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Design Guide: SLS 3D Printing

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Selective Laser Sintering (SLS) is an industrial 3D printing process printing process ideal for manufacturing end-use parts. Unlike other additive manufacturing processes, such as SLA and FDM, SLS does not require support structures, allowing for greater design freedom. For this reason, it is also one of the easier 3D printing technologies to design for.

SLS produces functional plastic parts with isotropic mechanical properties that can be used for detailed prototyping or end-use low-volume production parts.

To fully utilize the capabilities of this technology, it is important that your 3D model is designed according to a number of recommendations. In this article, we offer a comprehensive guide to the best design practices for SLS 3D Printing. It covers the characteristics of the process, our capabilities, design guidelines, summary of best SLS design practices and cost reduction tips.

SLS 3D Printing Process

In SLS, a laser selectively sinters polymer powder particles, fusing them together and building parts layer-by-layer. The unsintered powder provides the part with all the necessary support, hence support structures are not needed.

 

Disadvantages of the SLS Process

  • Shrinkage and Warping: SLS components are susceptible to shrinkage and warping due to the high temperature experienced during printing.
  • Oversintering: When radiant heat fuses unsintered powder around a feature oversintering can occur. This can result in loss of detail in small features, such as slots and holes.
  • Powder Removal: Powder left on or in the part have to be accounted for and cleaned off. Escape holes are needed to remove the unsintered powder from the inner sections of the component.

 

Characteristics of SLS 3D Printing

Maximum Build Size 400 x 400 x 450mm
Resolution ±0.2mm
Dimensional Accuracy ±0.3% (with a lower limit of ±0.3 mm)
Materials Nylon 12, Nylon 12+GF, TPU
Surface Structure Grainy structure
Support Not required

SLS Design Guidelines

Wall Thickness

For SLS designs, walls that are designed too thick or thin are at risk of warpage. To ensure successful print, walls should be at least 0.7mm. At HLH, 1mm wall thickness is recommended.

 

Designing unsupported walls for 3D printing

Holes 

The deeper the hole, the larger the hole diameter should be. All holes should be larger than 1mm in diameter. Blind holes — holes that do not extend completely through the wall — should be designed with an escape hole to allow for powder removal.

 

Designing escape holes in 3D printing designs

Hollow Parts

For hollow parts and designs containing blind holes, it is important to include more than one escape hole in your design to improve the ease of powder removal. At HLH, we recommend a minimum hole diameter of 3.5mm.

Slots

What size to design a slot is determined by the depth or thickness of the wall. We recommend a minimum slot size of 0.5mm. Keep in mind that if the depth or wall thickness is over 2mm, a minimum slot size of 0.5mm will be insufficient, and the print may fail.

 

Designing slots in 3D printing designs

Pins

The main difficulty when designing pins for SLS is the risk of small pins being too fragile and breaking off during post-processing. The minimum pin diameter is 0.8mm. The taller the pin, the greater its risk of breaking; increasing its diameter will give a tall pin more strength.

 

Designing pin diameter and height in 3D printing designs

Mating Parts

For SLS parts, adequate clearances must be designed between mating parts to prevent the assembly from fusing together into a single solid unit. To avoid this, models must be designed with a minimum clearance of 0.5mm.

 

How to design mating parts in 3D printing

Large, Flat Surfaces

Large, flat plains are susceptible to unpredictable ‘warping.’ Hence, such designs are not recommended. If they are a crucial feature of a part, ribs can be added to provide support; however, this may not always solve the problem, as such, such surfaces should be avoided whenever possible.

 

avoiding large flat plains in sls designs

Embossed Features

Embossed features must be designed using a minimum height, otherwise, it may not appear visible. They should be designed with a height of at least 1mm. To ensure such details come out nicely, make them larger than the indicated.

 

Engraved Details

An engraved detail is a recessed feature below the surface of a part. Because of the heat-dissipating from the laser into the surrounding powder during SLS, text and engraved details are at risk of closing up and won’t be visible if not designed with a minimum depth and width. Engraved details should be at least 1mm deep and 1mm wide.

 

How to design engraved text and details

Text

For readability, ensure minimum font height is 2mm (font size 14) and suitable for every direction. Always opt for larger fonts. Sans serif font is recommended not only for legibility but also to reduce details.

Text that is cut out of the material is much less legible than text that protrudes from the material, as it is harder to remove material in SLS. As such, choose embossed over engraved text.

Summary of SLS Best Design Practices

Wall Thickness 0.7mm-2mm; 1mm is recommended
Holes Greater than 1mm in diameter
Escape Holes At least 3.5mm in diameter; add multiple holes
Slots At least 0.5mm (depends on wall thickness and depth)
Pins At least 0.8mm (depends on pin height)
Mating Parts At least 0.5mm apart
Large, Flat Surfaces Avoid whenever possible
Text Minimum font height of 2mm; opt for larger fonts
Embossed Details Minimum embossing height of 1mm
Engraved Details Minimum engraving depth of 1mm and width of 1mm

 

SLS Cost Reduction Tips

In this section, we outline some simple tips and tricks to help reduce the overall cost of your SLS part. In 3D printing, there are three main drivers of cost to bear in mind: material, printing time, and post processing time.

 

  • Hollow Out Your Part — An effective way to reduce the amount of material used is to hollow out the center of your design to create an outer shell.
  • Use Scaling to Your Advantage — If you are manufacturing a design for proof of concept, consider scaling down the model size to reduce cost and lead time. This will allow you to get through the prototyping phase faster and cheaper than a full-scale design.
  • Limit Tolerances — When designing for SLS 3D printing, limit tolerances to areas where they are important such as mating parts, as excessive tolerancing would only increase the printing effort.

Getting Your Designs Printed

Use the 3D Printing Design Guidelines to help design your parts for machining then export your 3D CAD files in an STL format. Have your designs ready? Get an instant quote or use our get in touch form to get a manual quote. Our team of 3D printing experts are always available to provide on-hand support and recommendations to ensure your parts are print ready.

 

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