2. The distortion in either a DLP projected image or a laser galvanometer optical path leads to parts printed with inaccurate dimensions.

To make a higher precision, large format professional 3D printer, we have designed “Weaver”, the first desktop SLA printer to incorporate a F-theta lens.

Why is an F-Theta lens needed?

The Focusing Challenge (reference: Renishaw website)

A laser beam usually has a Gaussian intensity profile - intense in the center and reduces towards edge. Optical lenses are used to focus the beam from a few millimetres in diameter where it exits the fibre, down to a narrow waist where it intersects with the build plate. As we move away from this focal point, the beam's cross-sectional area increases, reaching twice the minimum size at a distance along the beam called the Rayleigh length (ZR in the diagram below):

The Rayleigh length is proportional to the square of the waist diameter. The smaller the waist diameter (which is desirable for producing detailed features) the shorter the Rayleigh length and the more sensitive the system will be to focusing precision. If our focusing is inaccurate and we are a Rayleigh length away from the true focal point, then our spot area will double and our energy density will fall by 50%. In Weaver's laser with a 50 micron diameter waist, the Rayleigh length is very small. This means that our optics must be able to maintain focus within this range as the laser spot moves across the build area.





Galvo Laser Positioning and Focusing

Laser SLA printers use galvanometric ('galvo') mirror systems to direct the laser beam to different locations across the build area. A pair of mirrors are positioned above the centre of the build area and these direct the beam at a range of compound angles to the required XY positions on the build plate. The beam therefore has to travel different distances before it strikes the build plate as these angles vary. The further away from the centre of the build area, the longer is the laser beam path from the galvo mirrors to the build plate. This means that the laser beam focal length has to be varied precisely with the beam angle - otherwise the laser spot size and shape vary over the build area, which is the case for all desktop SLA printers on market.

A laser spot projected by a galvo mirror system is round in the center of the printing area, but loses focus and becomes elliptical further to the sides. This cause two serious issues. First, you can never get a focused laser spot except right at the center of the area. Some printers list a 70 or 140 micron laser spot size, which might be true right at the center, but will be far-off at the edge. That's why a well-known SLA laser printer claims a 140 micron average laser spot size. Second, the non-uniform laser spot size also makes the light intensity non-uniform, leading to frequent print failures due to a lower light intensity at edges, a frustrating yet common issue for all SLA 3D printers.





Regular SLA printer and typical failure

High-end industry 3D printer use F-theta lens to address the focusing issue, such as metal 3D printer AM400 by Renishaw. It comprises a multi-element lens assembly that focuses an incident beam onto a flat plane. The F-theta lens has a focal length that varies with the angle at which the beam enters the lensing element. The intention is to keep WD constant across the complete range of incident beam angles:



The F-theta lens provides a focused 50 micron round spot profile at very wide angles, increasing light uniformity and enhancing the surface smoothness of printed parts. The Weaver is the first desktop SLA printer to use a F-theta lens which gives it the highest resolution of any SLA printer available today!