Sarah Jordan and Mark Debruin
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Lost foam casting presents significant advantages, yet it has not fully realized its commercialization potential. A major reason for this is the belief that the process necessitates substantial initial capital and tooling investments, which can make it seem suitable only for large-scale production.
However, diversifying into lost foam casting does not have to involve hefty investments in capital equipment or tooling. By investing in raw materials at a reasonable cost, foundries that produce castings in materials like iron, steel, aluminum, or brass can swiftly create prototypes and short runs using the lost foam method. This approach can serve as an alternative to additive manufacturing or assist in the production of tooling for prototypes, replacement parts, or small production volumes.
Benefits of Lost Foam Casting
The advantages associated with lost foam casting are well established. Known for its near-net-shape capabilities, this method allows for intricate designs. Research from the Department of Energy (DOE) indicates that lost foam casting can save between 25% to 30% on energy, while also offering a 46% improvement in labor productivity, a reduction of materials used by 7%, and cost savings on production of approximately 20% to 25%. Additionally, less solid waste is generated along with lower emissions of particulate matter and greenhouse gases compared to traditional methods. With effective gating design, yields exceeding 70% are common, and yields above 80% are achievable.
The authors' observations indicate that tolerances of +/- 0.003 in.-per-in. are standard (0.076 mm/mm), with +/-0.002 in.-per-in. (0.05 mm/mm) being viable in certain scenarios. In cases using the machined foam method, tolerances depend on machining precision, typically around 0.002 in. (0.05 mm) plus the machining tolerance. Recent DOE-funded studies concerning thin-walled ductile iron have shown that even a thickness of 0.040 in. (1 mm) achieves a tolerance of +/-0.039 in. (0.039 mm). Notably, these precision levels mean that lost foam castings can often be utilized with little or no needed metal machining, resulting in significant cost reductions that can offset the slightly elevated costs compared to conventional green sand or nobake casting.
As illustrated in Figures 1–3, lost foam is capable of producing net or near-net shapes with complex geometries, including interior channels, blind holes, and true position features. Components can be designed with zero or varying drafts (Figure 4). The authors have even pioneered a method for creating as-cast threads as shown in Figure 5, and the necessity for tooling can be eliminated, as foam can be machined and cast (Figure 6).
Steps to Implement Lost Foam Casting
The lost foam process is depicted in Figure 7. Achieving lost foam casting with minimal capital expense entails forgoing costly tooling and automated lines, allowing for machining the foam from stock and manually compacting flasks to create a fluidized bed. The subsequent steps align closely with standard lost foam casting procedures.
Step 1: Obtaining Foam
The initial step involves sourcing foam blocks for machining. The lost foam process typically employs expanded polystyrene (EPS) foam, ideally with a density of 1-1.5 lbs. per cu.ft. (0.016 g/cm³- 0.024 g/cm³). If controlling carbon is critical, particularly in iron casting, Clearcast—a co-polymer made of EPS and polymethyl methacrylate (PMMA)—is a favorable option and can also be procured in block form for machining.
A vital safety consideration is to ensure any foam utilized is free from flame retardants, as foams with these additives can explode during the lost foam casting process due to their design to prevent burning. Foundries should confirm with their foam suppliers that their products lack flame retardants and should perform flame tests independently.
Should the machined foam not be used promptly, foundries are recommended to let it age for a minimum of three weeks. Newly processed foam lacks dimensional stability due to moisture that gradually evaporates. Depending on environmental factors, the foam should stabilize within three weeks or potentially sooner.
Step 2: Pattern Machining
A CNC machine is essential for the foam pattern's machining. Various machine tools are viable, and trials may be necessary as cutting feeds, speeds, and depths will vary with machine type, foam, and cutting tools used.
The foam should be cut cleanly to form small chips. If the feed speed is excessive, cut depth too significant, or speed too low, tearing may occur, particularly with blown foam where beads pull free from the material rather than cutting cleanly. Typically, higher speeds paired with shallower in-feeds and cuts yield better results.
It is crucial to factor in the thermal contraction of metal when transitioning from CAD design to CAM and machining. Because of the absence of mold wall movement, only metal shrinkage requires consideration. Risers may be used with high-shrink materials like aluminum, while iron alloys typically do not use risers.
Step 3: Adhesive Application
One unique aspect of lost foam is that foam pieces can be bonded before casting, allowing the design to include multiple parts joined together rather than as a single machined or blown shape. Foam joints can employ simple adhesives, such as school glue or tape, but most foundries prefer Foam-Lok 70-12-11, a specialized adhesive made for lost foam that solidifies quickly upon cooling.
Step 4: Gating System Design
Lost foam primarily utilizes a consumable ceramic down-sprue funnel attached to a gating system that feeds the part through one or more smaller in-gates. These often feature notched designs for ease of breakoff during finishing. There are no strict gating ratios, and choke points should be avoided. While normal sand casting gating rules generally do not apply to lost foam, the gating system should facilitate proper gas flow.
Gating typically varies based on the type of alloy—aluminum castings are often top-fed while those in iron, steel, and brass typically receive bottom feeds. Minimized glue usage is essential to prevent excess gas production, which can otherwise compromise the casting.
Step 5: Coating Preparation
A variety of specialized coatings for lost foam casting are available. Achieving a uniform coating requires diligence. If a ready-to-use coating is not used, foundries will need a viscometer to ensure consistency and must mix the coating thoroughly for immediate use to prevent settling that can lead to an inconsistent final product. Care should also be taken to avoid over-mixing, which can introduce bubbles that lead to surface defects.
Step 6: Drying Process
Excess moisture in coatings can result in unwanted sand adhesion or, in extreme cases, steam explosions. Effective drying methods can vary by climate; some foundries utilize air circulation fans while others employ specially designed drying rooms.
Step 7: Coating Inspection
Careful inspections for cracks in the coating are vital, as these can lead to defects in the final casting. Components, along with their gating systems, should be scrutinized for any fractures and repaired with additional coating if necessary.
Step 8: Compaction Techniques
Although costly, automated compaction lines are available for lost foam casting, but manual options using flasks such as 55-gallon steel drums can also work effectively. Vibrating sand mimics water, allowing it to pack tightly around the foam.
Step 9: Casting Procedure
Lost foam casting follows the same melting processes as other foundry methods but requires higher temperatures to effectively vaporize the foam. Generally, pouring needs to occur quickly to prevent gas buildup that can lead to operational hazards.
Step 10: Finishing Details
After allowing sufficient time for solidification, castings can be removed from the sand, and traditional finishing processes similar to sand casting can be employed, usually requiring less time than typical sand casting.
In conclusion, for foundries needing rapid prototypes or smaller volumes, the machined foam and manual compaction method for lost foam presents an excellent opportunity for diversification with limited capital expenditure. This process retains similarities to existing operations while requiring only minor adjustments for temperature management.
With proper execution, foundries can benefit from the cost-efficiency of the machined foam method, especially for smaller production runs, and may consider larger investments as demand scales up.
Feel free to follow the link to view the article in the August digital edition of Modern Casting.
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