Fused Deposition Modeling (FDM) Design Guide

When people think of 3D printing, they usually think of fused deposition modeling (FDM). This technology has reached the mainstream due to its low cost and is also popular on an industrial scale.

FDM, also known as fused filament fabrication (FFF), produces parts from thermoplastics. The plastic material melts inside the printer’s nozzle. Then, the material is extruded layer by layer. As the layers cool down and solidify, they fuse with the existing layers. This process is repeated until the part is finished.

FDM technology is popular because it’s easy to use and flexible. An entry-level consumer desktop FDM printer costs only a couple hundred euros, and its use isn’t limited to cosmetic prototypes. Industrial-scale FDM printers are capable of sophisticated use cases involving high-performance materials. These printers make the FDM process ideal for functional prototyping – incredibly visual and geometric assessments.

The use goes beyond prototyping, as the technology and range of plastic materials are flexible enough for jigs, fixtures, and some end-use parts.

This guide provides design tips and best practices for designing parts for FDM technology.

Designing Support Structures 

During the design process, you must consider support. These support structures have vital functions:

• Supporting parts when there is an overhang
• Fixing and strengthening the part of the platform
• Preventing warping
• Preventing a complete build failure resulting from any of the above situations

The effects of not building supports

Designing the proper supports is essential. Here are some other critical considerations with FDM.

• The layer extruded at the nozzle can be printed as somewhat protruding from the previous layer. For this reason, you can print overhangs with angles up to 45 degrees and 5mm.
• Every area of the part with an overhang of 45 degrees or more requires, or more than 5mm length requires support. Support structures can take the form of lattices or be tree-like. Support should be 1.2 to 1.5 mm thick.

Print Orientation

Successfully designing a part for additive manufacturing won’t be possible unless you consider print orientation. The quality of every part (e.g., strength, material properties, surface quality, amount of support, etc.) depends on the print orientation. Most 3d printing processes result in anisotropic (direction-dependent) mechanical properties of the parts.

Due to the interlaminar bonding between the layers, mechanical properties are usually weaker in the build direction (z-height). So, if being mechanical in a specific direction is essential to your part, the part needs to be oriented so that the features requiring maximum strength are situated horizontally.

Although support material is essential to guarantee a design’s success, it should be limited to what’s necessary. The placement and amount of support material impact part quality and post-processing cost.

Orientation can also affect the part’s surface finish, especially when the part has rounded features. Not considering the orientation can cause the staircase effect. In this phenomenon, present throughout different additive manufacturing technologies, each printed layer becomes visible. Instead of the expected smooth surface, the damaged surface resembles a staircase. Upward-facing surfaces tend to have a better finish quality than sideward-facing surfaces. Don’t forget to consider the orientation, as the staircase effect depends on the nozzle diameter and layer height.

The staircase effect

Did you know that some Fused Deposition Modeling materials are flame retardants? Check out the guide to these materials.

Design Tips

These design guidelines below apply to FDM, but some specific guidelines might vary from machine manufacturer to model.

Fillet all corners. When designing a part, it’s generally good practice to fillet – or round – all sharp edges unless the design requires otherwise. Rounding internal corners reduces stress concentrations that might affect the overall strength of the object. The added benefit is that rounded corners need fewer materials, which means you can save some money. A good rule of thumb is to make the fillet ¼ of the wall thickness of ribs or pins.

Keep wall thickness constant. Designing a part that goes from thin structures to solid blocks generates additional stress, increasing the risk of warpage and bending. If making the wall thickness constant is impossible, avoid major transitions from thin to thick walls when possible.

Printing Holes

It’s best to print holes vertically with their axis if their roundness is critical to the part. Holes printed horizontally will experience the staircase effect and end up slightly elliptical. Holes up to 8 mm in diameter can be built horizontally without additional supports. Larger holes will require supports, although this diameter varies on machine and material.

Follow these guidelines depending on the shape of the hole.

• In elliptical holes, the ellipse’s height should be twice the width. The holes can be up to approximately 25 mm high.
• Teardrop-shaped holes can be almost any diameter if the top angle isn’t less than the minimum support angle (45°).
• Diamond-shaped holes can be almost any size, but it’s best to fillet the corners to avoid stress concentrations.

Next Steps

In short, every good FDM design contains these elements:

1. The lowest necessary volume, which reduces print time and material costs

2. The lowest possible build height that considers orientation

3. The least amount of support structure required