Preparation Of Polymer And Its Composite Powders And Their Selective Laser Sintering

Selective laser sintering (SLS) is a powder-based rapid prototyping (RP) process, which directly forms 3D solid components according to a three-dimensional CAD model by selective sintering of successive layers of powdered raw materials. One of the important applications of SLS process is to make high-quality functional parts using polymer and its composite powders. Although the researchers at home and abroad have deeply investigated the preparation of the polymer and its composite powders and their SLS process, there still exist many technical problems that need to be solved. Therefore, systematically investigations have been carried out, aiming to solve some of the key technical problems.The effects of polymeric characteristics including particle size, particle size distribution, particle shape, viscosity, aggregation structure and bulk strength on the SLS process were systematically studied. Thus, a theoretical basis for the selection and preparation of polymer powders for SLS was provided.The causes and influencing factors of secondary sintering and "Bonus Z" were analyzed, and thus the methods to reduce secondary sintering and "Bonus Z" were provided. Therefore, it gave important guidance to control secondary sintering and "Bonus Z", and improve dimensional accuracy of sintered parts.Polystyrene (PS) is the most widely used amorphous polymer in the SLS process; however, the mechanical properties of PS SLS parts are relatively lower, part accuracy is easy to be deteriorated when the parts are post-processed. Therefore, another amorphous polymer, styrene-acrylonitrile copolymer (SAN), was used as an SLS material. An SAN powder was prepared and formed by the SLS process, and then the SAN SLS parts were post-processed by inlfiltrating with an epoxy resin. In addition, the properties of the PS and SAN SLS parts were compared. The results show: the strength of the SAN parts is obviously higher than that of the PS parts; therefore, SAN can be used to manufacture the more complex and delicate parts via SLS; after infiltrating with the epoxy resin, the SAN parts have relatively high accuracy and mechanical properties and can be used as plastic factional parts.At present, the polymer coated metal powders for the indirect SLS process are usually prepared by spray drying process. However, the spray drying technique to produce polymer coated powders gives rise to some problems such as a labor-intensive preparation process, complicated technology and high requirements for materials and equipment. The dissolution-precipitation process was successfully developed to prepare nylon-12 coated metal powders. In this process, firstly, metal powders are uniformly distributed in the nylon-12 alcohol solution by intensively stirring in a closed vessel at a high temperature. Then the suspension solution is cooled gradually, and nylon-12 recrystallizes taking metal particles as heterogeneous nuclei and coats the surfaces of metal particles. Finally, nylon-12 coated metal powders are formed. The dissolution-precipitation process does no harm to environment, and has the characteristics of simple technology and low requirement for equipment. The nylon-12 coated metal powders were used in the following two applications.a) Aluminum powders were used to reinforce nylon-12 SLS parts, and nylon-12 coated aluminum powders were prepared by the dissolution-precipitation process. The effects of aluminum content and particle size on the properties of the SLS parts were investigated. The results show that: the tensile strength, flexural strength and flexural modulus increase by about 10.4%, 62.1% and 122.3% respectively when the mass fraction of the aluminum powder increases from 0 to 50%, while elongation at break and impact strength decrease by about 65.0 and 74.4% respectively; the tensile strength, elongation at break and impact strength all increase with decreasing the particle sizes of the aluminum powders.b) Nylon-12 coated metal powders for the indirect SLS process were prepared by the dissolution precipitation process. The coated powders have only 1.0 wt% nylon-12, and their SLS green parts have sufficient strength for features as small as 1.0 mm to be built and post-processed and high dimensional accuracy. The green parts were subjected to the post processing processes including binder decomposition and infiltration of epoxy resin, and then the metal/polymer composite parts with relatively high strength and accuracy were formed.At present, micron-scale inorganic fillers have been widely used to reinforce nylon SLS parts. Although those fillers can improve some mechanical properties such as the modulus and hardness, the impact strength is usually decreased greatly. Nanosilica was used to reinforce nylon-12 SLS parts, and a nanosilica/nylon-12 composite powder was prepared by the dissolution precipitation process. The dispersion of nanosilica particles in the nylon-12 matrix was examined by scanning electron microscope (SEM), and the effects of nanosilica on the thermal and mechanical properties of nylon-12 SLS parts were investigated. The results show: the nanosilica particles disperse uniformly on a nano-scale level in the nylon-12 matrix; at the nanosilica content of 3 wt%, the tensile strength, tensile modulus and impact strength of the SLS specimens increase by 20.9%, 39.4% and 9.54% respectively, and elongation at break only dereases by 3.65%; the initial decomposition temperature increases by more than 30°C.This study accelerates the development of domestic polymer and its composite powders for SLS by solving the key technical problems mentioned above, which has certain significance for promoting the whole progress of SLS technology and further entending SLS technology applications in various industries in china.