The New Model for Investment Casting

The chief selling point for investment castings has always been precision — thin walls, complex shapes and voids — and that is unlikely to change for many buyers. Many of the largest customers want precision parts in steady volumes produced by quality-certified processes, and often in difficult-to-cast materials. Investment casting foundries have all that covered.

But investment casting is more demanding of foundries’ time and expertise than most casting processes: an exact prototype of a finished part is formed in wax. This shape, or multiple versions of it, is attached to a “tree”, and then dipped in ceramic slurry that coats and hardens around the wax form. This sequence is repeated until a hard ceramic shell forms.

Next, the structure is heated to melt the wax cores, leaving a void that can receive molten metal. Once the metal is solidified, the ceramic mold is broken and the casting is released.

This explanation is simplified, and the actual procedure may take on much more effort. According to Jeremy Walker, prototype sales engineer for Invest Cast, the customary approach — "which involves cutting a tool from billet, providing a sample and FAI (first article inspection, a design record and verification file), all of which can take upwards of 12 weeks."

According to Walker, Invest Cast is an investment casting business "at our core," but it operates upstream (rapid prototyping) and downstream (finish machining), too. The rapid-prototyping division uses 3D printers to make the wax patterns of customers’ orders, setting in motion the investment casting process. "With 3D printing, the entire casting process can be done in as little as a few days."

Additive manufacturing (3D printing) still seems novel as an industrial process, but the applications in metalcasting are well established. "Our first move into additive (manufacturing) was in 1999," Walker explained, "when our CEO discovered the potential of 3D-printed patterns that could be cast on an expedited timeline."

AM/3DP describes various processes, two of which are in use by Invest Cast to support its prototyping operations: SLS (selective laser sintering) for metal parts, SLA (stereolithography) for plastic patterns and MJP (MultiJet Printing) for wax patterns. “These 3D printing technologies allow us to prototype quickly without tooling to expedite design iterations,” Walker reported. “The integration of additive manufacturing into our workflow helps us leverage our expertise more efficiently for our customers.”

Those customers are not simply large manufacturers (aerospace, oil-and-gas extraction and processing, agriculture/off-road equipment) but also include "customers who are just looking for a one-off piece that they designed themselves," he noted.

"We have the staff to support any need a customer may have regarding their cast parts," Walker said. "We can reverse-engineer from the crudest drawing and ultimately send you a casting of that product. Our engineers on staff are intimately familiar with the investment-casting process, and all of the specs and third-party processes that are common in this industry."

Invest Cast’s initial 3DP system was a Thermojet solid object printer, from 3D Systems Inc.  “We used it for the customers who wanted low quantities, wanted them fast, or had a part that was perhaps too complex to be cast using traditional tooling,” Walker recalled. "Customers were drawn to the short lead-time more than anything."

Currently, Invest Cast is operating seven SLS printers as well as two of 3D Systems latest models for printing parts in wax, the ProJet MJP 2500 IC. It was introduced last fall, building on several years of development and testing, according to product manager Mike Norkitis.

The ProJet MJP 2500 IC is based on 3D Systems’ MultiJet Printing technology for precisely placing a pattern wax (VisiJet^® M2 ICast) and a support wax (VisiJet^® M2 SUW) in place to build a three-dimensional part as indicated by a digitized model, "voxel by voxel and layer by layer," according to Norkitis. 

"The two waxes are jetted using a piezoelectric printhead that deposits tens of thousands of drops from each of the 880 individual jets each second," he detailed. "This allows for very fast build times while still maintaining great precision."

The ProJet MJP 2500 IC has a build area of 11.6x8.3x5.6 in, with a build resolution of 600x600x600 dpi, all in a machine envelope of 44.1x29.1x42.1 in. 

“The ProJet MJP 2500 IC is tailored to the needs of the investment casting industry to produce digital foundry wax patterns in a fraction of the time needed to build a traditional injection molding tool," he emphasized. 

The digital wax patterns produced by the new 3D Systems printers “fit seamlessly into a foundry’s existing wax pattern process, "without modification," Norkitis continued.  That’s important not only because it accelerates the implementation, but because it makes investment casters available to quote projects that previously had not been possible for them to consider.

"The ProJet MJP 2500 IC will open the foundry to produce parts that currently are not moldable even with soluble cores," he noted.

Every installation includes 3D Systems 3D Sprint^® software, with features developed especially for investment casting foundries, including tools to split large patterns and alignment features to facilitate joining. This program eliminates the need for investment casters to subscribe to third-party software in order to coordinate their customers’ orders to the prototyping process.

Invest Cast was looking for a system that that could produce a production-grade investment casting pattern, at a fraction of the lead-time, Walker recalled. "We knew that the market was there, and we just needed the machines to support it."

Several months since incorporating the new 3D printers, Invest Cast finds their expectations have been exceeded. It is able to convert CAD data instantly into wax patterns ready to implement in the shelling process.

Specifically, the fulfillment time for precision investment castings has been reduced. “After a customer accepts our quote and we receive the purchase order, we immediately send the build to one of our specialists who will load it into one of the ProJet MJP 2500 IC printers," Walker explained. "A matter of hours later, they pull the build out and deliver the parts to our (investment casting) facility in Minneapolis. Just like that, 5-10 days later, the customer has a casting in hand."

The positive reception was no accident. 3D Systems conducted an extensive development program for the ProJet MJP 2500 IC, including foundry customer feedback loops.

“We installed beta units at foundries to ensure the product and processes met the specific needs of the investment casting industry," Norkitis revealed. "As a result we gained insights into how the product will be used in a foundry setting, and further developed a sparse infill pattern that allows for patterns to be produced with up to 30% less build wax, saving the foundry money on each pattern."

With the ProJet MJP 2500 IC available, investment castes have no need to produce the tooling required to form wax patterns, which reduces customer delivery time by weeks, according to the developer.  It also eliminates the expense of the tooling, and allows for more design changes without the obvious cost or time necessary to modify the tool. 

"We are finding that the ProJet MJP 2500 ICs are geared towards the customer who needs tighter tolerancing, a better surface finish, and more repeatable castings, at a fraction of the production lead-time,” Invest Cast’s Walker observed.  “Smaller and more intricately designed parts seem to be the bread-and-butter for these printers."

"A foundry that adopts the ProJet MJP 2500 IC into its workflow will realize the ability to deliver finished parts to their customers substantially faster,” Norkitis emphasized. “It will enable those foundries to take on small jobs that were not previously economically viable, and make a profit doing so."