Methods for quantifying the stable sintering region in laser sintered polyamide-12

Manufacturing complex parts by the laser sintering process requires a minimum amount of energy input for consolidation of polymer particles to occur; however too much energy can result in a decline in mechanical properties. This decrease is thought to be the result of polymer chain degradation. A stable sintering region (SSR) has been proposed to describe the optimum temperature range for successful laser sintering. This article will aim to quantify the SSR for polyamide-12 by using thermogravimetric analysis (TGA) to provide a framework for identifying key laser sintering processing parameters. Weight loss with respect to temperature is the main measurement output of the TGA procedure. However, the precise temperature and thermal history of a material is difficult to quantify during the laser sintering process; instead an energy input approach has been developed. A degradation energy was calculated from the TGA data and was used in conjunction with a laser sintering formula called energy melt ratio to prescribe build parameters for laser sintered parts. The mechanical properties of these parts illustrated the effect of degradation at various levels of energy input. Implications for this work include optimizing the material selection process for polymer laser sintering materials beyond polyamide-12. © 2012 Society of Plastics Engineers

Optimum sintering region for laser sintered nylon-12

It is well known that the mechanical properties of laser sintered Nylon-12 improve with increased applied energy. However properties can reach a maximum after which, the application of additional energy can a lead to a decline in part properties. It is thought that the reason for this decline is that the additional energy causes polymer chain degradation or other changes in molecular structure. This paper aims to use thermogravimetric analysis (TGA) to investigate the thermal degradation of nylon-12 and explain the deterioration of mechanical properties when high energy density conditions are applied during processing. The key findings are the application of modelling methods to predict the temperatures achieved during laser sintering of nylon- 12. It is shown that temperatures in the laser sintering machine can achieve levels above 3008C. At these temperatures, TGA data show that mass loss occurs and could cause mechanical property breakdown. This practical work coupled differential scanning calorimetry and TGA as a means of identifying thermal transitions in the material. The term ‘stable sintering region’ is proposed as a novel concept for the laser sintering community, and can have implications for better understanding of how process parameters can affect parts built in the machine. In addition, the concept could be used in the material selection process when screening potential new polymers for the process. One limitation of laser sintering, compared to other polymer processes such as injection moulding, is the limited understanding of the connection between machine parameters and part properties. This work aims to improve that understanding by discussing the pattern of thermal behaviour, including degradation, seen in polyamide exposed to high laser parameters

Effect of mould temperature on high-resilience cold-cure flexible polyurethane foam surface texture

Flexible polyurethane foam is often molded directly into preheated tools for foaming reaction which expands to fill the mold cavity. The foam that is directly in contact with the mold surface cures as the foam skin. Parts frequently have surface defects ranging from shrink marks, to voids, to mottling and knit lines. There are many possible causes such as applying too much or too little release agent, or mold surface not cleaned and conditioned as required before the foaming process. Uneven mold temperatures are also suspected to be a cause of surface defects, especially in high-resilience (HR) cold cure polyurethane foam systems. A specially designed mold capable of maintaining tight temperature tolerance was built to produce foam samples at varying temperatures. The effects of mold surface temperature on the foam surface texture are studied and analyzed. The effect of processing temperature on the macro and micro surface texture is examined. It is shown that the processing temperature has a significant effect on the foam surface texture. 3D topographical analysis of foam surface texture discovered a trend from samples produced at varying temperature from 30 to 80 C

Shear viscosity measurements on Polyamide-12 polymers for laser sintering

Purpose – Laser sintering kinetics and part reliability are critically dependent on the melt viscosity of materials, including polyamide 12 (PA-12). The purpose of this paper is to characterise the viscosity of PA-12 powders using alternative scientific methods: constrained boundary flows (capillary rheometry) and rotational rheometry. Design/methodology/approach – Various PA-12 powders were selected and characterised by both techniques. Measurement of molecular weight was also carried out to interpret the viscosity data. Findings – Results demonstrate conventional pseudoplastic flow in all PA-12 materials. Zero-shear viscosity has been quantified by rotational rheometry; a notable observation is the striking difference between virgin/used PA-12. This is interpreted in terms of molecular weight and chain structure modifications, arising from polycondensation of PA-12 held at the bed temperature during laser sintering. Research limitations/implications – Accurate zero-shear viscosity data provide scope for use in predictive computational models for laser sintering processes. Careful sample preparation and equipment operation are critical prerequisites for accurate rheological characterisation of PA-12 powders. Practical implications – Differences in flow behaviour and molecular structure allow prediction and deeper understanding of process-property relationships in laser sintering, giving potential for further optimisation of material specification and in-process machine parameter control. Originality/value – This is believed to be the first time that techniques other than melt flow rate (MFR) have been reported to measure the viscosity of PA-12 in a laser sintering context, noting the effects of pre-drying and molecular weight, then predicting differences between virgin/used powders in practical sintering behaviour

Effects of electroplating on the mechanical properties of stereolithography and laser sintered parts

This paper provides a quantitative and qualitative assessment of the effects of electroplating on polymer parts made by stereolithography and laser sintering. A series of test samples were coated with copper and nickel with varying thickness. Thicker coatings (120μm) were reproduced with a repeatability that should not adversely affect the tolerances with which such parts may be produced given the tolerances of the initial rapid prototyping processes themselves. Thinner coatings (20μm) resulted in a smother surface finish than thicker coatings for stereolithography parts, however the converse was true for laser-sintered parts. Composite theory was used to predict that thicker coating would lead to higher Young’s modulus in parts and this was shown to be true in physical tests although the practical values were lower than the predicted values especially for thicker coatings. Physical tests also confirmed that thicker coatings increased UTS and impact energy but had a minimal effect on the ductility of parts

A targeted material selection process for polymers in laser sintering

Laser sintering (LS) of polymer materials is a process that has been developed over the last two decades and has been applied in industries ranging from aerospace to sporting goods. However, one of the current major limitations of the process is the restricted range of usable materials. Various material characteristics have been proposed as being important to optimise the laser sintering process, key aspects of which have been combined in this work to develop an understanding of the most crucial requirements for LS process design and materials selection. Using the favourable characteristics of polyamide-12 (the most often used material for laser sintering) as a benchmark, a previously un-sintered thermoplastic elastomer material was identified as being suitable for the LS process, through a combination of information from Differential Scanning Calorimetry (DSC), hot stage microscopy (HSM) and knowledge of viscosity data. Subsequent laser sintering builds confirmed the viability of this new material, and tensile test results were favourable when compared with materials that are currently commercially available, thereby demonstrating the efficacy of the chosen selection process