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  • Picking Up the Pace of Rapid Manufacturing

Photonics Spectra
Apr 2003
Both conventional and emerging laser-based rapid prototyping systems are undergoing somewhat of an industrial revolution to take more products from concept to reality faster.

Paula M. Powell

Both conventional and emerging laser-based rapid prototyping systems are undergoing somewhat of an industrial revolution to take more products from concept to reality faster.

By one estimate, rapid prototyping equipment generates more than 3 million models and prototypes annually. Even with the recent slowdown in the market, this figure will most likely continue to rise. Development work by equipment manufacturers and materials suppliers is bearing fruit in enhanced system uptime as well as in the use of rapid tooling and manufacturing — applications that go from computer-aided design (CAD) to functional tooling (injection molds, for example) or functional parts with minimal intermediary steps.

Automotive industry adapts

Nowhere are efficiency-related trends more obvious than in the automotive sector. Automakers were early adopters of rapid prototyping technology, particularly stereolithography. First commercialized by 3D Systems of Valencia, Calif., this process in its most basic form builds plastic parts one layer at a time. A UV laser traces across the surface of a vat of liquid photopolymer to solidify a thin layer. The worktable then lowers a fraction of a millimeter farther into the vat, and the process repeats several times to build a three-dimensional model from often fairly complex CAD data.

Although the result may require postprocessing, including curing, the process dramatically slashes the time to a physical prototype for automakers. These benefits extend to rapid tool manufacturing, at least in the case of injection molding requirements by suppliers to the automotive industry, as well as to rapid manufacturing environments that do not face the extremely high volume demands of traditional automotive components.

In line with this, 3D Systems recently partnered with the Oxfordshire, UK-based Mild Seven Renault Formula 1 racing team to launch an advanced digital manufacturing center to support aerodynamic development of single-seat race cars (Figure 1). This allows the Renault team to make design variants of every part on the car for wind tunnel testing and to choose the optimum design for the current model vehicle. The facility includes stereolithography and direct composite manufacturing technology, and there are plans to add selective laser sintering, which uses lasers to melt and fuse powder material into a 3-D object.

Figure 1.
The capability to prototype parts for aerodynamic wind tunnel testing could help Renault’s Formula 1 cars shave off seconds of lap time. Courtesy of 3D Systems.

Although 3D Systems, with its stable of rapid prototyping technologies, may have an edge in the competition in such a center, end users wishing to employ such a strategy are not limited to one system supplier but can play off the strengths and weaknesses of each process. Direct composite manufacturing, for instance, although based on stereolithography, eliminates the need for liquid resin and uses instead photosensitive paste to build up a part. The paste is pushed up through a cylinder onto a table where a special coating system smoothes it out into a solid layer. Computer-driven mirrors direct the laser beam to build the pattern. The process has the potential to be faster than traditional stereolithography, in which the resin has to settle before the next layer can form.

Near net shape

Another interesting addition to the commercial rapid manufacturing picture is the laser-engineered net-shaping process, which Sandia National Laboratories in Albuquerque, N.M., recently licensed to Optomec Inc., also in Albuquerque. Although similar to other laser-based rapid prototyping methods in that it relies on a layer-additive process to fabricate physical parts directly from CAD geometry, there are fundamental differences, especially in the outcome.

Its primary strength is its capability to produce a fully functional metal part instead of just a prototype, said Sandia researcher David Gill. “The component has material properties similar to or superior to a forged part of the same material. For example, one application might involve prototyping an injection mold directly from a CAD model that is near net shape. This mold could be made of tool steel that would normally be slow to machine.

“The second advantage comes from the capability to functionally grade the component, switching between materials either gradually or suddenly. In this way, the technique could build sections of highly thermally conductive material that would help cool selected areas of a mold.” The process can add this material while retaining the integrity of the parent build material, which promotes its use in repair applications.

The laser-engineered net-shaping system consists of a high-power Nd:YAG laser, a controlled-atmosphere glove box, a powder feed unit and a three-axis computer-controlled positioning system mounted in the box. The laser beam accesses the glove box chamber through a window at the top, and a planoconvex lens directs the beam to the deposition region.

Working on a flat solid substrate usually of the same metal as the one to be deposited, the system focuses the laser beam onto the surface to create a weld pool into which powder particles are injected to build up each layer. The positioning system moves the substrate beneath the beam, and a thin cross section of the geometry is deposited. As with other layer-additive processes, there is some movement in the Z-direction (in this case, the laser and focusing lens assembly) to deposit other layers.

Throughout processing, the build faces both a small heat-affected zone and rapid cooling of the weld pool. The limited heat-affected zone reduces the impact to the substrate. The cooling creates very fine microstructural features, delivering high tensile strength and ductility for most deposited metals. In the research stage, these features facilitated the fabrication of parts with thin walls and high depth-to-diameter aspect ratios.

Sandia has demonstrated parts that have 0.014-in.-diameter holes, are 1 in. tall and have an aspect ratio of more than 70:1 (Figure 2).

Figure 2.
The laser-engineered net-shaping process can build ultrathin parts more than 1 in. tall with depth-to-diameter aspect ratios up to 70:1. Conventional machining typically allows a maximum ratio of 10:1. Courtesy of Sandia National Laboratories.

Even with process commercialization, laboratory research continues. “We currently use a 1400-W Nd:YAG laser,” Gill said. “Other lasers have been used, though, including CO2. Diode lasers are expected to work well once they have enough power.” Current areas of research include true 3-D part building (builds are still 2.5-D, as with most rapid prototyping processes), precision repair and deposition, and the use of novel materials.

Making the industrial grade

Also of note is the direct metal laser melting process from Trumpf GmbH in Ditzingen, Germany. A variant of direct metal laser sintering technology from EOS GmbH in Munich, the process scans a laser across metal powder, melting and fusing layer after layer to build the part. As with the Sandia technology, the result is a complex part that retains the mechanical properties (i.e., density) of the original metal.

Unlike the EOS process, the work material is a one-component metal powder without any fluxing agents or binders. Materials used so far range from tool steel and stainless steel to light metals such as aluminum or titanium. The laser manufacturer, who expects to make commercial systems available by year’s end, is targeting tool and mold making, as well as lower-volume and one-off medical equipment manufacturing. The process can generate geometries impossible to produce with more conventional manufacturing techniques and provide flexibility to design and build special features into parts in one build, such as the addition of conformal cooling channels to an injection molding tool (Figure 3).

Figure 3. Based on a rapid prototyping process from EOS for applications such as rapid tooling (left), Trumpf’s direct metal laser melting technology can generate metal parts quickly with geometries impossible to produce with more conventional manufacturing processes (right).

The Trumpf focus is to mate the stringent requirements of the direct metal laser melting process with machine tool technology to make the technique economically viable for industrial manufacturing. Requirements of this “industrial grade” rapid prototyping process will most likely remain similar in some respects to that of the direct melt laser sintering technology.

Besides the recent introduction of new steel-based powders that allow precision builds of layers 20 μm thick, EOS has moved to a CO2 laser with more than 200 W of output and higher beam quality. The flat-field lens guiding the beam also has a coating redesigned to maximize transmission of the laser energy to the powder bed.

The company has enhanced its beam positioning system, which combines scanner heads with high-speed rotating mirrors driven by precision galvanometer scanners with temperature compensation and digital signal processing. To maintain good beam location under varying environmental conditions or high thermal loading from long build cycles, the positioning system includes active cooling of the mirrors as well as an integrated home-in sensor to detect and correct for scanner drift at regular intervals. Engineers also added a pneumatic lens-protection device to prevent dirt from settling on the lens surface, and boosted the efficiency of the beam calibration system.

Competition grows

With the entrance of Trumpf and Sandia technologies to the market, the competition among the laser-based systems continues to grow, and lasers used in some of the systems also face competition from other sources. For example, laser-engineered net shaping may also work with electron beam technology. At least within the rapid prototyping arena, other technologies — namely, the fused deposition modeling process from Stratasys Inc. in Eden Prairie, Minn. — also offer strong prototyping characteristics.

This competition extends into the realm of microstereolithography, one of the newest research fronts. Here, rastering laser systems will ultimately compete with layer-additive systems that cure a layer all at once by directing light onto the photopolymer through specially designed masks (Figure 4). In the case of the integral microstereolithography system developed at EPFL in Lausanne, Switzerland, curing is done with a broadband metal halide lamp.

Figure 4. Commercial microstereolithography technology will require work materials that can impart characteristics that allow components to act as functional parts. Courtesy of EPFL.

According to EPFL researcher Arnaud Bertsch, the holdup to commercializing the technology in the micro realm remains the work materials, including the resins. “As components are often very difficult to replicate by molding,” he said, “they must have sufficient mechanical properties, chemical resistance, etc., to be used as functional parts.”

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