Metal 3D Printing Alternatives: Investment Casting & Sand Casting Processes

While plastics compose a huge percentage of the parts in products we interact with every day, many applications still demand the strength and long-term durability of metals. 

Direct 3D printing of metal has been promoted as a way to quickly create near net shape parts in durable materials like titanium, but high equipment costs, the need for specially trained technicians, and a limited selection of alloys has meant limited use of the technology for all but the most exotic, high-value applications. 

Stereolithography (SLA) 3D printers are often thought of primarily as tools for creating plastic parts, but their high precision and broad material library is well-suited for casting workflows that produce metal parts at a lower cost, with greater design freedom, and in less time than traditional methods. 

Read our white paper to learn more about how to leverage the speed and flexibility of 3D printing without the expense of direct metal printers by using metal casting workflows. Continue reading for a basic overview of processes for casting from 3D printed patterns, including direct investment casting, indirect investment casting, and sand casting.

Metal Casting Basics

For applications where parts require fine features or complex geometries, casting remains a cost-effective and highly capable manufacturing process, producing critical components for aerospace, automotive, and medical applications. 

Metal casting dates back to at least 3200 B.C., progressing through many cycles of innovation to become the reliable, modernized process it is now. Today, industrial metal casting processes are used to make everything from knee implants to tractor parts.   

The basic process to create cast metal parts has a few steps common across different techniques: 

1. A manufacturer creates a pattern that represents the part, either as a removable element to make an impression in a material like sand, or left inside of the mold material and subsequently burned or melted out. 
2. In either case, a cavity in the shape of the pattern is left behind, and molten metal is poured in.
3. The molten metal cools, and the mold is either opened up or broken apart to retrieve the casting. 
4. Cast parts have vestiges of the process where vents, gates, and feeders that direct gases and molten metal during casting. To finish the cast parts, a foundry worker trims away excess material, and files, grinds, machines, or sandblasts parts to achieve final geometry and surface requirements. In some cases, the cast parts are also heat treated.

In any casting process, two basic forms must be fabricated: the pattern and a mold of some kind. The pattern is essentially a slightly modified version of the part to be produced. 

The design for the pattern differs from the final part geometry in a few ways: 

• Patterns are scaled up to compensate for shrinkage that occurs in casting. 
• Patterns often contain elements relevant for the casting process that will not be present in the final part (e.g., gates for metal to flow through at a controlled rate, vents for gases to escape through, etc.).  
• Patterns may have certain features oversized or filled in to accommodate secondary operations used to produce very tight tolerance features (boring, tapping, etc.). 

Patterns are typically made from wood, foam, plastic, or wax. Sometimes the pattern design will incorporate elements related to the casting process, like gates for molten metal to flow through. 

A mold includes the negative of the pattern along with sprues, gates, vents, risers, and other features to control the flow of metal and gases during casting. 

Molds are made from a variety of materials (e.g. ceramic, graphite, plaster, sand) and must be able to withstand the high temperatures and mechanical stresses of the casting process. 

Direct Investment Casting

The direct investment casting process moves straight from creating a pattern to surrounding that pattern with investment material. Since the wax injection method of creating a pattern requires multiple steps, it is considered indirect.

Direct investment casting is best for short runs of parts or initial testing of a part concept, as each printed part will require some finishing steps. Direct investment casting is also a good choice for large parts or parts with thick cross sections that may be more difficult to mold successfully in wax due to warping and shrinkage. 

Direct investment casting is valuable for producing parts with geometries that are too complex to be molded or for parts with extensive undercuts and fine surface texture details, where molding is possible but carries high tooling costs. 

Traditionally, patterns for direct investment casting are carved by hand or machined if the part is a one-off or expected to be only a handful of units. 

With the advent of 3D printing, engineers started experimenting with directly printing patterns in order to achieve shorter lead times and geometric freedom that exceeds the design for manufacturability constraints of molding processes. 

Indirect Investment Casting

The process of making patterns from molds or tooling is referred to as indirect investment casting because it requires creating molds for producing the patterns in addition to final investment molds.

Rigid molds for wax (often referred to as tools) are commonly fabricated by machining aluminum or steel. Machined metal molds cost thousands of dollars to produce and take weeks of machining and polishing work before first shots can be run and pattern parts evaluated within a casting process. By directly printing tooling, engineers and designers can reduce the time between concept and first tests from weeks to a matter of days.

Molds for producing wax patterns can be printed with High Temp Resin. For optimal surface finish of molded parts, treat the interior surfaces of the mold by sanding and polishing for a smooth look, or bead blasting if a uniform matte look is desired. 

To ensure the final cast parts are dimensionally accurate, compensate for shrinkage by scaling up the printed mold. The exact shrinkage of the wax and the casting process can be obtained from supplier specifications.  

While molded pieces must follow design rules for moldability (e.g., no undercuts, draft is beneficial, etc.), you can achieve increased pattern complexity by using assembly jigs to combine multiple components into a single structure. 

Sand Casting

In the sand casting process, a foundry worker fills containers known as mold boxes or flasks with a mixture of sand and binder, then packs sand around the pattern. The pattern is removed to leave a negative impression of the pattern behind, and molten metal is poured into the cavity. 

An open-faced mold may be used for parts with features on a single side. Parts with features on multiple surfaces require closed cavity molds, with upper and lower mold boxes, referred to as cope and drag. 

In a closed mold, metal travels through a gating system before reaching the part cavity. This gating system is carefully designed to minimize structural and aesthetic imperfections resulting from improper metal flow. 

Closed cavity sand molds sometimes make use of suspended cores to create internal cavities in the finished castings, as in the case of engine blocks or pump housings. 

Fabricating Metal Parts With 3D Printing

Desktop SLA printers provide a range of meaningful solutions for foundries for pattern production and rapid tooling at a low cost, with the accuracy and precision that modern cast part designs demand. 

By adding 3D printing to the traditional foundry workflows, manufacturers can be more responsive to customer demands, delay investments in hard tooling, and validate designs cost effectively. Additionally, the growing use of topology optimization in engineering and product development means an increased demand for geometries that can be achieved through direct printing of patterns. 

Comparing Pattern Creation and Casting Methods

Learn more about design guidelines for creating 3D printed patterns, the step-by-step direct investment casting process, guidelines for indirect casting and sand casting, and more in our white paper.