Making Investment Casting a Better Investment

May 16, 2014
Low-volume production of complex parts using investment casting is no longer associated with long lead-times and high cost, thanks to 3D printing.

To operate in some of the most important casting markets — automotive, machine tools — foundries have to be able to produce highly detailed designs — e.g., front-end (FEAD) brackets, gearboxes and other powertrain components — in comparatively low volumes. For their customers’ sake as well as their own, these project need to be completed as quickly and efficiently as possible. Thus, while the complex designs may be addressed effectively by investment casting, the process is not ideal for economic reasons. 

"The problem is the immediate production of the required wax patterns that result in expensive injection molding tools and wax presses,” explained Thomas Peipp, investment casting manager at Schmolz+Bickenbach Guss GmbH, Ennepetal, Germany. “These tool costs can become prohibitively expensive and time-consuming when used with small series or development, as they ultimately result in very high unit prices."

Voxeljet's additive manufacturing technology and service offers a work-around. Its 3D printers produce casting patterns quickly and cost-effectively, and with precision. The PMMA (polymethyl methacrylate, a synthetic polymer) models can replace wax models, and can be produced with speed and accuracy. No tooling is required —digital printing is based on CAD data — and the production process is repeatable regardless of the complexity of the design. In fact, designs can be developed regardless of current manufacturing techniques, and changes made as easily as revising the CAD data.

For example, voxeljet’s newest printers can produce a plastic model for a Francis wheel with a diameter of 500 mm in less than 24 hours. The material used in this application is PMMA, which softens at 73°C and burns without residue at temperatures over 700° C. This makes them well suited for use as investment casting models.

Schmolz + Bickenbach has experience with the 3D printing of plastic models. "In the past, we had the models printed at the voxeljet service center,” Peipp reported. “The advantages of the 3D print technology, along with continuously rising order volumes, prompted us to invest in our own voxeljet printer in 2013.

S+B Guss is a specialty steel foundry, producing castings ranging from 30 grams to 20 metric tons.

“With the VX1000 printer, our investment casting plant can print parts up to 1,060x600x500 mm and a maximum unit weight of 70 kg. Now we can offer our customers even more rapid and efficient support," he said.

Models are built in a layering sequence. Plastic powder is applied to a building platform in thin layers, and then selectively printed with a solvent. The solvent ensures that the particles adhere together over a locally restricted area. The desired object is created by repeated application and printing of layers.

The unprinted powder supports the printed structure, so that even complicated free-form surfaces with undercuts can be created without a supporting structure.

After the printing process, the residual particle material is removed from the finished model, which is then infiltrated with wax. This provides the plastic models with a sealed and clean surface, which enhances on the finished quality of the investment cast parts.

The subsequent steps are the standard investment casting routine, the same as if the model had been formed in wax. After it is inspected, ceramic coatings are applied the completed models are fired in a kiln. Since the plastic models shrink as a result of heating, there are no breaks in the ceramic shell. Once temperatures reach 700°C or more, the molds will burn out completely without any residue.

The molds must be blown clean before any metal is poured, but there is no requirement to wash them. After the metal is poured, solidified, and cooled, they are unmolded and any feeding and gating systems are removed. The castings are deburred, and any other surface defects are address in customary ways.

Finished castings are subjected to NDT and dimensional inspection, according to customers' specifications. "Whether prototype, individual part or small series–3D printing technology makes it possible to produce highly complex design and investment casting models quickly, easily and cost-effectively," concluded Thomas Peipp. “It’s an enormous advantage for investment casting specialists, and especially our customers.”

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