Mold for turned finial

[beacon14]

Ken Purcell, who posted about the .dxf import problems, has asked me to machine a sample of the type of product he intends to produce if/when he buys his new ShopBot. I'm happy to cut the file for him but it's too far into the 3D world for me to handle yet.

What he is looking for is a two-part form which he can use to cast concrete architectural details. The sample form would be a round, hollow casting with a profile like this:


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(Imagine rotating this profile around a vertical centerline.) What he needs is a female mold - the center section will be routed away leaving a bowl-shaped hollow form.

So my questions are two fold: the second question is what software would he need to do this sort of work and would anyone be willing to write the file for him (he's paying me and I think he'd be happy to pay for the file also)?

The first question is what's the best way to approach this job? The profile has some challenging aspects. I'm guessing we can separate the "collar" at the bottom, and divide the ball into 4 quarters, glueing two quarters up into each mold side. The material is low-density styrofoam, so piecing it back together is no problem.

I told Ken he could get a lot of answers here that will help him decide whether or not to take the ShopBot plunge. Thanks for any assistance.

[stevem]

Making the original is a job more suited to a lathe than a router. Once you have an original form, making a female mold from silicone is not very difficult.

Rhino would make easy work of the CAD file.

[stevem]

5 minutes with Rhino

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[Brady Watson]

David,
As with anything there is more that one way to skin a cat. This is how I would do it if I wanted to follow the path of least resistance:

Have someone with an indexer turn this for you & give you a solid 360° piece. Then mount the piece in a box with 4 sides and a bottom. Screw the bottom of the finial to one of the sides so that it is sitting at least 1" above the bottom. Coat the part & box with 2300 mold release. Pour in urethane mold rubber suitable for concrete; I would suggest PolyTek 75-80 RTV, up to the 50% mark of the model. Let the rubber cure. Then using a knife, create some 'keys' or divots in the cured rubber around the perimeter of the part. This will insure that you get proper alignment when it comes time to re-assemble the 2 halves of the mold. Spray 2300 release on the freshly cured rubber & try not to pool it. Then pour the 2nd half of the mold, covering the top of the part at least 1" above the model. After this has cured, you can now take apart the box and remove both halves of the mold.

To create the parts in the mold, you will need a release suitable for concrete. I would suggest PolyTek 2650. Apply this to the mold & prepare your substrate (concrete). Assemble the molds and sandwich the 2 parts in between 2 pieces of plywood using clamps. Pour in a little concrete and swish it around to fully coat the mold. Then fill the mold and use a concrete vibrator to help break up any bubbles that are in the mix & to avoid porosity problems. It's not any more difficult than that!

I suppose if you were not able to find someone with an indexer, the part could be modeled in ArtCAM, sliced and cut out on your bot. I do NOT recommend foam as a master because you will be disappointed with the results. You want something with substance that can be sealed...and something that isn't going to float away on you when you want to create a mold!

Here's a quicky of your finial in Rhino:


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Here's the same done in ArtCAM:


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Looks like Steve beat me to it...but I have to agree about the lathe or indexer idea as being the right tool for the job.

-Brady
PS - If you need help, send me an e-mail.

[billp]

David,
Here's a commercial item from Lee Valley which may give you some ideas;
http://www.leevalley.com/garden/page.aspx?c=2&cat=2,2180,33222,10337&p=10337 (http://www.leevalley.com/garden/page.aspx?c=2&cat=2%2C2180%2C33222%2C10337&p=10337 )

[Brady Watson]

Good point Bill...which brings up the idea that if the number of parts justifies the effort, a mold cold be thermoformed and used as well. I believe that HDPE could be used as the mold material and thermoformed...giving you a slick, easy to release surface to begin with.

-Brady

[stevem]

That Lee Valley mould looks like it's vacuum formed. That should have been my first thought, being in the plastics forming business. D'oh.

[Brady Watson]

Hey Dave...I missed the part about it being hollow. In that case, it can be done in a rotational molding machine...or slush cast by hand (this is where you continuously rotate the part by hand...a bit tedious for concrete)

You might want to talk him out of making it out of concrete if it is going to be hollow...or convince him that it needs to be solid. Just so you know, both molds (hollow or solid) would be identical. Only the hollow one will be a function of it being casted in a constantly rotating (XZ & YZ directions) to constantly coat the mold all over until it 'kicks'...

-Brady

[beacon14]

Thanks you guys,

Brady I mislead and/or you misread - it's the mold that's hollow, not the casting. But your suggestion of casting the mold from an original is the way to go. Much easier than what I was prepared to do. And since we can still use the ShopBot to make the original we can still show Ken what it can do. Maybe he'll buy an indexer and let me use it once in a while!

[btk]

David,

If you use a medium grade (45 shore) polyurethane rubber to make the mold, you can cast this part as a one piece mold (I.E. no need for two part with keyed mold). The re-entrant shape is not so severe that you will nto be able to de-mold (try to keep the side-walls to a minimum).

Brian

[drodda]

Just had to draw it in Autocad to see how fast I could do it. I have to admit that if Steve drew that in Rhino in 5 minutes he topped my speed. It took me almost 10 minutes in Autocad.

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Not that this adds to the post except to show that it can be done in almost any cad format.

[beacon14]

Just to prove that I'm not totally helpless, here's what Ken was asking me to produce in the first place.


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about 10 minutes in DesignCad - hopefully in a few months I can get this down to a couple of minutes. Now the SBP file, that's another story...