Improved Test Methods for Polymer AM Inter-Layer Weld Strength and Filament Mechanical Properties

Abstract

The international community is exploring many potential end-uses for polymer additive manufacturing. However, significant issues must be addressed before broad application can occur, in particular understanding the relationships between materials, processes, and final part properties. Key to these issues is having reliable test methods to measure properties of interest.

This work investigated Fused Filament Fabrication using an AON-M2 industrial printer. Other research has shown layer-to-layer weld strength (i.e. Z-direction strength) is typically the weakest property. Past work has also shown this property difficult to measure, with significant data scatter and poor failure modes common for tensile specimens printed vertically. Using ABS, the current work investigated in-plane shear testing to interrogate layer-to-layer weld strength, based on ASTM D3846 which uses a notched specimen in compression to load the gauge section. Further, a modified version of D3846 was investigated using much smaller “mini-shear coupons”. Both test methods were found to provide good failure modes and very consistent results, with Coefficients of Variation (CVs) of 5% or less. Also, the modified test method allowed direct measurement of gauge section strain, thereby providing shear modulus. These test methods will now permit detailed investigation of the effect of process parameters on layer-to-layer weld strength.

In assessing the effect of process on properties, choice of the basis of comparison, i.e. “maximum achievable property”, is important. While many researchers use injection molded properties, the authors believe this is misleading since the injection molding process itself affects properties. Instead, the authors investigated measurement of the polymer filament directly. New test methods for filament tension and shear were developed which provided very good CVs, and showed interesting relationships between “as printed” coupon properties and filament properties. The shear results show the “as printed” coupon layer-to-layer weld strength to be reduced approximately 10% from the filament strength.

Publication
ASTM International Conference on Additive Manufacturing

Presented by Richard G. Cole (National Research Council Canada), in collaboration with Kazem Fayazbakhsh (Ryerson University), Abraham Avalos (AON3D), and Nicholas Nadeau (AON3D), as part of the Polymers and Polymer Matrix Composites in Additive Manufacturing session at the ASTM International Conference on Additive Manufacturing (ASTM ICAM 2020).

Related