Additive Prototype Material Properties: A Well Kept Secret

Early in the design cycle, prototypes merely need to illustrate a new product's shape or form. Fast production speeds and low costs are paramount. If a product assembly has two or more mating pieces with matching hole patterns, its prototype requires dimensional accuracy and a good surface finish so the assembly fits together.

Before a product can go to market, however, the mechanical designer will need to produce a functional prototype. In a barrage of tests, this model must perform as well or better than the final manufactured product. It also needs to verify virtual testing to ensure that the final product will meet a host of FDA and other government regulations.

"Mechanical, thermal and electrical properties can be critical for testing prototypes," said Terry Wohlers, president of Wohlers Associates (Ft. Collins, CO). "If you don't know these physical properties, you will spin your wheels and make little progress."

While the selection of additive prototyping materials continues to grow, the engineering community still has little data about their mechanical, thermal and electrical properties. Additive manufacturers have not published these important data in their 20 years of existence. Currently, if a company wants to use an additive process to create a functional prototype, it must perform its own testing and certification of the material. This may work for major government contractors, but it's simply not feasible for most averaged-sized companies.

How much is the lack of published material properties holding back the additive RP community and vendor sales?

Government Regulations

The U.S. government has strict material requirements, especially for prosthetics and implants. Only approved materials can be legally placed within a patient. The vast majority of these approved materials are available in cast or extruded stock for milled prototypes or pellet for plastic injection-molding.

Food and biomedical products fall into the same category of government regulated materials. Liability concerns are a big issue in these industries. Imagine being poisoned from eating a hotdog or slice of cheese produced with a non-FDA approved material. Almost everything we eat is manipulated and processed by equipment that is heavily regulated by the FDA.

Unless the material specifications are reproduced, it's impossible to meet government regulations. It's particularly important for biomedical and food processing products, where testing for biocompatibility and chemically inert properties is critical. Meeting requirements such as these is a critical step in developing a product and reducing time to market.

"An additive RP system, for example, cannot produce a prototype out of NSF (National Sanitation Foundation) food-safe plastic," said Ed Tackett, director of the Saddleback College Advanced Technology Center. "Its material properties are unknown, making functional testing impossible. Only a subtractive process can use the actual plastic to make food processing prototypes."

Virtual Testing

Virtual prototyping helps designers optimize their designs before production. FEA (finite element analysis) is the most popular analytical method and known materials properties and boundary conditions can dramatically speed up the design process. But at the very end of the design cycle, FEA results must be verified against functional prototype testing.

Good product designers do not risk their reputation solely on virtual simulation. Verifying the simulation is often required by consumer protection laws. Designers need to double-check everything with hand calculations and years of practical experience.

Most importantly, accurate testing depends on producing a functional prototype with material properties and boundary conditions used in the virtual prototype. Functional tests should verify the virtual tests. If the material properties are unknown, however, the prototype will tell the designer nothing about the virtual tests.

Making It Work

Many designers use a two-step process to produce a prototype for functional testing. First, they use an additive system to create a form and fit model. Then, they use it to create a molded part with known material properties. This works nicely, but the secondary molding process of creating hard or soft mold tooling adds time to the development process.

Designers often choose a more direct subtractive rapid prototyping (SRP) process, which starts with a homogeneous, nonproprietary plastic and mills away unwanted material to reveal the desired part. The process produces low-cost prototypes that are structurally, thermally and electrically similar to the final production part/product. An extensive material selection includes popular engineered plastics such as ABS, Delrin, nylon that are often the exact material selected for the production part.

Known Facts

The mechanical, thermal and electrical properties of additive materials remain a mystery. This lack of information has forced mechanical designers to endure the extra time and expense of producing molded parts. Many designers have opted to purchase desktop SRP devices to quickly produce functional prototypes without secondary processes.

At the same time, some known facts about additive RP materials do exist. Regardless of the process, they all build prototypes one layer at a time. The prototypes end up with non-isotropic properties and residual stresses. While some residual stress can be removed though annealing, the part is still far from isotropic. As a result, parts often lack dimensional accuracy due to differing coefficients of thermal expansion in x, y and z directions.

The layer-on-layer build method also creates a stair-stepped finish. 3D printers, for example, use a thin layer of starch/sugar powder to build models with a particularly grainy and rough surface. For fit and functional prototypes, which require a smooth surface finish and tight dimensional accuracies, the designer is forced to perform a great deal of post finishing work. In addition to the extra time and energy, surface finishing additive prototypes lowers the dimensional accuracy of the part.

Demand More

The American Society for Testing and Materials (ASTM) has tested the vast majority of engineering plastics for material properties. Yet for additive materials, little has been done to test and publish results. Since additive technology came to the industry in 1986, not a single data sheet has been published on mechanical, thermal or electrical properties.

Product testing is expensive and time-consuming, but it's often required and always necessary. The product design community needs to come together and demand standardized materials with known properties. Until that happens, designers will never be able to use an additive system to produce a functional (testable) prototype.

"It would be helpful if material suppliers and universities would come together and address this problem," said Wohlers. "It would require funding, coordination and talent. It's all out there, but someone such as the ASTM needs to get it off the ground." Every university has testing equipment, and most are more than willing to help determine the physical properties additive materials. They researched and published the physical properties of all of the alloys of steel, aluminum and magnesium. Now, with the help of material suppliers, they need to do the same for additive materials.

At trade shows and in magazine advertisements, additive material vendors continually claim to offer functional prototypes. For a few months, the manufacturers described their materials as "durable," but now "functional" is their preferred descriptor once again. What do they mean by that?

"If a prototype does not have the same materials as the final part, it is worthless for functional testing," said Tackett. "RP material research focuses on the prototyping system and ignores what works for the design engineer. Material manufacturers need to rethink their business strategy and provide a full spectrum of data on material properties."

After 20 years, it's time to demand access to these vital data. Product designers ultimately shape the industry and its technology with their pocketbooks. In addition to being able to produce functional prototypes, buying only prototyping materials and processes with known physical properties would motivate suppliers to come clean. It's long overdue!

Will Curtis is the president of Curtis Communications (Lake Forest, CA) and a freelance writer who covers product design and manufacturing technologies.