As with any rapid prototyping (RP) technology, stereolithography’s (SL) versatility is dependent on both equipment capabilities and material offerings. In general, the broader the material offering, the broader the application range for that technology.

3D Systems, the only maker of SL equipment in North America and Europe, has been improving on SL capabilities by building machines with bigger build envelopes and better feature resolution. Their latest machine, the Viper Pro SLA, has a build envelope of 74cmx64cmx53cm and was designed with rapid manufacturing in mind.

Many engineers, however, believe that even more important than the equipment used is the SL resin selected-which will ultimately determine how the plastic part can be handled, tested and, in a growing number of cases, directly used.

Over the years, an ever-increasing range of SL resins has been developed by multiple suppliers, including those with physical properties mimicking engineering thermoplastics such as polypropylene, ABS and polycarbonate. These have allowed SL to expand from primarily the creation of concept models and master patterns to that of functional or fit/assembly models, even replacing cast urethanes or injection molded parts for low volume applications.

It is well known in the RP industry that when comparing stereolithography with selective laser sintering (SLS) and fused deposition modelling (FDM) methods, SL produces the most accurate parts with the best surface finish. However, for end-uses requiring high durability, SLS and FDM methods have so far been favoured over SL due to their use of engineering plastics, which tend to be tougher than SL resins.

Recently, however, with the introduction of highly durable SL resins such as Somos DMX-SL100, the differential between SL and the other two methods has been recalibrated (Fig.1).

The DMX-SL resin offers a combination of high modulus (2300MPa), high tensile strength (28MPa) and high impact strength (70J/cm by notched Izod). Its physical properties compare closely to other engineering plastics built via the SLS or FDM process (table top of opposite page). But with SL accuracy and sintered-like durability, DMX-SL is also being targeted for applications that would traditionally utilise cast urethanes, or in some cases SLS or FDM. The benefit in using SL combined with a durable resin such as DMX is that design engineers can have a faster turn around of durable parts and with a more accurate surface finish as compared to SLS or FDM.

Clear and functional

ABS-like resins are by far the most commonly used SL resin type in the marketplace today. Fig.2 represents flexural modulus versus elongation-to-break of various commercial ABS-like SL resins, highlighting the property range as well as the applications most commonly using them.

Within the ABS-like category, new materials continue to be developed with varying degrees of translucency/clarity. White resins (14120 and 18420, for example) produce parts resembling white injection molded plastic. Engineers often prefer these over translucent parts because they are aesthetically more pleasing and better at hiding minor flaws in the prototype.

Conversely, many rapid prototyping shops prefer using a translucent or clear SL resin for producing parts that must be highly finished (Fig.3), such as with automotive lenses, consumer and medical products, architectural designs, and even for the visual verification of what is occurring inside a product that typically has an opaque cover.

In the creation of master patterns, clear resins are highly useful for detecting flaws that would later be picked up by the silicone mold and transferred to the cast urethane. The need for smooth, unflawed surfaces is also one of the main reasons why SL is preferred over SLS and FDM in this application.

Whether clear or opaque, however, all ABS-like resins are valued for their functionality. WaterShed11120, currently the most widely used ABS-like resin on the market, produces parts durable enough to be used as functional models. Because of its low water absorption and accuracy, it is also widely used for creating investment casting patterns and master patterns.

To date, only stereolithography offers the ability to prototype highly functional parts that are also clear and colourless (often preferred by engineers). While other RP technologies such as inkjet printing and digital light processing do offer clear resins, they come with a yellow or reddish-orange colour.

Investment casting

To date, no other RP technology matches the wide build envelope of stereolithography coupled with SL’s accuracy and wide material offering. This is becoming more and more apparent in investment casting applications. Here, laser sintering can be used-but mainly where surface finish and high feature resolution are not required. By contrast, stereolithography can build highly detailed, extremely accurate parts as small as 1cm or as large as 74cm in one single piece.

Express Pattern, a rapid prototyping service bureau in Vernon Hills, IL, USA, has verified the metal part yield from stereolithography patterns can be as good as the traditional wax method (>93percent) but with a reduction in turnaround time. For this reason, SL investment cast patterns are increasingly being used in the aerospace and motor sports industries.

This trend will continue as better resins, specifically targeted for the investment casting industry, are developed. In 2006, for example, the Somos ProtoCast AF19120 resin was introduced, containing no Antimony – a heavy metal present in the photoinitiators used in all other SL resins. This elimination of antimony produces parts that, when burned out, result in a dramatic reduction of ash. As a result, one foundry has already reported a 75percent reduction of weld repair costs on the final metal parts.

Composite resins

The ongoing development of specialty SL resins such as highly filled composites are expanding stereolithography capabilities, particularly in applications where high stiffness, accuracy and heat deflection temperatures are important.

With modulus properties that can reach 11000 MPa and linear shrinkage of less than 0.025mm, these highly filled materials are opening the door to wind tunnel testing in the motor sports industry, master patterns for the aerospace industry, replacements for machined patterns, rapid tooling and even replacement for metal prototypes when the composite parts are plated.

While the high accuracy requirements of the aerospace industry have dictated that machined metal master patterns were necessary for certain applications, it was recently determined that certain composite SL resins produce patterns as accurate as the machined patterns, but in much less time.

The stiff, highly accurate properties of composite resins also make them excellent materials for metal plating-a growing application which saves time and money as an alternative to fully metal prototypes (Fig.4). To date, flexural modulus values >55GPa have been reached on a metal-to-resin volume ratio of 1:4. Metal plated composite resins are being used to replace die cast and sheet metal prototypes, since stereolithography can cost effectively produce complex shapes while the metal plating adds the strength required to mimic true metal parts.

Conclusion

Stereolithography is an increasingly useful rapid prototyping technology thanks to a broad range of advanced material offerings. While SL parts can always be used for concept models (just like any RP technology) SL’s value continues to be demonstrated in parts used for cast urethane replacements, fit and function models, master patterns, investment casting patterns, optically clear models, wind tunnels models, rapid tooling, metal prototype replacements and even direct manufacturing.

As SL resin suppliers continue to expand the technological capabilities of their materials, the expansion of stereolithography into new product development areas will also continue. Service bureaus are most familiar with the various RP technologies, the material offerings for each and where a particular application is best suited. Engineers are recommended to contact their service bureau and ask for updates at least annually, since new resins can save them turnaround time and produce better results.

Eva Montgomery is with DSM Somos, Elgin, IL, USA. www.dsmsomos.com