1. Introduction
Selective laser sintering (SLS) is one of the most efficient additive manufacturing techniques, which employs laser scanning to make a three-dimensional part from powder materials. Polymers are heavily utilized materials in SLS because of their thermal properties. However, there are several problems in polymer materials by SLS, including molding dimensional error, strength and post-processing technique. Proper process equipment and experimental conditions may lead to great differences in molding.
Many investigations on sintering molding of polymer have been conducted in recent years. It is important that SLS parts have high accuracy to ensure the functional requirements. Raghunath et al. investigated the relationship between shrinkage and SLS parameters of polyamide and optimum shrinkage conditions were obtained by Taguchi method [
1]. Senthilkumaran et al. [
2] presented an experimental study to understand the shrinkage in SLS polyamide 12 and the effect of building strategies on shrinkage behavior. Singh et al. [
3,
4] investigated the correlation between different SLS process parameters and dimensional accuracy by developing mathematical models. The above studies mainly focused on the semicrystalline polymer. Shi et al. [
5] investigated the sintering parameters and properties of the sintered parts of high-impact polystyrene and found the specimen had a good accuracy. Recently, Wei et al. [
6] investigated the effects of laser power, scanning speed and layer thickness on the dimensional precision and morphology of the polystyrene (PS) parts. It was indicated that the optimum dimensional accuracy could be obtained by suitable process parameters. In addition, warpage is also a key factor in part accuracy in the SLS process. Wang et al. [
7] studied the influence of process parameters on warpage when sintering PS in SLS. The results provided a better understanding of the influences of processing parameters on warpage in SLS process.
Besides the sintering part’s accuracy, SLS should be able to provide sufficient mechanical properties to meet the functional requirements. Beal et al. [
8] evaluated the effect of the variation of the energy density on the mechanical properties of polyamide. The research results indicated that laser power had more influence on the density and mechanical properties than the scan speed. Yan et al. [
9] investigated both the polystyrene and styrene–acrylonitrile copolymer SLS process. The influences of laser energy density on the sintering density, mechanical properties and dimensional accuracy of the SLS parts were studied. Singh et al. [
10,
11] carried out a set of experimental studies on optimization of shrinkage, mechanical properties and density. The effects of the different process parameters were also analyzed. Zhu et al. [
12] analyzed the tensile properties and thermomechanical properties of polypropylene samples fabricated by SLS. Bai et al. [
13] performed a study on the feasibility of processing a polyethylene by SLS. The effect of temperature on the mechanical properties of the sintered parts was also discussed. Recently, Pilipovic et al. [
14] expanded the process parameters that affect the mechanical properties and the process parameters depending on the manufacturing strategy and layer thickness was confirmed. However, the effects of process parameters on dimensional accuracy and mechanical properties were different.
Above all, the amorphous polymer SLS parts have very low relative density and much lower tensile strength than that of their fully dense forms, while the semicrystalline polymer SLS parts have higher relative density and comparable tensile strength compared to their fully dense forms. Yan et al. [
15] studied the difference in the part bed temperature, relative density, tensile strength and dimensional accuracy of the SLS parts between PS and nylon-12 (PA12). They found that the dimensional accuracy of the SLS parts of amorphous polymers was higher than that of semicrystalline polymer SLS parts with the same processing parameters. Therefore, post-processing methods were employed to enhance the mechanical properties of amorphous polymers SLS parts without damaging their relatively high dimensional accuracy. At present, infiltrating with resin or wax is the most commonly used post-processing methods. Shi et al. [
16,
17,
18] applied epoxy resin to improve the mechanical properties of PC SLS parts and found that the mechanical properties were improved remarkably by post-processing.
It is difficult to measure the real time temperature changes, melting zone size and the thickness of solid phase during the SLS process. Therefore, finite element (FE) simulation has been introduced to describe the SLS process. Dai and Shaw [
19] investigated the effect of the volume shrinkage during SLS using a three-dimensional finite element model, and indicated that the models with the volume shrinkage provided better prediction in the shape and size of the sintering parts. Dong et al. [
20] developed a transient three-dimensional finite element model, which could simulate the temperature and density distribution. The simulated results showed that the sintered depth was dependent on the laser power and scanning speed. Recently, Ganci et al. [
21] proposed a numerical approach to model the SLS of polypropylene. In their study, a 3D thermal thermomechanical model was set up to predict both the temperature fields and the distortion of the sintered parts. Lindberg et al. [
22] investigated SLS PA12 using FE method. It was found that FE simulations given good estimations for the location of a failure. Mokrane et al. [
23] developed a numerical tool for simulating SLS of polymer powders, which was validated at the numerical level and tested against the experimental study. These studies indicate that the FE model can provide good prediction of temperature and mechanical properties evolutions during SLS process.
Polystyrene has become a widely used polymer material for SLS due to its relatively low cost and good process ability [
24,
25]. However, few systematic studies on the whole molding process and post-processing of PS SLS process were carried out. The relationship between the different printing parameters and part properties can be investigated with application of Taguchi method [
26]. In this paper, orthogonal experiment was used to analyze the effects of laser power, scanning speed, layer thickness and scanning interval on sintered parts. Subsequently, the influences of different molding process parameters on the dimension, warpage and sintering density of the sintered parts were discussed. In addition, a FE method was employed to further study temperature evolutions and sintering mechanisms. At last, the infiltrated epoxy resin experiment was presented to investigate the effects of different epoxy resin systems on mechanical properties of the parts.
4. Conclusions
The effect of SLS processing parameters on the part quality of PS materials was investigated in this study, especially the dimensional accuracy and warpage. The influences of laser power, scanning speed, layer thickness and scanning interval on the part were analyzed by orthogonal tests. The range analysis indicated that the optimal combination of process parameters was laser power of 9.8 W, scanning speed of 800 mm/s, layer thickness of 0.24 mm and scanning interval of 0.28 mm, respectively.
An FE model was developed to analyze the temperature change and distribution of SLS processing to further understand the sintering mechanism. The results show that the greater laser energy density would produce the structural warpage. As a result of secondary sintering, the dimensional of height and the other direction may change in opposite directions. The numerical results also show that the small part size decreases warpage and shrinkage.
Post-processing methods of infiltrating epoxy resin are able to improve the mechanical properties of PS parts. Two different epoxy systems were selected to infiltrate the PS parts. The tensile and bending tests were conducted to the infiltrated epoxy parts, the results of which show that infiltrating epoxy resin can significantly increase their strength. The 593 epoxy system works better than the T31 epoxy system under the same ratio. The maximum tensile and bending strength were 36.81 Mpa and 52.22 MPa, respectively.