Stereolithography Answers Challenge Of 3D Manufacturing

The landscape of additive manufacturing has changed dramatically since the commercialization of stereolithography (SL) as the first viable 3D printing technology. In fact, the terms “3D printing” and “additive manufacturing” (AM) only entered the popular vernacular in recent years.

The scope of additive manufacturing has changed dramatically. Technologies encompassing thermoset and thermoplastic materials, along with metal, have proliferated. The 2017 Wohlers Report lists 96 different additive manufacturing equipment suppliers across a broad range of technologies. Substantial advances in equipment, software, and material are occurring concurrently with increasing computer power and growth in the 3D CAD installed base. Competition within the early technology supply base has increased as patents expire and new players come to the international market.

Today, the inherent characteristics of 355-nm laser based stereolithography (SL) technology characteristics leverage ever-expanding material capabilities to mature into one of the widest used and highest utility AM processes. The term SLA, a registered trademark of 3D Systems, is often used by some to encompass various 3D printing processes that fall within the ASTM grouping of AM processes as “Vat Polymerization.”

Stereolithography in this discussion will be focused on “industrial SL,” as the original technology has evolved and is differentiated from all other vat polymerization processes by:

Platform sizes ranging from 250 mm (9.5 in.) square to over 800 mm (31.5 in.) square.
Ultraviolet laser (355-nanometer wavelength) light source.
Materials formulated for 355-nm UV including clear, pigmented, and composite systems.
Imaging from above (build platform travels downward).

Figure 1: 355-nm stereolithography can create metal plated parts with high resolution and accuracy, as shown in this demonstration part.

The use of a laser to instantly cure a photopolymer using a UV laser with a nominal spot size less than 0.2 mm provides one of the highest combinations of accuracy and resolution of any AM process, especially considering the range of part sizes the process can handle. Today’s 355-nm SL materials can produce parts that have excellent dimensional consistency and surface aesthetics ranging from transparent to various colors resembling typical injection molded parts. These materials have overcome robustness and aging issues encountered in earlier generations that enables parts manufacture with a broad range of mechanical properties allowing functional applications in prototyping, patterns, and beyond.

Additive manufacturing processes utilizing thermoplastic materials are often cited for robust mechanical properties. Current generation SL materials can be selected to overlap the performance of common thermoplastics in other AM processes, while retaining all the accuracy and aesthetic benefits of the SL process.

Stereolithography is often typecast as a prototyping process sometimes based on an outdated understanding of material capabilities. The attributes of 355-nm SL equipment, combined with the latest photopolymers, enable applications that extend prototyping capabilities, as well as end uses. Opportunities in patterns for secondary forming operations include large-scale mass customization, low volume urethane part production, tooling for low volume injection molding, and metal clad composites.

Innovation via Technology Integration

The 3D printing process is often positioned as a disruptive technology, but it is better thought of as an enabling technology.

In the late 1990s, the founders of Align Technology imagined a different business model for correcting the alignment of teeth with a series of retainers. Today, this application is possibly the highest volume application example of mass customization. Converting the CAD images of individual patients to patterns used to thermoform the final aligners enabled what most would call a disruptive business model.

SL patterns for secondary thermoforming remain the dominant technology of this application today, based on the rapid processing times on large format machines optimized for this single application of mass customization.

Investment casting, one of the oldest known metal forming processes, has used the SL process for over twenty years. The ability to manufacture hollow smooth walled patterns for use in a foundry process coats the pattern with ceramic, then fully burns out the pattern in preparation for molten metals to be cast in the hollow form . While molded wax patterns dominant most high-volume applications, SL eliminates tooling costs for lower volume casting, but also facilitates sizes and part features not readily obtained in a molded pattern process. The latest SL photopolymers for this application have excellent dimensional consistency and contain no heavy metals often found in 355-nm photopolymers. This combination ensures accurate patterns, as well as minimal ash after burnout that can cause casting defects.

Similar hollow part methodologies used for investment casting can be applied to large parts, creating “lightweight” parts with tailored mechanical properties and reduced weight. Materialise, a global AM software company based in Belgium, has developed multiple software options to hollow and reinforce lightweight structures. This development has facilitated the cost-effective manufacture of point-of purchase displays, architectural models, and other art applications.

The ability to manufacture full density highly-accurate patterns also facilitates another well-established molding process known as urethane molding, RTV, or silicone molding. After careful secondary finishing, the pattern is embedded in a silicone rubber casing that becomes a two-part mold for the casting of urethanes. Polyurethane materials can be formulated to achieve properties consistent with levels of performance from injection-molded thermoplastics. The silicone tool can be used for low volume series when either multiple prototypes or low volume production is required.

Creating injection molding tooling has been an area of development interest since the earliest days of SL. The principal impediments to this potentially high-volume application for prototype and bridge parts include strength and temperature resistance of the 3D-printed tool, predictability of tool life, and compatibility with a large range of injection molding materials including glass-filled systems.

Like investment casting, electroplating of a substrate material to improve physical or mechanical properties is well known. All 3D-printed materials can be electroplated to improve strength, wear resistance, EMI/RFI shielding, flammability resistance, and aesthetics.

Photopolymer based printers; however, offer smooth, non-porous surfaces that plate readily with basic parts preparation. 355 production SL machines and state-of-the-art materials combine to provide the largest range of part size and substrate options. The same highly filled silica photopolymers used for injection molding tools can create extremely thin (0.010 to 0.040 in) substrates for a nickel/copper/SL composite with mechanical properties approaching die cast nickel.

This level of mechanical performance creates a significant bridge of opportunity between the gap of polymer additive manufacturing and direct additive manufacturing metal. Aside from mechanical properties, the design flexibility of SL combined with a copper coating can create a cost-effective wave guide or “antennae configuration.” There are other parts currently in production where the structural nickel creates a renewable wear surface.