Plastic Injection Molder – Part 1

When the weather started to change, I had a long list of projects I wanted to get done. So far, I’m not done with any of them, but I have gotten a reasonable start on some. The shop is looking a lot better, but is not where I want it to be. I got some of the “around-the-house” things done (like installing Rose’s new light switch). I haven’t had a chance to get any of the work on the motorcycle done, yet, but that’s next on the list. I’ve also gotten a good start on a plastic injection molding machine. What’s a plastic injection molding machine, you ask? Well, let me tell you!

A plastic injection molding machine (PIM) is a machine that, at its most basic level, melts plastic and squirts it into a mold. A significant portion of the plastic stuff in your life was injection molded: The case of your phone, every remote in your house, the knobs on your stove, the keys on your keyboard, the handles of your favorite scissors, your tupperware containers… Injection molding is fast, cheap and flexible, making it ideal for a wide variety applications. Personally, I have a handful of product ideas that I plan to try out, and the fact that I can use the machine to recycle plastics at home makes it a neat thing to show the kids. Also, none of my friends have one, so I can finally win at tools, at least for a moment.

Original heater assembly

My PIM machine is based on the design in Vincent Gingery‘s book Secrets of Plastic Injection Molding, plus some modifications from this guy, and some additional changes from Mike. The most interesting, and the most drastic, deviations from the original plan are the heating chamber and the temperature control. The original plans call for a rectangular chamber with a 250 watt cartridge heater to make the magic happen. Actual industrial PIM machines, though, use cylindrical heating chambers and band heaters for the heat. Mike suggested that I might do the same, and so we did some looking on the intertubes for band heaters and found one the will do nicely. After a slight detour toward a pneumatic piston (a detour that was abandoned after a short while) and a little time on ebay, I ordered a 3″, 500w band heater.

Band heater

At 500w, this will not only heat up faster than the original design, but by using a cylinder, I should get a very even heat in the chamber itself. Honestly, I’m not sure this will matter that much, but it feels a lot better than the rectangular chamber, and the price was right. Of course, using the band heater means the original chamber design is out the window. Instead of ordering the 1″ x 1 1/2″ CRS from Speedy Metals, I ordered a chunk of 3″ round bar and proceded to chop off a piece a bit more than 4″ long. Having never worked with steel in any significant way, I was not sure what to expect. My number 1 observation: Steel is different than wood. A few notes about that whole experience:

  1. This was my first order with Speedy Metals, and it went pretty well. It was a helluva drive out to their place, but they were helpful, and fast, and got me what I needed (once I found the door)
  2. Cutting 3″ steel bar with a hack saw sucks
  3. Using a liberal amount of WD-40 as you are cutting 3″ steel bar makes it a lot easier
  4. Even with the WD-40, it still sucks
  5. If you need a 4″ hole in a 4″ bar, you want a 4 1/2″ bar to start
Soon to be the heating chamber

Unfortunately, Mike’s Sherline Mill (on loan for quite a while, now) can’t finish the job up without the purchase of some additional tools. As I’m already at my budget for this project, and anxious to actually use it, Mike has agreed to finish up the chamber and the guide for me. Now I just have to get the frame built.

I also went with a proper temperature control module instead of the somewhat hacked together solutions others have used. I’ve got everything hooked up now, just to see it all go, and it’s awesome! Seriously, when you put something together, and turn it on for the first time, and it works, it is an exceptional feeling. When part of that success involves both green and red 7 segment displays and tells you the temperature of the room, it makes you a little giddy. In addition to the general awesomeness of the temp controller display, it will actually provide a much more stable, repeatable process for the molder.

Temperature controller and SSR

Initially, I expected basically a digital thermostat, but there seems to be a lot more going on. From what I can tell, the controller tracks the rate at which the chamber (the thermocouple wedged between the heater and a chunk of aluminium for testing) is heating and switches off the heater well before it actually reaches the set temperature. For example, I turned the system on and set the controller to 40 degrees. The temperature started going up as expected, but around 30 the power to the heater switched off and the rate of increase in temperature slowed. It still overshot the 40 degrees, but no by much. Pretty cool, I think.

So, next is the frame, or the enclosure, depending on how I’m feeling. Also, I’ve started to try to get some molding material together, and design my first molds, but I’ll get to that in another post.

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