Route 15 Overpass: Making the Mold

I mentioned last time that the main reason I wanted to learn how to do resin casting was so that I could duplicate the wonderfully-detailed bridge girders that Mike Redden made for me from photos I took of the Route 15 overpass. This overpass will anchor the north end of the Wethersfield scene and provide a nice - and prototypical - view block for hiding where trains head "to Hartford."

Well, after doing a test run using an old scrap load as a master (click here, here, and here for details on the steps involved), I decided it was finally time to tackle the overpass.

I started with Mike's Shapeways parts as masters, which I'd primed last October(!). Here I'm trying to arrange them as efficiently as possible for the mold. I determined that about 8.5" across and 4.5" front to back would make a good footprint/base. 

So I measured that all out on a piece of foamcore, cut out the base and then cut the sides at about 5/8" high. This would account for the thickness of the base (I glued the sides to the edges of the base rather than on top as I did before), the thickness of the parts, and still allow a 1/8-1/4" of rubber on top to cover everything adequately. In addition to hot gluing the sides to the edge of the base and to each other, I also ran a bead of glue inside each joint to ensure that the box was water rubber tight.

Next, instead of using hot glue, I decided to use quick drying Aleen's Tacky Glue to attach the parts to the base of the box. To (try and) ensure that no rubber would seep underneath the parts, I ran a bead of glue around the entire perimeter of each part to, hopefully, form a nice seal/barrier. I'm not too concerned about messing up the nice masters, since the Aleen's should just soak off with water. I guess we'll see.

After letting the glue cure for 4 hours or so, it was time to mix and pour the rubber. Here are the quick steps (but be sure to click here for a detailed post):

• Make sure the mold box is level in all directions
• Use rubber gloves
• (this time I didn't apply any mold release)
• Pour rubber mold material Part A into one cup and an equal portion of Part B in another cup
• Mix together in a larger, clear cup (so you can see that it's mixed thoroughly)
• Mix gently to reduce the number of bubbles
• Let the mixture sit to allow as many bubbles as possible to come to the surface (this material has a 10 minute work time from mix to pour)
• Pour the rubber into the mold from high up (12" +/-) if possible so you have a small, thin stream (again, to reduce bubbles), starting in one corner and covering the entire master. If you aren't comfortable pouring from that high - or if you can't maintain a steady stream - you can pour closer, just keep it flowing so as to reduce bubbles.

While I've enjoyed doing the videos, I wanted to try a new way to document these steps:

I set up my iPhone on a tripod and got a Bluetooth shutter release so I could take hands-free pictures during the process.

Here's the rubber mold material from the Micro-Mark resin casting starter set. Put some of Part A in one cup and an equal amount of Part B in another cup.

Then mix the two cups of material together in a third, clear cup. Be sure to scrape out all the material from each cup.

Then mix the two parts by slowly stirring them together. The clear cup helps you see how thoroughly you're mixing.

When it gets all thoroughly mixed together, you're ready to pour. Well, almost. Set the cup down and tap on the sides for a minute or so to encourage any bubbles to surface so they'll pop. Then pour.

This step is both the most fun - since it's cool to see it coming out of the cup and into the mold box - but also the most harrowing - since you're covering up, in this case, over $40 worth of parts with liquid rubber. Start in one corner . . .

and work your way around . . .

and all throughout the mold box, covering everything with hopefully enough material. You can see that I had extra cups waiting on standby in case I had to mix up more rubber quickly! You have about 7 minutes from mixing to pouring.

Use your stirrer to scrape out all the material - might as well be sure and use it all up!

The final step, like with the rubber in the cup, is to tap on either side of the mold box to encourage the bubbles to surface. And just because I have this handy-dandy camera & shutter release setup, I decided to do a video of this complicated process . . .

Tapping releases the bubbles and blowing across the surface helps them pop faster.

And here's what you end up with with your done - a nice, smooth swath of rubber. Whether it made a good mold or not remains to be seen. Until next time . . .