Study On The Polymer Materials Of Selective Laser Sinteringand The Properties Of Sintered Parts

Selective laser sintering (SLS) is a rapid prototyping technology in which a three-dimensional object is created layer by layer from heat-fusible powdered materials with heat generated from a laser. One of important application of SLS is to make high-quality functional parts with polymeric materials. As those kinds of parts need good properties in strength, dimensional accuracy, etc, it is very important to research and develop high-quality SLS polymeric materials. Some foreign companies have developed their own SLS materials. But, in China, as the research began late, the SLS polymeric materials, which can be used to make functional parts, had not been developed. In this paper, aiming at developing the SLS polymeric materials for plastic functional parts, the sintering mechanism, the recipe, the preparation method, the SLS process and the properties of SLS parts, etc. are studied The sintering behavior of different polymer is investigated and it is found that amorphous polymers and crystalline polymers have different SLS mechanism. The SLS mechanism of amorphous polymer is viscous sintering and the apparent viscosity is the key factor that effect sintering rate. It is difficult to produce fully dense SLS parts from amorphous polymer because of the high viscosity near the glass transition temperation, Tg, and the short laser heating time. For crystalline polymers, the part bed temperature can be kept near melt point; the sintering mechanism is melting-solidification. Fully dense parts can be produced because the powders under the laser heating can be melted completely. The recipes of SLS materials are studied. Polycarbonate (PC) and nylon 12 are chosen as the base polymer. The auxiliaries, such as stabilizers, dispersers, lubricants, fills, etc, are added to improve the processing technique and the properties of SLS parts. The particle size and shape of polymer powders are characterized by Scanning electron microscopy, (SEM). The apparent density and the whiteness of sintering polymer powders are measured. The mechanism and kinetics of thermal and thermo-oxidative decomposition of nylon 12 are investigated by Gas-chromatography and Mass-spectrometer (GC-MS ) , Thermogravimetry (TG), Infrared spectrum (IR), and by measuring the inherent viscosity and whiteness of oxidized nylon 12. The results show that nylon 12 has high decomposition activation energy and a good thermal stability. But in the presence of air, nylon 12 is liable to thermo-oxidative decomposition. Reducing oxygen concentrate and adding antioxidants are effective methods to inhibit oxidation. The inherent viscosity and the whiteness of nylon 12 decrease with increase of oxidation time. As the whiteness is much sensitive to oxidation, and easy to measure, it is a good specification to characterize the degree of oxidation of nylon 12. The effects of laser power on the physical density and mechanical properties of the PC SLS parts are investigated. It is found that some properties of the PC SLS parts can be improved at some extent by optimizing process parameters, but the SLS parts of PC cannot be used as functional parts directly because of the low strength. However, the mechanical properties of PC parts can be improved greatly with post-processed by epoxy resin and can meet the requirement of some functional parts. Warpage is easy to take place during sintering nylon 12. With strictly control of the part bed temperature and the laser power, the warpage of nylon 12 parts are minimized; the effects of process parameters, such as laser power, scan speed and layer thickness, on the strength of the nylon 12 SLS parts are investigated. The parts with 44Mpa of tensile strength, 50.8MPa of flexural strength, 37.2KJ/m2 of impact strength have been fabricated; the effects of fills, such as glass bead, talc, calcium silicate, on the SLS process and the properties of nylon 12 parts are also studied. The result shows that the glass bead makes the sintering powders a relatively wide processing window, good flowability, and makes the SLS parts a good mechanical properties and dimensional accuracy, but, at the same time, glass bead accelerates the oxidation of nylon 12. The experiments also show that calcium silicate reinforces the SLS parts; talc improves the parts appearance; fills increase the thermal distortion temperature of SLS parts greatly, but decrease the impact strength. The blended powder of nylon 12 and organic rectorite is used as SLS material. The structure of sintered nylon 12/rectorite composite was characterized by (X-ray diffraction) XRD ,Differential scanning calarimetry (DSC) ,SEM and Transmission electron microscopy (TEM), its thermal stability and mechanical properties were studied. Theresults show that the organic rectorite is intercalated by nylon12 during the sintering process, the thermal stability and the mechanical properties of the nylon12/rectorite nanocomposite are better than that of sintered nylon12. With 5wt% OREC, the tensile strength,flexural strength,impact strength are improved by 14.3%,22.8%,40.3%, respectively, the thermal decomposition temperature increases by 27℃, and the thermal distortion temperature increases more than 68℃. The nylon 12/rectorite composite can be used to fabricate high-quality functional parts. The study in this paper, on many aspects, supplies gaps of SLS field in China. The development of polymeric material and related process for SLS will meet the requirement of the market in making high-quality functional parts by SLS technology, and will promote the applications and development of SLS.