Sometimes there’s a tool you want to buy, but you can’t justify the cost. In that case, why not make it? For me, a plastic injection molding machine fell into that category — and it turns out they’re not hard to make.
I wanted to make solid plastic parts for some of my amateur science experiments. There are a number of ways to make things out of plastic, each with its advantages and disadvantages. Often just cutting raw material to the desired shape works best. Some plastics can be cast by pouring a liquid resin with hardener into a mold (Make: Volume 41). Vacuum forming works well for making things out of thin sheets of plastic (Make: Volume 11). I considered making a 3D printer, but for the few plastic parts I envisioned needing, it wasn’t worth the time and money.
Plastic injection molding has been around since thermoplastics were invented. It’s a great way to make many copies of a part quickly, and what I like best is that it’s easy to reuse old plastic objects to make new ones.
So I built an injection molder based on the plans in Vincent R. Gingery’s book Secrets of Building a Plastic Injection Molding Machine. David Gingery could be considered a forerunner of the Maker Movement — he and his son Vincent have written a whole series of books on building tools for the machine shop.
This project should cost between $100 and $200. It depends a lot on where the metal is purchased. I had a lot of the metal already, left over from other projects. Try to find a friendly local iron dealer, rather than getting the metal online or from a hardware store. They’ll often let you pick through their cut-off pile and sell it for almost scrap prices.
I wouldn’t recommend this project for someone who has no metalworking experience. I made full use of my machine shop: mill drill, lathe, stick welder, horizontal band saw, ½” reamer, and various taps. You can get by with a drill press and various hand tools. I welded most of the assembly together but the book calls for screws, except for one weld (which you could have someone else do if you don’t have a welder).
Being an engineer, I couldn’t resist making improvements. The plans call for a 1″×1½”×4″ piece of cold rolled steel for the heater block (A), where the plastic is heated before being injected into the mold. I used a leftover piece of 1″×2″ hot rolled steel instead. The wider block allowed me to add a second cartridge heater (B), so my machine warms up quicker and can get hotter.
The frame (C) in the Gingery design is mostly angle and flat iron held together with bolts and nuts, but two connections needed to be welded. So I decided to weld most of the frame and avoid drilling so many bolt holes.
The injection lever (D) pivots on a½”-diameter steel rod (E). I beefed this up by drilling larger holes in the frame and turning some steel bushings (F) on my lathe to support the rod. The bushings also keep the lever centered over the injection piston (G).
My favorite fastener for projects is Allen head cap screws so I used #10-24 cap screws instead of 1/4-20 bolts to mount the heater block and guide block to the frame. They’re at least as strong as ungraded 1/4-20 bolts and should conduct a little less heat from the heater block to the frame.
(One modification I tried didn’t work out so well. The heater block and guide block need to be spaced out from the frame a bit, to isolate the hot heater from the frame and put the parts into proper alignment with the piston. The plans call for washers, but I tried using some leftover fiber ceramic insulation sheet instead. It was too soft — it allowed the heater block and guide block to move out of alignment, preventing the piston from operating smoothly.)