First off, we turned to our friend Nick Starno for a design review of the 3d printed model. Even though we designed it to be straightforward to manufacture (no overhangs or complicated geometries), there were a few things that needed to be tweaked to transition from 3d printing to molding. First, the design had to be made thicker to ensure even flow of the material. Second, the snaps needed to be elongated to reduce the stress on them as they are attached to the BlinkyTile. After making the changes, he performed a Finite Element Analysis (FEA) to validate the design, and it was ready to submit to the factory.

Simulating snap design using Finite Element Analysis (FEA)
Simulating snap design using Finite Element Analysis (FEA)

After receiving the design, the next step for the factory was to cut the mold. They use a traditional CNC machine to make the rough shape, then a neat process called electrical discharge machining (EDM)  to machine the fine details (square corners, etc) that are difficult to machine using spinning tools. EDM uses tiny, high voltages sparks of electricity to wear through a piece of hardened metal, rather than cutting it with a spinning blade like a drill or end mill. The EDM process looks like this:

EDM tool sparking away metal
EDM tool sparking away metal

And the resulting metal mold looks like this:

Metal mold ready for injection
Metal mold ready for injection

The green stuff on the mold is a grease that protects the metal from oxidation while it is in storage.

After finishing the mold, the factory loaded it up into the machine and produced samples (first shots) of the design for inspection. The purpose of this was both to evaluate the physical shape and condition of the parts (do they fit in the PCB tiles? are there any ugly surface features?), as well as test that the material produces the right amount of diffusion.

Once the samples were ready, we went to visit the factory and inspect them in person. Over some delicious tea, we inspected the samples and found the design to be spot on.

First shots of the BlinkyTile diffuser
First shots of the BlinkyTile diffuser

We had 5 different material types to choose from, from transparent to full white opaque. After some careful evaluation using our Disco Party app, we settled on the ‘B’ variety:

Looks good!
Looks good!

A few days later, and we’ll have more pieces then we can count! Don’t worry, we can determine the amount by weighing them.

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