Just like any other additive manufacturing process, the object you want to print starts off as a digital file in 3D modeling or CAD software. If we stick to the example from the introduction, the design of an optical lens may be derived in a program specific to optical designs, its model however can then simply be created in the users CAD program of choice.

The 3D data is then imported into the slicing software. For TPP printers, however, this software is often specific to the individual printer. The slicing software is where you set print parameters, such as speed, laser intensity, and layer height. What parameters you select in part depends on the object you’re printing, its size, the laser focusing objective used, and the material you’re using.

To print a micro-optical lens, it is key that the entire body is clear and contains no artifacts. Print defects, such as bubbles, can happen if the laser power is set too high, the printing speed is too low, or the printing area is too small for the previous two parameters.

There are a handful of manufacturers for TPP print systems. What many of the machines have in common, is that they have a gantry system which allows them to manipulate the printing area in x-, y- and z-direction. This system alone would be sufficient for printing. However, manufactures often additionally employ galvanometer laser scanners. These are made up of two motor-controlled mirrors which can very quickly deflect the laser beam in the x- and y-direction. This is beneficial as the print itself can stay static while achieving high print speeds. As the scanning area of such galvanometers is limited, the two systems are usually used sequentially. Meaning the gantry is responsible for choosing the rough print location and the laser scanner is used for the actual print process. This lets you print many parts sequentially.

A key difference among printers is the shape of the print platform and how the laser light is coupled into the resin. The print platform can be a flat piece of metal or glass, onto which a drop of resin is placed. The resin usually has a viscosity similar to that of honey and thus stays in place. Due to the typically small part size, a drop of that resin is also sufficient for the entire print.

Alternatively, the resin can be stored in a small vat which may have a translucent floor. The laser light can then be coupled into the resin in at least three ways. One, the laser objective can make direct contact with the resin; two, focusing the laser through a translucent build plate; or three, focusing the laser through air and then into the resin. All of these options have their pros and cons, but we’re not going to dive into those details here.

Just like any 3D printing process, you need to prepare the vat or surface you are going to print on. The glass on which you print has to be very clean to ensure proper adhesion. One cleaning approach would be to sequentially clean the glass with acetone, isopropanol and distilled water before removing residual moisture. In a laboratory context, the glass may additionally be treated with oxygen or plasma to achieve even more reliable print adhesion.

You also need to house your printer in an environment that is not subject to vibration caused, for example, by trucks passing by on a nearby road. Also, a cleanroom environment would be ideal, but newer print systems are more enclosed and thus lessen the need for this.

Before we hit the “start print” button, let’s have a look at some of the special materials that can be used in the TPP print process.

As with all 3D printing processes, the range of usable materials plays a significant role in the possible applications of the individual process. In theory, commercial resins used in SLA 3D printers would work for TPP but they are not optimized for the process. Many commercial UV resins contain color pigments that inhibit the laser from entering into the inner parts of a resin drop.

A variety of materials can be used in the TPP 3D printing, but vary depending on application. For example, a photoresin for optical elements would vary greatly from a biofabrication resin for tissue engineering.

Printer manufacturers usually offer a set of materials for which they supply machine-specific parameters, so that users need to do as little as possible trial and error work tuning in the right parameters like print speed, laser power, etc.

You’re not limited to the manufactures offerings and researchers have come up with a wide variety of materials which offer specific: optical-, mechanic-, electric-, chemical- and magnetic properties.

Postprocessing TPP

After printing, the build plate is removed from the printer and depending on the material one or more solvents (e.g. isopropanol or acetone) are used sequentially to wash off the residual uncured resin. To ensure that all of the resin is fully cured, the print may additionally be heat treated on a hot plate at around 70 °C or subjected to a UV treatment.

The TPP 3D printing process is a relative newcomer compared to other additive manufacturing methods. Research on the matter started appearing around the 1980s and the basics physics fundamental to the process were only established in the early 1900s.

At the Institute for Control Engineering of Machine Tools and Manufacturing Units (ISW) of the University Stuttgart in Germany, where I work, one area of research focuses on additive manufacturing processes.

The TPP process is of special interest, and, in one of our current research projects, we’re exploring multi-axis (5D) capabilities within the TPP process as these have received almost no attention. For example, in the case of printing optical lenses, it is unknown how the properties of a lens are affected if one creates the lens by stacking vertical layers as opposed to printing spherical layers. A 5D printing process would also allow users to vertically print onto free-form surfaces and would thus enable further research on this topic. However, as such machines currently don’t exist it is of interest to create such printers.

The ISW’s specific research project is motivated by the previous capability of printing onto free form shapes. The basic idea behind the project is increasing the productivity of the TPP process while printing aspheric lenses. These lenses are particularly interesting because they can eliminate or minimize optical distortions normally present when using mass produced spherical lenses. To achieve this increase in productivity, cheap spherical lenses are to be used as the starting point of the print process. The desired aspheric contour is then printed onto the spherical lens. This reduces the required print time and material.

In this case, it is again interesting to examine how the optical properties of a such a lens are influenced by stacking voxels vertically relative to the build platform or by perpendicularly stacking these onto the spherical lens. Yet, printing on the glass lenses creates another necessity for the 5D printing process. When curing resin near the edge of the glass lens using the highly focused and thus conic laser beam, the beam will be refracted by the lens itself. This will impair the curing process. With a 5D printer one can simply tilt the lens and avoid refraction issues.

It is for these reasons why we are developing a 5D TPP print system. However, there are a couple of major challenges linked to this idea. One is that the position of the spherical lens blank has to be perfectly in line with the center of the to be printed aspheric contour. Second, the system has to be capable of nm precession, and third a TPP specific multi-axis slicing process has to be established to generate machine code which can take advantage of the 5D capabilities.

To help overcome these challenges, we are collaborating with the university of Stuttgart’s 4th Physical Institute, which has extensive experience in additive manufacturing micro optics. Our research is into TPP is ongoing and there’s no doubt you’ll be hearing more about 5D TPP and other TPP printers and applications in the future.