Additive Manufacturing II

Question 1

Selective Laser Sintering - SLS

Answer 1

• A moving laser beam melts and fuses (sinters) a heat-fusible powder one layer at a time to build solid 3D parts.
• Category - Powder Bed Fusion

Question 2

SLS Method

Answer 2

• Scanning & sintering metallic & non-metallic, thermoplastic powder using a CO2 laser beam that causes the powder to sinter and solidify in the shape of a layer of the prototype.
• Powder bed moves up & she 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 & supporting the model as the build porgresses.

Question 3

SLS - Process Chracteristics

Answer 3

• 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.
• Sintered at operating temperatures of up to 385C; or just below the melting point of the polymer.
• Build speeds of 7-48mm/hr (material dependent)
• Build of parts stacked on different planes (build packet).
• Parts can be built in sections & rejoined.

Question 4

SLS - Process Chracteristics (cont.d)

Answer 4

• Surface finish & accuracy:
• Powdery like the base material with granular, porous texture.
• Smooth when using crystalline powders (wax)
• Typical tolerance 0.4mm
• Recommended wall thickness (min) 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 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

Desired properties:

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

Question 6

SLS - Applications

Answer 6

• Functional prototypes - durable prototypes without tooling, withstand form, fit & functional testing.
• Functional parts (e.g. dental)
• Pattern for investment casting
• Capable of living durable hunges, snap-fits & high-flex snaps.

Question 7

SLS - Variation

Answer 7

• 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 8

SLS- Advantages

Answer 8

• More versatile than SLA as more variety of materials; including metal 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 or with adhesive (thermoplastic like properties).
• Nontoxic process.

Question 9

SLS- Limitations

Answer 9

• Slow cycle times; but as little preparation time is required a rapid turnaround is possible.
• Prototypes require cooling and increase build time.
• Surface finish is inferior to SLA because of granular texture and porous surface (rougher surface finish); can be sealed for a 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.