Selective Laser Sintering - SLS

Question 1

Selective Laser Sintering - SLS?

Answer 1

A moving laser beam melts and fuses (singers) a heat-fusible powder one layer at a time to build solid 3D parts.

Question 2

SLS category?

Answer 2

Category - Powder Bed Fusion

Question 3

SLS - Principle/Method?

Answer 3

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.

Question 4

SLS - Process characteristics?

Answer 4

• 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.

Question 5

SLS - Process Characteristics?

Answer 5

• 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.

Question 6

SLS - Materials?

Answer 6

• Variety of Polyamides (Nylon-based polymers)
- Glass - Carbon - Aluminium- fibre- filled.

• Polyaryletherketone PEAK
• Thermoplastic elastomers
• Polystyrene

Question 7

SLS - Desired Properties?

Answer 7

• Sterillisable, biocompatible, Flame retardant.
• High stiffness, toughness, elevated temperature resistance.
• Anisotropic mechanical properties (fibre-filled)

Question 8

SLS Applications?

Answer 8

• 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.

Question 9

SLS Variation?

Answer 9

• 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.

Question 10

SLS - Advantages?

Answer 10

• 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

Question 11

SLS - Limitations?

Answer 11

• 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.