Connecting pieces for casting in brass?

See Rhino file cross_demo.3dm (99.6 KB). The object is less than a centimeter high, and the wires making up the cross have a thickness of just 1 mm. Now the challenge is to arrange several of these objects so:

  • it is easy to separate them later, for example with a wire cutter, or with a small circular saw,

  • molten metal can find its way,

  • the number of sprues needed is minimized.

I first tried it as illustrated below, but the design was rejected by Shapeways: “The amount of sprues and supports necessary to cast this part would make finishing impractical. Metal will not flow freely as currently designed. Pieces should be separated.”

What do you think? Should I arrange objects on a sphere? On a torus? Make connections thicker? Where would spruces be connected?

I am aware that this is not strictly a Rhino question. However, there are jewelry makers here, and maybe someone has some advice. :slight_smile: That being said, my object is not jewelry. It’s a accessory for a toy.

I can’t tell from the drawing but it looks like there are only two unique designs. Why not just upload those and print multiples? If you want them all as one piece I would lay them out in a radial array. That way the molten metal could flow from a central point to all the pieces equally.

Wouldn’t be economic:

  1. They are all slightly different. I need to find dimensions as the pieces are supposed to plug into another system, a toy.

  2. As far as I understand it, there will be a handling fee of about 10 EUR per printed piece. At a material cost of just a few cents, that just doesn’t make sense. I’d rather spend a little time to separate the pieces later.

The standard way of doing this would be make a runner to which all the pieces are connected by ingates. This system of feeding metal to the casting is called gating.


A runner in this case could be just a long rectangular tube next to the castings with smaller tubes connecting to the castings.


Thanks for the link, @jim! I wonder if there is some plugin for Rhino to automatically create a gating system. Anyhow, new attempt:

model.3dm (3.0 MB)

The technicians at Shapeways are free to decide how to best orient the model, to get pieces filled yet minimize turbulence.

That looks like it will work as a gating system, but you also have to consider the 3d printing operation. I don’t know that much about 3d printing but they may want something that will support itself and be stable so that it doesn’t fall over while printing.

Anyway, they will tell you if that will work.

What exactly is Shapeways doing? Printing a wax pattern and casting it straight?

If yes, the standard in jewelry investment casting is creating trees (don’t know the right English terms, it’s how we call it in Portuguese).

But usually we would print the patterns with only a stump that would be attached to a wax three with hot wax. We’d do that to avoid the 3d printing costs of the three, usually way highier than the cost of the isolated pieces.

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Their wax printers add soluble support where needed.

They build up the tree manually:

So I’d recommend you to print only the parts. Upload it as a multi-part file with separate volumes. No need paying extra for the work they’ll do anyway. Besides that, if you submit it like this they won’t split the tree, they’ll deliver you the complete tree and you’ll have to cut it an finish yourself.

Or are they stating that they’ll interfere with the geometry you send?

Not permitted, unless all pieces are the same, which they aren’t in my case.

The material cost of the main trunk in my file is way below the handling fee of even one part.

So you are aware you’ll have to clip it later, right?

Make the main sprue base a conical shape, place the parts pointing towards the thin end of the trunk, like in the picture. Do cylindrical sprues, 2mm or 3mm in diameter for a ring sized part (If the 3d printer material holds - that’ll be try and error). No need to go fancy there, I’ve printed jewelry for ten years and only saw cylindrical sprues the whole time.

I’d keep it flat like you did, it will cut the support printing time & material to a minimum. What you can consider is adding patterns to both sides of the main sprue.

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One last thing: what shrinkage they told you to use?

They don’t tell. That’s why I need to make tests with slightly different sizes.

Thanks for the suggestions!

If memory serves me we’d use 3% for a two step casting, silver master pattern and gold product. I’ve seen brass listed as 1.4% so I guess it would be a good start.

It’s a good idea to add a cube with known measures and X/Y/Z axis markings on the face to your test castings, that’ll give you a good idea of what is going on. Sometimes geometry interferes.

This line bellow drove me crazy with the thinner valleys being overstretched too thin by the bigger masses pulling material while cooling. We’d never get the expected thickness and the customer kept pointing his finger at me, that the prints where wrong.

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Thanks @RicardoAmaral for all the advice! Meeting an experienced jewelry designer was what I was hoping for when posting in this forum.

Shapeways rejected my recent attempt. I am now in contact with support trying to resolve the issue.

Good luck there.

It seems that chariot’s standard pole must go to at least 1mm diameter, change the sprues to 2mm cylinders.

Right you are! I was looking at “min unsupported wall thickness” when I should’ve looked at “Min unsupported wires” in the design guidelines for brass. Still it passes automatic checks, and the technicians don’t complain about it. So maybe I’ll keep it like that. If it breaks, then I learn something. The chariot is kind of an add-on for now, anyhow. It’s unfinished.

At what angle are your feed sprues, relative to the main sprue?

FWIW, we get some parts directly 3D printed in steel and inconel. I see there are printers that do brass as well. Perhaps worth looking into for the future?

Most of that work was done by hand. It will depend much on part shape, how much you need to open it so they stack neatly. Anyting from 15 deg to 45deg most of the time, if you do an image search on google you will see the trend. You need 2mm diameter at least, more if they get long. Lest they break when the mold ceramic is poured over the patterns.