It’s been suggested to me that I should avoid injection molded plastics wherever possible, and try to go with thermoformed plastic. Generally I research and validate advice before following it, so I started researching when thermoforming should be used over injection molding.
For the newbies, injection molding is when you build a metal tool that is hollow where you want the plastic to be. You put the tool in a large press that holds the two halves of the tool together. The other part of the press has a hopper with a bunch of small plastic bits. Those plastic bits get fed through a heater and then pumped at high pressure into the mold. A few seconds later, the plastic cools inside the mold, the press separated the halves, and the part is popped out of the mold. This is great for smaller parts.
Thermoforming is when you take a plastic sheet, put it in a square frame to hold it, put it near a flat heating element until the plastic reaches a certain temperature, at which point it will sag in the middle. Then you drop it onto a prepared mold. The mold will have lots of little holes drilled into it, and those holes all lead to a vacuum. The vacuum is turned on and the melty plastic is sucked up against the mold. It spends some time cooling, and then can be popped off the mold. It requires another step of cutting the extra plastic off. This method is good for enclosures or large pieces, and it’s a fraction of the cost.
But there are a LOT of considerations, including what material to make the mold out of, what specific method to use for thermoforming (there are lots of fancy variations), and even what kind of heater to use to melt the plastic, and how to move it from the heater to the mold.
But for learning everything there is to know about the process, the materials, the tools, and the molds, this free resource was a fantastic and entertaining read (where entertaining means marginally better than reading a book on accounting methods). It’s available as a PDF here: