Materials for high speed sintering
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gh Speed Sintering (HSS) is a novel additive manufacturing technology which currently uses Nylon 12 as the standard feedstock material. To expand the number of processable materials, the preferred characteristics of polymeric powder as a feedstock powder are presented, appropriate materials identified, parts made, and mechanical properties measured. Two commercially available laser sintering (LS) grade powders previously untested for HSS were selected, DuraFormÒ HST10 and ALM TPE 210-S. Tensile test specimens were manufactured using each material and mechanical properties analyzed and compared to the manufacturers’ specification for LS. Tensile test specimens built using DuraFormÒ PA show higher tensile strength and elongation at break than LS whereas DuraFormÒ HST10 shows somewhat reduced tensile strength but slightly increased elongation at break. ALM TPE 210-S shows elongation at break of more than double that of LS demonstrating the capability of HSS to process viscous materials. The results indicate that HSS is capable of processing LS grade polymeric powders and may extend beyond.
I. INTRODUCTION
High Speed Sintering (HSS) is a novel, layer by layer, powder-based additive manufacturing (AM) process. The process retains the benefits of AM processes whilst simultaneously eliminating drawbacks such as machine cost and process speed (Fig. 1).1,2 Similar to laser sintering (LS) in the powder deposition stage of the process, the powder must also be able to flow at elevated temperature effectively. This is required to produce a layer of powder with uniform thickness, typically 100 lm. Despite requiring similar powder flow properties, the mechanism by which sintering is initiated is markedly different. Rather than using a laser to scan a cross-sectional area, HSS uses an InkJet printer (Huntingdon, Cambridgeshire, UK) which deposits radiation absorbing material (RAM) on the surface of the powder which is followed by irradiation of the entire build surface by an infrared (IR) lamp. Figure 2 depicts this schematically and also shows how the HSS process is a self-supporting manufacturing technique with the unsintered powder acting as a support structure. The area on the powder surface printed with RAM absorbs IR significantly more than areas without, this absorption of energy is sufficient to elevate the temperature of the underlying powder to its melting point and allow sintering to occur. Therefore, to prevent sintering of the entire build bed there must be a significant difference in IR absorption between the RAM and the powder. Examples of HSS parts are shown in Fig. 3.
Once sintering has taken place it is imperative that the polymer does not cool down below its crystallization temperature as this will cause the polymer to solidify, induce thermal stresses and warp causing the build to fail. Therefore, it is preferable for the polymer to possess supercooling behavior, that is, the crystallization temperature is preferably lower than that of the melting temperature.4 This behavior can be identified by differential scanning c
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