Rapid prototyping leverages computer aided designs (CAD) create in a relatively short time a three-dimensional object that once may have taken months to fabricate. Now, advances in lasers and optics are making such systems easier and faster to use.

The secret behind rapid prototyping is the use of a laser to “print” a three-dimensional figure one layer at a time, building up material with each step. Several technologies are in use for rapid prototyping, the most common of which are selective laser sintering and stereolithography. Both use a high-powered laser to change material from a fluid to a solid state. Before going into the optic advances, a closer look at the laser operation in each approach is necessary. 

A stereolithography prototyping system uses a photopolymer and scanning UV laser to build solid objects one layer at a time.

From raw material to a prototype
Selective laser sintering uses thermoplastic powder as the material from which it creates three-dimensional shapes. The prototyping system spreads the thermoplastic powder in a thin layer. A high power infrared (IR) or near-infra-red (NIR) laser traces out on the surface of the powder a cross
section of the object derived from a CAD file. The laser energy fuses the thermoplastic particles together to make a solid mass and the process repeats by adding layers of powder until it has created the desired three-dimensional shape.

The long wavelengths of the NIR or IR lasers, however, limit the resolution at which three dimensional shapes can be printed. Also, the thermal effects of the laser energy spread from the target point, which can affect the surface quality and finish of the final prototype and limit the ability of selective laser sintering to prototype small components or products with high levels of detail.

Stereolithography uses a photopolymer fluid and an ultraviolet (UV) laser. Rather than melting due to heat, the polymer hardens (cures) due to activity at the molecular level when exposed to high enough levels of UV. The system is thus able to create a three dimensional figure in the midst of a volume of fluid by focusing the laser at a point in the volume to deliver the energy density needed to stimulate curing. Structural support for the object must be available during the construction process to insure its dimensional integrity.

The process for building a prototype using stereolithography is relatively simple. A model’s CAD program is converted into files that will control the lasers. Mirrors mounted on a galvanometer position the laser beam at points along a plane within the fluid as directed by the CAD files, exposing the photopolymer to UV light and hardening the polymer at each spot. Scanning the beam in two dimensions creates all the solid areas within the layer. Once all sections of a layer are completed, a platform holding the container of photopolymer drops to bring the next layer into the laser’s focal plane.

The short wavelength of UV provides very high resolution and minimal thermal damage to the prototype surface, making it ideal for parts needing relatively high levels of detail and excellent surface finish and quality.

The main disadvantage of stereolithography is the need for post processing of the prototype. The building process solidifies the photopolymer but not to full hardness. Once the parts are fully developed, they must be placed in a UV oven for final curing. In addition any structural support used or created in the building process must be removed, which adds to the prototyping time beyond that needed for simply “printing” the object.

For lasers, optics are key elements
The key elements of a stereolithography system are the UV laser, alignment optics, laser beam expander, galvanometer, scanning lens, and the vat containing the UV-sensitive photopolymer. In addition, most systems include a method for characterizing the laser beam to insure optimal performance, and a closed loop feedback system with either a Helium Neon laser or a laser diode module to determine the vat platform’s exact position.

The function and efficiency of the system particularly depends on the laser, the optical components that manipulate and shape the laser beam, and the electronically controlled mirror system that directs the beam in the appropriate pattern.

The optical components of a stereolithography system fall into two main sections: the laser system and the scanning system. There are several types of lasers used in stereolithography, ranging from 193 nm to 355 nm in wavelength. Bulky and expensive gas lasers have dominated the stereolithography field for years, but newer and more efficient Diode Pumped Solid State (DPSS) lasers are now finding wide use. DPSS lasers are efficient, consume little power, have long life, and a compact size. The most common DPSS used in stereolithography is a frequency-tripled Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser with a 355 nm wavelength. New photopolymer resins have appeared on the market that work well with this wavelength.

The delivery of laser energy in a stereolithography system can be Continuous Wave (CW) or Q-switched pulses. CW lasers provide continuous laser energy and can be used in a high speed scanning process. However, their output power is limited which reduces the amount of curing that occurs during object creation. As a result the finished object will need additional post process curing.

Q-switching is a new technique that allows the laser to produce very high peak power pulses at a relatively low repetition rate. The technique uses an attenuator to prevent stimulated emissions in the gain medium of the laser so that the pump energy accumulates. When the stored energy in the gain medium reaches saturation, the attenuator quickly changes state so that the stimulated emission process can occur. Due to the high energy level stored in the laser cavity, amplification by stimulated emission occurs quickly resulting in a short pulse with very high intensity. Thus, the main advantage of Q-switching is high energy levels that induce substantial curing, reducing the need for lengthy post processing. However, due to the low repetition rate of Q-switched DPSS lasers, scanning speeds are reduced and the object takes longer to create.

Whether CW or Q-switched, the laser is generally aligned and fixed in place. Then, fine alignment of the laser beam is done using additional optical components. Optical mirrors mounted in highly stable and accurate holders, such as Edmund Optics’ TECHSPEC kinematic mounts with 100 TPI resolution, enable the precision alignment of the laser beam with the input aperture of the laser beam expander. Because these mirrors and the optics of the beam expander are subjected to the high energy density of the raw laser output, they need both a high damage threshold and excellent optical characteristics.

The laser beam expander plays two key roles in the scanning system. The first is spreading the laser beam to reduce the beam power density. This step helps minimize damage that the high power laser beam may cause to the scanning system components. The second role is collimating the laser beam. The better the collimation the tighter the scanning system lens will be able to focus the beam at its target point. The tighter the focus the higher the energy density and the faster the photopolymer will cure.

These roles place significant demands on the beam expander’s optical characteristics and call for high precision and quality. The Edmund Optics BeamX expander, for example, meets these demands through wave-front testing at its specified operating wavelength to insure accurate alignment and excellent surface quality of the multi-element lens system. In addition, the expander offers adjustable focus to insure installation flexibility and beam divergence correction. It has a large input aperture to eliminate the need for critical alignment. The BeamX laser beam expander also features high damage threshold rating – up to 15 J/cm2 – so that it can handle the peak power levels of the Q-switched laser. Such performance is typical of what a rapid prototyping system will require.

Following expansion, the laser beam enters the scanning system that will position and re-focus the beam to create a high-intensity spot at a precise location within the photopolymer to build the object. Early designs of such scanning systems used one of two methods. The first was to place the work piece on a movable, motorized X-Y stage and position it appropriately relative to a fixed laser focus point. The second method was to use a robotic articulated arm to adjust the focus point of the laser on a fixed work piece.

A scanning lens should minimize distortion and form an image on a flat plane. Singlet lenses from an image on a curved plane (Figure 1). Flat field scanning lenses compensate for this aberration with a flat-surface scanning field. The output image size is proportional to the object size. However, the scan length is proportional to the tangent of the incident beam angle (Figure 2) and nonlinear to the angle itself requiring complicated control electronics.  F-Theta lens addresses this problem and satisfies the linear relationship between the scan length and the incident beam angle (Figure 3).

Mirrors speed and simplify scanning
As new applications emerged, however, new machines with smaller, faster and less expensive scanning systems evolved. They use galvanometer-rotated mirrors instead of an articulated arm to position the beam. The small size and light weight mirrors also allow high speed scanning with long mechanical life. However, such systems can no longer use a simple lens. Simple singlet lenses create a curved focal surface, which greatly complicates the control system that is trying to trace out a flat layer within the polymer.

To eliminate this complexity, systems must use special scanning lens that provide a flat field at the image plane. Ordinary flat field scanning lenses, however, focus the laser beam a distance from center that is proportional to the tangent of the beam’s incident angle on the lens. As the scanner changes the beam angle the focal point’s motion on the target varies in a non-linear proportion to the angle itself. Thus, a constant angular-rate scan would result in a variable linear scan rate on the target, resulting in uneven hardening of the object. Compensating for this non-linear motion so that each point on the scan receives the same energy dosage requires complicated algorithms that vary the speed of the galvanometer mirrors to correct the target scan rate.

Another option now available is the F-Theta scanning lens, such as the Edmund Optics TECHSPEC F-Theta lens, that both minimizes distortion and provides flat field at the image plane, addressing two common optical design criteria of laser scanning systems. Such lenses also, however, contain additional optical elements that linearize the relationship between incident angle and spot position. The linear relationship eliminates the need for complex scan correction algorithms, thus simplifying the electronics and reducing cost in the scanning system.

As with other optical elements in the scanning system, the F-Theta lens needs to be high damage threshold rated and available in a wide variety of scan fields and operating wavelengths to meet specific system needs.

Together with the DPSS UV lasers, the availability of F-Theta lenses has transformed the established technology of rapid prototyping by making it faster, more accurate, and less expensive.

Edmund Optics
www.edmundoptics.com

MPF

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