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Casting of cannon barrels of polyurethane resin


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First I decided not to issue this at all, but after having seen here elsewhere Chuck`s tutorial about casting the thin carvings out of PU, I decided to publish also my method to cast cannon barrels. I have used this method many times before when building my car & tank models, so I had a little experience about the chemicals and working methods also before starting to cast cannons.

 

The first thing was naturally to turn a pattern for the barrel in the lathe:

 

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Next thing to do is to make a spruce, a channel through which the liquid resin is poured into the mold. For this a piece of 3 mm iron wire was used:

 

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And then we can start making the mold. First a small box was made of 3 mm MDF. The pattern together with spruce was installed into one of the walls of this box, approximately at half height of the short wall. Then liquid silicone rubber was poured into the box so that the level is at the middle of the barrel. Sorry that there is no picture of this phase.

I was using Oomoo 25 silicone rubber, made by Smooth-On. It consists of two parts A and B, small amount of each is measured into a small container, mixed thoroughly and poured into the box. When cured, releasing agent was applied to the lower part of the mold and new batch of silicone rubber was made and poured to the box and let cure.

 

This is how the first half of a mold looks like, when dismantled from the box. The pattern with spruce is still on its place:

 

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And here you can see the both halves of the mold. A cone is provided at the top of mold where resin is poured. Small air venting channels are also cut at the top of the barrel to have air to escape when resin is poured into mold:

 

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The mold is closed and polyurethane resin is prepared by using equal amounts of part A and B of the Smooth Cast 305 resin from the same manufacturer as the rubber. Parts are stirred very carefully and poured into the cone of the mold. It would have been possible at this phase to add some metal powder to the resin, to have the barrel really look like cast out of metal. I have however found out that if metal powder is added into the resin, it will become very stiff and doesn`t flow very easily into the spruce & fill the barrel cavern completely.

 

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Resin has been poured and it can be seen that it has raised into the air venting channels too, which proves that the barrel cavern is filled as well:

 

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After the resin has cured and mold opened, the spruce and barrel look like this:

 

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Now it remains only to clean the barrel, drill holes for the axle pin, ignition powder and naturally for the cannonball, and glue a small glass pearl into the back of the barrel:

 

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And finally paint the barrels. I wanted them to look like old patinated bronze, so I mixed Model Master gloss black enamel paint with AK Interactive`s old bronze metal color. Diluted them so that the paint could be airbrushed on the barrels. And finally, when dry the barrels were wiped lightly with a soft cloth to get the look of old patinated bronze:

 

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I was thinking this a lot, and decided to make casting without the trunnions, because I wanted to keep the cast as simple as possible. With trunnions in place there is a risk that the resin does not fill the cavern completely because there always remain pockets filled with air. And it is relatively simple work to drill the holes and add them afterwards.

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Moxis

 

I have used a similar method with great success and I too add the trunnions separately.  I do leave a slight indentation in the master so the casting has the exact same drill spot on each piece, slight off the center line.  Drilling without the indentation tends to make the drill want to wander or skip a bit.  I also include the cascable and cascable loop where appropriate, but it takes some cleaning up and yes, there is the occasional air pocket to be filled. The main reason I did not include the trunnion is that I used a single piece mold so there is no seam and removing the barrel from a single piece mold is easy if there are no trunnions to grab at the rubber and tear it on the way out of the mold.   The seams were always a pain in the neck to clean up so the single piece mold eliminated the problem totally.

 

Allan

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Thanks Allan for your message, and all "likes" as well.

 

It is true that the seam which builds up if you use a two part mold, is a nuisance. So far however I have managed to remove it by gently cutting it away with a sharp blade and sanding the rest. The single piece mold is very interesting solution, of which I would gladly like to know more. Like which rubber are you using, and how do you manage to take a casting away without breaking the mold.

A good idea is also to provide a small indentation for trunnion drilling. So far I have managed to drill the holes with careful starting of the drilling.

I have left a small indentation only to the business end of the barrel to be able to drill the hole for cannonball accurately centric, that was easy to make when turning the master in the lathe.

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One thing you did, but didn't mention, is that you used location bumps and indentations in your molds. The readers can see them in the corners of the RTV molds, in several pictures. For those who don't know what they are for, it is to align the molds. If you leave these out, getting them lined up so the casting will not have the two halves skewed, when you take them out of the mold, is extremely difficult.

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Well done....excellent reference for casting cannon.   I also find that sometimes air gets trapped int the trunnion pin of the molds even when vented.  But a good solution I found was to make the trunnion much longer as the air seems to be trapped on the extremities of the pin mostly.  Making the pin about 1/8" longer on each side does the trick.   Then you can just cut them off to the proper length before you use them.

 

Chuck

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  • 2 weeks later...

And here they are finally on the deck of Le Cerf. Not yet rigged but otherwise finished:

 

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Carriages made. I know that there should be a thin chain at the trunnion fastening piece, but I couldn`t find chain thin enough. The smallest chain I found was a silvery one made of 0,3 mm wire, but that looked like anchor chain at this scale. So instead of chains small rivets:

 

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