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An Investment in Wax

Some rambling on the art of lost wax casting.

Wax masters ready for casting

There is a basic or fundamental law in lost wax casting. The law says,”The casting is only as good as the model.”

Every and I mean EVERY flaw in the model will be duplicated in the cast. There is no such luck as expecting it to “look better” once it is cast. Because tiny details are easily overlooked in the model. The casting almost always has a flaw in some detail.

Time spent on the finish of the wax is time well spent. Wax on a model can be added as well as subtracted to “fix” a flaw or blemish. Once the cast is made any fix is subtractive using filing or grinding away hard metal.

I thought 3 dimensional printing would be a great boost to my designs and investment castings. I can print designs that can be cast that I can’t carve by machine or by hand. So I tried very hard to produce and investment cast using 3D printed models.

Depending on the user skill in using CAD, drawings can be produced that are technically perfect. A good starting point. But that has problems too. Not every CAD drawing can be printed or machined. It is a user skill to design the possible.

First issue was the quality of the printed model. 3D printing is a layer by layer process. The layers produce visible lines. Undesired in the casting. The smaller the layers the less visible the lines. But they are still there. They don’t “go away” in the casting process. Re-read the first line in the second paragraph above.

The photosensitive resin process produces the best and smoothest 3D prints. Smoothness is a relative term. Jewelers must do additional finish work on the cast model if the layers are not desired. I don’t use an abrasive tumbler (or have one) to surface finish my silver castings. Perhaps this is the solution larger production shops used for the large volume they produce.

I do use a planishing tumbler with steel shot. That is a final step polishing process for a hard, shiny surface finish. It does not remove flaws.

But there is a second issue that has stopped me in my 3D print efforts. This is the burn out of the resin from the investment material. The investment material degrades (crumbles) inside the mold form, from the resin burnout.

The culprit seems to be the incomplete cure of the printing resin in thicker sections of the model. I have produced good casts by deploying extra ordinary processes like heated vacuum curing of the model and ultrasonic cleaning.  Good casts are usually from models with thin sections rather than heavy.

Bulky designs must be printed with a hollow core. That core must be completely free of uncured resin so drainage must be included in the print process. The hollow core can be sealed for casting to produce a solid cast (if desired). (Hollow investment casting are quite difficult.) That requires extra steps to seal the drainage holes.

I don’t do investment casting because it is easy. I do it for the results. But I don’t want to do more work when I have another process (wax) that produces outstanding results with less effort.

I have used wax to repair defective resin 3D prints. That is a good solution. Printing defects are never mentioned by the printer vendors as they pretend in their marketing that every print is perfect for casting direct from their systems. Actually, they don’t make that claim, but they never mention anything but necessary minor repairs where supports attach. Usual admission is, “a light sanding” at those points and all is perfect. Reality is sometimes a touch of wax to fill a pin hole is necessary. Or closing of drain holes from internal voids.

Cast silver with glass enameling. (Champlevé)

The truth is hand finishing any model is always a part of the investment casting process. The cast is only as good as the model. I have said it again…

I have not totally abandoned resin 3D printing as a model source. But I have returned to wax carving as my most successful process for the majority of “lost wax” investment casting.

I now have 4th axis rotary CNC machining and two sided 3D CNC carving available for creating detailed and reliable wax models. I can hand finish wax adding and subtracting if necessary.

I do silver casting because I like the creative effort and the results that are of the highest quality. Not because I need an income for my next meal or a roof over my head. It’s a nice place to be at this point in my life.

 

 

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