Iron Castings - Sep 28, 2020
Advancing 3D Printing for MetalcastingJ. Thiel, N. Bryant, K. Woods, Eerson Process Management | Casting Source
Advancing 3D Printing for Metalcasting
While 3D sand printing has seen a drastic increase in use over the last five years, the technology is primarily used for small- to medium-scale prototypes and high-mix, low-volume tooling. As new technologies are developed, the speed and scale of printing is believed to increase exponentially. Two recent studies on 3D printing for metalcasting are shared.
Making Headway to High Speed Sand Printing
Jerry Thiel, University of Northern Iowa (Cedar Falls, Iowa); Nathaniel Bryant, IMERYS (Atlanta); Kip Woods, Emerson Process Management (Marshalltown, Iowa)
The number of companies both producing printed sand molds and cores and the companies using the technology has quadrupled in the last three years.
The technology has allowed improvements in the casting process in several ways. The cost of hard tooling is eliminated. Conventional castings are produced by forming sand around a positive shape and bonding in place through inorganic minerals or chemical polymers. For example, the tooling for the positive shape is very costly compared to the value of the individual castings and must be amortized over the life of the casting design. This high cost often precludes the use of castings without sufficient future production volume requirements, and the components are produced using subtractive methods. This subtractive method is costly and less capable of producing complex internal geometry, which in turn requires multiple machined components. This is neither cost effective nor without large amounts of energy consumed and waste generated. Additive manufacturing for metalcasting eliminates the costly tooling and allows for better utilization of floor space and capital funds, all while reducing the overall component cost and material waste, particularly for prototypes and very low volume work.
The critical obstacle to the adoption of additive manufacturing in metalcasting has been the production rate possible and the subsequent cost of manufacturing. The cost of printed molds and cores can be as much as 20–30 times more expensive than conventional mold and core production, which can make it less cost effective for production volumes higher than a few dozen.
This prevents its usage on high production jobs that have the highest potential for utilization of the technology and subsequent improvements in quality and competitiveness. The resulting return on investment for an operating foundry purchasing the equipment is often much longer than conventional equipment, putting it out of reach for small to medium size enterprises (SMEs), which comprise over 80% of operating U.S. foundries. Current additive manufacturing technologies are targeted at the service centers where utilization rates can be kept high and that can find benefits in high-priced 3D equipment, which dictate high printed sand costs. This has stagnated the industry, keeping it viable for only low production and prototype casting.
The next generation of equipment is already being conceived and will be produced by a U.S. domestic supplier of foundry equipment. Domestic manufacturing allows for the use of regionally available components that tend to be more available and allow for a faster response time with machine maintenance and repairs. Using domestic components also allows for a lower cost of equipment, elimination of shipping, and OEM stocking requirements.
The new printer will target the operating foundry, rather than service centers, and will allow the SME foundries to advance their technology. Consequently, they will become a more valuable part of the supply chain. The printing equipment being conceptualized will be designed for affordability, facilitate short return on investment, and provide high operational speed in order to use the technology for production rather than prototypes. This technology will provide a paradigm shift in producing cast components that are both cost competitive and take advantage of the design freedoms additive manufacturing technology can provide.
In sand printing, the process starts and fails with the sand layer. The iterative creation process must be repeatable under many different environmental conditions that are present in small to medium foundries to be successful, all while trying to use the most cost-effective printing media available. The selection of sand media requires the understanding of how it will spread, impact printing resolution, and long-term reusability. A slight change in the screen distribution for a printing media can have a large impact on the final printing even if the pH and acid demand value are the same for furan printed parts.
Extensive studies were conducted by researchers at the University of Northern Iowa (UNI) to determine optimum characteristics of 3D sand printing materials. These studies have resulted in the development of many regionally available materials that both reduce the cost of sand printing while extending its capabilities. The result of lowering the operational cost while extending the capabilities has an indirect effect of increasing the rate of industry adoption. Understanding the characteristics of the materials used in sand printing will allow future increases in speed and efficiency, moving the technology from service centers to typical smaller business foundries.
Previous research by UNI has drastically reduced the cost of 3D sand printing by developing several domestic sources of aggregate material, bonding resins and printing methodology. The university is continuing to work with the equipment manufacturer to refine the design and facilitate construction of the printer. The design will incorporate technology that allows for production rates of up to 10 times the current best available technology. The speed increases will come from new high-speed print heads, advanced software and machine design improvements. Variations in the equipment design will be investigated to allow for large format printing and the greatest range of material usage, including environmentally friendly inorganic resins.
This paper can be read in its entirety at www.afsinc.org/2020-056.
Producing Large-Scale Castings With 3D Sand Printing
Andrew H. Pike, Joel C. Busler, and Alex R. Uchida, Mueller Co. (Chattanooga, Tennessee)
Patterns for large nobake molds have traditionally used wood as a material for making cavity impressions. This has posed challenges to producing consistent castings, as wood will swell and shrink due to different levels of temperature and humidity, leading to a phenomenon of dimensional seasonality. As polymers are typically more resistant to these effects, they have been used by patternmakers for some time; however, urethanes have usually been limited to small- and medium-sized work.
Within the past few years, developments in 3D printing have scaled up both the size and speed of prints.
To produce large-scale 3D printed parts effectively, two primary process steps are required: additive (printing) and subtractive (machining). Large scale print heads are based primarily on screw extruders common in the plastic injection molding industry.
With the advent of large-scale 3D printing, there is now the possibility to replace wood patterns with plastic, increasing patterns’ strength and dimensional stability. However, the larger printers do not have the same resolution of smaller printers, which necessitates the need for CNC machining to net size.
When creating the print file for large-scale printing, the model needs to be larger than the target geometry to ensure adequate machine stock. Due to the intricacies of the different slicing software, this is a tricky step. Tapered walls are especially difficult to maintain consistent machine stock and fusion and often require individual layer adjustments to the print file.
Material properties also must be considered regarding managing the heat in the printed part. Careful consideration must be given to the print details to provide resistance to cracking and dimensional stability. Each material has a unique temperature threshold that will cause material degradation above that threshold and delamination between layers below. Time and heat management between layers is critical.
In terms of material selection, large 3D printers use a pelletized polymer, with different polymers having different properties and processing parameters such as melting temperatures and pressures. The application and properties of the different materials vary as widely as their compositions. Early development focused on fiber-filled ABS plastic, but more compounds are becoming available.
While 3D printing 100% of the tooling as a single piece may be technically feasible, it would not be economically prudent to do so. As with smaller 3D printing, holding flatness over very large areas can be tricky, which would be detrimental to the parting line.
Gating and runner systems can be printed as integral to the casting, especially when the casting layouts were simulated with various software. However, doing so would sacrifice flexibility and reparability. Having separate pieces for gates, runners, filter prints, slag traps, or other features allows for them to be replaced as they age or wear. It also allows for changes without needing to separate them from an integral piece.
3D printing will increase in use within the metalcasting industry, and the definition of what defines “traditional” patternmaking will continue to change as more technologies and skill sets are available to be used. More research is needed into this area, particularly in wear resistance, behavior and durability of the different materials.