Calling Harold V - 50Kw induction furnace

Home enthusiasts discuss their Foundry & Casting work.

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Harold_V
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Re: My home monitor brightness must be set real high..

Post by Harold_V »

If you can lighten up the image such that we can see the gun a little bit better, that might be nice, but I can honestly say that the sparks and smoke make a great picture. It's pretty obvious you're using black powder! Nice pic, George.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
GeorgeGaskill

That image is too dark to see the gun. And ...

Post by GeorgeGaskill »

in any event, that gun is not impressive as it was a field expedient marriage of the barrel to a mount designed for another barrel. I'll take a picture of its sister and post that. By the way, we can move this thread over to the Gunsmithing/Shooting forum if it has drifted to far away from Casting & Foundry. :-)
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Harold_V
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Re: That image is too dark to see the gun. And ...

Post by Harold_V »

The new pic would be most welcome. Regards moving this to the gun forum, I had the same thoughts and have absolutely no objections. As you know, Marty is trying to encourage all of us to try to keep things in the proper forum so they are easier to find when doing a search, and to spare those that have no interest from having to check them to discover their content. Feel free to make the move.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
GeorgeGaskill

I am putting this pic here since it is really ...

Post by GeorgeGaskill »

about casting. These are the last two barrels I cast; the one on the right was No. 3 of 3 cast in Petrobond. The left one is the first one done as a shell mould (remnants are still stuck to the casting.)

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Harold_V
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Re: I am putting this pic here since it is really ...

Post by Harold_V »

George,
While it's hard to see detail, I get the idea that the petrobond casting is a little better quality than the shell mold variety. Is that the case? Considering you machine them, that may not be an issue, but if one were to cast some high detailed barrels such as the ornate Spanish one I spoke of, which would not get machined, might the petrobond sand be a better choice? Shell molding is supposed to superior, but is it? I'd appreciate your views on this.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
jpfalt
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A little confused....

Post by jpfalt »

I looked over the photos and wonder if that really is a shell mold.

A shell mold uses a dry sand and bakelite plastic mix that is a powder. A metal pattern is used and the pattern is heated. The sand/bakelite mix in contact with the metal pattern cures and forms a shell up to 1/2" thick. The excess powder is poured off and saved for a future mold.

The mold material in the photo looks like a plaster mold. The plaster is actually a high temperature alumina plaster mix. The mold has to be baked to remove all moisture. The shell mold casting in your photo looks like there were gas problems in the mold, either from decomposition of the plaster or from water boiling off to steam when the mold was poured. It's also possible that some of the surface flaws are cold shuts from metal chilling on the mold surface before it has fully flowed. Plaster molds are usually heated up to nearly red hot before pouring.

The plaster molds I have seen were used in lost wax casting and the mold is first held at about 250 degrees to drive off excess moisture and then heated to about 1500 degrees to preheat the mold and thoroughly remove any volatile material from the mold before pouring.

One big difference between a plaster mold and a petrobond sand mold is permeability, or the ability of gas to pass through the mold media. I periodically have problems with steam blows and trapped gas in molds I make in green sand molds if I ram the mold too hard and don't put in enough vents. Petrobond is difficult to over ram, vents well and generally doesn't generate very much gas when hot metal hits it. Plaster has almost nil permeability and must have vents and "pop-offs" to get trapped gas out of the mold.
GeorgeGaskill

The left casting was made from a wax ...

Post by GeorgeGaskill »

pattern which itself was made in the attached fiberglass mold. The molding process is an investment process that uses a colloidal silica slurry which is sprinkled with fine, medium and coarse fused silica in succeeding layers (2 or 3 fine, 2 or 3 medium and 5 or 6 coarse to build up the mold strength.) We may just have a terminology difference here.

The suface is not very good because I painted wax over the pattern surface to fill up pin holes quickly rather than try to fill (and find) each hole individually. The petrobond casting probably should have been rammed harder but I don't have a lot of experience with it so it got what I thought was best. The surface finish for these castings is really irrelevant as they will be machined all over and have a machining allowance built in.

Regarding a decorated barrel, I would do that in a shell mold as the process is designed to pick up fine detail without having to make multi-part patterns. Mine is ugly because of the over-paint. The art class does do some plaster block investment casting but this is not one of them.

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GeorgeGaskill

This is the match plate for the petrobond mold ...

Post by GeorgeGaskill »

but it's slightly modified to use as a pattern for the fiberglass mold shown in the previous post.
jpfalt
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Re: This is the match plate for the petrobond mold ...

Post by jpfalt »

Technically speaking your process is not a shell mold. It's actually closer to an investment casting because of the medium you are using for the mold.

To get vastly improved results bake the mold to at least 500 degrees before you pour into it and add vents liberally to help gas get out of the mold.

The petrobond casting looks pretty darn good. No arguing with success.
GeorgeGaskill

After the dipping/sanding process is complete, ...

Post by GeorgeGaskill »

the colloidal silica/fused silica mold is de-waxed in an large autoclave and then heated to about 1600 deg F to burn out the remnant wax and fire the mold. It is taken from the kiln glowing orange, put into the pit and poured. Usually they have stopped glowing by the time they are poured but they are still quite hot.
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Harold_V
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Re: After the dipping/sanding process is complete, ...

Post by Harold_V »

Nice explanation, George, in keeping with what I had suspected.

Just last Friday there was an article in our local newspaper about a fellow in a near town that is casting bronzes. He uses the exact process you described and appears to welcome visitors. Guess who's going to make a trip in the near future. [img]/ubb/images/graemlins/cool.gif"%20alt="[/img]

Harold

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
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