Effect of the properties of the polymer materials on the quality of selective laser sintering parts

Abstract

The effect of the properties of the polymer materials, such as molecular weight, molten viscosity, crystallization rate and the particle size of the powder, on the quality of selective laser sintering (SLS) parts is researched. The results indicate that the molecular weight affects the quality of the SLS parts through the melting viscosity. SLS parts of higher density can be fabricated with polymer materials of lower melting viscosity. Crystallinity largely affects the precision of the SLS part-shrinkage is more serious with increasing crystallinity. SLS parts sintered with polymer powder materials, whose melting peak and crystalline peak differ greatly, have high dimensional precision. The particle size of the powder affects not only the precision but also the density of the SLS part. The appropriate particle size is about 75-100 mm.

Effects of processing on microstructure and properties of SLS Nylon 12

“…However, shrinkage during recrystallisation hinders production of accurate parts [10]. For this reason it is essential that for materials to be processed by SLS the melt temperature be considerably higher than the crystallisation temperature so that crystallisation can be delayed and reduced during the build process [11] and to allow new layers to bond to previous layers with a more homogenous microstructure.…”

“…Molecular weight is a critical characteristic when describing polymers in general [12] and the weight-average molecular weight (M W ) directly affects the melt viscosity [10]. The Melt flow procedure is used to characterise melt viscosity by the melt flow index (MFI).…”

“…Low MFI is associated with high melt viscosity and high molecular weight. Gornet et al [13] and Shi et al [10] investigated the effects of powder melt viscosity and related lower MFI melt flow (and therefore higher melt viscosity and M W ) to improved mechanical properties particularly elongation at break (EaB). It is standard practice in industry to blend virgin powder with used to maintain acceptable part properties which deteriorate with continued use of used powder [13].…”

Polymer viscosity, particle coalescence and mechanical performance in high-temperature laser sintering

“…In the case of the LS process, a powder with a good flow and appropriate particle size distribution leads to good powder spreading performance and particle packing, this subsequently leads to more dense parts and less porosity [10].…”

“…A limited number of studies investigated these interactions and defined these polymer characteristics for polyamides only [1,2,10,12]. According to Kruth et al [2] the ratio between surface tension and melt viscosity, velocity parameter, defines the limit for a successful sintering in polymers.…”