Stereolithography (SLA) is a 3D printing process with photopolymer resin solidified by laser system. It is most suitable for small size parts with fine details and high tolerance. We will introduce SLA printing process, SLA advantages and limitations, and finally SLA design principles.
SLA printing process
A SLA machine is normally consist of a UV laser system and a photosensitive resin tank. The tank is transparent in bottom for UV laser trance, which make laser system control 2D contour of printing parts.
The later system cures the resin to form a solid layer with each pass. This thin slice get stuck on build plate or tank bottom. Then newly printed layer separate form tank bottom, build plate move as 1 layer thickness and repeat this process until complete parts.
It is important to reduce newly printed layers force in separation stage, which determine success of SLA process. In separation stage, high stress will occur in area of razor thin edge, it will give rise to part failure and warping. Some time parts will stick to tank bottom, not build plate.
SLA printing orientation
Z-axis cross-section area should be the most concern in SLA part orientation. Which is proportional to force between printed parts and tank. In reason of this issue, we also print parts with angle to plate, rather than support consideration. Reducing cross-section are in Z-axis is the most suitable way for SLA printing orientation.
It is significant to understand the part orientation impact of parts quality. In order to reduce z-axis sectional area, amount of support should be added to printing model. In special instance, too many support will make SLA no longer cost-effective, or affect parts appearance once remove support. All these issues will encourage us optimize our SLA design, limit number of horizontal members, hollow out components and reduce cross-section area.
SLA 3D printing produces isotropic parts, in reason of all layers com-bond with each other in both x,y and z direction, so SLA parts have near identical physical properties. No matter printed pars are printed parallel or perpendicular to build plate, there will be no affection on final material properties.
SLA printing design
The laser spot size and resin properties in SLA machine will determine final parts level of detail. SLA design general guideline as follow:
Supported walls: Thin walls should be connected with other structures on both side, in order to reduce warping chance. Minimum wall thickness in design should be 0.4mm.
Unsupported walls: Thin walls only connect to on side will be at high chance of warping or detaching in printing process. Minimum size of unsupported wall is 0.6mm. In addition, we recommend fillet base design at bottom for connection between wall and rest print area, this will reduce stress concentrations along joint area.
Overhangs: SLA printing has no issue for overhang features unless model without adequate internal and external support structures. Printing structure without support will always give rise to warping. Any unsupported should be less than 1.0mm length and more than 19° from level.
Embossed detail: At least 0.1mm height features above surface will ensure all details can be visible.
Engraved detail: Too small details are easy to fuse with model in printing process, we recommend detail size of at least 0.4mm in width and 0.4mm in thickness.
Horizontal bridges: As bridges can be printed between two points in SLA process, we should consider that wider bridges must keep less than 21mm, and shorter than thin bridges. In addition, wider bridges will increase z-axis contact area, which will give rise to printing failure rate during peeling process.
Holes: We recommend holes diameter more than 0.5mm, less than this size in any direction will be close off during printing process.
Connections: Moving parts clearance should be 0.5mm. Assembly connections clearance should be 0.2mm. Push or snug fit clearance should be 0.1mm.
SLA has higher resolutions than FDM in reason of laser system solidification. SLA printing resolution in horizontal (X, Y) direction is form 30 to 140 microns, which is determined by laser spot size. The minimum feature size should be bigger than laser spot size, this is not adjustable.
Resolution on vertical (Z) direction varies form 25 to 200 microns. Vertical resolution is determined by parts quality requirement or speed requirement. There will be invisible difference between printing parts of 25 microns and 100 microns, if there are few curves or fine details in parts design.
Hollowing and cupping
SLA always produces solid and dense parts, once these parts are not as functional parts, we recommend hollowing parts structure to reduce material amount and print time. In addition, hollowed parts thick should be at least 2mm, avoid failure risk in printing process.
In hollow parts, we need to prevent resin trapping by adding drainage holes. these uncured resin give rise to imbalance in hollow chamber, and finally cause small failures as cracks or holes. This cupping will propagate throughout all parts and cause complete failure or explosion. Drain holes should be at least 3.5mm in diameter, and at least one hole for each hollow section.
SLA printers generally produce smaller parts volume than FDM printers. Common desktop SLA printer has build volume of 145mm×145mm×175mm, while common desktop FDM printer provides 223mm×223mm×205mm. For parts geometries exceed SLA volume capacity, we always print smaller sections and then assemble. The best way of SLA parts bonding is 5-30 minutes epoxy.
SLA parts are not suitable for loading function parts. SLA resins nature properties determine SLA parts are brittle, unstable and creep deformation in long periods of time. Most SLA parts need post-print curing in UV chamber, in order to increase the parts strength and stability.