Lost-foam casting

Type of evaporative-pattern casting process

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Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of polymer foams to simplify the investment casting process by removing the need to melt the wax out of the mold.

Process

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First, a pattern is made from polystyrene foam, which can be done by many different ways. For small volume runs the pattern can be hand cut or machined from a solid block of foam, or a sheet of foam core board if the geometry is simple enough it can even be cut using a hot-wire foam cutter. If the volume is large, then the pattern can be mass-produced by a process similar to injection molding. Pre-expanded beads of polystyrene are injected into a preheated aluminum mold at low pressure. Steam is then applied to the polystyrene which causes it to expand more to fill the die. The final pattern is approximately 97.5% air and 2.5% polystyrene. Pre-made pouring basins, runners, and risers can be hot glued to the pattern to finish it.[1]

The foam pattern does not need to be coated with investment if high detail is not needed, simply putting the foam pattern in a box, filling with sand and vibrating will do. However, when detail is needed, the foam cluster is coated with ceramic investment, also known as the refractory coating, via dipping, brushing, spraying or flow coating. After the coating dries, the cluster is placed into a flask and backed up with un-bonded sand which is compacted using a vibration table. The refractory coating captures all of the detail in the foam model and creates a barrier between the smooth foam surface and the coarse sand surface. Secondly it controls permeability, which allows the gas created by the vaporized foam pattern to escape through the coating and into the sand. Controlling permeability is a crucial step to avoid sand erosion. Finally, it forms a barrier so that molten metal does not penetrate or cause sand erosion during pouring. [1][2] Once the sand is compacted, the mold is ready to be poured. Automatic pouring is commonly used in LFC, as the pouring process is significantly more critical than in conventional foundry practice.[citation needed]

There is no bake-out phase, as for lost-wax. The melt is poured directly into the foam-filled mold, burning out the foam as it pours. As the foam is of low density, the waste gas produced by this is relatively small and can escape through mold permeability, as for the usual outgassing control.

Details

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Commonly cast metals include cast irons, aluminium alloys, steels, and nickel alloys; less frequently stainless steels and copper alloys are also cast. The size range is from 0.5 kg (1.1 lb) to several tonnes (tons). The minimum wall thickness is 2.5 mm (0.098 in)[citation needed] and there is no upper limit. Typical surface finishes are from 2.5 to 25 µm (100 to 1000 µin) RMS.[3] Typical linear tolerances are ±0.127 mm/mm (0.005 in/in).[4]

Advantages and disadvantages

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This casting process is advantageous for very complex castings that would regularly require cores. It is also dimensionally accurate, maintains an excellent surface finish, requires no draft, and has no parting lines so no flash is formed. The un-bonded sand of lost foam casting can be much simpler to maintain than green sand and resin bonded sand systems. Lost foam is generally more economical than investment casting because it involves fewer steps. Risers are not usually required due to the nature of the process; because the molten metal vaporizes the foam the first metal into the mold cools more quickly than the rest, which results in natural directional solidification.[3][5] Foam is easy to manipulate, carve and glue, due to its unique properties. The flexibility of LFC often allows for consolidating the parts into one integral component; other forming processes would require the production of one or more parts to be assembled.[6]

The two main disadvantages are that pattern costs can be high for low volume applications and the patterns are easily damaged or distorted due to their low strength.[3] If a die is used to create the patterns there is a large initial cost.[5]

History

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Lost-foam casting was invented in the early fifties by Canadian sculptor Armand Vaillancourt. Public recognition of the benefits of LFC was made by General Motors in the mid 1980s when it announced its new car line, Saturn, would utilize LFC for production of all engine blocks, cylinder heads, crankshafts, differential carriers, and transmission cases.[7]

See also

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• Full-mold casting – evaporative-pattern casting process

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References

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Bibliography

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• Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4
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• Kalpakjian, Serope; Schmid, Steven (2006), Manufacturing Engineering and Technology (5th ed.), Pearson, ISBN 0-13-148965-8
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Marshall Miller

The use of foam patterns in metalcasting first began in 1958. While it is not as widely used as other processes like bonded sand, permanent mold or diecasting, lost foam casting provides advantages for casting intricate patterns using precision molding tools to totally pattern-less castings using machined foam patterns that are not feasible with other processes.

Designing for other casting processes, such as sand casting, has restrictions. Because most casting methods require reusable patterns that must be withdrawn from the mold prior to casting, the removal of the pattern from the mold must be taken into consideration when planning the pattern layout. Contrarily, using foam patterns that remain in the mold during casting and are evaporated helps reduce some of these limitations.

In the lost foam casting process, polystyrene or co-polymer beads are expanded in an aluminum tool and bonded together to form complete patterns or sections that are assembled into a foam pattern. These pieces are then glued together to form a replica of the final cast component to be made. This foam pattern with rigging is then coated with a refractory, placed in a flask and surrounded in unbonded sand and compacted. Molten metal is poured onto the foam pattern, evaporating the foam and forming the part.

With lost foam assemblies, major opportunities for mass reduction, cast-in inserts and component integration are offered. Further, metalcasters can cast components with complex shapes unthinkable in other processes. For instance, engineers can create designs with little to no draft and it is common to see uniform wall thicknesses and excellent surface finish on the end product.

Although the lost foam process may be more expensive than other casting methods, it leads to reduced costs over time, as extra labor in casting and machining processes is eliminated, as is the cost for tooling.

Advantages

The lost foam process holds unique advantages over other manufacturing and casting processes. 

The freedom to design complex internal passages that are impossible or not easily manufactured in other processes is a primary advantage guiding customers to select the lost foam process. It is useful for part consolidation and the elimination of finishing and machining steps. 
In the case of an iron stalk roll for 360 Yield Center, lost foam casting process was the only option (Fig. 1). This component features backdrafted teeth that come to tiny points. The teeth also have a small cup under the backside. The casting supplier, AFS Corporate Member Grede Columbiana (Columbiana, Alabama), made prototypes using a cut foam process where it had foams and patterns machined out of foam billet and used for test castings just to prove they could cast the product. 

However, lost foam castings do not have to be complex to be successful. For example, one part for an oil drain was relatively simple. Originally made with green sand molds and a core used to make the vents, the component ran into issues during machining, when the vents were obliterated. It was a part with a high scrap rate. Converting it to lost foam casting eliminated the issue and was less expensive than the green sand cast part. 

Beyond the design advantages, lost foam has process advantages, as well. The typical lost foam casting line is usually about 100 feet long. It’s a compact, usually highly automated process that has the capability of single part flow. Plus, process control is simpler. However, some of the largest lost foam foundries in the world were able to design and construct their own systems that are incredibly simple but effective.
Benchmarked vs. sand casting, lost foam achieves energy savings, improved labor productivity, material reduction, reduced stock, reduced scrap, and improved safety. Lost foam tooling can last 400,000-750,000 cycles. In iron lost foam foundries, four-week lead times are normal since patterns can be stored for months and clustered when needed. Core and mold manufacturing do not have to occur ahead of time.
The process advantages lead to cost savings. Lost foam tooling has an extremely long life, labor costs are lower, and the aggregate used is easily recycled and lends itself too ready adoption of synthetic aggregate eliminating silica issues. Waste disposal is inexpensive and clean. Finishing costs are reduced with the major effort being gaging, and lower capital cost is required. It’s a high-productivity process with lower energy consumption and higher yield.

Quality improvements include no binder-related defects, long-term repeatability, no core defects, no mold shift, close dimensional tolerances, excellent surface finish and improved pressure tight castings.

Tackling the Cost of Tooling

One of the negatives about lost foam is the cost of the tools. The automotive industry has used the lost foam process heavily at high volumes, and those companies are less sensitive to tooling costs. However, the average customer is put off by the high upfront tooling cost. A current research project funded by the American Foundry Society is exploring how to reduce the cost of a tool to compete with a typical matchplate automated sand casting line. First cut tests of 3D printed aluminum molds yielded a functional tool at half the manufacturing cost. Now, AFS is funding printed titanium tools to create a lost foam tool that is expected to eliminate stress cracking and corrosion cracking.
The project entails producing and testing printed lost foam molds and expanding patterns. First, the project examined laser-printed aluminum tools using a 12 x 15 in. piece. Based on preliminary results, it appears that using an additively manufactured tool would reduce the cost about 50% (Fig. 2).

Taking the project a step further, the research team used software to redesign the tool and reduce the thickness of the tool by half. They added ribbing on the back for strength. There were no negative issues in the pattern molding process.
The same Software also was used to maximize heat transfer and reduce cycle time. The target is to halve the typical cycle time. Simulations were run to ensure strength was not lost. Titanium tools are now under design for printing.

AFS is also jointly developing printed lost foam polymer tools in various materials. The effort is to produce functional tools at 25% of the cost of conventional tools. This project utilizes the same software to design for Additive Manufacturing. Molds will begin printing in April with testing following. 

To further reduce the barrier of entry to lost foam casting, researchers are also looking at eliminating the need for tooling altogether by directly printing the patterns. Castings could be ready in hours without expensive molding equipment. The industry has produced castings on a test basis successfully in bronze, aluminum, ductile iron and grey iron. Carbon and Stainless steel are next.  CS

Click here to see this story as it appears in the March/April 2020 issue of Casting Source.

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