The sand-casting process has undergone dramatic changes in recent years thanks to the benefits and advantages offered by additive manufacturing (AM). Traditional sand casting processes are enhanced by making foundries faster to market, more cost-efficient, and more agile.
Enhanced by AM methods, the fundamental sand-casting process remains essentially the same:
• Patterns are still required
• Molds are still produced
• Prepared sand is compacted against the pattern face
• Gating systems continue to be utilized
• Metal is also melted and poured into the mold.
Exclusive of sand printing, the only step that changes because of 3D printing is how the actual pattern equipment is fabricated (see Figure 1.)
Subtractive vs. AM patterns
Conventional methods for fabricating patterns involves subtractive machining of metal or plastic basic shapes. This approach manipulates a shape of material, often assembled, into the end shape by removing unwanted material.
Conversely, the AM process builds only the material required, layer-by-layer, within a build volume. The additive process delivers products in a wide range of polymers, resins, or even metal. AM patternmaking is recommended over traditional manufacturing when:
• Pattern equipment is worn out. 3D scanning can capture the original part or pattern geometry and then it can be amended using CAD software and 3D-print patterns to correct tolerances and shrinkage factors.
• Alloys for a casting design change. Shrinkage factors change with alloys. 3D-printed tools can be reproduced quickly to properly produce castings to the correct shrinkage factor, eliminating lagging and pattern damage.
• Short lead time. 3D-printed patterns can be produced in hours, which is a major advantage when prototyping, or delivering a batch of short-run parts or full production orders.
• Multiple revisions to the pattern are required. It is generally faster, more cost-effective, and more automated to adjust a pattern’s CAD file and print it than to significantly adjust or remake traditionally fabricated pattern equipment.
• Complex cored castings. Combined additive processes of printed polymer patterns and printed sand cores allow for rapid production of castings without producing expensive and time-consuming core equipment. If for some reason the casting is scrapped, it is less expensive and quicker to make conventional molds with printed patterns and purchase/produce more additive sand cores.
• Deep-pattern feature machining. To create narrow, deep pockets on patterns and core boxes, and long tool lengths are required or additional stock needs to be assembled to the pattern while machining due to spindle interference, additive in situ machining eliminates the issue as segments of the pattern are printed and machined allowing standard tool lengths.
Not all 3DP is the same
There are a wide number and range of 3D printing technologies, including binder jetting (aggregates and metals); Laser Powder Bed/Electron Beam Melting; Stereolithography (polymer and ceramic); and Polymer extrusion (filament or pellet.)
A large-format 3D printer will serve the larger and smaller size requirement of many sand casting patterns. Pellet-fed 3D printing allows use of a wide range of materials with deposition rates up to 10 times higher than filament 3D printing and 10 times lower material cost.
While 3D-printing technologies bring significant advantages to the casting process on their own, there are several advantages for sand casting applications when combining additive and subtractive manufacturing into a hybrid platform. For example, each of these key methods — pellet extrusion, filament extrusion, and spindle subtractive tooling — have advantages and limitations when considered in isolation. However, when combined in a hybrid platform solution, all the benefits can be realized without limitations. (see Figure 2.)
To illustrate the effectiveness of a hybrid platform for pattern applications, we can explore a real-world use case (see Figure 3.) Mexican manufacturer Proveedora de Servicios y Suministros Industriales (PSSI) wanted to adopt AM to reduce the cost and save time in manufacturing pattern equipment. The target was to reduce both time and cost by 50% while maintaining pattern dimensional quality.
Using a hybrid platform with a Titan pellet-extrusion 3D printer, PSSI was able to eliminate wood assembly and machining of typical wood pattern equipment. Unlike conventional CNC machines, the Titan 3D printer does not require continual attention while operating the equipment enabling lights-out manufacturing — from 7 weeks down to 2.5 weeks – and enhancing productivity by more than 60% — from 7 weeks down to 2.5 weeks and lowering total cost by 50%. Additionally, carbon-fiber-reinforced ABS plastic not only has better durability than wood but has equivalent benching properties for final post-processing. This yields a faster, lower cost, near-net-shape solution directly off the printer (see Figure 4.)
“3D Printing has helped us accelerate our manufacturing process of our products due to the confidence we have for the 3D printer to work 24/7 without the need for special care during printing,” PSSI CEO Alonso Alvarez said.
AM excels at reducing assembly in the pattern-making process because the pattern plate and impression can be combined in one setting, which eliminates labor and machining time (see Figure 5.)
Eliminating labor and time
Regardless of the additive manufacturing technology used to produce sand casting patterns, there are some important tips to keep in mind (see Figure 6.)
For sand casting patterns with improved surface finish: use carbon-fiber or glass-filled ABS or PC for high durability, heat resistance, and good benching properties (see Figure 7.) Consider that primer/paint adheres well to most high-performance polymers. Coatings designed specifically for printed objects are available.
For solvent resistance and chemical sand resistance, there are no known reactions to no-bake, oil sand, green sand, phenolic urethane, CO2, or Isocure.
For low wear, high-throughput patterns using stronger polymers, AFS studies have documented cases involving >30,000 cycles with insignificant wear.
For equipment exceeding the build area, print in segments and assemble. This also will reduce print issue impact and allow manufacturing in parallel over multiple printers, rather than serial.
Always design for additive manufacturing (DfAM) to minimize material, print time, reduction of support, and print defects such as curling. Remember the 45-degree rule and design it. Guidelines are available for DfAM.
For high-wear areas, design the prone area to be assembled to the pattern and replaced when excessive wear occurs.
Gating is also an excellent additive application. It is fast (overnight), precise and repeatable.
The technology is reasonably simple to integrate into an organization’s processes using open market slicing software. CAD capabilities and human resources are essential, as experience with designing and producing pattern equipment improves integration of the technology
By integrating additive manufacturing into the foundry process, foundries and pattern shops are becoming more agile and competitive, bringing a product to market faster at a lower molding equipment cost compared to traditional methods.
Marshall Miller is an Applications Engineer with 3D Systems.