SCANNING ELECTRON MICROSCOPE STUDY OF A GLASS-FILLED CRYSTALLINE POLYMER SINTERED PART A.E. Tontowi*) and T.H.C. Childs**) *)
Department of Mechanical Engineering, UGM, Jl.Grafika 2 Yogyakarta, Indonesia **) School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK Http://www.geocities.com/menaet; Email:
[email protected]
ABSTRACT A scanning electron microscope study of a composite-crystalline polymer has been conducted to observe its densification during sintering. It highlighted how densification occurs and how glass particles have given contribution in their influence density. A crystalline polymer, nylon-ll, with 30 % volume of glass particles has been used as a specimen in the experiments. Five laser energy densities have been applied to create densification: 0.008, 0.11, 0.14, 0.20 and 0.25 J/mm2. The observation results showed that the density of nylon-11 SLS parts depends on the laser energy density absorbed by the powder bed. The densification significantly began at the energy density of 0.11 J/mm2. It is suggested that the glass powder fraction in the laser beam's path is heated to a higher temperature than the polymer powder fraction, leading to superficial sintering before temperature equalisation by conduction takes place.
Keywords: Selective laser sintering, SEM, polyamide, glass-filled polyamide.
INTRODUCTION Selective laser sintering, known as SLSTM, is one of these technologies in which shaped parts are built up layer by layer of powder [5] following the computer images without human intervention [7]. The computer image, normally three-dimensional, is created applying CAD or CAE software. It is numerically sliced into series of twodimensional images in the form of STL files. The SLS machine then reproduces each of these 2D images by scanning the patterns onto the thin layers of powder. One sliced image is for one thin powder layer. Thickness of the powder layer typically is 0.125 to 0.3 mm. As the process of SLS basically is a sintering process, it is one which can be identified by the rate of increase of contact area between powders. This rate depends on the mechanisms of material transport [6]. There are four mechanisms summarized by Thummler and Thomma [8] including viscous flow or plastic flow, evaporationcondensation, volume diffusion and surface diffusion. However, in the context of SLS of polymers, it is believed that viscous flow mechanism is important. Frenkel [4] has the first to model viscous sintering shown in equation (1). He studied the coalescence of a pair of spheres as representative of the initial sintering of a body of packed powder. In his study, he supposed that densification occurred by viscous flow driven by capillarity. He formulated this by equating the energy release due to the decrease in surface area to the energy dissipation due to viscous flow. Equation (1) describes the resulting relationship x2 3 σ = t (1) a 2 η where x is the sintering neck radius, a is the radius of the particle, σ is the surface tension of the material, η is the viscosity and t is the sintering time. This model is applicable to amorphous materials because their sintering is driven by viscous flow [2]. However, under high temperatures, crystalline materials may
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sinter as amorphous materials since the viscosity of a crystalline material then becomes low enough to flow under the action of surface tension forces [1]. As the average particle size is only about 50 micron, therefore, sintering process among the particles and the porosity which determines the final density of the sintered part may only be studied using scanning electron microscope. EXPERIMENT METHOD Material The material that has been used in the research was a glass-filled crystalline polymer, Nylon-11 with 30 % volume of glass particles with the commercial brand name is Protoform Composite [3]. Specimens of the experiment were in the form of powder and sintered parts. Table-1 shows the properties of the powder. Figure 1 depicts the laser scanning strategy and the shape and dimensions of the specimen. Tabel-1: Properties of Protoform composite powder [3] Parameters Particle Shape Particle Size, µm Tap Density, kg/m3 Spc.Gravity @ 20 oC Melting Temp., oC Volume % Polymer Volume % Glass
Materials Protoform Composite Irregular+spherical (glass) 15-92 (ave :50) 780 1.47 193 70 30
Equipment Two equipments have been used sequentially. The first is EMSCOPE SC500 for gold coating of the specimen and the second is Cam Scan CS-44-EX with Link Systems EDX for capturing images.
50 mm
y
10 mm
z y 15 mm
x
x
Figure 1. Laser scanning direction strategy and shape of the specimen. Specimen and Preparation Specimens used in this experiment are powders and sintered part. The sintered part specimen shape is shown in Figure 1. Various laser powers: 1.62 (0.008 J/mm2), 2.10
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(0.11 J/mm2), 2.69 (0.14 J/mm2), 3.89 (0.20 J/mm2) and 4.89 watts (0.25 J/mm2) have been applied to build the sintered part specimens. As the nylon composite, both powder and sintered part block, is not conductive material, gold coating is needed to improve the electron conductivity before capturing images into the Cam Scan. a. Powder In preparation of specimens to the SEM, a small part of the powder was stuck on an holder using glue. It is then coated with gold in the EMSCOPE SC500. From this step, the specimen is ready to be fixed in the Cam Scan CS-44-EX. b. Sintered part Before gold coating, an interesting area that shows sintering between the powder particles was taken in the y-z plane. To obtain this area, the sintered part was cut by breaking it in the y-z plane. As similar to that of the powder specimen preparation, a small part of the sintered part was stuck on the specimen holder using glue. Subsequently, it was coated with the gold and ready for image capturing into the Cam Scan CS-44-EX. RESULTS AND DISCUSSION Table 2 shows the results of the observation of the Protoform composite for various energy densities or laser powers. Here, scanning speed (U) and scan spacing (s) were set to be 1257 mm/s and 0.152 mm, respectively. Table 2: SEM’s images of Protoform composite [9]. P(W) Powder
P/Us (J/mm2) 0.0000
1.62
0.0085
2.10
0.011
2.69
0.0141
3
3.89
0.0204
4.89
0.0256
As seen on the Table 2 above that sintering process has begun at the laser power of 1.62 watts which is indicated by forming a neck for part of the nylon particles. Whilst glass particles are still solid, in the form of sphere, and separated from the nylon particles. However, when the laser power increases to 2.1 watts, necking process occurs to almost the whole nylon particles. Melted nylon starts to flow filling the gaps between the glass particles and even covers the glass particles. At this stage, voids are still exist and the glass particles are still solid in the form of sphere. By continuing the increase of the laser power to 4.89 watts, the number of voids reduces due to the melted nylon fills the voids. It is obvious that, the high density of the final sintered part is due to the melted nylon fills the gaps between the solid glass particles and the density contribution of the glass particles. This densification process, however, can only be observed using SEM based method as the average of the particles size is only 50 micron. CONCLUSIONS • The density of the sintered part is a function of the laser power. It increases when the laser power increases. • Sintering occurs starting at the laser power of 1.62 watts, the nylon begins to melt and fills the gaps between glass particles. Here, it can be seen that glass particles are still solid. It is clearly why the solid glass particles contributes in improving the final density of the sintered part. • The highest density can be achieved at the laser power of 4.89 watts. However, at the laser power higher than that the density of the laser part may reduce due to burning out of the nylon particles. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Childs,T.H.C. and Tontowi, A.E., IMechE Journal, Part B, (2001) Childs, T.H.C., Berzins, M., Ryder, G.R. and Tontowi, A.E., IMechE Journal, 213, Part B, 333-349 (1999). DTM Corporation, Sinter Station-2000 System User’s Guide, Austin, Texas (1995). Frenkel, Journal of Physics-Academic of Science (USSR), 9, No. 5, 385-391 (1945). Kruth, J.P., Annals CIRP, 40, No. 2, 603-614 (1991). Kuczynski, G.C., Transactions Metall. Society A.I.M.E., 185, 169-178 (1949). Nelson, J.C., Xue, S., Barlow, J.W., Beaman, J.J., Marcus, H.L. and Bourell, D.L., Industrial Engineering Chemical Research, 32, 2305-2317 (1993). Thummler, M. and Thomma, W., Metallurgical Review, The Metals and Metallurgical Trust, 69-108 (1976). Tontowi, A.E., Selective Laser Sintering of Crystalline Polymers: Modelling and Experiment, Ph.D.- Thesis, University of Leeds, pp. 181-183 (2000).
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