Additive Manufacturing 101: Part III

By Terry Wohlers, President, Wohlers Associates

[Editor’s note: This is a continuation of a series started in the January/February 2010 issue of Time Compression that provides information on the various types of additive manufacturing processes.]

The "Wohlers" column is authored by Terry Wohlers for Time Compression.
This column was published in the May/June 2010 issue.

Additive manufacturing (AM) systems that build parts in photopolymer were among the first to emerge in the late 1980s. Stereolithography (SL) was the very first commercially available AM system and was considered the “gold standard” for many years.

SL technology uses one or more lasers to harden ultraviolet-sensitive liquid photopolymer layer upon layer. One type uses a vat filled with photopolymer. A build platform mounted on an elevator mechanism is positioned near the top of the vat when the first layer is constructed. Cross sectional data of the parts being built drives the position of the laser beam as it strikes the uncured resin, hardening it as it moves across the surface. After the first layer is complete, the build platform lowers the thickness of the layer and a fresh layer of liquid resin is spread across the first layer. This process repeats, one layer adhering to the next, until the build is finish. When the parts are complete, the platform is elevated and most of the uncured resin drains away.

The parts are manually removed from the build platform and cleaned with solvents to remove any remaining uncured resin. Protective gloves and lab coats are worn to prevent skin from touching the resin in its uncured state, which can be harmful. Also, a ventilation system is important. The cleaned parts are then placed in a UV light chamber where final curing occurs. After removing the parts from the light chamber, the support structures are manually removed from the parts. These structures are necessary to secure the parts to the build platform and support the build of overhanging features of the parts. The removal of the support structures can be time-consuming and require a level of skill, especially for parts that include fine and delicate features. If done improperly, the features of a part may be removed or broken inadvertently. The surfaces where the supports are attached are usually sanded smooth by hand. All of this requires labor and can potentially impact the dimensional accuracy and esthetics of a part.

Experienced modelmakers can produce a fine finish on an SL part by hand-sanding and polishing the surface. This, coupled with a good paint job, can make SL parts look indistinguishable from painted parts made with conventional methods of manufacturing. In fact, it is not unusual for SL parts to be used to prepare print and television ads before the production parts are available.

SL parts are also used extensively as master patterns for silicone rubber molds. The high-quality surface finish of an SL part results in a mold surface that is also of high quality. Rubber molds can be used to cast as many as 50 urethane parts that can then be used as prototypes or sometimes final products. Urethane materials are available in a wide variety of colors and durometers. Clear urethanes are used to produce prototypes of lenses for automotive and other products. 

As with most AM technologies, there are downsides to using SL. In addition to the labor involved in removing the support structures, the technology comes with some health risks. Working with and potentially coming into contact with uncured resin and cleaning solvents are considerations. SL equipment can be expensive to purchase and maintain, and it requires highly trained technicians, compared to some newer-generation methods of AM that also produce parts in photopolymer.

Another consideration of SL is the material. Some SL photopolymers do an excellent job at mimicking popular thermoplastics. This means that they can work well for making rigorous prototypes. However, the physical properties of photopolymers change and often decline over time. Sunlight, temperature, and humidity can impact the physical characteristics. Consequently, they are not stable long-term, unlike most industrial thermoplastics. This means that photopolymers may not be a good candidate for part manufacturing, especially when it is important for mechanical properties to remain stable over a product’s life.

3D Systems (3dsystems.com) was the first to commercialize SL in 1987 and continues to sell systems and materials. SL systems from the company range in price from about $185,000 to more than $800,000. Customers can expect to pay about $165 to $295 per kg ($363 to $649 per lb.) for photopolymer from suppliers including DSM Somos (dsm.com/en_US/html/dsms/home_dsmsomos.htm), Huntsman Advanced Materials (huntsman.com/advanced_materials), and 3D Systems. 

The Japanese have a long history in developing and commercializing stereolithography. In fact, one Japanese company, at the time called NTT Data CMET, began to sell and ship SL systems in 1988, the same year 3D Systems sold and shipped its first, non-beta systems. Sony/D-MEC manufactured and sold systems the following year and six additional Japanese companies produced and sold SL systems after that. Most of these companies have since stopped its SL manufacturing and sales or have had little commercial impact. 

The only SL manufacturer that competes with 3D Systems in Japan is CMET (cmet.co.jp/en/company), a company that sold 20 systems in 2008. Most of the systems available from it and the other Japanese manufacturers have been sold and installed in Japan. Sony Manufacturing Systems sold systems in the U.S.—the only Japanese company to do so. It ended its sales activity in May 2006 after selling only four systems over a three-year period.

Some of the low-end and mid-range SL systems from Japan do not work the same as those from 3D Systems. Rather than the parts resting on top of a build platform, with the bottom-most layer being made first, the parts are attached from beneath a build platform. After structures are produced that attach the parts to the underneath side of the platform, the machine builds the top-most layers of the parts first and finishes with the bottom-most layers of the parts. The laser is directed upward from the bottom of the machine.

DWS is an Italian company that has manufactured and sold SL systems since 2005. The company uses a build approach similar to some of the low-end and mid-range SL systems from Japan. The build volume of the machines are relatively small, with the jewelry industry as the company’s primary market. The systems from DWS range in price from 16,000 euros to 82,000 euros.

Chinese companies have also developed and sold SL systems, almost entirely within China. The companies include Shaanxi Hengtong Intelligent Machine Co., Ltd., Shanghai Union Technology Co., Ltd., Wuhan Binhu Mechanical & Electrical Co., Ltd., and Guangzhou Electronic Technology Co. Ltd. Combined, these companies have sold fewer than an estimated 400 systems total over a period of about 12 years. The list prices of the systems are $110,000 to $260,000, although the systems often sell for half the list price.

The Jetting of Photopolymer

Israeli-based Objet Geometries (objet.com) began to manufacture and sell its PolyJet systems in 2001. The systems use inkjet print heads to deposit liquid photopolymer, layer by layer. A UV lamp is a part of the print mechanism and cures the layers of photopolymer as they are being deposited. The machines are capable of producing layers as thin as 16 microns (0.0006 in.), which results in fine features and a surface finish that is arguably the best in the industry. In comparison, SL creates layers in the range of 0.025 to 0.25 mm (0.001 to 0.010 in.), although this can vary from machine to machine. A layer thickness of 0.1 mm (0.004 in.) on an SL machine is typical.

PolyJet systems use a special material that resembles a relatively soft wax to support the parts and their features as they are being built. Most customers use a high-pressure water-jet system to remove this wax-like support material; the hand-sanding typically used for smoothing the artifacts left by the support structures on SL parts is not necessary with PolyJet parts. Deep holes and slots require additional jetting to remove the PolyJet support material, but it is generally easier and less time-consuming than removing SL supports.

Objet offers a wide range of materials. However, they do not match the number and types of materials available for stereolithography. Objet has been developing its materials for about 10 years, whereas SL material suppliers have been at it for more than 23 years. Competition among 3D Systems, Huntsman Advanced Materials, DSM Somos, and others over the years has created some impressive materials.

A development that sets Objet apart is the “digital materials” with its Connex machines. These materials are made possible by simultaneously jetting two model materials, resulting in a new material based on the amount and kind of each material deposited. This method, which is unique to the Connex machines, makes it possible to build parts that would be difficult or impossible to make any other way. These materials can simulate the materials of overmolded parts that contain two materials. For example, a Connex machine can produce a rigid white cell phone housing and rubbery black buttons within the same build. This opens up many possibilities for modeling and prototyping products with rubbery grips, such as toothbrushes and razors, that are typically overmolded and difficult to prototype. 

Except for other SL machines, PolyJet has presented the only serious threat to photopolymer parts from mid-range and high-end SL systems. A number of companies, especially service providers, have purchased PolyJet systems. In fact, PolyJet systems represented only 3% of the installed base among service providers in 2008, but they accounted for 12% of the systems added by service providers the same year. The systems from Objet range in price from $40,000 to $250,000.

3D Systems also offers systems that use inkjet printing technology and UV light to deposit and harden photopolymer. The company’s first product of this type was introduced in 1996 with the Actua system. The company has since introduced the ThermoJet, InVision, and now ProJet. The ThermoJet system gained some popularity among a few companies for the production of wax patterns for investment castings. The product was instead positioned as a machine for producing early concept models. The ProJet systems, which have replaced all of the others, sell for about $60,000 to $140,000.

Flash Imaging of Layers

The Perfactory systems from Envisiontec (envisiontec.de) use photopolymer and digital light processing (DLP) technology from Texas Instruments to harden an entire layer at once. The image of a cross section is projected from below onto the bottom surface of a build platform. The parts attach to the platform similarly to some of the low-end and mid-range SL systems from Japan and the SL systems from DWS. The primary difference is that Perfactory systems image an entire layer at once, making the process faster. Also, the Perfactory systems are capable of producing very fine and delicate features and an excellent surface finish. These Perfactory systems, which are developed and manufactured in Germany, are priced from 40,000 euros to about 75,000 euros.

Envisiontec offers much larger systems that are developed and manufactured in the U.S. Unlike the company’s Perfactory systems, their vat system resembles traditional stereolithography machines, with the build platform moving downward into a vat of liquid photopolymer with each layer. The primary advantage to these systems is the speed of the DLP technology, which images an entire layer with a flash. These systems range in price from $185,000 to $310,000.

V-Flash, another photopolymer-based system from 3D Systems, was introduced in mid-2009 after the company initially rolled it out in 2007. The machine uses film transfer imaging technology and operates in an inverted fashion similar to Envisiontec’s small Perfactory systems. A transparent film with a thin layer of photopolymer is pulled across the exposure area, the underneath side of the build platform lowers to the surface, and a full layer is imaged using a deformable mirror array. The platform rises, the transparent film is rewound into the material cartridge for recoating of photopolymer, and the process repeats. The system is priced at $9,900.

A Class of its Own

Z Corp. (zcorp.com) offers systems that jet a liquid binder onto the surface of composite powder. The material is neither a photopolymer nor thermoplastic. In fact, it consists mostly of plaster. The parts being produced are fully embedded in the powder when the build process is finished, similar to laser sintering. The powder surrounding the parts serves as the support material, so support structures are not attached to the parts. The fine powder, however, must be removed and cleaned from the parts. The newest systems from Z Corp. automatically remove up to about 80% of the loose powder after a build is complete. The part or parts are then manually moved to an adjacent powder-recycling chamber and the remaining powder is removed with an integrated, pen-sized airbrush. 

The parts straight out of the machine are not particularly strong. Most parts are infiltrated with a resin, such as epoxy or cyanoacrylate, and then left to dry. This step produces parts that can be handled and shipped. The powder-recycling chamber of the newest machines from Z Corp. can then be transformed into a station for part infiltration, eliminating the need for a post-processing apparatus or another work area.

The primary advantages to the machines from Z Corp. are speed and multiple colors. The build speed of the machines are believed to be the fastest in the industry. Also, they are the only systems that can produce multiple colors within the same part. In fact, it is possible to wrap JPG images onto the surface of an STL file and print them. Color is especially important when printing video game avatars, 3D maps from satellite imagery, detailed architectural structures, and industrial parts that require color to convey important information. The systems from Z Corp. range in price from about $20,000 to $60,000.

Voxeljet Technology GmbH of Germany (voxeljet.de) offers large powder-based systems that use 3D printing technology originally commercialized by Z Corp. Several years ago, Voxeljet purchased a nonexclusive patent sublicense from Z Corp. As part of the sublicensing agreement, the Voxeljet systems can be sold for making casting patterns and models. The Voxeljet focus is to deliver large-capacity, high-throughput systems for industrial manufacturing environments, including the option of multiple semi-palletized build chambers. The systems from Voxeljet are priced at about 270,000 euros to 530,000 euros.

This concludes the AM systems that used photopolymer materials, as well as the systems from Z Corp. and Voxeljet Technology. Part IV will discuss the systems that build parts in metal.