Investment Casting for Prototypes and Low Volume Production

BY DAVID DOLATA, PRECISION METALSMITH INC.

	Recommended Supplies 1. Acrylic (clear) paint 2. Solvent (support removal) 3. Cyanoacrylate (optional)
Finished casting, made from a PolyJet pattern, is ready for use in 10 days.

Abstract

Investment casting has many advantages over other metal casting and machining processes. With investment casting, a wide variety of metal alloys are available, and the cast parts have exceptional surface finish and dimensional accuracy. The process also delivers complex parts that are difficult, if not impossible, to machine, forge, or cast.

For precision metal parts, investment casting often offers a lower total cost solution when compared to machining, die casting, or forging. However, often the tooling necessary to produce patterns is too costly and too time consuming to make prototyping practical. With PolyJet,™ tooling is eliminated and investment casting becomes practical for prototype and bridge-to production applications.

Introduction

Investment casting, also called lost wax casting, is widely used for producing ferrous and non-ferrous metal parts. Unlike other casting processes, investment casting produces net shape parts with excellent surface finish and dimensional accuracy. Investment casting offers a broader range of alloys and significantly lower tooling costs compared to die casting, which is the only casting process that yields better tolerance and surface finish. Because most applications have low volume production quantities (10 to 10,000 pieces) and rapidly changing products, investment casting is ideal.

Nearly 200 alloys are available with investment casting. These metals range from ferrousstainless steel, tool steel, carbon steel, and ductile iron-to nonferrous-aluminum, cooper, and brass. When cast in vacuum, super alloys-nickel, cobalt and magnesium-based alloys-are also available. The only process that matches this breadth of materials is machining, but it cannot produce the complex geometries that investment casting can deliver.

Investment casting uses expendable patterns and shells, which makes it excellent for complex and detailed part designs. Examples of these designs include internal passages and ports of a valve body, curved vanes of an impeller, and internal cooling channels of a turbine blade [figure – cross section of casting showing internal detail]. In addition, the smooth surface of the ceramic shell delivers “cast-in” details, such as lettering and holes.

Typically, investment castings have a dimensional tolerance of ± 0.005 inch/inch and a surface finish between 60 and 125 μin (RMS). Both variables are alloy dependent. The dimensional accuracy is also geometry dependent. The critical barrier in prototype development is the time and cost for the molds, as tooling is needed for the injection molding of a wax pattern. The PolyJet rapid prototyping process eliminates this barrier, which makes investment cast prototypes feasible and affordable.

PolyJet and Investment Casting

The key advantage of a PolyJet pattern (figure 1) is that it eliminates the need for tooling. Injection molds for wax patterns range from $3,000 to $30,000, and completing the tools can take 4 to 6 weeks. Time and cost can increase even more with complex geometries, as separate tools may be required to produce soluble wax cores or ceramic inserts. With the tooling cost eliminated, the lead-time for a cast part is slashed to just ten days on average. This yields a savings of $30,000 and 2.5 - 4.5 weeks, making investment casting viable for prototype quantities.

	Other advantages with PolyJet are that there is no need to add draft to the design, and the investment casting is made without parting lines. Without adding time or cost, the design can be extremely complex, such as a network of small internal passages.
Figure 1: PolyJet patterns can slash investment casting lead times and cost.

 

 

Creating casting masters with PolyJet is superior over other rapid prototyping processes in many ways. Most importantly, polyjet masters for investment casting have similar dimensional accuracy (± 0.008 inch/inch) as those made from the conventional lost wax process. Hands-on experience with PolyJet and other rapid prototyping processes shows that the PolyJet patterns offer superior surface finish and detail. And unlike other processes, PolyJet does not require sanding or filling to repair surface defects. Also, eliminating support structures with PolyJet allows complex passages to be clear; other technologies rely on difficult rigid supports.

Process Overview

The investment casting process (Figure 2) begins with creating the pattern. The pattern represents the cast part with the addition of modifications, such as shrink compensation gates and sprues. In traditional investment casting, the pattern is injection molded in foundry wax. With PolyJet, there is no need for injection molds because the pattern is made directly from the PolyJet system.

Patterns are then assembled to make a casting tree. Gates are attached to the pattern and the gates are then attached to the sprue. After all patterns are mounted to the sprue, the casting tree is ready for shelling. The casting tree is repeatedly dipped in ceramic slurry to create a hard shell. The patterns are then melted out of the ceramic shell, also called the investment, in a pressurized steam autoclave or furnace.

Figure 2: The investment casting process. 1) injection molding wax patterns 2) marking wax sprue for assembly locations 3) slurry coating of casting tree 4) refractory grain stucco coating to make ceramic shell 5) burn out of patterns 6) casting of alloy 7) removal of ceramic shell 8) inspection of finished casting.

 

 

The alloy is melted, often in an induction furnace, and poured into the preheated investment. After cooling, the shell is broken away, the metal parts are cut from the tree, and the gates are ground away. Investment castings can be used as is, or they can be modified with secondary operations.

Pre-Process

Investment casting is a combination of science, experience and art. Prior to final design and pattern construction, it is important to select an investment casting foundry and initiate communications. Each foundry will typically have unique capabilities, processes and requirements. In addition, pattern specifications will vary with the selection of the metal alloy and the geometry of the part.

If producing patterns for the foundry, it is critical that the foundry reviews the design so that it can recommend necessary design modifications to produce the highest quality part. The foundry can also make recommendations that reduce cost, time, and weight, while improving castability and product performance. Finally, the foundry will provide shrinkage compensation and gate design information.

Process
1. Investment Pattern Making

A. Pattern Design
Beyond good design practices, (figure 3), the key consideration is pattern modification to prevent shell cracking and to minimize residual ash. Ceramic shells have a very low coefficient of thermal expansion, so any expansion of the pattern during the burn out cycle will cause the shell to crack. PolyJet material does not melt like wax; it burns and leaves a slight amount of ash in the shell cavity. To address these constraints, the pattern is modified to reduce the amount of material and to minimize expansion forces. This is achieved by hollowing the STL file for the master part(s) to yield thin, yet sturdy, wall sections (figure 4).

Casting Design • Fillets: 0.030 inch min. • Edges: sharp to 0.010 inch min. radius • Wall thickness: 0.050 inch - 0.080 inch min Casting Deliverables • Angles: +/- 1.0 degree • Tolerance: +/- 0.005 inch • Surface finish: 60 - 125 _in RMS • Flatness: +/- 0.005 inch/inch • Size: < 1 oz to 20 lb. • Quantity: 1 to 5,000 typical

Figure 3: With properly designed fillets, edges, and wall thicknesses, investment casting delivers net shape parts.

Research has shown that the upper limit for wall thickness is 0.100 inch, and the ideal wall thickness is 0.050 to 0.060 inch At 0.100 inch, the shell may develop hairline cracks, but will not fail. To achieve the ideal wall thickness, CAD or STL modification tools can be used to hollow all features and walls to 0.050 inch When built with PolyJet, this will yield a part that has support material inside rigid walls. The next step is to flush the support material from the pattern.

Figure 4: Solid sections of the STL file (top) are hollowed out to yield a wall thickness of 0.050 - 0.100 inch	Figure 5: End gating is added to the part (gray) on both sides of the casting.

 

Hollowing the model reduces the amount of material, which reduces the amount of residual ash in the shell. The PolyJet FullCure™ 700 resin has an ash content of just 0.05 percent when fired at 2000° F. However, even this small amount of ash will lead to inclusions and surface defects in the castings. Hollowing the part reduces the ash content in the shell making it easier to flush the residual ask in the shell-washing step.

A recommended option is adding foundry-defined gating to the CAD model and constructing it as an integral part of the pattern (figure 5). The design of the gates should be both generous and conservative. To facilitate shell washing, gates are added to opposite ends of the pattern. This gate design provides a flow path for water to flush out the residual ash from within the shell. Like the pattern, the gates must have hollow walls with a thickness of 0.050 inch The hollowed areas of the gates must connect with those of the pattern to provide a flow path for the evacuation of the support material. Alternatively, the foundry can add wax gates to the pattern.

The final optional step is to add machine stock and shrinkage compensation to the model. For machined surfaces, 0.020 to 0.030 inch machine stock is added to the CAD model. Shrinkage compensation, ranging from 0.007 to 0.020 inch/inch., is then is applied to the CAD data. The material shrinkage may vary in the X-, Y-, and Z-axes and will be dependent on model geometry and the selected alloy. Therefore, the foundry must supply these parameters.

2. Pattern Construction

PolyJet investment casting patterns require no modification to the build parameters and build styles. The only consideration is to orient the model to achieve the best resolution for fine features, such as lettering.

3. Support Material Removal (External)

External support material is removed with a water jet. Ensure that all material is removed from narrow pockets, channels, or sharp inside corners, because this will affect the casting quality. No additional removal is necessary.

4. Support Material Removal (Internal)

Like the model material, the support material expands during burnout, which can crack the ceramic shell. Therefore, the support material contained between the hollow walls must be removed prior to shelling the pattern.

To facilitate support material removal, the pattern is soaked in a tetramethylammonium hydroxide solution (25 percent concentration). The solution liquefies the support material, allowing it to be flushed from within the pattern. The pattern is placed in an agitation bath filled with the solution for a minimum of two hours. If a pattern has long, thin channels or complex passages, it may require soaking for a day or more.

Following the solution bath, the pattern is flushed with water to evacuate the support material. Using moderate velocity and pressure, water is introduced into one of the gates and exits through the other. During this process, the pressure and velocity must be high enough to flush the support material. However, excessive pressure may cause the pattern to balloon outward. Therefore, the pressure and flow is monitored and adjusted as needed. After flushing, a small amount of support material (less than 10 percent) remaining in the pattern is satisfactory.

5. Pattern Finishing

Unlike other rapid prototyping investment casting patterns, the PolyJet pattern requires no additional sanding or part finishing. There is no need to sand away stair stepping or artifacts from rigid support structures. There is also no need to fill stair stepping with body fillers. The surface finish and detail of the original PolyJet pattern are consistent with the output quality of the investment casting process (figure 6).

	Prior to shelling, a surface coat of clear, fast-drying acrylic is applied. Krylon® Crystal Clear is recommended, but many other surface treatments are compatible with investment casting. As long as the surface treatment does not prevent the ceramic slurry to adhere to the part, the coating will be suitable. One consideration is to avoid pigmented primers or paints.
Figure 6: Example of PolyJet part.

Following burnout, the pigment will leave a powdery residue in the shell that will affect casting quality. Though this residue can be flushed from the shell, the pigmented surface treatment adds an additional and unnecessary consideration.

With modeling clay, define the remainder of the parting line. To prevent inhibition of the silicone rubber's curing, use a non-sulfur based modeling clay, such as "Kleen Clay". Apply the clay by hand to all features that are captured in the opposite side of the tool. Also, use the clay to build up from the mounting board to elevated areas of the parting line. Work and smooth the modeling clay with tools such as artist's spatulas, modeling picks, or tongue depressors.

6. Pattern Assembly

If the pattern size exceeds the build envelope of the PolyJet system, the pattern can be split into multiple pieces and assembled. Cyanoacrylate (“super glue”) is a good choice for bonding the pieces of the pattern, as it works well with the PolyJet material and is compatible with investment casting.

When bonding parts, note that the hollow areas within the pattern must not be sealed off. The mating surfaces at the bond must allow an unimpeded flow path for the water stream to flush the support material.

7. Packing, Storing, and Shipping

Protect the pattern from exposure to moisture, excessive temperatures, and ultraviolet (UV) light. This is especially important if the pattern will be stored for more than a few days.

8. In the Foundry

Once the PolyJet pattern has been built and prepared, the work now turns to the foundry.

9. Casting Tree Assembly

If gates were not built as part of the PolyJet pattern, they are added at this time. These gates will be molded in foundry wax and wax welded to the pattern. The gated patterns are then attached to the wax sprue, once again through wax welding. When all patterns are attached to the sprue, the casting tree is ready for shelling.

10. Shelling

A ceramic shell, approximately 0.375 inch thick, is created around the patterns. The result is the investment into which the alloy is cast. The casting tree is dipped in a face coat of agitated ceramic slurry (figure 7) and then coated with a stucco of fine sand (figure 8). The face coat process is repeated and is then followed by four more coats of slurry. The shell is then dried under controlled conditions.

Figure 7: The casting tree is coated with a fine-grain ceramic slurry to create the investment casting shell.	Figure 8: Between coats of ceramic slurry, refractory grain stucco is added to the shell for reinforcement.	Figure 9: The ceramic shell is fired in a furnace to burn out the PolyJet patterns

11. Burnout

The critical modification to standard foundry procedures is in the burnout of the pattern from the ceramic shell (figure 9). The standard processes of steam autoclaving and flash firing used with wax patterns are not suitable for evacuation of the PolyJet pattern. Instead, a higher temperature, longer duration furnace cycle is required. Testing has shown that two burnout options are available, but one is optimal.

Option 1 (Non-Optimal)

When a programmable, high temperature furnace is unavailable, flash firing may be used if the pattern's wall thickness does not exceed 0.050 inch The shell is flash fired at 1600° F for a minimum of four hours. The shell is then inspected for complete burnout and firing is continued as necessary. While the flash fire option is not optimal, it is compatible with solid sprues and gates made of investment casting wax.

Option 2 (Optimal)

The optimal burnout cycle involves temperature ramping in two to four hour cycles over a 20-hour period.

Using a programmable furnace, burn out the pattern with the following cycle:
1. 200° F - 2 hours
2. 250° F - 2 hours
3. 300° F - 2 hours
4. 350° F - 2 hours
5. 425° F - 4 hours
6. 500° F - 4 hours
7. 1600° F - 4 hours

Following the four-hour, 1600° F cycle, the shell is inspected to determine if the pattern has been completely burned out. If not burned out, the shell is fired at 1600° F until evacuation is complete. Note that the low temperature cycles can cause foundry wax to expand, which will cause shell cracking. To avoid cracking, all wax gates, sprues, and runners are hollowed to a 0.050 inch wall thickness.

12. Shell Washing
To remove ash and ceramic dust, the shell is washed with a forceful stream of water. If using fused silica for the shell, it may be washed immediately after removal from the furnace. The water stream is allowed to enter one gate and exit through the other. During washing, the water is agitated by shaking the shell vigorously. The shell is inspected to ensure that all residual material has been removed.

13. Casting
The completed shell is now ready to receive the molten alloy. The shell is preheated, and the alloy is cast into it according to the foundries operational procedures (figure 10). After cooling, the shell is broken away and the castings are cut from the sprue (figure11). The gates are then ground off. The casting is now ready for use (figure 12) or for secondary processes, such as heat treatment.

Figure 10: Molten metal alloy is poured into the ceramic shell.	Figure 11: After the metal has cooled, the ceramic shell is broken away from the casting tree.	Figure 12: The finished casting, made from a PolyJet pattern, is ready for use in only 10 days.

14. Conclusion
With PolyJet patterns, investment casting is practical for prototype and bridge-to-production applications. In less than two weeks, prototype castings in stainless steel, ductile iron, or aluminum are ready for testing and evaluation. Applying PolyJet to the pattern making process validates both the functionality of the part and the quality of the casting design.

With only minor modification to the pattern design and the burnout process, PolyJet eliminates the costly and time consuming tool making step needed for lost wax casting. Applying PolyJet to the creation of investment casting patterns preserves the cast-in detail, accuracy, and surface finish that distinguish investment casting from all other processes. Requiring no labor in pattern finishing, PolyJet expedites the entire process.

With this process guide and the skills of a qualified foundry, companies in all industries can capitalize on the efficiency, capability, and quality of investment casting.

For information on Precision Metalsmiths, visit their web site at
www.precisionmetalsmiths.com.

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