Steriolithography (SLA)

Question 1

What aer the basic principles of steriolithography (SLA)?

Answer 1

• Based on phase change of photo-curable resins
• Uses a scanning laser to selectively solidify liquid resin to form a thin horizontal layer of required geometry
• Forming multiple layers on top of one another to gradually grow the part

Question 2

What is Polymerisation and how is it exploited in the SLA process.

Answer 2

• polymerisation is where small molecules (monomers) are linked together to form larger molecules (polymers)
• SLA process exploits a PHOTO-POLYMERISATION reaction
• Incident laser light is absorbed by a liquid resin and initiates a polymerisation reaction to solidify the resin

Question 3

How is photo-polymerisation used in SLA process?

Answer 3

• photopolymer resins comprise a liquid monomer mixed with an initiator
• initiator is formulated to casue polymerisation of the monomer under the incidence of light of the required wavelength
• In SLA resins: 1. a free radical os produced when two photons are absorbed by the initiator. 2. The free radical causes monomer molecules to link together into polymer molecules. 3. one free radical will join up to 1000 monomer molecules together

Question 4

What are the 5 phases of photo polymerisation?

Answer 4

1. Photoinitiator is mixed with a monomer
2. Photonic excitation and free radical generation (from laser)
3. Chain initiation
4. Chain propagation
5. Chain termination

Question 5

What are laser bean characteristics?

Answer 5

• Circular cross-section
• Collimated, but not quite parallel (small divergence)
• single wavelength, SLA now uses ‘vanadate’ (solid-state) laser = 355 nm (UV)

Question 6

What is Beer Lambert law of absorption?

Answer 6

Irradiance at any point can be calculated as a function of irradiance incident on the resin surface = (H(R,0))

Question 7

What is critical exposure?

Answer 7

If the resin exposure is:

1. less than E_c: polymer will remain liquid
2. More than E_c: polymer will solidify

• E_c is the point at whcih solidification occurs (called the “Gel Point”)
• As laser penetrates the resin, there will be locus of points below the surface where E=E_c, energy absorbed allows the resin to reach the ‘gel point’ there will effectively be an edge to the solidifcation process
• this locus defines the geometry of a single cured voxel (volume element) of resin.

Question 7

Why is cure depth important in SLA?

Answer 7

Must be suffcicent to create a solid layer of the required thickness to bond that layer to the previoulsy solidified layer

Question 7

Why is cure linewidth important in SLA?

Answer 7

• In a parabolic cure pattern the maximum cured line width L_w will occur on the surface (Z=0)
• Cured linewidth is directly proportional to the laser spot diameter
• When cure depth is increased, the cured linewidth increases by the square root of the cured depth

Question 8

What are the 5 different builds types?

Answer 8

Tri Hatch - Laser is scanned in one direction per layer, layer tends to shrink in one direction

Weave - scans in either x or y direction (alternates for each layer), Hatch spacing is 250 microns greater than line width

Star weave - start point moves around four corners in turn

Aces - most accurate style, accurate, clear, fully solidifies layer boundary with large cure depth

Quick cast - 30% solid, build thin wall parts with honeycomb internal structure