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Sand casting and industrial 3D printing

3D printed sand casting molds and cores are changing the way today’s high-performance, large metal parts are manufactured. Additive manufacturing enables modern foundries to quickly produce complex metal parts within lead times. This article takes the redesign and manufacturing of an approximately 1-meter-long robotic arm as an example to illustrate the advantages of combining advanced casting design with 3D printing sand molds.

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 currently is sand casting; more than 70% of metal parts are manufactured using this production process.

The sand-casting process first requires the creation of a mold

                                                   Large sand-casting molds are ready for use after 3D printing and cleaning

Sand casting dates back to the 1st century BC. Over the centuries, this technology has evolved into the industrial process we know as such. However, the emergence of digital manufacturing and 3D printing technologies has enabled the further development of modern foundry companies.

Sand casting using 3D-printed sand molds and cores is becoming a key industrial application for additive manufacturing. Until recently, design engineers and foundries used this hybrid manufacturing technology primarily for prototyping. Today, more and more foundries are adopting this manufacturing technology to enhance their internal processes.

3D printed sand molds and cores are prepared layer by layer directly using digital files in an industrial 3D printing system

Benefits of using 3D printed molds and cores for sand casting

• 3D printed sand molds and cores help create a reasonable pouring and riser system, allowing the preparation of high-performance metal parts with fewer internal defects, and the material strength of the parts can be increased by up to 15%;

• Additive manufacturing eliminates the need for process equipment and casting molds and associated geometric constraints. This facilitates the production of high-performance, optimized parts with complex geometries;

• 3D printing and other digital manufacturing technologies can help change the image of traditional foundries, attracting young talent and a new workforce to the field.

Limitations of Sand Casting Using 3D Printed Sand Molds and Cores

However, 3D printing is just a tool. Limitations of this new technology in sand casting include:

• Part design still needs to adhere to the limitations of the casting process and 3D sand printing system. These design considerations include changes in wall thickness, piece cross-section, and wall-to-wall spacing;

• Currently available industrial sand 3D printers are limited, and the manufacturing cost of 3D printing molds is relatively high. For reference, sand 3D printing costs about $0.10 per cubic inch, while traditional foundries typically charge between $10,000 and $20,000 for a mold;

• As with every new technology, access to sand 3D printing knowledge and design skills remains limited. The difficulty in finding the best design practices and design guidelines prevents engineers and manufacturers from taking maximum advantage of this new technology.

Case Study: Topology Optimized Robotic Arm

To demonstrate the benefits of using 3D-printed molds and cores for sand casting, nTopology, Penn State University, Flow 3D and Humtown

engineers 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 manufacturing.

Exploded view of metal casting mold assembly

The team combined advanced design techniques such as topology optimization with advanced casting features that can only be manufactured additively, including gates, runners, and risers. Using this approach, the team managed to achieve several goals:

• Reduce part weight by 40%

• Avoid common casting defects

• Directly 3D print the entire sand mold

• Manufacture the part within a week

The first step in the project was to optimize the geometry of the robotic arm. Using topology optimization software, the team reduced the part’s weight by 40 percent—from 240 pounds to 165 pounds—while still meeting functional requirements for specified load conditions.

Of course, the engineering team considered the part’s manufacturability during the design phase. The final metal part weighed 165 pounds (or approximately 75 kilograms) when cast in aluminum and had dimensions of 39″ × 16″ × 16″ (or 1.0 m × 0.4 m × 0.4 m). The size of the robotic arm limited the team’s production Selection of this huge part.

Following the traditional method of pattern making (using wooden molds) comes with some complications. Due to the complexity of the geometry, the design team will

Many compromises had to be made, which reduced the performance of the part.

To demonstrate the capabilities of the technology, the team decided to 3D print the entire mold directly. Usually, a common production method is to print only a part of the mold, such as the core of the mold or other key parts.

This decision allowed the team to optimize other key features of the mold, such as the geometry and location 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 between Penn State University and Flow3D. The team considered two main design requirements during the design process:

• The molten metal must fill the cavity as smoothly as possible. Research shows that flow velocities below 0.5 m/s are necessary to minimize turbulence and reduce the possibility of material defects due to oxide layer shedding and pores;

• The riser must solidify after the part. Uneven solidification is another common cause of internal defects, shrinkage, cracking, and part distortion. For this reason, the parts that will be machined away after casting must solidify last.

The mold is manufactured in separate pieces and then assembled before pouring the molten metal. The spiral gate design cannot be manufactured with traditional patterns.

To ensure that 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. They chose to have a spherical or hemispherical riser rather than a cylindrical riser.

This optimized gate and riser geometry ensures that the flow rate of molten metal is below the required threshold and that the molten metal solidifies uniformly. Furthermore, these features can only be fabricated using additive manufacturing techniques, as such complex gate and riser systems are not possible using traditional fabrication processes.

Casting process simulation helps team ensure velocity flow remains below critical value of 0.5mm/s

To determine the best part casting direction and optimal locations for runners, gates and risers, the team conducted multiple design iterations using casting simulation software. The purpose of the simulation is to optimize riser performance, minimize porosity, and verify gate flow rates. The simulation phase ensures that the part is produced successfully the first time and reduces development time from months to weeks.

The unique direct production capabilities of the 3D printing process enable the application of these advanced mold design methods. And can produce significant performance improvements. Research shows that compared to traditional methods, castings produced using this model have:

• The total content of internal non-metallic inclusions is 0.02%, and defects are reduced by 99%;

• Strength can be increased by 8%-15% when cast from the same material.

The improved properties of the cast material make this process most suitable for foundries manufacturing high-performance or custom parts.

Streamlining the manufacturing process and the ability to quickly produce the complex shapes and structures required is fundamental to part manufacturing to meet the project’s goals.

Humtown used one of four ExOne SMax binder jetting 3D printing systems to create the mold. Once Humtown engineers receive the final mold design, they can print the mold in less than 24 hours.

The mold is destroyed after casting to remove the metal parts, 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 by optimally designing casting molds, and create a leaner, more flexible manufacturing supply chain.