Direct Metal Laser Sintering (DMLS) is a process in which metal powder is sintered by using laser energy without melting the metal. The best way to describe sintering is by looking at a ball full of ice cubes in a room temperature and notice that the ice cubes are attached to each other, forming a large solid structure.
In the printing process of DMLS, one of the challenges is to move the laser beam over the bed surface printing area (X,Y). The printing of the X,Y surface is done either by line vector drawing or line raster drawing by moving the laser beam over a powder in a line pattern. The Laser will turn the energy on, on the dots that need to be sintered and off in the spots that are to be cavities; as a result the area where the laser is off will keep the powder un-sintered while the spot that was heated by the laser will sinter and therefore attach bunch of small powder balls together.
As the 3D industry expands, more vertical industries are in search of the best 3D printer which delivers a suitable print for their market, hence new requirements are pushing the performance envelope of a metal 3d printer to the limit.
This article will focus on the one of optics limitation and challenges a 3D metal printer manufacturers are facing. This is not about printing a decorative 3D lamp, this is about printing a replacement part for your antique car or printing of a rocket part.
Fig .1 typical DMLS printer
Galvanometer in the SLS service
Most of today SLS printers are using Galvanometer (Galvo) for X and Y. A Galvo is a mirror driven by a motor where instead of moving the Laser we are deflecting the light from the Laser into a mirror. When the mirror rotates, it will reflect the laser in a straight line. An X,Y Galvo is a Galvo that has a set of two mirrors where one covers the X direction and the other covers the Y direction. Now the two Galvs can control an area (X-Y).
Fig 2. One dimension Galvanometer scanner (image courtesy of Thorlabs)
Fig 3. Two dimensions X-Y Galvanometer scanner
The galvo is controlled by servo motor. Most servo motors use a closed loop motor and capable to position the mirror within 10 Microradian of motor rotation; this 10 microradians will be reflected on print surface error of L*tan(aiming_angle ±10 microradian) will translate to line accuracy of L*tan(10 microradians) where L is the distance to the print surface. If our print surface is 500mm then it will reflect an error of 0.005 mm. which is 5 micrometer. Although, this number is small relatively to the print size. This number is not small relatively to the Laser beam diameter, therefore, limiting the print beam diameter. In other word, the beam diameter needs to be much larger than 5 micrometer not to be affected severely by this error. The laser beam diameter will set the print resolution. Printer manufacturers will be using a beam diameter in the range of 50-300 micrometer. Tecnica defined this error as the Glavos Positioning Error.
Fig. 4 (notice the distortion)
Galvs deflection Errors
The deflection of the Laser beam bouncing of two mirrors causes three type of distortions on the reflected image:
- The arrangement of the two mirrors leads to a certain distortion of the image field; see Fig. 2 above. This distortion is due to the fact that the distance between mirror 1 and the image field depends on the size of the scan angles of mirror 1 and mirror 2 . A larger scan angle leads to a longer distance (when y=max, x travel longest distance).
- The flat field distortion: The distance in the image field is not proportional to the scan angle itself, but to the tangent of the scan angle. Therefore, the marking speed of the laser focus in the image field is not proportional to the angular velocity of the corresponding scanner.
- Focus curve distortion: If an ordinary lens is used for focusing the laser beam, then the focus lies on a sphere. Projecting the image on a flat field will result in varying spot size (a circle beam will be more elliptical as it gets far from the origin).
Distortions 2 and 3 can be corrected by using f-theta lens; f-theta lens is transforming the focus from a sphere to a plan. Mathematically, f-theta lens is transforming the tan(theta) to theta itself, resulting in L*theta projection in oppose to L*tan(theta).
Input aperture (input to the galvanometer):
Considerations of Small or large input aperture input to the galvanometers is selected by trade offs. If a large input aperture is used then larger mirrors are needed and therefore stronger (larger) motors are needed (more below).
Pros (using small aperture):
– the smaller the input aperture the smaller the mirrors. Smaller mirrors will reduce motor inertia load and decrease air resistance which will require smaller motor. Smaller mirrors will also reduce the distance between the mirrors and as a result will contribute further to smaller distortion (See Fig 4 ∅2 is smaller).
– small input aperture will also utilize larger f-theta space
Cons (using small aperture):
-the mirrors’ coating needs to handle higher energy density as the irradiance intensity is high (W/cm^2)
-the f-theta coating needs to handle higher energy density (W/cm^2)
-there will be a need for additional beam expander lens and beam collimator lens prior to feeding the the beam to f-theta
The galv is using a closed loop servo motor that run back and forth. Galv are limited to a rotation speed of about 2500 Hz. This translates to a print bed scan speed of about 6 m/sec. In addition, there is a 5 microradian location error which translates to 300 micrometer error when using a 360 mm print bed. The larger the print bed the larger the error.
f-theta lens factors:
f-theta lens main function is to focus the laser energy from collimated wide beam (about 10-20mm) to about 100 micrometer. It’s also responsible to correct the flat field distortion by moving the focus point from a circle to a line (sphere to flat surface). f-theta itself produce a distortion due to the imperfectability of the lens:
One of the main distortion is Angle Non linearity error: this error is a function of the angle and can range from 0 to about 850 microradians and it’s a function of the angle.
We will discuss f-theta in a dedicated article.
When using Galvos system for SLS, the system is limited by definition to a beam diameter larger than 50 micrometer using the best Galvo in the market. Hence, limiting the part resolution.