Preparation and selective laser sintering of nylon-12 coated metal powders and post processing

https://doi.org/10.1016/j.jmatprotec.2009.06.010Get rights and content

Abstract

A dissolution–precipitation process was successfully developed to prepare nylon-12 coated carbon steel powders. The SEM and laser diffraction particle size analysis results show that the metal particles are well coated by nylon-12 resin; therefore, an effective method for preparing nylon-12 coated metal powders is provided. Green parts were formed from the coated powders by selective laser sintering (SLS) process, and when the nylon-12 content in the coated powder was 1.0 wt% and the applied laser energy density was 0.06 J/mm2, the SLS green parts had sufficient strengths for features as small as 0.1 mm to be built and post-processed, and relatively high dimensional accuracy. The SLS green parts were post-processed by binder decomposition and epoxy resin infiltration, and the obtained epoxy-infiltrated parts have the dimensional errors in the X, Y and Z directions of −0.30, −0.32 and −0.25% respectively, and the bend strength, bend modulus, tensile strength and impact strength of 93.4 MPa, 14.7 GPa, 70.3 MPa and 12.4 MPa respectively.

Introduction

SLS is a powder-based rapid prototyping (RP) process, which directly forms solid components according to a three-dimensional CAD model by selective sintering of successive layers of powdered raw materials (Kumar, 2003). While the capability of SLS to produce functional objects directly from metals is still under development, indirect methods of producing functional objects from metals have been widely used. In an indirect method of SLS process, the laser is used to melt a polymer binder and produce a “green” part in which the metal particles are bound together by polymer–polymer bonds. The green part is usually porous, has low mechanical properties and need to be post-processed. This green part is processed in a high temperature furnace to remove the polymer binder, and then the binder-free part is sintered or further post-processed to form a nearly full density part with geometric precision that is comparable to that of the green part (Beaman et al., 1997).

At present, two methods have been used to add polymer binder to metal powders. One method is to mechanically mix polymer binder powders with metal powders. Liu et al., 2006, Liu et al., 2007 manufactured metal parts via the indirect SLS process, using admixtures of metal and epoxy resin powders. Epoxy resin contents in these admixtures were 4–5 wt% (about 24–29 vol%). Sercombe and Schaffer, 2003, Sercombe and Schaffer, 2004 formed the green parts by laser sintering the blend of aluminium alloy and 4 wt% (10 vol%) copolymer polyamide 6–12 binder. In general, the mechanical mixing method has the advantage of simple process. However, Beaman et al. (1997) revealed that the mixed powders have obvious disadvantages of powder segregation during shipping and poor binder efficiency. The other method is to prepare polymer binder coated metal powders. Joel and Neal (2000) reported that coated powders are readily handled, conveniently shipped and may be stored for long periods of time without the separation of components encountered in mixed powders. More importantly, at the same binder content, the green strengths of coated powders are found to be much higher than those of mixed powders. Balasubramanian (1995) indicated that SLS green parts made from coated metal powders were found to yield higher strengths than those made from mixed powders at the same polymer content. Subramanian et al. (1995) revealed that the bend strengths of the spray-coated alumina powders are three to four times those of the mixed powders at the same binder volume fraction. Nowadays, the most commonly used coating technology is a spray drying process. The materials used in this process are a class of emulsified amorphous polymer binders, based on poly(methyl methacrylate) (PMMA), copolymers of MMA (methyl methacrylate) and BMA (butyl methacrylate) monomers, which have been developed at the University of Texas to permit the formation of metal parts by the indirect SLS process. In the spray drying process, polymer emulsion is first synthesized by emulsion polymerizaion, and then the binders are coated on to metal particles by spray drying a slurry of metal powders in the polymer emulsion. Agglomerated, porous particles are formed by this coating process, which typically contain about 20 vol% polymer (Beaman et al., 1997). The PMMA-based binder coated powders has been successfully used to prepare metal and ceramics shapes by the indirect SLS process. For instance, Subramanian et al. (1995) formed alumina parts by the indirect SLS process from PMMA coated Al2O3 particles; Vail et al. (1996) developed a thermal of polymer degradation during selective laser sintering of PMMA or poly(MMA–BMA) coated ceramic powders; Balasubramanian (1995) prepared injection molds by the indirect SLS process from PMMA or poly(MMA–BMA) coated metal powders. However, the polymer binder contents in these coated powders are relatively high, which goes against accuracy of final parts, and Chua et al. (2004) pointed out that the spray drying technique to produce polymer coated powders gives rise to some problems such as a labor-intensive preparation process and high requirements for materials and equipment.

In this study, nylon-12 resin was used as a polymer binder, and a dissolution–precipitation process was developed to prepare nylon-12 coated carbon steel powders. In the dissolution–precipitation process, the metal powder was firstly dispersed in the nylon-12 solution by stirring intensively at about 160 °C. Then, the suspension liquid was cooled gradually and nylon-12 recrystallized to uniformly coat on the surfaces of the metal particles. Finally, when the solvent was recovered, nylon-12 coated metal powders were prepared. The green parts made from the coated powder with 1.0 wt% (about 7 vol%) nylon-12 have sufficient strength for features as small as 0.1 mm to be built and post-processed and relatively high dimensional accuracy. After the binder in the green parts was removed by a decomposition process, the binder-free parts were infiltrated with a high temperature resistance epoxy rein, and fully dense carbon steel/epoxy functional parts were manufactured.

Section snippets

Materials

The nylon-12 pellets were obtained from Degussa Co. in Germany. The properties of the nylon-12 resin according to its specification are listed in Table 1. Selecting nylon-12 as the polymer binder for the indirect SLS process was largely governed by the previous works (e.g. Caulfield et al., 2007), which suggested that nylon-12 SLS parts have relatively high densities and strengths, and nylon-12 has good interfacial adhesion with metals. The carbon steel powder was provided by Powder Metallurgy

Powder morphology

The SEM micrographs of the carbon steel powder and CP1.0 are shown in Fig. 2(a) and (b), respectively. As shown in Fig. 2(a), the particles of the carbon steel powder have a spherical shape and smooth surfaces. By comparing Fig. 2(a) and (b), it can be found the particles of CP1.0 become less regular and have rougher surfaces than those of the carbon steel powder. This result indicates that the carbon steel particles are coated by nylon-12 resin in the dissolution–precipitation process. At

Conclusions

Nylon-12 is used as a polymer binder for making parts by the indirect SLS process. The dissolution–precipitation process is developed to prepare the nylon-12 coated carbon metal powers. The SEM and laser diffraction particle size analysis results show that the metal particles are well coated by nylon-12 resin. Therefore, this study provides an effective method for preparing nylon coated metal powders. The nylon-12 content and laser energy density are determined as 1.0 wt% and 0.06 J/mm2,

Acknowledgements

This work is financially supported by Opening Project of the Key Laboratory of Polymer Processing Engineering, Ministry of Education, China (20061006) and Postdoctoral Foundation in China (20070410277). The authors greatly thank the Analytical and Testing Center of HUST for the measurements.

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