3D printed sand casting molds and cores are changing the way high-performance, large-scale metal parts are manufactured today. Additive manufacturing enables modern foundries to quickly produce complex metal parts within lead times. This article illustrates the advantages of combining advanced design of castings with 3D printed sand molds using the example of redesign and fabrication of a robotic arm about 1 meter long.
Sand casting and industrial 3D printing
Metal casting is a common and well-established manufacturing method used to produce goods that are integrated into our daily lives. Today, 90% of manufactured products and machinery use cast parts. The most popular metal casting process today is sand casting; more than 70% of metal parts are made using this production process.
The sand casting process begins by creating a sacrificial mold consisting of compacted sand mixed with a binder. The cavity of the mold is filled with molten metal through the gating system, and then the mold is broken to remove the casting.
Sand casting dates back to the 1st century BC. Over the centuries, this technology has evolved into the industrial process as we know it. However, the advent of digital manufacturing and 3D printing technology has allowed the modern foundry to develop further.
Sand casting using 3D printed sand molds and cores is emerging as a key industrial application for additive manufacturing. Until recently, design engineers and foundries used this hybrid manufacturing technique primarily for prototyping. Today, more and more foundries are adopting this manufacturing technique to enhance their internal processes.
The benefits of sand casting with 3D printed molds and cores
• 3D-printed sand molds and cores help create a rational gating and riser system to produce high-performance metal parts with fewer internal defects and up to 15% higher material strength;
• Additive manufacturing eliminates the need for process equipment and casting molds and the associated geometric constraints. This facilitates the production of high-performance optimized parts with complex geometries;
• 3D printing and other digital manufacturing technologies are helping to change the image of traditional foundries, attracting young talent and a new workforce into the field.
Limitations of sand casting with 3D printed sand molds and cores
However, 3D printing is just a tool. The limitations of this new technology in sand casting include:
• Part design still has to adhere to the constraints of casting processes and 3D sand printing systems. These design considerations include wall thickness, variation in piece cross-section, and wall-to-wall spacing;
• Currently available industrial sand 3D printers are limited, and the manufacturing cost of 3D printed molds is relatively high. For reference, sand 3D printing costs about $0.10 per cubic inch, while traditional foundries typically charge between $10,000-$20,000 for a mold;
• As with every new technology, access to knowledge and design skills for sand 3D printing remains limited. The difficulty of finding the best design cases and design guidelines prevents engineers and manufacturers from making the most of this new technology.
The following case study will try to address this last point. By documenting the design approach and practical considerations in every decision, we hope to make this technology more accessible to manufacturers, designers, and engineers.
Case Study: Topology-Optimized Robotic Arm
To demonstrate the advantages of using 3D printed molds and cores for sand casting, engineers from nTopology, Penn State, Flow 3D, and Humtown teamed up to redesign a one-meter-long robotic arm. Together, they created an end-to-end digital casting workflow – from part optimization to design for manufacturability and finally fabrication.
The team combined advanced design techniques such as topology optimization with advanced casting features, including gates, runners, and risers, that can only be additively manufactured. Using this approach, the team managed to achieve several goals:
• Reduced part weight by 40%
• Avoid common casting defects
• Direct 3D printing of entire sand molds
• Manufactured the part within one week
The first step in the project was to optimize the geometry of the robotic arm. Using topology optimization software, the team reduced the weight of the part by 40 percent — from 240 pounds to 165 pounds — while still meeting the functional requirements for specified load conditions.
Topology optimization is a simulation-driven design technique commonly used in aerospace and automotive engineering where the optimization goals are usually to maximize stiffness and minimize weight. Automatic smoothing and model rebuilding in nTopology software enabled the team to make design changes quickly and easily.
Of course, the engineering team considered the manufacturability of this part during the design phase. The final metal part weighed 165 pounds (or about 75 kilograms) when cast from aluminum and had a boundary dimension of 39″ x 16″ x 16″ (or 1.0 m x 0.4 m x 0.4 m). The size of the robotic arm limited the team’s production The choice of this huge part.
Following the traditional method of moulding (using wood moulds) introduces some complications. Due to the complexity of the geometry, the design team would have to make many compromises, reducing the performance of the part.
To demonstrate the technology’s capabilities, the team decided to 3D print the entire mold directly. Usually the common production method is to print only a part of the mold, such as the core of the mold or other critical parts.
This decision allowed the team to optimize other key features of the mold, such as the geometry and placement of gates, runners and risers. These optimizations will result in metal castings with minimal internal porosity and high material properties.
The mold was designed in collaboration with Penn State and Flow3D. The team considered two main design requirements during the design process:
• The molten metal must fill the cavity as smoothly as possible. Studies have shown that flow velocities below 0.5 m/s are necessary to minimize turbulence and reduce the likelihood of material defects due to oxide peeling and porosity;
• The riser must solidify after the part. Inhomogeneous solidification is another common cause of internal defects, shrinkage, cracking and part deformation. For this reason, the part to be machined away after casting must solidify last.
To ensure no turbulence was introduced when filling the mold, the team redesigned the gating system and risers. They used a spiral gate instead of a downward gate. Instead of cylindrical risers, they chose to have spherical or hemispherical risers.
This optimized gate and riser geometry ensures that the flow velocity of the molten metal is below the desired threshold and the molten metal solidifies uniformly. Furthermore, these features can only be fabricated using additive manufacturing techniques, as such complex gating and riser systems cannot be fabricated using conventional fabrication processes.
Casting process simulation helps team ensure velocity flow remains below critical value of 0.5mm/sec
To determine the best part casting direction and the best locations for runners, gates, and risers, the team performed multiple design iterations using casting simulation software. The purpose of the simulation was to optimize riser performance, minimize porosity, and verify gate flow rates. The simulation phase ensures that the part is made right the first time and reduces development time from months to weeks.
The unique direct production capabilities of the 3D printing process enable these advanced mold design methods to be applied. And can yield significant performance improvements. Studies have shown that, compared to traditional methods, castings produced using this mode have:
• The total content of internal non-metallic inclusions is 0.02%, and defects are reduced by 99%;
• When cast from the same material, the strength can be increased by 8%-15%.
Improvements in cast material properties make this process most suitable for foundries that manufacture high-performance or custom components.
Streamlining the manufacturing process, the ability to rapidly produce the complex shapes and structures required is fundamental to the fabrication of parts to meet project goals.
Humtown used one of four ExOne SMax binder jet 3D printing systems to make the mold. Once Humtown engineers received the final mold design drawings, they were able to print the mold in less than 24 hours. Thanks to the close cooperation between the Trumbull foundry and Humtown, the foundry was able to complete the casting of the parts in one day.
The process of pouring molten metal into a 3D printed mold is the same as sand casting in any traditional foundry
Humtown is one of the leading 3D printing sand mold solutions providers in the United States. They have embraced new technology and now they are encouraging other foundries to adopt new casting methods to create a lean and agile supply chain ecosystem.
The mold is broken after casting to remove the metal part, which can then undergo post-processing operations
3D printing technology is changing the face of metal casting. 3D sand printing enables design and manufacturing engineers to produce optimized large parts with complex geometries, minimize internal material defects through optimized design of casting molds, and enable a leaner, more flexible manufacturing supply chain.