Investment casting produces ferrous and non-ferrous metal parts with excellent surface finish and dimensional accuracy. This manufacturing process is ideal for applications that have relatively low production quantities or rapidly changing product designs.
Investment casting, also called lost wax casting, is a process in which a master pattern traditionally made of wax, is covered with a ceramic slurry. The wax pattern is normally produced with injection molding. The wax is melted out of the ceramic shell, which is pre-heated before molten metal is poured into the pattern. After the metal cools, the ceramic is vibrated and blasted from the metal casting. Investment casting generally provides higher accuracy and better surface finish than other casting processes. It is generally used in applications with moderate production quantities that require the accuracy and surface finish benefits provided by the process.
Since investment casting uses expendable patterns and ceramic shells, it is excellent for complex and detailed part designs. It can produce intricate parts that are difficult, if not impossible, to machine, forge or mold. But this ability comes at a cost. Investment casting patterns are typically injection molded, and when designs are complex, the time and cost to produce tooling increases.
The cost of injection molds for wax patterns usually ranges from $3,000 to $30,000. The typical lead-time is one to four weeks.
3D Printing the pattern
Investment casting wax patterns are typically produced using an injection molding process. A big problem for many investment casting foundries is that they are unable to produce prototype castings until the injection mold is completed. At this time, problems are often discovered, such as interference with other components in an assembly. Additional time and money must be invested to fix the injection mold. In a worst-case scenario, it may be necessary to scrap the original tool and start over, which is a big setback in cost and time. Another problem with wax patterns is that the complexity is limited by what is feasible from an injection mold design.
Additive Manufacturing (AM), both FDM® (Fused Deposition Modeling) and PolyJet™ (PJ), provide an alternative method for producing investment casting patterns that provide dramatic time and cost savings. Both of these technologies build plastic parts layer by layer using data from CAD files.
FDM Patterns
With FDM, an added benefit is that the pattern can also serve as a prototype for form and fit evaluation. In one day, the foundry can provide the customer with a prototype that’s a perfect match to the casting. The strength of the materials used in the FDM process also makes performance testing possible. If any problems are discovered, a new pattern can be produced in about 24 hours.
PolyJet Patterns
PJ masters are best suited for dimensional and visual inspection prior to casting since the pattern tends to be more fragile than FDM patterns. As soon as the customer approves the prototype, the foundry can move into production by using the AM parts as patterns for investment casting. Since both FDM and PJ are additive processes, the patterns can incorporate more complex design features without any impact on cost. This is not true for injection molding, where increasing pattern complexity requires a more complex and more expensive tool. AM patterns are also more durable than wax and allow for greater accuracy by avoiding the dimensional compensation needed for the creation of the wax parts during molding. The durability is important because it prevents damage that often occurs in the transportation and handling of patterns made of wax or other fragile materials.
Process Overview
Both FDM and PJ patterns are direct replacements for traditional patterns that are injection molded with foundry wax. One significant difference is that both FDM and PJ materials do not melt like wax. These materials burn, which leaves a small amount of ash in the shell cavity, typically between 0.01 – 0.26%, depending on the process and material.
The ash is later removed in a shell-washing operation. Venting should be added to the ceramic shells to speed the burnout of the pattern and ease the shell-washing step. Investment casting patterns need to be produced to close tolerances and with an excellent surface finish because any defects are reproduced in the finished part. PJ parts have a very fine finish directly out of the printer and FDM parts can easily be post-processed to get the desired surface finish. Using the Stratasys Finishing Touch Smoothing Station, a semi-automated process to improve the surface finish of FDM parts, near-injection molded quality can be achieved without the labor or cost associated with traditional finishing. Advancements in finishing technology have improved FDM parts to the point that hand finishing is no longer needed, making FDM a much stronger competitor in the investment casting market.
Substituting 3D printed fused deposition modeling (FDM) patterns for the molded wax patterns eliminates the time and expense incurred with the injection molds while preserving the ability to produce complex metal castings.
Download the application brief to learn more about the process and how TriMech FDM 3D Printing Services can help you to save molding costs.
