Additive Manufacturing - Selective Laser Sintering

Question 1

What is SLS

Answer 1

Selective Laser Sintering

A moving laser beam melts and sinters a heat-fusible powder one layer at a time to build 3D parts

Question 2

SLS- Method

Answer 2

• Scanning and sintering metallic and non-metallic thermoplastic powder using a carbon dioxide laser beam (CO2) that causes powder to sinter and solidify in the shape of a layer of the prototype
• Powder bed moves up and delivers polymer which a roller spreads across the surface of the build area, forming an even layer of powder
• Non-sintered powder forms a “cake” encapsulating and supporting the model as the build progresses

Question 3

SLS- Process Characteristics

Answer 3

• Layer thickness 0.06-0.15mm
• Resolution x/y 0.8-1.3mm
• Process takes place in an inert nitrogen atmosphere <1% oxygen to stop the powder oxidising when heated by the laser
• Sintered at operating temperatures of up to 385°C, or just below melting point of the polymer
• Build speeds of 7-48mm/hr (depending on material)
• Build of parts stacked on different planes (build packet)

Question 4

SLS Process Characteristics (Surface finish and accuracy)

Answer 4

• 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 (min) 1.0mm
• Natural radius 0.4mm
• Good accuracy; problem if the temperature of uncured powder gets too high, excess fused material can collect on the part surface

Question 5

SLS- Materials

Answer 5

-Variety of polyamides (nylon-based polymers)
[-Glass-, carbon-, aluminium-, fibre-filled]
-semi-flexible (rubber-like)
-Polyaryletherketone PEAK
-Thermoplastic elastomers
-Polystyrene

Question 6

SLS- Desired Properties

Answer 6

• Sterilisable, biocompatible, flame-retardant
• High stiffness, toughness, elevated temperature resistance
• Anisotropic mechanical properties (fibre-filled)

Question 7

SLS- Applications

Answer 7

• 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 8

SLS- Variation

Answer 8

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 loosly 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 9

Pros of SLS

Answer 9

• More versatile than SLA as more variety of materials including metal and ceramic powders
• Materials less expensive than with SLA
• Most materials are recyclable (minus 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)
• Nontoxic process

Question 10

Cons of SLS

Answer 10

• Slow Cycle times, however little preperation time needed a rapid turnaround is possible
• Prototypes require cooling meaning higher build time
• Surface finish is inferior to SLA because of the granular texture and porous surface (rougher finish also, 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 in the SLS process