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Rapid Prototyping: Ink Jet Printing
(By C. Kaan Senol)
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The Ink Jet Printing technology is also sometimes called Ballistic Particle Manufacturing. Other systems providers use considerably different techniques, but they all rely on squirting a build material in a liquid or melted state which cools or otherwise hardens to form a solid on impact. One example of the technology variations available in these so-called phase change inkjets is provided by 3D Systems. This company produces an inkjet machine, called the ThermoJet Modeler (formerly Actua), based on technology from Spectra, Inc. which utilizes several hundred nozzles.

By contrast, the Solidscape machine uses a single jet each for build and support materials, and it serves as an introduction here. Plastic object, wax and support materials, are held in a melted liquid state at elevated temperature in reservoirs (A). The liquids are fed to individual jetting heads (B) through thermally insulated tubing. The jetting heads squirt tiny droplets of the materials as they are moved side by side in the required geometry to form the layer of the object. The heads are controlled and only place droplets where they are required to. The materials harden by rapidly dropping in temperature as they are deposited.

After an entire layer of the object is formed by jetting, a milling head (C) is passed over the layer to make it a uniform thickness. Particles are vacuumed away as the milling head cuts and are captured in a filter (D).

The operation of the nozzles is checked after a layer has been fabricated by depositing a line of each material on a narrow strip of paper and reading the result optically (E). If all is well, the elevator table (F) is moved down a layer thickness and the next layer is begun. If a clog is detected, a jetting head cleaning cycle is carried out. If the clog is cleared, the problem layers are milled off and then repeated.

After the object is completed, the wax support material is either melted or dissolved away. The Solidscape system is capable of producing fine finishes, but to do so results in slow operation. Thus, there is a tradeoff between fabrication time and the amount of hand finishing required.

Fused Deposition Modeling

Figure 1 is a schematic of this method. A plastic filament, approximately 1/16 inch in diameter, is unwound from a coil (A) and supplies material to an extrusion nozzle (B). The nozzle is heated to melt the plastic and has a mechanism which allows the flow of the melted plastic to be controlled. The nozzle is mounted to a mechanical stage (C) which can be moved in horizontal and vertical directions.

As the nozzle is moved over the table (D) in the required geometry, it deposits a thin bead of extruded plastic to form each layer. The plastic hardens immediately after being squirted from the nozzle and bonds to the layer below. The entire system is contained within an oven chamber which is held at a temperature just below the melting point of the plastic. Thus, only a small amount of additional thermal energy needs to be supplied by the extrusion nozzle to cause the plastic to melt. This provides much better control of the process.

Support structures must be designed and fabricated for any overhanging geometries and are later removed in secondary operations. Several materials are available for the process including a nylon-like polymer and both machinable and investment casting waxes. The introduction of ABS plastic material has led to greater commercial acceptance of the method. It provides better layer to layer bonding than previous materials and also has a companion support material which is easily removable by simply breaking it away from the object.

Laminated Object Manufacturing

Figure 2 presents a schematic of this method as implemented in systems sold by Helisys. Profiles of object cross sections are cut from butcher paper using a laser. The paper is unwound from a feed roll (A) onto the stack and bonded to the previous layer using a heated roller (B). The roller melts a plastic coating on the bottom side of the paper to create the bond. The profiles are traced by an optics system that is mounted to an X-Y stage (C). The process generates considerable smoke. Either a chimney or a charcoal filtration system is required (E) and the build chamber must be sealed.

After cutting the geometric features of a layer is completed, the excess paper is cut away to separate the layer from the web. The extra paper of the web is wound on a take-up roll (D). The method is self-supporting for overhangs and undercuts. Areas of cross sections which are to be removed in the final object are heavily cross-hatched with the laser to facilitate removal. It can be time consuming to remove extra material for some geometries.

The finish and accuracy are not as good as with some methods, however objects have the look and feel of wood and can be worked and finished in the same manner.

Variations on this method have been developed by many companies and research groups. Kira's Paper Lamination Technology (PLT) uses a knife to cut each layer instead of a laser and applies adhesive to bond layers using the xerographic process. Other variations include Thick Layer Lamination from Stratoconception of France, Precision Stratiform Machining from Ford Research, and Adaptive-Layer Lamination developed by Landfoam Topographics. These are hybrids of additive and subtractive CNC technologies which seek to increase speed and material versatility by cutting the edges of thick layers to avoid stair stepping.

Solid Ground Curing

The early versions of the system weighed several tons and required a sealed room. Size has been decreased and the system has been sealed to prevent exposure to photopolymers, but it's still very large. Please see the discussion on stereolithography for a description of photopolymers.

Instead of using a laser to expose and harden photopolymer element by element within a layer as is done in stereolithography, SGC uses a mask to expose the entire object layer at once with a burst of intense UV light. The method of generating the masks is based on electrophotography (xerography).

This is a two cycle process having a mask generation cycle and a layer fabrication cycle. It takes about 2 minutes to complete all operations to make a layer:

1. First the object under construction (A) is given a coating of photopolymer resin as it passes the resin applicator station (B) on its way to the exposure cell (C).

2. A mask is generated by electrostatically transferring toner in the required object cross sectional image pattern to a glass plate (D) An electron gun writes a charge pattern on the plate which is developed with toner. The glass plate then moves to the exposure cell where it is positioned above the object under construction.

3. A shutter is opened allowing the exposure light to pass through the mask and quickly cure the photopolymer layer in the required pattern. Because the light is so intense the layer is fully cured and no secondary curing operation is necessary as is the case with stereolithography.

4. The mask and object under fabrication then part company. The glass mask is cleaned of toner and discharged. A new mask is electrophotographically generated on the plate to repeat the cycle.

5. The object moves to the aerodynamic wiper (E) where any resin that wasn't hardened is vacuumed off and discarded.

6. It then passes under a wax applicator (F) where the voids created by the removal of the unhardened resin are filled with wax. The wax is hardened by moving the object to the cooling station (G) where a cold plate is pressed against it.

7. The final step involves running the object under the milling head (H). Both the wax and photopolymer are milled to a uniform thickness and the cycle is repeated until the object is completely formed within a wax matrix.

Secondary operations are required to remove the wax. It can either be melted away or dissolved using.

Stereolithography

The implementation shown in figure 3 is used by 3D Systems and some foreign manufacturers. A moveable table, or elevator (A), initially is placed at a position just below the surface of a vat (B) filled with liquid photopolymer resin (C). This material has the property that when light of the correct color strikes it, it turns from a liquid to a solid. The most common photopolymer materials used require an ultraviolet light, but resins that work with visible light are also utilized. The system is sealed to prevent the escape of fumes from the resin.

A laser beam is moved over the surface of the liquid photopolymer to trace the geometry of the cross-section of the object. This causes the liquid to harden in areas where the laser strikes. The laser beam is moved in the X-Y directions by a scanner system (D). These are fast and highly controllable motors which drive mirrors and are guided by information from the CAD data.

The exact pattern that the laser traces is a combination of the information contained in the CAD system that describes the geometry of the object, and information from the rapid prototyping application software that optimizes the faithfulness of the fabricated object. Of course, application software for every method of rapid prototyping modifies the CAD data in one way or another to provide for operation of the machinery and to compensate for shortcomings.

After the layer is completely traced and for the most part hardened by the laser beam, the table is lowered into the vat a distance equal to the thickness of a layer. The resin is generally quite viscous, however. To speed this process of recoating, early stereolithography systems drew a knife edge (E) over the surface to smooth it. More recently pump-driven recoating systems have been utilized. The tracing and recoating steps are repeated until the object is completely fabricated and sits on the table within the vat.

Some geometries of objects have overhangs or undercuts. These must be supported during the fabrication process. The support structures are either manually or automatically designed.

Upon completion of the fabrication process, the object is elevated from the vat and allowed to drain. Excess resin is swabbed manually from the surfaces. The object is often given a final cure by bathing it in intense light in a box resembling an oven called a Post-Curing Apparatus (PCA). Some resins and types of stereolithography equipment don't require this operation, however.

After final cure, supports are cut off the object and surfaces are sanded or otherwise finished.

Stereolithography generally is considered to provide the greatest accuracy and best surface finish of any rapid prototyping technology. Work continues to provide materials that have wider and more directly useable mechanical properties.

Selective Laser Sintering

The process is somewhat similar to stereolithography in principle as can be seen from figure 4. In this case, however, a laser beam is traced over the surface of a tightly compacted powder made of thermoplastic material (A). The powder is spread by a roller (B) over the surface of a build cylinder (C). A piston (D) moves down one object layer thickness to accommodate the layer of powder.

The powder supply system (E) is similar in function to the build cylinder. It also comprises a cylinder and piston. In this case the piston moves upward incrementally to supply powder for the process.

Heat from the laser melts the powder where it strikes under guidance of the scanner system (F). The CO2 laser used provides a concentrated infrared heating beam. The entire fabrication chamber is sealed and maintained at a temperature just below the melting point of the plastic powder. Thus, heat from the laser need only elevate the temperature slightly to cause sintering, greatly speeding the process. A nitrogen atmosphere is also maintained in the fabrication chamber which prevents the possibility of explosion in the handling of large quantities of powder.

After the object is fully formed, the piston is raised to elevate the object. Excess powder is simply brushed away and final manual finishing may be carried out. No supports are required with this method since overhangs and undercuts are supported by the solid powder bed. This saves some finishing time compared to stereolithography. However, surface finishes are not as good and this may increase the time. No final curing is required as in stereolithography, but since the objects are sintered they are porous. Depending on the application, it may be necessary to infiltrate the object with another material to improve mechanical characteristics. Much progress has been made over the years in improving surface finish and pororsity. The method has also been extended to provide direct fabrication of metal and ceramic objects and tools.