Sand such as is used for casting is a good medium for 3D printing. The green sand already used for casting is a combination of ingredients that help it to hold the shape of the pattern placed in the sand. The pattern is a reproduction of the part to be cast so that the sand forms a negative image of it.
In 3D printing the mold is built up layer by layer in typical 3D printing style. Binder chemicals are used to lock down the sand in the proper shape. Casting hollow parts requires cores, which can also be 3D printed at the same time. Best of all, the sand without the binding chemicals supports all these parts as they’re being made, not unlike the laser sintering process. The molds and cores come out of the sand just as they’ll be used at the foundry.
Here’s a video of the whole process using an ExOne machine to make a bunch of molds in sand:
You’ll note the work area for the 3D printer is quite large–like a gantry router. This allows either creation of large molds or packing a bunch of smaller work together. Filling the entire work volume of the machine requires up to a 24 hour 3D printing job.
Why use 3D printers for pattern making?
With a pattern, new molds can be made over and over again to support large production runs. Not so much with this 3D printing process. So why use 3D printers for pattern making? Clearly, it’s not the best answer for a high volume process. But it has its place in other ways.
For example, 3D Printers can be used for initial test runs to make sure all the mold and core designs will work well before put into high volume use. It’s less trouble to produce one mold + core via 3D printing than by making patterns. And of course, for low volume parts, the 3D printing process is ideal.
Another advantage is that 3D Printing can be used to produce molds with shapes that are difficult or impossible with traditional pattern making. There’s no need for draft angle to make sure the mold releases the pattern cleanly. Undercuts are fairly straightforward. The blacksmithing-style twisted bar cage in the video is a great example of a type of part that would be difficult for traditional patternmaking. Some industries need parts with these shapes. When the material being cast is very expensive, it can also make sense to use these techniques to reduce the amount of the material needed.
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