A subject that I had to learn about in the course of engineering the chumby is injection molding. For an electronics guy with little mechanical background, this is not a small hill to climb. The concept seems simple: you make a cavity out of steel and you push molten plastic into it at high pressure, let it cool, and voila, a finished part comes out. Just like the Play-Doh molds from elementary school!
Oh if only things were that simple.
There’s all kind of nuances to the process. Plastic flows, but it’s not particularly runny. So it moves slowly, and it cools as it flows. The color of the plastic is impacted by this, and you can see flow lines and knit lines in an improperly designed mold. There’s also a whole assortment of issues with how the part is pulled from the tool, how the tool is made and finished, where the gates and runners are for getting the plastic in there…
Fortunately, we have experts in China who know all about this, and I get to learn mostly by watching.
If I were to summarize injection molding with a single adjective, it would be “precision”. The molds are precise to better than hair-thin tolerances, yet they are made out of hard steel. Achieving this level of precision out of such a durable material is no mean feat. It’s impressive to see a mold being cut out of raw steel.
The machine above is moving around a block of steel that weighs probably several hundred pounds in quite a hurry! Below is a photo of the whole machine, so you can get a sense of how big it is.
However, this is only the roughest step in mold making. After the rough shape is cut out, the mold is put into a machine called an “EDM”–Electron Discharge Machine–where a burst of electrons are used to knock microscopic chunks off the steel surface. If this sounds like a terrifically tedious process to you, it is. I’ve watched many EDMs do their job and it’s like watching paint dry (which is why I don’t have a video of it here). However, these things are wicked precise and they yield spectacular, repeatable results. All told, a mold transforms form a block of raw steel into a first-shot tool in about 4-6 weeks. I think, from a project management standpoint, the phenomenally long lead times of production-quality injection molded plastics was the biggest eye opener for me. I had to go to China and see the tooling shop do its work before I was convinced that there wasn’t some gross amount of schedule padding going on. What’s even more harrowing from the risk management standpoint is that there are no good simulation tools (to my knowledge) to predict how plastics will flow through the mold. So those visible blemishes like flow lines and knit lines? Well, you have to wait 4-6 weeks to see if you got it right. Ouch! Fortunately, the toolmakers we use in China anticipate this and they make the tools so as to err on the side of having excess steel, because it’s much easier to remove material to fix a problem, than it is to add material. Sort of like the old carpenter’s saying of measure twice, cut once, and if you have to cut wrong, cut long.
Here’s what a final mold looks like in the injection molding machine:
This is the mold that is used to create Chumby’s back bezel. It’s actually a very complex mold, because it involves a process called overmolding. If you happen to buy a chumby (I hope you do!), take a look at the back side and you’ll see what I mean. There is a rubbery TPE surrounding the hard ABS bezel. Many people assume this was glued on rubber band. In fact, the TPE is molded in-place on the back piece. This requires a two-shot mold. Watch in the video below what happens:
You can see that there are actually two molds, and one side of the mold spins around so that the alternating material systems can be molded at the right points in the process. Just as a bonus, I swiveled around at the end of the video to show you an unrelated product where a robotic arm is doing the mold extraction. The part is a very high-gloss part and a robot arm is used to prevent getting fingerprints or nicks on the work piece as it is still soft from the heat of the mold.
High-gloss steel molds are really impressive to see in real life, because the exposed mold surfaces are mirror-polished. I don’t have a video of this, but all of the steel surfaces that are exposed to the end users’ eyes are hand-polished by a crew of women. They spend days rubbing and picking at blocks of steel using specially cut bamboo rods, cotton swabs, and very fine grit polish. This step happens after the EDMs are done doing their thing, or anytime the mold has sat unused for a long time or has been opened for rework.
There is a lot of hard work that goes into those humble plastic parts!