Selective Laser Sintering (SLS) is a suitable manufacturing technology for functional products with complex geometries. It has fewer design limitation than other 3D printing technologies. In addition, this technology is also suitable for batch production.
We will introduce the advantages of SLS, present common SLS materials, and finally provide clear design rules.
Selective Laser Sintering (SLS) is a powder fusion technology, which use a laser system to siner powder layer by layer, finally create desired structure parts.
Powder material will be heated to elevated temperature, then deposited on build platform with thin layer of typical 0.1 mm by a recoating blade. A leaser beam will scan deposition layer surface, and sinter powder selectively to solidify cross-section area. When entire cross-section is sintered and solidified, building platform decline one layer thickness in height. Meanwhile material powder without sintering will stay as subsequent layer support. Recoating blade will deposit a new layer on top of scanned layer for next scanning and sintering. This process will repeat until final parts are fully manufactured, the final consolidated parts will contain with powders. This technology can produce multiple products as batch manufacturing, optimization of parts placement and orientation will maximize powder usage during each print.
Once printing process finish, powder container and products need to cool down. Solid products will part form unsintered powder, then clean with compressed air and blasting medium. These parts can be ready to use or post operation for mechanical properties and appearance improvement. Remaining powder can be collected for re-usage.
Designing for SLS
There are several design recommendations in SLS for quality, surface and functionality improvement. The most advantage of SLS designing is no need of support structures. Unsintering powder in container surrounding SLS parts eliminate support requirement, which encourage SLS parts to be high complex and intricate.
SLS designing guideline of features:
Wall thickness: minimum wall thickness varies form 0.7m for PA12 to 2.0mm for carbon filed poly-amide.
Hole size: minimum hole diameter is 1.5mm
Escape holes: minimum escape hole diameter should be 3.5mm, which is convenient for further powder removal.
Feature size: we recommend minimum size of features are 0.8mm.
Embossed and engraved details: minimum depth of engraving is 1mm, minimum height of embossing is 1 mm.
Text: minimum font height of 2mm is suitable for each direction.
Tolerances: typical tolerance is±0.3mm to±0.5mm.
Common design application
SLS common design applications rules will ensure SLL parts quality.
Nylon has low friction mechanism for low load, low-velocity application, which is natural bearing material. In running axles design, we recommend a bearing surface clearance of 0.3mm. In order to guarantee smooth running shaft, it is possible to design escape holes of minimum 3.5mm, and 2mm holes between running shaft axle and clearance shaft for powder removal.
SLS nylon can work well with integrated hings in proper design. Semi-spherical balls can be accepted by trapezoid shape pocket for low friction and good stability. Recommendation of clearance between sphere and pocket is 0.2mm, and 0.3mm between other gaps.
In reason of excellent chemical resistance of SLS nylon, it is widely applied in custom tank design. It is used to coating or lining tanks for extra water tightness or aggressive fluids. We suggest minimum wall thickness of 1 mm, and extra powder can be removed from inside tank.
SLS is the only method of functional living hinges manufacturing in 3D printing technology. Anneal the heating SLS hinges and then flex hinges back and forth, normal dipping in boiling water is sufficient. We advice living hinges to be 0.3-0.8mm thick and at least 5mm long.
Shrinkage and wraping
Because of high temperatures in SLS printing process, we need to cool SLS parts slowly to eliminate impact of warping and shrinkage. Most design for SLS have 0.3-0.5% increase in overall dimension to accommodate shrinkage. Add ribs to large flat surface to increase stiffness, select good orientation in printing process also reduce warping.
Product size: part size is limited by material container size in SLS machines. The average volume is around 300mm×300mm×300mm. Bigger machine provide volume of 700mm×380mm×580mm.
Consistency: small variations like dimension, surface quality will occur in products. Minor variations as small color or coating will occur in post manual process.
Surface finish: SLS parts has satin-like matte finish and slight grainy on touch, shiny and smooth finish need post-processing.