Machined Tooling or 3-D Printing?
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The 3-D printing of cores and complete molds, without any requirement for tooling, is revolutionizing the industry of fast prototype casting manufacturing but it also offers distinct advantages for certain production castings.
A recent case study of a complex thermal command center casting for a high performance marine powertrain illustrates the calculation metalcasters and end-users may take in determining the cost factors that would promote the use of 3-D printed molds over conventional soft tooling.
The thermal command center, which combines a water pump, thermostat housing, and a series of bypass valves into one single casting was required for early mechanical development testing (Fig. 1). The part’s overall dimensions were 29 x 13 x 6 in. (736 mm x 330 mm x 152 mm), and it weighed 14 lbs. (6.35 kg).
Two castings were needed on an accelerated timeline. The first cast and machined part was needed in four weeks. More parts would be required after development testing, and it was likely the design would be modified based on the results of dynamometer testing. The casting was to be made in A356 alloy and heat treated to a T6 condition prior to CNC machining. The designer communicated information to the metalcasting facility strictly through 3-D data in order to accelerate the manufacturing process by eliminating two-dimensional drawings.
3-D printing the entire sand mold holds many advantages:
- Backdraft and zero draft present no obstacles and considerably reduce time for the product designer.
- The ability to make a casting, quickly evaluate the casting size and alter the shrink factor for the second casting is possible with 3-D printing, but not with tooling where remanufacturing the tools at great expense is necessary.
- Complex in-gate and riser geometries can be created more easily with 3-D printing than tooling.
- Design features can be quickly revised without any cost of tooling updates.
- Internal core positions are accurate as cores can be grouped together or printed directly into the cope and drag, eliminating core clearances and the resulting core shift in the casting.
- Reduction in weight or heavy sections of the casting is possible because it is not necessary to be able to draw sand components from molds. This leads to improved casting quality by eliminating heavy sections vulnerable to porosity.
However, 3-D printing holds some disadvantages compared to conventional tooling:
- The cost per sand mold is much higher, and the time to manufacture is generally much longer.
- The cost per printed sand component generally is linked to the size of the cuboid containing the printed part. As is the case for the core depicted in Figure 2, a small 3-D printed component occupying a large volume of unprinted sand would be expensive. This is because current 3-D printing machines use activated sand for which only a small percentage can be recycled. Similarly, large molds will be much more expensive than their tooled equivalents if the cost of the tooling is eliminated from the comparison. Future developments with phenolic binders may improve the cost effectiveness of low printed density cores since the sand can be recycled.
- Once soft tooling is made, it is possible to manufacture castings at a much quicker rate for most cores and mold components than by 3-D printing.
Because only two parts were required at first, it was decided to manufacture these castings using 100% 3-D printed sand molds.
Computer Aided Engineering Crucial
The cost of 3-D printed sand is high in comparison to sand molds and cores produced by soft tooling, making the use of simulation upfront critical. Simulations assist the engineer in gating and chilling strategies for complex castings. This strategy leverages preemptive casting development rather than trial and error development.
For the thermal command center, a variety of analysis techniques were conducted through the use of solidification modeling software. First, a 3-D mesh of the casting was built, and a natural solidification simulation was run. Natural solidification illustrates how the casting would solidify if it were filled with aluminum at a uniform temperature of 1,292F (700C). The natural solidification provides critical information such as which parts of the casting will solidify first and which will solidify last. Figure 3 shows what parts of the thermal command center will solidify last. The engineer used this information to determine the orientation in which to cast the part, as well as locations to feed the casting with in-gates and risers.
After an initial gating system was designed, the mold filling was simulated. This analysis provided invaluable information regarding the metal temperature and velocity during the filling process. The part was cast using a low pressure sand casting process in which the sand mold was filled from below. In this process, nitrogen gas is used to pressurize the aluminum in the furnace and force it up a ceramic tube into the mold. Low pressure casting ensures tranquil fill and prevents oxidation of the aluminum by eliminating turbulence during the filling process. It also was possible to examine the predicted solid fraction to ensure the metal was not going to solidify with cold shuts prior to filling the part.
The gating system for the thermal command center was adjusted more than 10 times over a one-week period and the analyses re-run until the engineers were convinced a satisfactory gating system had been designed.
The sand molds were printed using silica sand and furan binder. The machine had a build volume of 59 x 27.6 x 27.6 in. (1,500mm x 700mm x 700mm) and was able to print the complete mold and internal core during one build cycle that took approximately 30 hours. Figure 4 depicts the three printed sand components.
The subsequent de-powdering clean-up process took another six hours before the mold and single internal core were ready to assemble.
The total timescale from receipt of 3-D data, through gating design and extensive fill and solidification analysis, 3-D printing of molds and casting was 15 days. The casting was heat treated to the T6 condition and inspected prior to machining. A white light scan of the casting compared it to the machined part model, and x-ray inspection revealed no visible defects. It also was leak checked at 30 psi under water with no leaks.
Analysis of Cross-Over
Point for Tooling
The cross-over point for any casting is defined by the quantity at which the cost of manufacture by tooling is equivalent to the cost of manufacture by 3-D printed sand. This can be expressed in the number of parts. Figure 5 shows the cross-over point for the thermal command center. Similarly, as shown in Table 1, it is possible to determine the lead time for both routes of manufacture.
The conventional way to manufacture a small number of prototype castings would be to make urethane soft tools by CNC machining blocks of urethane board. In this project the conventional tooling was designed in detail, ready for manufacture, but because of the success of the 3D printed route, it was never made.
For the thermal command center, where just two parts were required in a compressed timescale, there were clear advantages from both a cost and timing standpoint to using 3-D printed sand to manufacture the molds. The only downside of using 3-D printing was that the surface finish was approximately 400 microinches RMS while with soft tooling the surface finish would be approximately 200 microinches. The increase in surface roughness was not an issue for the function of the component.
The graph in Figure 5 shows that the cross-over point when soft tooling would become more cost-effective for the thermal command center is 22 parts. At that point, the unit casting price with soft tooling is approximately half of the cost of the 3-D printed casting.
For all cast parts there is an equivalent graph, and the cross-over point moves to the left or the right depending on the overall size of the casting (large castings are expensive in 3D printed sand) and the number of cores (tooling and piece price increase).
In another example, a complex pump housing has a cross-over point in excess of 150 castings (Figure 6). This example shows the potential for 3-D printed cores to enter serial production in relatively low volume applications. Volumes of fewer than 500 units a year are common in high performance vehicles and aerospace applications.