Challenges, opportunities, and limitations in 3D printing

Computer-aided design—cost production

More specifically, the pre- and postprinting cost amount to a significant proportion of total cost per printed part. So, even when the cost for printer materials decreases, the labor-cost penalty will remain. A “simple” rule says that if no computer-aided design (CAD) exists, then no 3D print could be done. Getting a CAD is challenging and may require several man-hours in order to produce basic results. It requires persistence and discipline to develop the necessary skill. Therefore, there are many 3D model libraries on the internet for e.g., GrabCAD, Pinshape, Thingiverse, and several others. But here (until today), you can only 3D print what you can find on the sites and with many more limitations existing regarding medical applications.

Types of materials—strength-printing techniques

There is a limited set of materials to print. Most of them are thermoplastics. The ability to print in only a few materials is a major setback in the FDM 3D printing industry. However, plastic may vary in strength capacity and may not be the best for some components. Popular and low-cost 3D printers use a plastic filament. The “first” selection is the biodegradable polylactic acid (PLA), but the ABS (acrylonitrile butadiene styrene) filament is still the most commonly used type of plastic. Some companies offer metal as a material, but final product parts are often not fully dense. Other materials like glass, carbon fiber, and nylon are being used, but have yet to enter commercial production.

Summarizing the current printing technologies, these are found to FDM, SLA, SLS and Polyjet. A fifth CLIP is still currently not as widespread as these four.

Stereolithography or SLA is one of the older and more widely used 3D printing technologies. SLA has a smooth surface finish and allows for fine detail. SLA is limited in terms of materials though, and despite increasing progress on this front, the ones available are still pretty far from production materials. The parts are generally quite brittle and will discolor over time. They are also susceptible to damage by moisture, heat, and chemicals. These specifications, limit the potential applications of parts made by SLA, which means that for the most part it is only useful for prototypes and models.

Selective laser sintering or SLS is a great choice for functional models and even production parts. Their materials are tough and durable. That said, you are again confronted with a lack of choice with SLS; the nylon PA materials are essentially all you have. The powder sintering process also means fine details are difficult to realize via SLS. The parts are porous and have a rough surface which is difficult to polish and paint (though not impossible).

Polyjet is a Stratasys technology ( https://www.stratasys.com/ ) that shoots very thin layers of liquid photopolymer to build complex, detailed parts with smooth surfaces. Polyjet also allows you to combine multiple colors and materials in one print so you can create overmolded parts or detailed display models. Despite the multiple material choices available, they all remain of the same ilk and as such are limited in terms of the properties, and like SLA materials not very durable. The build accuracy is different in different directions, and due to the layered build process, there is also a stepping effect on some surfaces.

We have presented above the basics of the technology specific limitations and there is also one major limitation facing all current 3D printing processes—the build size. This is also a critical factor in medical surgery. All of these technologies are limited by the size of the machines, and the larger they get, the more issues you have with accuracy across the whole build table. It is possible to split and glue parts produced from FDM or SLA, for example, but again you are losing strength and accuracy. Compact printing objects may more easily produced by the metal 3D printing. This will probably only pay off as a substitute for casting at the earliest by 2020, metal printers are already profitable for small batch sizes. This is because molding is still much cheaper.