“Any sufficiently advanced technology is indistinguishable from magic,” wrote Arthur C. Clarke, and Computational Axis Lithographyor Caland finally the magic was born.
CAL is a layerless 3D printing process invented at the University of California, Berkeley in 2019. There is no growth or stacking, parts are made all at once. After trying for a while to understand what was just said, the common question that comes to mind is “how?”
How to use
First, there is the print material. Typically a UV light-curable resin with similar chemistry to that used in optical lithography (SLA) printing. The simplest materials consist of a monomer and a photoinitiator. A monomer is a molecule that reacts to grow long molecular chains that entangle with itself to create a polymer. The reaction is started by a photoinitiator, a specialized molecule that reacts to specific wavelengths of light. Materials can be much more complex than this and can have multiple monomers, or many more molecules that react differently to light, or even tiny silica/glass particles. CAL has already been demonstrated on over 60 different materials.
Next is the mechanical setup of the printer. The illustration above is almost identical to the one in the patent. The printing material is A transparent cylindrical glass container (D). there is Rotational element (C) Rotate the container Projector (A) Shine Image (B) These are the parts that make the part. Everything else is extra and is only used to improve the printing process. This includes things like lenses to better control the quality of the light beam, and index-matching boxes to stop the light from refracting when it hits the edge of a round glass bottle.
The magic of CAL exists almost entirely in the images projected by the projector, and its roots lie in Computed tomographyAlso known as a CT scan, in a medical CT scan, a 2D x-ray fan is aimed at a slice of the patient’s body. The x-ray source is then rotated slightly and the process is repeated. After several rotations and complex calculations, a clean slice of the person is produced. Rebuilt — It determines how much energy the x-rays absorb inside the patient’s body, which is then calculated using filtering techniques. A rotating x-ray source is then moved up and down along the patient’s body, taking multiple slices. Once the person’s full length has been scanned, the slices can be put together to create a complete 3D model of the patient’s body, including inside.
CAL is essentially a reverse version of a CT scan: it starts with a 3D model and calculates the slices needed to form that 3D model from multiple angles. The process starts by breaking down the model. Voxelwhich are little cubes that are essentially 3D pixels. Then, like a CT scan, slices are taken through the model. Starting at 0 degrees, 1-dimensional rows of pixels of different intensities are calculated based primarily on the density of the object viewed from that angle. This is rotated a little, and then repeated for 180 degrees (not 360 degrees; a row of pixels is calculated the same way when viewed from 180 degrees, so it doesn’t need to be calculated twice). The combination of these 1D rows of pixels from different angles is SinogramThis process is then performed on multiple slices of the model, similar to a CT scan, to create sinograms of all the slices.
Now we can create a first version of the projected image. By taking all the 1D pixel rows from a certain angle (say 30°) and stacking them, we create a 2D image that takes into account the 3D space. However, this first version of the image will be improperly blurred and distorted. Filtering, optimization methods, and other mathematical tricks will create a high-quality 2D image. This image can be projected onto a vial to start forming the part.
However, many angles of these 2D projections are needed to form the desired shape, which are then combined into a 3D projection video (above). A typical video created for projection will have 180 unique frames/angles. This is just the beginning of how these videos are created. CAL also calculates many of the physical phenomena involved in the printing process (such as how light refracts when it hits a surface, or how molecules move within the printing material as the part is formed), and can alter and distort the video to account for these effects. There are very few physical effects that cannot be corrected for.
Print in seconds
Now that the three core parts of CAL are in place – the material, the mechanical setup, and the special projected video – the printing begins. First the vial starts to rotate on its central axis, then the projected video is shined through the vial. The focal point is in the center of the vial as before, but as the light passes through the entire volume of the vial at each angle, it starts to form everywhere in the part. It takes a certain amount of light for the material to start to become solid. As the vial rotates, only the parts that have had enough light pass through become solid. This allows the part to actually form all at once, sometimes in as little as 10 seconds.
CAL parts require post-processing: they are removed from the vials with tweezers, washed in solvent to remove excess resin, and then further exposed to light under a powerful UV LED lamp to achieve full mechanical properties.
The unique way the parts are formed allows CAL to do several unique things: Overprint. Overmolding This is a method of molding plastic around an existing part, such as a screwdriver bit, to create a screwdriver handle. CAL does the same thing with printing, forming 3D geometry over an existing part. The example above printed in just 20 seconds (instead of a minute), sacrificing quality for speed.
CAL also does not require support structures to create a part; once formed, the part is supported by the print material itself. As such, CAL typically uses materials that are much more viscous than SLA. Typical CAL materials have a viscosity similar to or greater than that of molasses.
CAL in zero gravity
High viscosity materials tend to have stronger and more desirable printing properties. However, if a low viscosity material is required, several different techniques can be used, the most exciting of which is to simply remove gravity using CAL in space. CAL has already been demonstrated in microgravity (a more scientific term for “weightlessness”) on a parabolic aircraft flight (above), which printed over 400 parts. The microgravity environment on the parabolic plane lasts only 20 seconds, yet CAL is able to fully form the parts. The next step for the space version of CAL is to test it in ballistic space and eventually on the space station.
What’s next?
Developed in 2019 by Brett Kelly and colleagues under the guidance of Dr. Hayden Taylor, the technology has borne much fruit since then. As well as testing the technology in microgravity, pure glass components have been produced with dimensions one-fifth the width of a human hair. Many more projects are in the works, from printing optical lenses to printing on 50-centimeter diameter vials to recyclable materials.
CAL has grown beyond UC Berkeley, with more than a dozen institutions researching and extending the technology. And because it’s an open source project, Try it for yourselfThe future of CAL seems limitless.
This article make: Vol.88.
