Optimising SLS Builds for Cost Efficiency and a Perfect Finish

Selective Laser Sintering or SLS is a familiar additive manufacturing process to the UK’s manufacturing industry. Indeed, Nylon PA 12 prints produced through SLS were some of the first to find real-world applications, taking 3D printing out of the prototyping and into the production arena. SLS provides amazing design flexibility, high strength and functional end-use parts. 

In an increasingly competitive additive manufacturing world, where SLS vies for position with similar, occasionally faster, proprietary branded processes such as MJF (multi-jet fusion), it seems pertinent to fly the flag for this versatile and dependable process. The most common materials are tried and tested in real-world settings, it remains one of the most cost-efficient additive manufacturing methods, and also one of the faster processes. 

How, then, can improvements be made? The obvious answer is to reduce finishing times and ensuring parts come out of the powder right the first time. Here are some factors to consider.

1. 1. Reducing layer lines, improving repeatability

Layer lines really aren’t a significant issue when it comes to SLS printing, so most engineers will be wondering why this is the first step. For the uninitiated, the reasons why layer lines don’t impact production projects in the same way as, say, FDM, are as follows:

• Self-Supporting Powder Bed: The unsintered powder surrounding the printed part acts as a natural support structure, meaning there are no harsh layer lines caused by support removal.

1. Fine Powder & High-Power Laser Sintering: SLS typically uses fine polymer powders (like PA12, PA11, TPU, etc.) that, when sintered, create a more uniform surface finish compared to FDM, where extruded filament creates visible ridges.
2. Heat Fusion Instead of Extrusion: Instead of depositing material layer by layer like FDM, SLS fuses layers together thermally. This means there’s less distinct separation between layers.
3. Post-Processing Enhancements: SLS parts often undergo media blasting, dyeing, or vapor smoothing to eliminate even minor surface roughness, making layering even less of a concern.

While it’s much less of an issue, layer orientation still affects mechanical properties:

• Z-axis (vertical) strength is weaker than X-Y plane (like all 3D printing)
• Surface roughness can vary based on orientation and part geometry

Researchers have mentioned that layer line prominence can be controlled with strategic orientation planning. 

2 Build Orientation

Build orientation stands for the angle at which a component is positioned on the print bed during the SLS process. This orientation influences the formation of layer lines as well as the overall surface texture. Different orientation produces different effects on the finished part quality. 

Horizontal Orientation (Flat on the Build Platform):

• Produces fewer visible layer lines on flat surfaces.
• It is ideal for components with large flat sections where surface finish is crucial.
• Can result in anisotropic strength distribution, meaning the part may be weaker along particular axes.

Vertical Orientation (Standing Upright):

• More pronounced layer lines on curved and side surfaces.
• This may lead to better feature resolution for fine details.
• Often, it results in higher surface roughness due to the stair-stepping effect.

Angled Orientation (450 or other inclinations)

• The middle ground between vertical and horizontal orientations. 
• Eliminates the accuracy of layer lines across various surfaces. 
• Helps balance aesthetics and strength. 
• Can require additional support structures that can increase post-processing efforts. 

A supplier who fully supports your ideation to creation cycle will advise you on the best orientation for your parts. You will need to design your part depending upon the quality, speed and finishing criteria you have selected.  

Build Orientation and the Effects on Surface Finishes

The surface finish of an SLS-printed component is important, particularly in industries where aesthetics as well as functionality go hand in hand. Of course, orientation will also affect final surface finish, which in turn will impact the amount of post processing required.

• Roughness and texture

• Components printed in a vertical orientation tend to have a harder texture due to the stair-stepping effect. 

• Horizontal orientation offers smoother surfaces but can still need post-processing for a refined finish. 

• Angled orientations may help distribute roughness more evenly across the part. 

• Powder adhesion and sintering quality

• Downward-facing surfaces: tend to trap extremely unsintered powder, leading to rougher textures. 
• Upward-facing surfaces: Commonly have a smoother finish, as they are exposed to the laser directly without interference. 
• The selection of material can also affect how well the powder adheres, influencing the requirement for additional finishing methods. 

• Support requirements and post-processing

Unlike SLA printing or FDM, Selective Laser Sintering does not need support structures, however orientation still affects post-processing requirements: 

• Parts with deep recesses or overhangs may need additional cleaning to remove excess powder. 
• Orientation that eliminates layer lines also eliminates the requirement for extensive polishing or sanding. 
• Tumbling and vapour smoothing can help achieve a superior finish if required. 

Best practices for optimising build orientation in SLS printing

To achieve the best balance between mechanical properties and surface quality, UK manufacturers can adopt the following strategies:

• Assess stress distribution and part function: Orientation can be selected to increase strength where required while eliminating surface flaws. 
• Experiment with various angles: Conducting test prints at different angles can help identify the optimal build orientation for particular parts. 
• Utilise post-processing techniques: SLS parts can be polished, chemically smoothed or bead-blasted to increase their surface finish. 
• Use advanced software tools: Modern slicing software can simulate surface quality variations depended on orientation, assisting manufacturers make informed decisions. 

The impact of SLS on UK manufacturing

The UK manufacturing sector continues to integrate SLS into aerospace, medical, and automotive production, leveraging its ability to produce high-performance parts with minimal post-processing. Optimizing build orientation is critical to controlling surface finish and mechanical properties, directly impacting efficiency and cost. Advances in SLS technology, alongside ongoing research into material properties and build strategies, are further expanding additive manufacturing capabilities, keeping UK manufacturers competitive on a global scale.

FAQs

1. How can I reduce layer lines in SLS-printed parts?

employ finer layer thickness settings, pick the best build orientation, and employ post-processing methods like vapour smoothing or bead blasting to minimize layer lines.

2. What is the best orientation for achieving a smooth surface in SLS printing?

When compared to vertical positioning, a horizontal or slightly tilted orientation (such as 45°) frequently produces smoother surfaces.

3. Does build orientation affect the strength of an SLS-printed part?

Yes, mechanical strength is impacted by build orientation. Because of layer adhesion qualities, parts are often stronger in the XY plane and weaker in the Z direction.

4. What post-processing methods improve SLS surface finish?

To improve surface quality and lessen roughness, common post-processing methods include chemical finishing, tumbling, bead blasting, and vapour smoothing.

By leveraging the right build orientation and surface finishing techniques, UK manufacturers can optimise SLS printing for high-performance applications, ensuring superior part quality and enhanced efficiency in production.

Reference:

https://www.sciencedirect.com/science/article/pii/S2212827123003463