Rapid Prototyping Roundup

DN Staff

February 16, 1998

9 Min Read
Rapid Prototyping Roundup

Rapid prototyping machines convert the CAD file which defines a part into a solid three-dimensional object that the designer can touch, hold, and show to colleagues. The first stereolithographhy (SLA) machines, pioneered by 3D Systems, (Valencia, CA), built up prototype parts from successive layers of liquid resin.

Since the introduction of SLA in the late '80s, many companies have entered the field. They are developing ever-improving materials and faster processes. Prices are also coming down. The following examples reflect some of the most recent advances in rapid prototyping and its application.

UV light, a photomask, and wax

Ranaana, Israel--Solid Ground Curing (SGC), the technique behind Cubital's Solider machine, involves shining ultraviolet light through a photomask to harden successive layers of resin. Here's how it works:

The system creates a photomask from layered CAD data using electrostatic toner on an electrostatically charged plate. The operator aligns the photomask with the build platform on which a layer of liquid resin has been spread.

Applying a burst of UV light exposes and hardens those areas of resin which the photomask leaves clear, while the remaining liquid resin is vacuumed off. Cavities are filled with molten wax; once the wax hardens, the entire surface is precisely milled to give a defined layer thickness and smooth surface finish.

While the first layer is being constructed, the system cleans the electrostatically charged plate and prepares a photomask for the next layer. Build time per layer? About 70 seconds. Largest working envelope? 500 x 350 x 500 mm.

Ford Motor Company reports developing a new air intake manifold for a 4.6-l V-8 engine with the aid of SGC. The company claims a cost savings of 33% compared to less accurate manual methods; time savings totaled four months or more.

An emphasis on speed

Banbury, UK--Based on technology acquired from IBM, the new L51,000 Genisys machine from Stratasys focuses on high speed. Build material is a plastic polymer, initially supplied as a small solid wafer.

Delivered one-by-one into a liquefier, the wafers form a reservoir of liquid polyethylene. An Archimedes screw drives the liquid into an extrusion nozzle for deposition onto the build table. Simultaneous nozzle motion in the X-axis and platform movement in the Y-axis helps optimize cycle times.

Using Genisys, prototypes measuring up to 203 x 203 x 203 mm can be produced to an accuracy within 0.0356 mm. The machine has sufficient capacity, once fully loaded with polymer wafers, to build a model weighing 2.3 kg.

Michael Turner of Laser Lines, the company that markets the Stratasys range in the UK, says Stratasys AutoGen software automatically prepares a user's CAD file for the Genisys machine, making the system quick and simple to operate. Turner adds that Genisys complements the company's Fused Deposition Modelling (FDM) technology, which emphasizes accuracy and surface finish, rather than speed.

FDM is similar to Genisys technology though it works with a polymer filament. Parts measuring 254 x 254 3 254 mm, accurate to (plusMinus)0.127 mm, are possible. A larger, FDM 8000 has also been introduced for dimensions to 457 x 457 x 609 mm.

From CAD to sand

Munich, Germany--Cylinder heads are extremely difficult to prototype due to their complex internal channels. A rapid prototyping system that makes models directly from sand changes the situation. Developed by EOS GmbH, the EOSINT S has already earned praise from car companies across Europe.

EOS sand sintering machines let engineers go direct from CAD drawings to a sand core or mold without tooling. As the EOSINT S scans each cross-section of the geometry, its laser system builds the part in thin layers by locally binding foundry sand. Once the part is complete and has been through a post-process operation to cure the sand, it can go straight to a foundry to be cast in metal.

"While it is possible to make prototypes of single cylinder heads using investment casting and existing rapid prototyping technologies," says Chris Ryall, operations manager at the Rapid Prototyping & Tooling Centre, a joint facility between Rover and Warwick Manufacturing Group, "multi-cylinder heads are a bigger challenge."

Ryall explains that complexity of the internal water cores makes it difficult to create a consistent ceramic coating of the prototype's internal geometry. In addition, he points out that it is difficult to remove the ceramic shell from inside the head once the prototype has been burned out and an investment casting made.

Using the EOSINT S700, Ryall claims he can build a "ready-for-casting" three-cylinder-head mold in 52 hours--approximately 1/6th the time and 1/7th the cost of conventional tool making techniques.

Toner plus paper=prototype parts

Hazu-gun, Japan--Essentially a 3-D photocopier, the KIRA Solid Center rapid prototyping machine automatically converts a 3-D computer model into a solid part using sheets of ordinary paper. As with a photocopier, the KIRA machine takes one sheet of paper at a time and applies toner to a selected area of the paper. This area corresponds to one particular cross-section or layer of the model being built.

Printed sheets assemble one on top of the other. Pressed together at a temperature which melts the toner resin powder, they form the solid model. The toner, therefore, acts as the adhesive.

Parts made from KIRA Solid Center have the consistency of wood, and can be used as an investment die casting pattern. Maximum model size: 400 x 280 x 300 mm.

A more productive ModelMaker

Wilton, New Hampshire--ModelMaker II, an upgraded and larger version of the MM6-Pro, builds parts three to five times faster than its predecessor. Developed by Sanders Prototype Inc., ModelMaker II incorporates an air conditioning system for quicker cooling cycles, and faster servomotor speeds for the x-y plotter.

Like the MM6-Pro, MMII uses a patented ink-jet process to build up models layer-after-layer. Two jets are involved. One jet deposits droplets of a green thermoplastic material; the other deposits droplets of a red wax material.

The actual model is constructed from thermoplastic while all supports for overhanging parts and for cavities inside the model are made from the wax. After deposition of each layer, the top surface is milled to remove excess height.

Immersion in a solvent bath dissolves the wax elements once the model is complete. The resulting part requires no finishing and can be directly investment cast. Maximum model size is 15.24 x 30.48 x 22.86 mm--three times the build volume of the previous generation machine.

One platform, many materials

Hilden, Germany--The unique feature of DTM's selective laser sintering (SLS) technology is that a single sinterstation can process a variety of plastic, metal, and sand powders, depending on the user's need at the time. Thus, by purchasing just one platform, a company can make sand casting cores, conceptual models, functional prototypes, patterns for secondary processes, and metal mold inserts.

During operation, SLS builds up a part layer-on-layer, using a heat-generating CO2 laser to selectively "draw" each cross-section on a fine layer of powder. The laser causes the particles to melt and fuse, forming a solid mass. Once the part is complete, any loose powder simply falls away.

A choice of 11 different powder materials is presently available for use with the SLS process. The newest material available is Somos 201, an elastomeric polymer allowing flexible parts with rubber-like characteristics. Interest in the Dupont-developed material comes from the automotive, medical, sports equipment, and toy markets.

An additional benefit of Somos 201: resistance to elevated temperatures and harsh chemicals such as gasoline or automotive coolants.

Plastic sheet for LOM

Birmingham, UK--Laminated Object Manufacturing (LOM) is usually associated with prototypes made from layers of paper. Now, Helisys has introduced a plastic sheeting for use with its existing range of LOM machines.

In principle, LOM machines use a laser to cut successive layers of a part from a coated sheet material, and heat to bond the layers together. With LXP 050 LOMPlastic sheeting, LOM users can produce either paper or plastic prototypes, or a mixture. Changeover from one material to the other requires no mechanical adjustments, says Simon Graham, specialist engineer at Umak, the company responsible for UK sales.

The new material comprises a polyester film coated with a proprietary polyethylene co-polymer adhesive. Sheet thickness is 0.125 mm. Compared to models made with paper, Graham claims parts made from the plastic sheet have better moisture resistance, greater flexibility, and a higher mechanical strength.

Better moisture resistance means that parts can be used as patterns, masters, and molds for regular tooling and liquid-based secondary processes. In one application, LOMPlastic was used to make a water pump impeller measuring 300 mm in diameter and 100 mm in depth. The LOM model was run at 1,200 rpm for a day, then stripped and examined. As a result, it was possible to identify where cavitation had occurred on the impeller and to carry out the necessary design modifications.

Further new materials for the LOM machine are expected in the near future. Graham anticipates the next development will be a composite sheet made from randomly woven glass fiber. The bonding agent will be a thermoset glue to give improved temperature resistance. "Prototypes will be able to withstand higher surface temperatures making them suitable for vacuum casting processes," he predicts.

3-D Printing

Valencia, California--Pioneer of rapid prototyping, 3D Systems has broadened the range of technologies it offers. Following the success of its early stereolithography (SLA) machines, the company has now developed a complementary 3-D printing process which avoids the need for costly and complex laser equipment. It can be located in an office environment and is as easy to use as a printer, the company claims.

Known as the Actua 2100, 3D Systems' new 3-D "printing" machine employs multi-jet modeling (MJM) technology to construct models in successive layers. Its "print" head comprises 96 jets oriented in a linear array.

Activation of individual jets instantaneously dispenses a special thermopolymer material on demand. The raster action of the building process means that a complex model takes no longer to build than a simple model of similar overall size. Maximum model dimensions are 254 x 203 x 203 mm.

Intended for quick look-and-see models, the Actua 2100 aids concept visualization and communication before releasing the design data to other rapid prototyping tools.

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