Investment casting yields accurate metal parts with detailed features, thin walls and smooth surfaces. This is achieved by using wax patterns that replicate the part being made. These patterns can now be made by 3D printing, which saves time and money and enables production of even more complex parts. This blog explains how.

Wax Pattern Production

Investment casting molds are made by coating a wax pattern with ceramic slurry. Once the slurry has dried the wax is melted out, which leaves a cavity for metal to be poured into. Investment casting therefore needs one pattern per part produced.

In volume production these patterns are made by injection molding. It’s a fast and repeatable process but the molds are expensive and can take many weeks to design, machine and prove out. Likewise, any changes needed can take weeks to implement. In addition, customization to satisfy specific customer requests is difficult and expensive.

3D Printing Patterns

Additive manufacturing technologies take many forms, not all of which are considered printing. The 3D printing technologies used for patterns are:

• Stereolithography (SLA) – one of the first “rapid prototyping” methods, this is a vat polymerization technique that produces clear plastic patterns
• Fused deposition (FDM) – this is where a polymer is extruded and the part built up layer by layer
• Binder jetting – the pattern is “printed” by spraying polymer droplets from a print head, similar in concept to an ink jet printer

The time needed to print a pattern by any of these methods depends on part size and complexity. However, it’s usually measured in hours rather than the seconds needed for injection molding.

Important considerations when 3D printing wax patterns for investment casting are:

• Characteristics of the material used – it’s important that it “burns-out” of the mold cleanly with no residual ash. Especially for aerospace applications, it should not contain or leave traces of antimony.
• Finishing needed before “investing” with the ceramic coating – surface lines, (common with FDM), need sanding away otherwise they will appear in the final cast metal part.

The Impact of 3D Printing on Investment Casting

Slow build rates mean 3D printing is not a replacement for injection molding of wax patterns. Rather, its impact is in small quantity production and the geometries that can be produced. Ways in which 3D printing is transforming investment casting are:

• Dramatic reductions in tooling lead time and cost
• Shorter time-to-market/faster new product introduction
• Bridge production
• Increased part complexity

Injection molding tools can take eight weeks or more to manufacture and cost thousands or a few ten thousand dollars. If, after an initial run of parts are cast it’s found that changes are needed, this can add more weeks to the lead time. In contrast, 3D printed patterns can be modified and new patterns produced in just hours. This allows more development iterations and lets the manufacturer get new products to market in less time and at lower cost.

3D printing also supports “bridge” production, providing patterns so that parts may be cast while injection molding tools are being produced. This makes parts available for testing and showing to customers and supports a ramp-up of production quantities.

Injection molding imposes constraints on the shapes that can be investment cast. This results from the need to eject the molded pattern from the tool. (Patterns can be assembled from components, but this adds cost and there are limitations in the amount of detail achievable.) In contrast, 3D printing can create small features like meshes, galleries and holes, that can improve in-service performance, eliminate machining operations, and support light weighting goals.

A Wider Range of Use Cases for Investment Casting

The ability to produce wax patterns by 3D printing is expanding the use of investment casting. It supports cost-effective small quantity production, customization, and feature enhancement while also saving on tooling costs and reducing time-to-market.

As a leader in investment casting, Impro has the expertise and capabilities to help manufacturers realize complex part designs in a wide range of alloys and production quantities. Contact us to learn more.