Patterns for Investment Casting

New Foam Pattern Material Replaces Wax

FOPAT Production Inc. for INCAST Magazine, March 2011 Issue

Although wax, in one form or another, has been used for investment casting patterns for centuries, a new inovative foam material is proving itself as a viable replacement for as a pattern material for everyday production use– even in demanding military applications.

FOPAT Production Inc. of Miamisburg, OH has developed an advanced, dimensionally accurate, temperature-stable, energy-efficient and cost-effective material and process to manufacture patterns for the investment casting industry.

This new foam pattern material (patent pending process/formulation) can be produced in a variety of mold shapes, sizes, and complexities that offer solutions to multiple technical problems common to investment casting foundries.

As a comprehensive solution and direct replacement to traditional wax patterns, FOPAT (FOam PATtern) uses a formulation of various polymers in combination with a modified reaction injection molding (RIM) process and alternate tooling methods to achieve production of investment casting patterns.

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Figure 1: Humvee male hinge. No chill, made from SLA tooling
 

The accuracy and manufacturability of various aerospace, military & defense, and other industrial components are highly dependent on the investment casting process capabilities. Throughout the years, the investment casting industry has had several major limitations and potential problems with using wax to manufacture investment castings, especially when it comes to the highly complex metal castings.

Dimensional control and accuracy, especially with regards to surface cavitations resulting from solidification shrinkage during and after pattern injection is limited due to dimensional changes that occur in processing of wax. In addition, the plasticity of wax requires the material to be stored at controlled temperatures to prevent distortion, as well as handling issues with its brittleness after the wax solidifies.

Other areas relate to higher wax costs due to extra melt cycles; scrap rates due to leaks and reaction with shell materials; large and heavy structural parts, as well as the thin wall parts or complex crosssections that pose cost and processing challenges for wax patterns.

FOPAT’s technology has demonstrated through foundry trials and bench-scale testing several major advantages over the current wax injection pattern process.

This foam material was evaluated by producing patterns using investment casting molds of various aircraft engine and military and defense vehicle components then processing them through traditional investment casting process to cast them as metal parts.

These patterns were made successfully by FOPAT Production Inc. The evaluation established the baseline comparisons with the present wax process for investment casting to provide the following advantages:

  • Reduced tooling costs
  • Superior dimensional accuracy
  • Durable patterns for thin sections
  • Elimination of need for chills
  • Improved manufacturability & ergonomics
  • Shippable patterns & handling/storage stability
  • Energy efficient & cost effective-ness

The Process

FOPAT patterns are typically produced using traditional aluminum tooling, along with less expensive and less time consuming materials, which has demonstrated bench-scale feasibility. The FOPAT reaction injection mold (RIM) operation does not require high-cost steel and/or high-strength aluminum tooling typically used in wax patterns for heat extraction. Actually just the opposite occurs with this material – there is an exothermic reaction upon injection and the heat retained in the mold helps the process cycle time of the material.

In addition, the new foam material is injected at a very low pressure (less than 8 psi), thus allowing the use of lower cost, less rigid mold materials. As a result, foam patterns can be produced from tooling made of epoxy resin, stereo lithography (SLA) resin, liquid silicone rubber (LSR), as well as low-cost aluminum and/or other engineered composites.

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Figure 2: Humvee male hinge. SPC study conducted on pattern length demonstrates repeatability in pattern to die production with dimensional accuracy. Less than 0.001” overall variation.

The viability of the FOPAT product has been demonstrated not only in lab tests, but also in actual foundry production trials.

This testing has shown that foam patterns are compatible with the investment casting process in yielding good quality castings with very stable dimensions.

Also demonstrated were the product’s ability to reduce lead times, reduce costs and even increase the number of potential investment castings suppliers.

As FOPAT opens the door for more complex and thinner patterns, many parts currently fabricated by machining, for instance may be converted to a much less costly casting process.

The Results

Many foundries have used this foam pattern material without significant increase in costs or major changes to current processes and/or operations. During one of several production runs, SLA tooling was injected with the new foam material without the use of heat or holding of the pattern material at elevated temperatures. This provided an early verification of the dimensional stability (Figure 2) and the minimal energy needed in the FOPAT process.

Foam patterns also eliminate the use of chills, typically used in wax patterns of similar thickness to reduce sink, which provided further cost savings. In addition, the tightness of the resulting FOPAT pattern demonstrated that the foam pattern material does not shrink after it has been taken out of the die. For years, shrinkage of wax patterns in regards to dimensional changes between pattern tooling and its corresponding cast part has been a critical factor in this process.

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Figure 3: 12” (L) X 6” (W) X 0.5” (T), FOPAT test block made from aluminum tooling. This foam pattern demonstrated shrinkage less than 0.0012” over time, and 1/20” the dimensional variability of wax on large parts (1/10” on small parts < 11” in length).

Most wax patterns experience long cure times (typically 24 hours or longer); which results in shrinkage ranging from 0.4% to 1.1% respectfully once the wax pattern is extracted from the tool. However, with FOPAT just the opposite occurs. Tests results have shown these foam patterns with yields less than 0.1% shrinkage from 12 minutes to 3-6 months after they come out of the tool and/or die.

Thus, the foam patterns can be processed at a faster rate while maintaining dimensional control and meeting the tight tolerance requirements.

A statistical process control (SPC) analysis conducted on foam patterns has shown minimal variation providing very stable pattern dimensions that were repeatable throughout the process.

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Figure 4: Single vane strut with ceramic core inserts injected with FOPAT material then molded and cast into 3-Vane Segment for Aerospace application

In addition to FOPAT‟s dimensional stability and minimal shrinkage, this pattern formulation allows the material to slowly expand into the cavity of the die providing a pattern creation process that requires lower pressure (4 psi nominal pressure) than the traditional wax injection process (typically 40 psi to 1500 psi based on die size). Because of this low pressure, FOPAT is able to reduce the chance for core movement and/or breakage when using ceramic inserts in the die to provide internal passages and/or cavities inside a metal casting.

This process was verified through radiographic inspections conducted by an independent labs during several projects involving aerospace turbine frame struts and single vane struts produced using the foam pattern material.

The struts were injected with FOPAT around three ceramic cores then processed through an investment casting process of pattern assembly, shelling, and burnout resulting in a 3-vane segment cast to dimensional specifications (Figure 4).

The FOPAT material also has the ability to continue pushing the design envelope as far as the dimensional range of length, width, and cross section extremes in regards to production of thin parts.

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Figure 5: Long thin pattern made from epoxy tooling using FOPAT material. The size of the die cavity was 6” wide X 24” long with a thickness of 0.040” on one end and 0.060” at the other end. This demonstrates FOPAT’s capability to fill a long and thin cross-section along with ability to remove such a pattern successfully in one piece.

A long, thin airfoil (approximately 24”X 6” X 0.040” to 0.060” cross-sections), is shown in Figure 5. The generic airfoil die was constructed in epoxy from a prior wax pattern. This die was injected with the foam pattern material to demonstrate the versatility of the FOPAT process to easily inject long thin sections in less expensive epoxy dies as opposed to using higher cost machined aluminum dies that are currently being used in the wax pattern process.

The injection of such a long thin section was a major accomplishment for this industry, since similar patterns have been nearly impossible to produce as a wax pattern, mainly due to the inability to get the wax pattern out of the die in one piece and/or handle it without deforming or breaking the wax pattern.

FOPAT now provides the ability to create more complex and geometrical shaped patterns with thin wall sections as low as 0.015” to 0.030” ±0.005” that will hold their shape and/or dimensional tolerance once they come out of the die.

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Figure 8: Military and defense handle made from RP tooling – silicone rubber mold to FOPAT pattern to metal casting

Since FOPAT patterns are not affected by environmental conditions such as temperature and humidity, they have a long shelf life and maintain dimensional stability. Thus, they are easily shipped and can be stored anywhere. Wax patterns, on the other hand, can be very difficult to ship since they are relatively brittle, especially in complex/thin size patterns.

Energy & Cost Savings

Today, manufacturers seeking a competitive edge in this economy are constantly looking for new innovative methods to streamline their processes, reduce tooling costs and bring their products faster to market. Since the 1990s, the investment casting industry has gone from traditional wax patterns to rapid prototype (RP) patterns for many applications. Despite the earlier problems faced in using stereolihography (SLA) within the traditional wax process, technology has improved allowing RP patterns to be suitable for low production use.

Although RP technology has eliminated the need for expensive tooling typically associated with wax patterns, there still is a significant cost in producing patterns that is higher than molded wax patterns, especially in large volumes.

The FOPAT material, on the other hand, requires inexpensive tooling compared to traditional wax patterns. Because of this lower cost, alternative tooling methods can now be realized in production of patterns. Since the foam material is injected at very low pressure and uses an exothermic process, little to no heat is required to be added or removed from the tooling material allowing for less energy-intensive and less expensive tools than those traditionally used. Where the FOPAT material really becomes cost effective over wax patterns and RP pattern production is in the manufacturing of short-run patterns (in the range of 5 to 200 pieces).

Part Tooling Material Used Pattern Quantity Price Per Pattern Tooling Cost Total Cost (w/Tooling) Timing (Tool Lead-time)
Handle SLA* 5 $1,426.80 $0.00 $7,134.00*
10 $1,233.00 $0.00 $12,330.00*
50 $1,233.02 $0.00 $61,651.00*
100 $1,233.01 $0.00 $123,301.00*
Handle (Figure 8) Silicone Rubber 5 $150.00 $3,700.00 $4,450.00 5 days
10 $120.00 $3,700.00 $4,900.00
50 $80.00 $3,700.00 $7,700.00
100 $50.00 $3,700.00 $8,700.00
Handle Aluminum $8,500.00 4 weeks

Table 1: Tooling & Pattern Cost Analysis between SLA Patterns & FOPAT Patterns

* Does not require tooling

Illustrated in Table 1 is an economic cost analysis and comparison between patterns made from SLA (stereolithography) and FOPAT patterns made from lower cost RP (Rapid-Prototype) tooling that demonstrates the cost effectiveness between FOPAT and RP (SLA) patterns.

Foundry Participation

As a result of winning two contracts from the DLA (Defense Logistics Agency) in September of 2009, FOPAT entered a new phase of its development and commercialization processes. Through the IBIF (Industrial Base Innovation Fund) program, the company were able to identify multiple foundries that demonstrated their capability to provide military & defense quality hardware, using FOPAT patterns instead of conventional wax patterns.

Tooling was built for numerous military & defense parts for vehicles and machine gun turrets that were deemed appropriate by the Cognizant Engineering Authority.

FOPAT produced patterns and sent them to various investment casting foundries for qualification trials. Investcast (Minneapolis, MN), MCM Precision Castings (Weston, OH), Kovatch Castings (Uniontown, OH), Precision Castings of Tennessee (Gallatin, TN), Net Shapes (Ontario, CA), EPS Industries ( Spring Lake, MI), and Metaltek Investcast (Watertown, WI) all participated in this program to demonstrate the feasibility of building one tool and producing multiple patterns that could be used throughout the industry. This saved money and time.

In tandem with the IBIF program was the SBIR (Small Business Innovation Research) program that emphasized identification of insertion opportunities in the DLA supply chain and quantifying energy savings over the traditional wax pattern production process.

Parts identified for use in this program typically were conversions from weldments and other casting processes, but also new designs with no designated tooling or process. Emphasis was on optimizing the tooling and material injection process and documenting the energy savings.

Researchers were also able to demonstrate the efficacy of foam patterns in producing a casting with a final weight of 165 pounds, in this case a 165-pound military artillery casting.

An SLA (stereolithography) master was generated from an electronic data file, and sent to a tool builder who produced an aluminum-filled epoxy tool for injection. The pattern was sent to Metaltek for solidification modeling, assembly, shell building, burnout and casting. No issues were reported and the casting is undergoing NDTE (non-destructive test equipment) evaluation.

A tool for another weldment-to-casting conversion was built and foam patterns sent to Investcast where castings were produced and shipped directly to the end user for evaluation.

Other parts were converted from traditional alloys to titanium, again using FOPAT patterns in place of wax. Both Ti Squared and Flowserve produced titanium castings that originated from an SLA model, an aluminum-filled epoxy tool, and FOPAT patterns.

More than 16 parts are currently included in the programs. The reasons for choosing the new foam process were usually driven by quality issues, cost, and time to produce. FOPAT has consistently shown the ability to produce tooling in 2-3 weeks for a fraction of the cost of typical investment casting tooling.

These programs are also helping to define the magnitude or degree of improvement in the castings produced using FOPAT in place of wax. Some of the parts have verified the ability to cast thin sections more accurately and with much less variation than wax.

During the course of these programs hundreds of FOPAT patterns were sent to MCM and Investcast for casting military & defense parts to fulfill production orders. FOPAT Production Inc. has successfully scaled-up and optimized its manufacturing ability and is now in the process of producing over 12,000 machine gun turret parts to fulfill government contract order by the end of February 2011, as well as production runs on defense vehicle hinges and military artillery handles respectfully.

Conclusion

FOPAT’s foam pattern material is a proven and viable replacement for wax in the investment casting process offering superior dimension control, energy savings, lower cost tooling, pattern ship ability, less core breakage and cost competitive patterns.

With the development of FOPAT moving from experimental to a now state-of-the-art production at its facility in Miamisburg, OH, the company now has the capability to produce hundreds of thousands of patterns per year, supported by a national distribution network. The foam product is now being used to for prototype, short run or high volume production castings.

References

Department of Defense (DOD), U.S. Air Force, Small Business Innovation Research (SBIR), Phase I, Phase II & Phase II Extension Program

Defense Logistics Agency (DLA), Small Business Innovation Research (SBIR), Phase I, Phase I Option, Phase II Program

Defense Logistics Agency (DLA), Industrial Base Innovation Fund (IBIF) II Program

Department of Energy (DOE), Inventions & Innovation (I&I) Program

Ohio Research Commercialization Grant (ORCGP) Program