3D printed PMMA conquering the North American investment casting industry

Thomas Müller first encountered industrial 3D printing in the early 2000s while studying mechanical engineering at the University of Augsburg. The Binder Jetting was still in its infancy when he wrote his thesis at Generis GmbH in 2001. In the years that followed, Müller developed his knowledge and skill in additive manufacturing for metal casting as a service technician and maintenance expert.  After receiving a job offer, he relocated to the United States in 2008. He worked as a production manager with sand 3D printing, remaining dedicated to the industry and technology. During his time there, voxeljet introduced the first PMMA printer for the investment casting industry in North America, which fascinated Thomas from the start.  In 2012, he made the decision to concentrate solely on this market segment and established his own business.

As the investment casting process has existed for several thousand years, the question arises as to how such an established process still offers opportunities for innovation and optimization. To answer this question, it is worthwhile to gain a better understanding of the process and the steps involved in investment casting.

He bought a VX500 from voxeljet and installed it in an investment foundry. From that point on, he provided the foundry and his trusted customers, spanning various industries, with quick prototypes, simplified handling during the shell building process, and top-notch quality, even for intricate designs.

If multiple design drafts or variants need testing simultaneously, 3D printing and casting can be done in one go, saving the expense of producing numerous patterns with tooling that, in the worst case, will never be used again because the design will be discarded. 3D printing-aided investment casting can boost and speed up product development cycles and spare parts acquisition. Thomas already knew most of his customers from previous projects. These were mostly companies in the automotive, agricultural, and aerospace industries. However, Express Prototyping has also taken on some unique projects, including producing visors for football helmets. The use of 3D-printed PMMA models was in high demand due to their exceptional burn-out properties, which led to a constant growth in capacity until the VX500 was no longer enough.

Thomas quickly realized the solution and upgraded from his VX500 to a more powerful VX1000. A Second VX1000 followed shortly and a third one in 2021. Why is that? Why is there an increasing demand for 3D-printed investment casting models?  According to Thomas Müller, three main factors contribute to this trend:

In this case, we are discussing Stereolithography (SLA) and wax-based additive manufacturing technologies used to manufacture parts. In the SLA process, liquid plastic resin is used to create the parts, which are cured layer by layer using UV light. These parts have a very smooth surface finish. Alternatively, 3D wax printers offer an alternative to SLA. This material is well known and established in the investment casting industry.

Both SLA and other technologies encounter molding difficulties when transferring designs to a ceramic mold. Specifically, SLA-printed parts often crack the shells during pattern burnout. Typically, the material side is to blame, as the resin tends to expand when heated. Wax-printed investment castings do not face this issue. However, they tend to be challenging to manage due to the need to also print and remove support structures. The material used is costly, and there are severe restrictions on the size of parts that can be printed with the available printing systems.

“PMMA presents an optimal solution with quick and cost-efficient production of patterns. Its exceptional burn-out behavior in shell building includes no fractures, even with thinner ceramic shells,” explains Müller. This is due to PMMA’s negative coefficient of thermal expansion; the material shrinks when heated instead of expanding.

“When SLA is fully printed, i.e. there are no cavities in the part, every shell breaks during burnout,” explains Müller. “PMMA has found the perfect niche, allowing wall thicknesses < 1mm without any problems. However, the surface of PMMA patterns has slightly higher roughness compared to other technologies. This can be significantly improved with wax infiltration.

Until 2021, Thomas Müller and his team printed almost 100% technical parts. Engine components, joystick housings for agricultural machinery, impellers and spare parts. Since 2022, however, Express Prototyping has utilized 70-80% of its production capacity for artistic parts, while still utilizing the technology for prototype production in technical fields. Batch sizes of up to 100 units are regularly produced in our everyday business.

“It is surprising to note that PMMA is increasingly popular in the art scene, particularly in recent years. With many artists now designing entirely in 3D, the PMMA material set is becoming more and more popular in the creative scene because of its size and ease of use. The growing popularity of PMMA as a standard material for art casting has led to an ever-increasing demand,” notes Müller.

As in technical industries, art also requires careful attention to detail and creative thinking. This enables artists and engineers to create complex, lightweight structures through topology optimization and interwoven designs.

“PMMA 3D printing enables the creation of intricate designs and geometries that cannot be achieved through other additive manufacturing technologies. The key factor behind this is the outstanding burn-out properties of PMMA. According to Müller, with other methods, such as thin-walled propeller production, the likelihood of shell breakages is drastically increased.”

Last but not least art foundries are capable of reducing wall thicknesses with the help of PMMA patterns, saving costs through less poured metal in the end.

Using 3D printing for pattern making can result in up to a 75% reduction in time and costs compared to traditional methods. Because 3D printing is tool-free and uses CAD data, adjustments to the design can be made digitally, and a new pattern can be printed immediately.

“The cost-effectiveness of PMMA 3D printing is influenced by various factors. Thanks to the large build volume of 1,000 x 600 x 500 mm, multiple product variations and larger individual parts can be printed together, thus decreasing the cost per part. The absence of tool production using milling or similar methods results in the biggest savings. Additionally, the ability to recycle and reuse the unused material should not be overlooked, which ultimately helps optimize powder consumption and operational expenses. This also allows powder consumption and running costs to be optimized,” concludes Müller.

Also, when considering the steps after 3D printing, PMMA has the potential to simplify and reduce processes, thereby saving costs. The aforementioned shell structure forinstance:  Thanks to PMMA’s burnout behavior, fewer ceramic layers can be placed around the model, allowing it to dry faster and ready for molding.