Effects of energy density on morphology and properties of selective laser sintered polycarbonate

Abstract

Selective laser sintering (SLS) was used to prepare components from polycarbonate (PC) powder. The effects of energy density of the laser beam, ranging from 0.036 to 0.12 J mm⁻², on the physical density, tensile properties and morphology of the sintered specimens were investigated. At low energy density levels, the physical density and tensile strength of the specimens were found to increase with the increase in energy density. Smoke was observed when the components were sintered under an energy density above 0.09 J mm⁻², which suggests degradation of the PC powder. These specimens appeared slightly yellow and there was a reduction in the density and tensile strength. In specimens built under a low energy density, scanning electron microscopy (SEM) showed that slight fusion occurred at the surface of the PC particles. The individual particles could be identified and the voids between the particles were interconnected. The crack propagated between the particles during tensile fracture. When the energy density was high, the powder was fused thoroughly which allowed a more dense structure to be built. SEM also showed that fusion was more effective within the individual layers of the PC powder, whilst considerable amounts of voids were present between the layers. Therefore, the strength of the components normal to the layers is expected to be lower than the in-plane strength of the layers. Some wavy circular features were observed on the fracture surfaces of the specimens built under a high energy density. Such features are commonly found on the fracture surfaces of brittle plastics. The work forms a basis for the production of selective laser sintered PC functional products.

Introduction

Selective laser sintering (SLS) is an advanced rapid prototyping technology which can shorten the design–manufacturing cycle, hence reducing the production cost and increasing the competitiveness. In the SLS process, a three-dimensional object is created layer by layer from heat-fusible powdered materials with heat generated from a CO₂ laser. A very thin layer of heat-fusible powdered material is delivered onto the top of the build cylinder by a roller. A heat-generating CO₂ laser beam traces across this layer, sintering specific areas according to the instructions of the CAD file. The platform lowers slightly and another thin layer of material is delivered. Then, the laser scans selected areas of this layer which bond to the previous layer. The process continues, layer by layer, until the three-dimensional object is complete. The unsintered powder stays in the container during the process, serving as a natural support for the object [1].

Gibson et al. [2] studied the effects of fabrication parameters on the properties of selective laser sintered nylon components. A number of researches on the thermal modelling of selective laser sintering of polycarbonate have been carried out [3], [4], [5]. Nowadays, the SLS process is not only used for rapid prototyping but also for producing functional graded components and tooling. Hence, the properties of the sintered components must be considered according to their applications. In this study, polycarbonate specimens were prepared under different energy densities. The energy density is a function of the beam speed, the fill laser power, the scan size and the scan spacing. The energy density was varied by setting different values of the fill laser power and keeping the other parameters constant. The effects of the energy density on the physical density and the tensile properties of the components were studied. Furthermore, the morphological development and fracture behaviour of the specimens were studied by scanning electron microscopy (SEM).