Making a Conoco Gas Truck, Part I

Truck 4_400I’ve been making injection molds and parts for Ragg’s…To Riches? for a little over a year now. I’m pretty new to all of this, being self-taught. So there’s been lots of trial and error. But I’m slowly getting better and trying more difficult molds.

Right now I’m working on a new set of molds for an S-scale gas tank that will fit on the back of a PBL 1934 Ford Pickup. The PBL kit is very close in appearance to the truck shown in the Rio Grande Southern Story, Volume VII on page 123. And I wanted to design a tank that would look much like the one in this photo. It’s very hard to find prototype information about these trucks, so I used some conjecture to create a “close” match.

My first step is to use a program called SolidWorks that allows me to build 3-dimensional models, like the one on the right. I like to use a different color for each part to help me remember how everything will fit together when it’s all done.

Making the Mold

Tank Core

Once I have the model, I then have to design a mold, again in SolidWorks. Molds consist of two parts—a core and a cavity. Generally, but not always, the core is the mold half like the one on the right where there is metal projecting out of the mold. The design at the right is a core that I can use for both the top and bottom half of the gas tank itself.

Making a mold like this is a little tricky, as you’ll see in the sequence below. I could have started with a thick piece of metal and then cut away all the parts I didn’t need. But that’s a lot of work, so I used another approach—I created an insert, as I’ll describe and show below. Inserts were a technique I’d never tried before.

Tanks Support and Trough Molds

I’m going to switch now to using photographs of actual aluminum parts to show you how I’m making the mold for the tank supports (the green parts in the first photo) and the troughs (the red part in the first photo) that are on either side of the tank. First I milled the cavity out of a block of 1/2 inch thick aluminum:


Next I milled a pair of inserts for the core side of the mold. To create the inserts, I needed fairly small blocks of aluminum (these are 0.5 by 0.7 by 2.5 inches) and I had a long 1/2 by 2.5 inch bar. In the past I would have cut this with a hack saw, but that’s not much fun at all. So I broke down and purchased a metal-cutting band saw. Boy, am I glad I did—it makes quick work of cutting the smaller bars, which I then finished off in the milling machine.