Machine Design

Rapid Prototyping Hits Light Speed

Believe it or not, flashing light on photopolymers lets one RP machine build 50 parts in the time conventional machines build one.

Efrem V. Fudim
Light Sculpting Inc.
Milwaukee, Wis.

A few years back stereolithography manufacturers set out to end the agonizingly long process of building machined prototypes or wooden mockups. Stereolithography machines jolted engineering departments by slashing months, if not years, from their design cycles. Since the advent of SL, several manufacturers have produced rapid-prototyping (RP) machines that also harden material one point at a time. One machine now available aims to turn conventional RP on its ear.

The new quick-layering, or QL, technology is exemplified by the LSI1212 from Light Sculpting Inc. It hardens entire layers of polymer at once, making high-resolution parts at speeds unattainable by other RP machines. Build time is just one critical factor for rapid prototyping. Others include machine price, and part strength and resolution. Although several RP designs may score in one or two of these categories, few machines rate well in all of them. What’s more, most rapid-prototyping systems cannot be used as production machines due to such drawbacks as low productivity, high cost, or weak parts.

QL technology, however, clears conventional hurdles by quickly building strong parts that often require no lengthy postprocessing. Parts only need washing after they’re built. The machine builds complex parts directly from their cross-sectional images by shining light on liquid photopolymer. Although this sounds similar to other RP methods, QL differs because it hardens entire layers at once, instead of the point-by-point process. This builds parts fast and inexpensively. The machine can build individual parts up to 8 3 10 3 12 in. and can just as easily build multiple smaller parts, regardless of complexity.

QL builds parts on an open platform. In contrast, other machines progressively lower parts into a vat of polymer to cover each new layer with raw material. Building on an open platform, however, lets designers access parts as the machine builds them. This allows on-line cleaning or lets users insert components, such as electronics or integrated supports that do not require removal. QL machines can be built with options to change materials as parts build. For instance, the machine could add glass or other powders to increase part strength.

Unsolidified material on the face of each layer hardens after contacting adjacent layers. This forms parts as one large molecule, rather than parts with multiple layers that might split apart. Machines could use multiple nozzles to deposit several materials within each layer. This lets engineers design multimaterial objects, such as devices with flexible diaphragms or aircraft-instrument panels with optoelectromchanical assemblies.

The light method
The system consists of an industrial printer, a punching unit, model-slicing software, and the LSI1212. After engineers design a part, system software slices the model. The slicing software analyzes the part geometry and varies layer thicknesses depending on geometry and required accuracy. For example, the software cuts thinner layers in part regions with shallow angles. This reduces “stair stepping” and improves surface smoothness. In steep or vertical regions, on the other hand, the software maximizes layer thickness to decrease build time. The software then outputs an image file for printing masks and a machine control file specifying thicknesses for each layer.

The printer uses transparencies to print masks. Printed areas correspond to voids in each cross section and transparent areas correspond to part cross sections. The standard printer has 300:1 contrast ratio and 600-dpi resolution with options for higher contrast ratios or resolutions. Contrast ratio measures how clearly the printer defines borders between printed and nonprinted areas. Higher contrast ratios let the machine build smaller parts because the borders are clearer.

A punching unit cuts holes in the printed masks and loads them in a stack. The printer applies alignment marks on each mask to ensure they are properly stacked in the machine. The masks are printed off-line before the machine builds actual parts. As a result, the machine is not burdened with processing images or calculating slice thicknesses. It performs simple motions and can build parts with the mask resolution. With an upgraded printer, resolution can jump to 1,200 dpi.

Users load the machine control file to an on-board industrial computer and place the stacked masks in the machine. The fully automated QL machine builds parts using forced deposition, in which a wide nozzle applies a thin layer of liquid polymer onto the underside of a separating film. Light shines through the mask for that layer onto the film of polymer. This irradiates the raw polymer. Liquid solidifies behind transparent areas and remains viscous behind printed areas, forming part layers. The layers require no postcure.

A thin film of liquid polymer remains on each side of the hardened polymer, which bonds part layers together. Air prevents hardening on one side of the layer and a separating material between the mask and polymer prevents hardening on the other side. The separating material also keeps the masks clean, dry, and reusable. The separating material easily detaches from the mask by sliding or peeling. The film of liquid polymer allows intercrosslinking between layers. In other words, layers form molecular bonds, effectively building single-molecule high-strength parts.

The software adds support structures to model features that overhang. The software also adds hatching to strengthen outer walls of hollow parts. If designers plan postprocessing, such as sanding or machining, they add a skin to allow for material removal. Engineers can also scale parts to compensate for errors, such as printer inaccuracy or shrinkage in the photopolymer and molding material. The software also prompts for options to rotate part orientations in the build envelope, build negative part shapes for mold cavities, or fill the platform with multiple parts.

High-speed building
The new machine provides high resolution and speed at low cost by irradiating entire layers at once. This makes QL suitable for volume production, especially of smaller, complex parts that may need periodic upgrades. Designing the system with separate components that provide imaging and energy actually simplifies part production.

The unit’s printer uses conventional laser toner. Industrial high-contrast printers typically cost less than 1% of many single-scanner RP systems and print much faster. Although standard printers for the new prototyping technology have either 600 or 1,200-dpi resolutions, printing images off-line also allows using photoplotters with resolutions over 9,600 dpi (3 µ) and 3,000:1 contrast ratio. But even 600 dpi is over eight times higher than the 67 dpi possible from SL machines with upgraded lasers.

Simultaneous irradiation also reduces energy consumption and requires low-cost radiation sources. Hardening materials point by point dissipates much more energy than hardening whole layers at once. Furthermore, oxygen inhibits polymer hardening so machines that build parts in air, such as SL units, require substantial energy to overcome the effects of oxygen.

Quick-layering technology, however, uses “contact irradiation” by spreading polymer on transparent material that separates the polymer from air. The short distance between the light source and polymer eliminates the need for collimated radiation, which is a light that follows a straight path instead of diverging. Lasers, for instance, provide collimated radiation. The QL machine can instead flood irradiation with less-expensive fluorescent or mercury bulbs and harden layers in a few seconds. This requires about 1% of the energy consumed by other RP machines that need accurate, powerful lasers. One clever idea for QL technology suggests replacing masks with on-line direct electronic imaging through liquid-crystal or similar multipixel displays.

The new layering method is in stark contrast with conventional techniques. For instance, most rapid prototyping machines use a scanning element, such as a laser beam, to harden raw part material or a small nozzle to apply polymers. Although customers can upgrade a machine’s scanning element to build parts faster, the most advanced lasers can still take hundreds of hours to build complex parts. This is because conventional RP machines harden part layers one point at a time. After tracing all points that comprise one layer, the platform lowers. This covers the newly hardened material with raw material and the scanning element begins hardening the next layer.

The size of the scanning element creates an inverse relationship between two of RP’s fundamental parameters, resolution (quality of imaging), and build speed. Smaller scanning elements increase resolution but lengthen build times. This also requires more energy to maintain the original speed.

The more-recent technology avoids this limitation and quickens part production by treating the entire layer at one time. For example, an 8 3 10-in. layer of any content (filled with individual or multiple parts) hardens with a 2 to 3-sec flash of light so each layer takes about 20 sec to complete. This allows building eighty 1.2 3 7 3 0.3-in. parts in 2 min, regardless of complexity. The same parts could take about 15 hr each to produce on CNC equipment. QL machines can fill an 8 3 10 3 5-in. envelope with 600 dpi or higher resolution in 51⁄2 hr. SL machines (which are typically at least three times more expensive than QL devices) can be upgraded with 75-W lasers to produce parts with 67 dpi. It would take over 190 hr for such a machine to fill an 8 3 10 3 5-in. envelope.

Furthermore, material for QL machines is relatively inexpensive. Simultaneous low-energy irradiation and forced deposition do not require polymers with low viscosity or other special properties. Basic raw material costs of about $200/gallon are one-half to one-quarter the material cost for laser-scanned systems. The QL polymer provides 165,000-psi flexural modulus with 65 Shore D hardness. Designers can add a variety of fillers to improve mechanical, electromagnetic, or optical properties.

Open-platform building helps reduce material expenses. SL machines, in contrast, require a tank filled with raw polymer worth up to $30,000. Because open-platform machines do not need a tank, each part only requires enough material to fill the part volume.

Machines with larger build envelopes present no serious design challenges. The LSI1212 can be supplied with 12 3 12 3 12-in. envelopes, up to 50% faster build times, or 1,200-dpi printers. The basic 8 3 10 3 12-in.-envelope QL machine, however, meets demanding part-production requirements.

For example, a recent job required 1.25-in.-diameter 3 0.5-in.-wide gears with 0.01-in.-thick layers. The machine built 42 of these gears in 16 min and 40 sec. If a company ran its machine for 4,000 hr/yr it could build over 604,000 gears, which would require about 1,200 gallons of polymer. What’s more, the QL machine could build over 4 million 0.5-in.-diameter 3 0.5-in.-wide gears in the same amount of time.

A recent RP-machine comparison study demonstrates the build-speed differences between QL and other RP methods. Although QL technologies were not tested, the study timed several machines as they built speedometer parts that comprised about twice the volume of the QL machine’s 1.2530.5-in. gears. Extrapolating the results shows the fastest SL machine building 845 speedometers (similar to about 1,690 gears) in a 4,000-hr year. This falls far below QL’s 604,000 gears.

1 The source begins on the right.
2 The platen, with the stack of masks, moves up and the top mask attaches to the plate.
3 The platen drops and the light source with attached mask moves left.
4 At the same time, a nozzle deposits a layer of liquid polymer onto the underside of the film.
5 The platform moves up to a distance from the film equal to the layer thickness.
6 Light irradiates the polymer for 2 to 3 sec, hardening it and adhering the layer to the platform.
7 The platform drops, holding the newly formed layer, which detaches from the separating material.
8 The source moves to its original position, drops the mask if the next layer has a different geometry, and the process repeats.
9 If the next layer has the same geometry, the source holds the same mask and the process repeats.

© 2010 Penton Media, Inc.

TAGS: 3D Printing
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