Designing for Additive Manufacturing

Background

When designing a part for production, its means of manufacturing must be considered. Utilizing additive manufacturing opens a world of possibilities when it comes to design. Additive manufacturing can achieve complex geometries not attainable by injection molding or subtractive manufacturing. In fact, designing for additive manufacturing may result in a part looking more organic than parts made with traditional manufacturing processes. Let us investigate what that means for design. Although additive manufacturing can achieve complex geometries, it still must abide by a set of rules to optimize the outcome of a design. The design guidelines of additive manufacturing include considerations such as flat surfaces, wall thickness, standalone features, overhangs, support structures, infill, and lattices.

Flat Surfaces

Large parts often have a lot of surface area. Some parts often have one big face that presents itself as an ideal adhesion surface for 3D printing. However, this idea that “the more surface area in contact with the print platform the better” is not always true. As features are added onto the first adhered layer of a print, internal stresses can develop. The addition of such features causes tension to form in the adhered layer. This phenomenon causes buckling of the adhered surface away from the printing platform, often at corners of the part. To avoid this issue, a design must follow guidelines for wall thickness as to not overwhelm the adhered layer of a print. When possible, lattices or infill should be used rather than solid walls or features to help avoid buckling.

Wall Thickness

Just as wall thickness is an important factor in injection molding, it also plays a role in additive manufacturing. Too thick of walls, and the part could have sink marks or cause warping. Too thin of walls and the structure may not resolve at all. In addition, thin walls are susceptible to rupture given internal pressures within the part. Also, wall thickness and cross-sections of the part should not change too quickly. Sudden changes could lead to defects. Any changes in wall thickness or cross-section should be smooth and gradual. The use of fillets helps ease the transition between features of varying thicknesses.

Standalone Features

A standalone feature in a 3D print can easily become bent or deformed during the printing process. This has to do with the forces exerted on a part during printing. This could include mounting holes, walls, and other extrusions. Features should always be equipped with fillets or chamfers to help support them. In addition, the use of ribs also helps aid in supporting a feature.

Overhangs

Overhangs are an important consideration when designing for additive manufacturing. Like every other manufacturing process, additive manufacturing does have its limitations. Although it can achieve complex geometries, it is important to note that a 3D print in process is subject to its environment. In the case of overhangs, the consideration is gravity. If a part is to have an overhanging feature, that is not supported by say support structures, then that feature should not exceed a 45° angle normal to the printing platform. Anything less than that angle should be supported with support structures.

Support Structures

The next consideration when designing for additive manufacturing is the utilization of support structures. Much like the 3D print itself, structures meant for supporting overhanging features can also be 3D printed simultaneously. These supports can help maintain part geometry and then can be removed later as a post-process. Often, the machine’s software provides a way to add supports to the part after it has been designed. It should be noted that support structures often leave relics, small nubs, when removed from the surface of a part. If surface finish is of importance than post-processing may be necessary to remove such features.

Infill and Lattices

Infill and lattices are features easily attainable by additive manufacturing. Such features cutdown on material use for the part, decrease processing time, and aid in support and structure for the part. In fact, a part designed with an appropriate lattice structure does not need support structures at all because it is self-supporting. These features can come in many shapes and sizes. They can also either be designed directly into the part or added to the part STL when being prepared for additive manufacturing.  Infills and lattices both come in different densities and patterns. The density and pattern used with largely depend on desired mechanical properties, weight, and budget of the product.

Escape Holes (For SLA and DLP)

Some guidelines are specific to certain types of additive manufacturing. For example, parts produced with SLA or DLP are developed in a vat of liquid resin. It is important that the liquid resin captured within a part during its processing is somehow released. If not, the pressure will build up in the part leading to warping or rupture.  An efficient way of releasing the pressure of a part during processing is adding escape holes. These are simply holing near the printing surface of the machine that will allow the inner pressure of the part to equalize during processing.

Overview

If done correctly, additive manufacturing can be a fast, cost-effective means of manufacturing.

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