What are the engineering design rules for AM? Explain their effect.

Engineering design rules for additive manufacturing (AM), a practice known as Design for Additive Manufacturing (DfAM), are crucial for ensuring a part is printable, functional, and cost-effective. Unlike traditional methods, AM has a unique set of constraints and opportunities that must be considered during the design phase. Violating these rules can lead to print failure, poor part quality, or a part that doesn't perform as intended.

Here are some of the key design rules for AM and their effects:

1. Overhangs and Support Structures

An overhang is any part of a model that extends outward from the layers below it. Since AM processes build a part layer by layer, any section that is not supported by a previous layer will likely fail (sag, curl, or collapse).

• Rule: For most AM processes, an overhang angle steeper than 45° (from the vertical) requires a support structure. The exact angle can vary by material and technology (e.g., SLA often requires supports for even gentler overhangs due to peeling forces, while some powder-based systems can handle steeper angles).

• Effect: The use of support structures adds to material consumption, increases the overall print time, and requires a time-consuming post-processing step to remove them. The removal process can also leave marks or damage the surface of the final part, affecting its aesthetic and functional quality. Designing a part to be self-supporting or optimizing its orientation to minimize overhangs is a key DfAM practice.

2. Wall Thickness

The thickness of a part's walls is a critical factor for successful printing and part durability.

• Rule: Every AM process has a minimum wall thickness requirement, determined by the machine's resolution and the material's properties. Additionally, there are recommendations for maximum wall thickness.

• Effect: If a wall is too thin, it may not print successfully or be too fragile. If a wall is too thick, especially for processes that use heat, it can lead to thermal stress buildup, which can cause warping or cracking. In powder bed fusion, thick walls can also trap unfused powder, adding to the part's weight and complexity.

3. Part Orientation

The orientation of a part on the build platform is one of the most critical decisions a designer must make. It affects everything from strength and surface finish to print time and cost.

• Rule: The part should be oriented to optimize for key requirements. For example, orienting a part to minimize its Z-height will reduce the number of layers, thereby reducing print time and cost.

• Effect: AM parts, especially those made with Fused Deposition Modeling (FDM), are anisotropic—meaning their mechanical properties differ depending on the direction. They are generally strongest along the X-Y plane (within the layers) and weakest along the Z-axis (between layers). Therefore, orienting a part so that the direction of mechanical load is parallel to the layers can significantly increase its strength and durability. Orientation also affects surface finish, as sloped surfaces will show a "stair-stepping" effect.

4. Holes and Internal Channels

Designing holes and internal channels requires special consideration to ensure they are accurately printed and can be cleaned out.

• Rule: In powder bed fusion, holes and channels should be designed with an exit or "escape" hole to allow for the removal of trapped, unfused powder. For FDM and SLA, internal channels may need support if they are not self-supporting.

• Effect: A hole's dimensional accuracy is heavily influenced by its orientation. Vertical holes tend to maintain their roundness, while horizontal holes may appear elliptical or distorted. For internal channels in powder-based systems, an inadequate design can lead to trapped powder, which can add unnecessary weight, affect part performance, and make the part unusable. A teardrop shape is often recommended for internal channels as it is self-supporting and can be printed without supports.

5. Warping and Thermal Stress

Warping occurs when a part's layers cool and shrink at different rates, causing the part to curl or deform. This is a common issue in processes that involve heating and cooling cycles, such as FDM and powder bed fusion.

• Rule: Designers can mitigate warping by:

  • Adding fillets and chamfers to sharp corners to distribute stress.

  • Ensuring an even wall thickness to promote uniform cooling.

  • Reducing the size of large, flat surfaces in contact with the build plate.

  • Using a heated build plate or enclosed build chamber to maintain a stable temperature.

• Effect: Uncontrolled warping can lead to print failure, poor dimensional accuracy, and a part that cannot be used. By following these design rules, the internal stresses that cause warping can be managed, resulting in a successful and dimensionally accurate print.