Measurement of the welding strength of a two-beads foam

Abstract

Bead foams are ubiquitous and versatile materials that are used, among others, in packaging, automotive, and sports applications. The overall performance of the finished part made from bead foams is crucially dependent on the welding strength. The techniques proposed to measure welding strength typically involve mechanical tests of multiple-beads samples, that is, samples composed of a large number of beads. However, in most cases, (i) the effective welding interface area cannot be properly quantified, (ii) the welding interface is subjected to complex load conditions also due to the random orientation of the interface, and (iii) spatial uniformity is not easily achieved in macroscopic parts. In this framework, we develop a measurement technique of the welding strength of bead foams produced via in-mold foaming and molding technology that is based on dogbone-shaped two-beads samples. The samples produced present the welding interface perpendicular to the tensile direction and within the gauge length of the sample. With this approach, the welding interface is fixed and subjected to pure tensile stress, so that the welding strength can be rigorously characterized. We tested the proposed measurement technique on two-beads samples made of polystyrene. To verify the reliability of the proposed measurement technique, we compare the measured strength of the two-beads samples in a wide range of foam densities with literature data of the tensile strength of multiple-beads samples which are typically obtained from manufactured bead foams. In addition, data are compared with the tensile strength values of monolithic (i.e., not composed of beads) polystyrene foams, pointing to a large room for improvement.