Additive Manufacturing of Ultra-High Molecular Weight Polyethylene Under Applied External Pressure

Additive Manufacturing (AM) has been widely applied to many material classes and economic sectors. AM is of special interest to the medical device industry because of the need for complex geometries, the economics associated with tool elimination, and the ability to democratize production. Unfortunately, AM materials currently are lacking with respect to wear resistance.

In orthopedics applications, ultra-high molecular weight polyethylene (UHMWPE) has emerged as the preferred bearing material. Typically, UHMWPE is produced through compression molding or sintering, followed by machining to final tolerances. Producing UHMWPE components with AM would represent a major accomplishment that has been elusive to date. The main problem is the very high viscosity of UHMWPE upon melting, which limits reflow that can take place in a powder bed process.

This thesis demonstrated in-situ pressurization in AM of UHMWPE for the first time, leading to increases in strength and wear resistance even under moderate applied pressures.

A novel additive manufacturing machine was fabricated, complete with conventional selective laser sintering controls and a pneumatic mechanism for applying pressure through a glass plate onto the powder in the build chamber. By using glass, the powder is sintered while pressurized. The apparatus produced in this research used a fiber laser and commercial toner (Clearweld^®) added to the UHMWPE powder to produce AM components.

A design of experiments approach demonstrated that improvements in strength and density could be achieved. Even more impressive were the gains in wear resistance, evaluated by using an AMTI OrthoPOD. The research demonstrated the scientific hypothesis that selective laser sintering combined with in-situ pressurization is a viable approach for producing high-performance polymers like UHMWPE. The pressures applied were only 0.015 MPa, due to limitations in the pneumatic design and compressed air available. Much higher pressures could easily be designed into the system using hydraulic or mechanical compression means. Mathematical models suggest the higher pressure will have an even greater effect on material properties.