Saturday, April 3, 2010

Making a Conoco Gas Truck, Part V

Since my last post, I’ve been mulling over how best to remove the tank end part from the mold. The “right” way is to add ejector pins. The ejector pins are attached to an ejector plate. And the ejector plate also has two push-back pins that move the ejector plate away from the mold when the mold halves are closed. I’ve never added ejector pins before, and it seemed like a lot of work.

Then an idea struck me last night. Since I’m only making a small number of these, all I really need is the ejector pin and nothing else. I have some 1/4” K&S brass rod that I measured and came out to precisely .250 inches. Plus I have a little bench-top cut-off saw I got from Harbor Freight with a 2” diameter blade. So, first it was back to SolidWorks to add a hole in the middle of the mold, and then back to the milling machine to add this hole.

Holes cut by a milling machine or a drill usually aren’t completely smooth, so I use a reamer from and over/under set. These are reamers that are either .001” over or under the target size. Using the over 1/4” reamer makes the hold nice and smooth, and the rod will slide easily, but still with a tight fit so plastic won’t work it’s way between the rod and the hole.

I carefully put the rod into the mold so it’s flush with the back, and the put the mold into the machine. Once I’ve injected the plastic, I pull the mold out of the machine, put it upside down between two blocks, and then tap with a hammer on another piece of rod:


It worked like a charm! Below you can see the end glued onto the tank truck, along with a casting still attached to the sprue:


Now I only have two more molds to make: the tank bottom, and a mold for the tank covers. Both of these will be straight-forward to make, so I’m getting to the home stretch.

1 comment:

  1. More fantastic work John, thanks for sharing this with us, it is very interesting and something I've always wanted to learn.

    J. Motts