Selective Laser Sintering - SLS?
A moving laser beam melts and fuses (singers) a heat-fusible powder one layer at a time to build solid 3D parts.
SLS category?
Category - Powder Bed Fusion
SLS - Principle/Method?
1) Scanning and sintering metallic and non-metallic thermoplastic powder using a carbon dioxide laser beam (CO2) that causes the powder to sinter (fuse or melt) and solidify in the shape of a layer of the prototype.
2) Powder bed moves up and delivers polymer which a roller spreads across the surface of the blind area forming an even layer of powder.
3) Non- sintered powder forms a ‘cake’ encapsulating and supporting the model as the build progresses.
SLS - Process characteristics?
- Layer thickness 0.06-0.15mm
- Resolution x/y plane 0.8-1.3mm
- Process Takes place in an inert nitrogen atmosphere <1% oxygen to stop the powder oxidising when heated by the laser.
- Sintering at operating temperatures of up to 385 degrees or just below the melting point of the polymer.
- Build speeds of 7-48mm/he (Material dependent)
- Build of Parts stacked on different planes (Build packets) trying to use as much space as possible.
- Parts can be built in sections and rejoined.
SLS - Process Characteristics?
- Surface finish and accuracy
- Powdery like the base material with granular, porous texture.
- Smooth when using crystalline powders (wax)
- Typical tolerance 0.4mm or 0.1mm/mm
- Recommended wall thickness 1.0mm
- Natural Radius 0.4mm
- Good accuracy; problems if the temperature of uncured powder gets too high, excess fused material can collect on the part surface.
SLS - Materials?
• Variety of Polyamides (Nylon-based polymers)
- Glass - Carbon - Aluminium- fibre- filled.
- Polyaryletherketone PEAK
- Thermoplastic elastomers
- Polystyrene
SLS - Desired Properties?
- Sterillisable, biocompatible, Flame retardant.
- High stiffness, toughness, elevated temperature resistance.
- Anisotropic mechanical properties (fibre-filled)
SLS Applications?
• Functional Prototypes - Durable prototypes without tooling, withstand form, fit and functional testing.
- Functional Parts (e.g. dental)
- Pattern for Investment Casting
- Capable of living Durable hinges, snap fits and high-flex snaps.
SLS Variation?
- Selective Laser Sintering of metal Powders.
- Indirect Sintering of metal powders that are coated with a thermoplastic Binder using a CO2 laser.
- Melting Binder Material loosely binds the desired shape to what is called the “green part”
- Burning off the Binder in a furnace with the metal powder bonding by traditional Sintering mechanics shaping the “brown part”
- Second material (Copper, bronze) is added to the furnace to infiltrate the porous brown part via capillary action.
SLS - Advantages?
- More versatile than SLA as more variety of materials; including metal and ceramic powders.
- Materials are less expensive than with SLA
- Most materials are recyclable (except carbon - filled ones)
- Self-Supporting powder allows a large number of designs to be built around each other; higher efficiency.
- Parts can be machined fairly easily, readily joined mechanically or with adhesive (thermoplastic like properties)
- Non toxic process
SLS - Limitations?
- Slow cycle times; but as little prep time is required a rapid turn around is possible.
- Prototype require cooking and increase build time.
- Surface finish is inferior to SLA becasue of granular texture and porous surface (rougher surface finish). Can be sealed for better surface finish.
- Accuracy is inferior to SLA as excess fused powder can collect on the part surface leading to dimensional problems.
- Many variables to be controlled.
