Flammability of 3-D Printed Polymers – Composition and Geometry Factors

The focus of this paper is to evaluate the comparative flammability of additively manufactured (AM) and conventionally molded polymers. Flammability of objects is dependent on two main factors: material composition and object geometry. To evaluate effects of material composition, experiments on polymer samples made via conventional molding and via AM were performed using an ASTM E1354 cone calorimeter to measure and compare material ignitability and heat release rate. ULTEM™ (amorphous thermoplastic polyetherimide) and PPSF/PPSU (polyphenylsulfone) heat release rates were about 10 times lower than ABS (acrylonitrile butadiene styrene). This was in part due to the large char layer formed by these materials during burning. Comparisons between conventional molded and AM materials revealed slight differences in heat release rate. Additively manufactured ABS sheets had about a 17% higher mean average heat release rate (MAHRR). Conversely, the characterization of ULTEM 9085™ sheets revealed the MAHRR of the AM samples were 13% lower than the molded samples. This is attributed to additives in the material used for extrusion AM as well as the build process itself. Effects of geometry were assessed using material cribs, which were composed of layers of rectangular prisms separated by air gaps, with prisms on consecutive layers being orthogonal. Cribs were constructed with three to ten prisms per layer to evaluate the effects of varying the internal material surface area. Below a specific threshold, the burning mass loss rate per unit area of the cribs decreased with an increase in internal material surface area; this agrees with trends predicted using a theoretical model previously developed for wood cribs.