Vat Photopolymerisation Flashcards
What is vat photopolymerisation?
Liquid photopolymer in a vat is selectively cured by light activated polymerisation
What are the 2 different types of vat photopolymerisation?
Top down SLA process
Bottom up SLA process
What is the top down SLA process?
Uses a UV laser above the vat of photocurable resin to trace a pattern on the surface
Exposure to the UV cures the pattern to the layer below
The platform drops a layer and recoated with resin
Process repeats
Once complete the part rises from the resin and needs to be cleaned in a solvent bath
Support material removed
Placed in an UV oven to cure
Post processing
What is the bottom up SLA/DLP process?
A DLP projector projects an image of the first layer form below
Exposure to the light solidifies the layer joining it to the one below
Table is raised and the process repeats
Solidification due to polymerisation
Washed in solvent bath
Cured in UV oven
Post processing
What are the 3 methods of patterning with light?
SLA - stereo lithography - laser beam cures the resin spot by spot
DLP - digital light processing - projected light over the entire layer to cure all at once
LCD - liquid crystal display - an LCD screen either masks a projectors light or uses its own light to cure a layer all at once
Advantages and disadvantages of top down SLA?
Adv
Higher resolution parts, the current layer only adheres to the previous one
Dis
Surface characteristics of the resin can affect part quality
Longer process due to re coating step
Viscosity of resin determines part thickness
Advantages and disadvantages of bottom up SLA/DLP?
Adv
No recoating stage
Ultra thin layers possible
Reduced material wastage
Dis
Light must pass through an increased number of surfaces reducing intensity
Current layer may adhere to the surface of the vat damaging the part
What are the resins like that are used for SLA? Why might additives be added?
Light curable resins that are typically UV spectrum
They consist of a monomer, photo initiator and an inhibitor
Additives are used to enhance materials for specific applications and to change colour/stability
E.g. bluestone, Accura 60
What is photopolymerisation?
Causes the linking of small molecules (monomers) to larger chain-like molecules (polymers). It is initiated by the excitement of the photoinitiator by UV radiation which produces free radicals
Why must SLA parts be UV cured?
To complete cross linking, parts can be weak if not cured properly
What affects cure time?
Mass and material
What are the benefits of UV curing?
Improved finish
Better scratch, chemical and hardness resistance
Better elasticity
Improvement in general strength
Advantages of vat photopolymerisation?
Accurate Good surface finish Large volumes Reliable technology Low maintenance cost Can be mechanically and chemically smoothed
Disadvantages of photopolymerisation?
Parts need to be cured Expensive machines and materials Toxic resins Can only use photocurable resins Parts age in sunlight Require supports which waste material and affect surface finish Weaker parts in z axis Parts are hygroscopic (dimensionally unstable over time) Denser prints take longer Higher accuracy requires longer print times Part orientation affects surface finish Stair stepping Trapped volumes
Example applications of SLA
Wind tunnel testing models to help with the aerodynamic design of F1 cars
Patient ear canal impressions to produce custom hearing aids
Invisalign use a intra oral scanner to produce a STL file of patients teeth which is then printed to form a SLA mould which can be used in the final thermoforming step
High loading of ceramic into a photopolymer. Polymer acts as a binder and parts are fired to remove binder and sinter ceramic
