Manufacturing and Evaluation of Kenaf/Polypropylene Honeycomb Cores

Abstract

Light weight and sustainable sandwich panels are increasingly sought for an extensive range of applications, from structures in airliners to wall linings in buildings. A detailed study on the development and mechanical performance of honeycomb cores manufactured from very thin extruded kenaf/polypropylene sheets is presented here. The manufacturing process involves production of flat composite sheets using an extruder, thermoforming the flat sheets into half-hexagonal corrugations and joining of the corrugations using an ultrasonic welder to form honeycomb cores. The effects of kenaf and maleated polypropylene proportions, fibre length, polypropylene melt flow indices and die temperature on tensile and flexural properties, as well as in-plane and out-of-plane shear properties of the flat sheets have been analysed by conducting experiments through ‘design of experiment’ methodology. Since melt flow indices and die temperature appear to be critical factors influencing the average and variation in the properties of the sheets, a sweep to characterize varying die temperatures (185 C and 195 C) for several melt flow indices (1.3–11 g/10 min) has been performed. The thermoformability of the sheets has been examined in the context of single curvature V-bending using an Instron machine. The effects of pre-heat temperature, forming rate, punch tip radius and bending direction on both shape conformity and tensile/compressive instabilities have been examined, both instantaneously and over a short period of time. A method to produce good quality corrugations using a hydraulic press has been developed. The best combination of ultrasonic weld time, amplitude and trigger force in terms of the joint strength of flat sheets has been determined using a lap shear test. This combination has been applied in joining multiple corrugated sheets to form core materials. The influences of directionalities, cell wall thickness and core depth on the stiffness and strength of the structures under flatwise bare compressive and shear loadings have been examined to produce desirable core properties for manufacturing sandwich panels.