Pontoon Bridge

Joined: 13 Apr 2007 Posts: 89 Location: Naples, Fl

I was flipping through a Napoleonic 'arms and equipment' book the other day, and ran across a description of a typical pontoon bridge of that era. There were no illustrations, but the article did have dimensions. I converted all the feet and inches to centimeters, then reduced those numbers to millimeters, giving me an exact 1/100th scale. I laid out the dimensions on graph paper, then transferred the digrams onto styrene sheet. After a little work with the knife and file, I ended up with this.

The Pontoons

The Bridge Deck

And Together

Some things you may have noticed:
1) The plastic I used is two sizes of scribed styrene. I used an exacto knife and a scribe to distress the divisions between the boards, as well as chop up the ends of the deck. (I wanted these to look well-used.) As a final step, I worked the plastic over with a metal suede brush to give it some wood grain.
2) The bridge deck you see in the picture is actually two sections, as per the article's dimensions. I just glued them together to make it easier for the next step, which is........

3) (No, the bridge deck in the last picture is not levitating above the pontoons, it is sitting on the wires that will form the air channels because I decided to make this)........ my first resin casting project. Yep, I had purchased silicon rubber and resin from Micro-Mark some time ago, and thought this would be the perfect project to try them out on. I made high-temperature molds and cast in hot metal years ago, but it always made me nervous. I wanted something a little safer.

Making the Mold

As I said, I got my supplies from Micro-Mark. They have a big selection moldmaking and casting materials. I picked the ones that were two-part, mixed by equal volumes. I didn't want to get into weighing the liquids. (Many of the best resins are measured by weight, so if you already have a digital scale, this is the route you may want to take.)

There are lots of articles out there on moldmaking, so I suggest you use Google to find them. There is even a thread category on TMP to check out. I made my mold box with wood and plastic 'hot-glued' together. (Suffice it to say, I will go buy a cheap Lego set before my next mold project.) I laid a thin layer of 'clean' clay at the bottom of the box, then embeded my masters in them. As you can see, I used electrical wire nuts to form the pour holes.

I mixed the rubber according to the instructions, and slowly poured it over the masters. I used a power sander on the mold base to help vibrate air bubbles away from the masters. ( Note: Try to buy your rubber and resin shortly before needed. My rubber said 'best when used by 12/07,' and I think it is a little fragile.) I let it cure overnight.

Here is the mold with the clay removed and the masters still in place. As you can see, there are two pontoons, two bridge decks (four actual sections), and a small bridge deck to attach to the bank.

I put this assembly back in the mold box, and brushed on several coats of 'mold release.' (Silicon rubber will stick to itself.) I mixed another batch of rubber, and poured it over the top. This is what that half of the mold looked like after curing. As you can see, it picked up even the smallest detail of the master.

And here is the other half.

Now it's time to make some duplicates.

Casting

I put the two halves together on my desk, mixed the two-part resin, poured in the mold, and waited the one-hour demold time. I carefully pried the mold apart and.............

.......what a disaster!

One of the bridge decks was 90% complete. None of the rest came close to being completely cast. The only bright spot was that wherever the resin did flow, it picked up the detail quite well. So, what went wrong?

First, I was trying to pour out of a round plastic cup, and it was hard to hit the pour holes accurately. Second, the pour holes were way too small. Third, the air escape holes were also too small, or in the case of the pontoons, forgotten. Embarassed

Fourth, there was rubber 'flash' inside the mold that kept the resin from flowing properly.

So, I used a hot air gun to shape a pour spout on the pour cup. I enlarged the pour holes with a knife. I drilled out larger air escape holes. Cuticle scissors took care of thin rubber 'flash.' OK, let's try this bad baby again. I mixed up another batch of resin ( less this time, I wasted way too much the first time). I actually hit the pour holes this time, and when resin came up through the air escape holes, I knew the molds were full. I waited an hour, held my breath, pulled apart, and..........

YES!

You can see a thin layer of resin flash, and what didn't break away with gentle flexing, was easily removed with a knife. There are a few voids from air bubbles, but I've seen worse from commercial resin kits. Here they are next to their corresponding master and in use. That is an AB 18mm Russian 20lb gun for size comparison.

I will be casting some more, and will post pictures of the completed, painted model.

Don

Joined: 16 Feb 2008 Posts: 55 Location: Newcastle Australia

Very well done. Bridges and field engineering could be made in a similar way.