Study On The Selective Laser Sintering Materials For Plastic Functional Parts And Complex Castings

Selective laser sintering(SLS) is a rapid prototyping technology.Traditionally,these techniques have been focused on areas where the primary interest was in generating a physical model of a component or system for the purpose of visualization.More recently, the most two important applications for SIS are manufacture of plastic functional components with sufficient mechanical properties and dimensional accuracy,and making complex castings combining with foundry technology.Some foreign companies have developed their own SIS materials.But,in China,as the research began later,the only commercial material is polystyrene(PS).The foreign materials are not only expensive, but not external sell.More important,as the laser sintering technology is so complex and strict,it can not be used in homemade SLS equipment in China.In this paper,aiming at developing the high-quality SLS materials and expanding the applications of SLS, indirect and direct preparation plastic functional components,investment casting of complex cast using SLS mould,laser sintering of pre-coated sand and casting technology are studied.The laser sintering thermodynamics behavior of thermoplastics is investigated and it is found that laser sintering is a spontaneous process inclining to decrease surface area driven by surface tension.The factor in which affect the surface tension such as particle size,particle geometric pattern etc.,would also affect the laser sintering process.The three sub-model of laser sintering are developed:optical model,thermal model and sintering model.It would provide therory sustaining to develop materials and laser sinering technology.The laser sintering behavior and mechanical properties of amorphous polymers are studied,containing polystyrene(PS),high-impact polystyrene and polycarbonate(PC). In order to further improve the properties of the prototype,the post-processing—infiltrating with reinforcing resin—must be implemented.The compatibility and permeability of the reinforcing resin and prototype are studied.The ideal curing model is also developed.Through reinforce,the mechanical properties of the prototype are much improved with good precision.The method of preparing nylon12 powder by using a dissolution-precipitation process was investigated in this paper,the factors that affect the process of solvent,stir speed,dissolution temperature,cooling method and technology to particle geometric pattern,particle size and its distribution were studied.In order to obtain narrow particle size distribution and near spheric shape powder,this paper proposed that adding a nucleation stage at cooling process and using 0.1%SiO₂ as nucleating agent.The powder can be used in SLS need not classify.In the presence of air,nylon 12 is liable to thermo-oxidafive decomposition,the result shows that adding compound antioxidants in the process of prepare nylon 12 powder are effective methods to inhibit oxidation.The laser sintering behavior of the nylon 12 powder prepared by dissolution-precipitation is studied.The result shows that although the main shrinkage in SLS process is crystal shrinkage,the warp is relative to melt viscosity but not crystallization temperature.The SLS process is greatly affected by particle geometric pattern,particle size and its distribution,oxidation,nucleating agent,and laser sintering technology.At ideal conditions,the sintering window is only 1～2℃.With strictly control of the part bed temperature and laser sintering technology,the SLS nylon 12 prototype with 41 MPa of tensile strength,47.8 MPa of flexural strength,39.2 MPa of impact strength,1.30GPa of flexural modulus have been fabricated.In order to improve the mechanical properties of SLS nylon 12 parts and avoid the disadvantages of enhancer to laser sintering process.Dissolution-precipitation was introduced to prepare SLS nylon 12/potassium titanium whiskers(PTW) composites powder.The results indicate that when the PTW content of the composites is small,the shape of the powder becomes more regular and the particle diameter distribution becomes narrower.The mechanical properties are greatly improved by PTW.The tensile strength,bending strength,and bending modulus of the composites containing 20 wt% PTW are 68.3 MPa,110.9 MPa,and 2.83 GPa,respectively,which are much higher than those of nylon-12 and glass-filled nylon-12.But it can not be used to prepare nylon 12 nanocomposite of nano-SiO₂ and clay,as the low density and high viscosity of the powder.As the high pre-heated temperature and narrow sintering window,the SLS process is strict and complex for nylon 12,more important it can not be used in SLS equipment with large working face.This paper proposed and proved that it is possible to achieve low temperature sintering for SLS in theory.The experiments results indicate that:high refractive glass bead can increase air temperature of working chamber,as a result it have a wide sintering window;plasticizing agent can decrease the pre-heated temperature in SLS process,but the plasticizing agent tend to volatilize,as a result,the SLS technology is not stability;,thermoplastic elastomer modified nylon 12 alloy can deeply decrease the part-bed temperature.This paper prepared the alloy powder by dissolution-precipitation. In order to solve the conglobation of the alloy powder,solvent substitute is used.The sintering window is 37℃at the range of 123～160℃.The structure of sintered nylon 12 alloy was characterized by(X-ray diffraction) XRD,and SEM,transmission electron microscopy(TEM),the mechanism of low temperature sintering was proposed.The nylon 12 alloy can be used to fabricate large scale prototype in SLS equipment with large working face.As high melting temperatures and melting viscosities,it is very difficult to dewax completely for the SLS pattern.In order to avoid carbonization residues,the proper dewaxing technology is developed in this study:(1) the wax is removed first at low temperature,(2) the temperature is then increased to 200-250℃to allow most of the HIPS to flow out and finally(3) baking is performed at a temperature higher than 500℃. Furthermore,examples are used to illustrate the settings of slag extraction risers,riser vents and associated running channels for producing complex parts.The precoated foundry sands can be used to make the complex sand model and sand core.This paper researched the sintering mechanism of precoated foundry sands as thermosetting.The result indicate that the curing of the resin is notably affected the sintering characteristics.The post curing technology was also studied.At last the eligible casting part is obtained.The study in this paper,on many aspects,supplies gaps of SLS field in China.The development of plastic function parts and casting complex cast related process for SLS will meet the requirement of the market in making,and will promote the applications and development of SLS.