POLISHING OF CERAMIC TILES

Transcription

1 MATERIALS AND MANUFACTURING PROCESSES, 17(3), (2002) POLISHING OF CERAMIC TILES C. Y. Wang,* X. Wei, and H. Yuan Institute of Manufacturing Technology, Guangdong University of Technology, Guangzhou , P.R. China ABSTRACT Grinding and polishing are important steps in the production of decorative vitreous ceramic tiles. Different combinations of finishing wheels and polishing wheels are tested to optimize their selection. The results show that the surface glossiness depends not only on the surface quality before machining, but also on the characteristics of the ceramic tiles as well as the performance of grinding and polishing wheels. The performance of the polishing wheel is the key for a good final surface quality. The surface glossiness after finishing must be above 208 in order to get higher polishing quality because finishing will limit the maximum surface glossiness by polishing. The optimized combination of grinding and polishing wheels for all the steps will achieve shorter machining times and better surface quality. No obvious relationships are found between the hardness of ceramic tiles and surface quality or the wear of grinding wheels; therefore, the hardness of the ceramic tile cannot be used for evaluating its machinability. Key Words: Ceramic tiles; Grinding wheel; Polishing wheel INTRODUCTION Ceramic tiles are the common decoration material for floors and walls of hotel, office, and family buildings. Nowadays, polished vitreous ceramic tiles are more popular as decoration material than general vitreous ceramic tiles as they can *Corresponding author. cywang@gdut.edu.cn 401 Copyright q 2002 by Marcel Dekker, Inc.

2 402 WANG, WEI, AND YUAN have a beautiful gloss on different colors. Grinding and polishing of ceramic tiles play an important role in the surface quality, cost, and productivity of ceramic tiles manufactured for decoration. The grinding and polishing of ceramic tiles are carried out in one pass through polishing production line with many different grinding wheels or by multi passes on a polishing machine, where different grinding wheels are used. Most factories utilize the grinding methods similar to those used for stone machining although the machining of stone is different from that of ceramic tiles. Vitreous ceramic tiles are thin, usually 5 8 mm in thickness, and are a sintered material, which possess high hardness, wear resistance, and brittleness. In general, the sintering process causes surface deformation in the tiles. In the machining process, the ceramic tiles are unfixed and put on tables. These characteristics will cause easy breakage and lower surface quality if grinding wheel or grinding parameters are unsuitable. To meet the needs of ceramic tiles machining, the machinery, grinding parameters (pressure, feed speed, etc.), and grinding wheels (type and mesh size of abrasive, bond, structure of grinding wheel, etc.) must be optimized. Previous works have been reported in the field of grinding ceramic and stone [1 4]. Only a few reports have mentioned ceramic tile machining [5 8], where the grinding mechanism of ceramic tiles by scratching and grinding was studied. It was pointed out that the grinding mechanism of ceramic tiles is similar to that of other brittle materials. For vitreous ceramic tiles, removing the plastic deformation grooves, craters (pores), and cracks are of major concern, which depends on the micro-structure of the ceramic tile, the choice of grinding wheel and processing parameters, etc. The residual cracks generated during sintering and rough grinding processes, as well as thermal impact cracks caused by the transformation of quartz crystalline phases are the main reasons of tile breakage during processing. Surface roughness R a and glossiness are different measurements of the surface quality. It is suggested that the surface roughness can be used to control the surface quality of rough grinding and semi-finish grinding processes, and the surface glossiness to assess the quality of finishing and polishing processes. The characteristics of the grinding wheels, abrasive mesh size for the different machining steps, machining time, pressure, feed, and removing traces of grinding wheels will affect the processing of ceramic tiles [9]. In this paper, based on the study of grinding mechanisms of ceramic tiles, the manufacturing of grinding wheels is discussed. The actions and optimization of grinding and polishing wheels for each step are studied in particular for manualpolishing machines. GRINDING AND POLISHING WHEELS FOR CERAMIC TILE MACHINING The machining of ceramic tiles is a volume-production process that uses significant numbers of grinding wheels. The grinding and polishing wheels for

3 POLISHING OF CERAMIC TILES ceramic tile machining are different from those for metals or structural ceramics. In this part, some results about grinding and polishing wheels are introduced for better understanding of the processing of ceramic tiles. Grinding and Polishing Wheels Ceramic tiles machining in a manual-polishing machine can be divided into four steps each using different grinding wheels. Grinding wheels are marked as 2#, 3#, and 4# grinding wheels, and 0# polishing wheel; in practice, 2# and 3# grinding wheels are used for flattening uneven surfaces. Basic requirements of rough grinding wheels are long life, high removal rate, and lower price. For 2# and 3# grinding wheels, SiC abrasives with mesh #180 (#320) are bonded by magnesium oxychloride cement (MOC) together with some porous fills, waterproof additive, etc. The MOC is used as a bond because of its low price, simple manufacturing process, and proper performance. The 4# grinding wheel will refine the surface to show the brightness of ceramic tile. The GC#600 abrasives and some special polishing materials, etc., are bonded by MOC. In order to increase the performance such as elasticity, etc., of the grinding wheel, the bakelite is always added. The 4# grinding wheels must be able to rapidly eliminate all cutting grooves and increase the surface glossiness of the ceramic tiles. The 0# polishing wheel is used for obtaining final surface glossiness, which is made of fine Al 2 O 3 abrasives and fill. It is bonded by unsaturated resin. The polishing wheels must be able to increase surface glossiness quickly and make the glossy ceramic tile surface permanent. Manufacturing of Magnesium Oxychloride Cement Grinding Wheels After the abrasives, the fills and the bond MOC are mixed and poured into the models for grinding wheels, where the chemical reaction of MOC will solidify the shape of the grinding wheels. The reaction will stop after 30 days but the hardness of grinding wheel is essentially constant after 15 days. During the initial 15-day period, the grinding wheels must be maintained at a suitable humidity and temperature. For MOC grinding wheels, the structure of grinding wheel, the quality of abrasives, and the composition of fill will affect their grinding ability. All the factors related to the chemical reaction of MOC, such as the mole ratio of MgO/MgCl 2, the specific gravity of MgCl 2, the temperature and humidity to care the cement will also affect the performance of the MOC grinding wheels. Mole Ratio of MgO/MgCl 2 When MOC is used as the bond for the grinding wheels, hydration reaction takes place between active MgO and MgCl 2, which generates a hard

4 404 WANG, WEI, AND YUAN XMgðOHÞ 2 YðMgCl 2 Þ ZH 2 O phase. Through proper control of the mole ratio of MgO/MgCl 2, a reaction product with stable performance is formed. The bond is composed of 5MgðOHÞ 2 ðmgcl 2 Þ 8H 2 O and 3MgðOHÞ 2 ðmgcl 2 Þ 8H 2 O: As the former is more stable, optimization of the mole ratio of MgO/MgCl 2 to produce more 5MgðOHÞ 2 ðmgcl 2 Þ 8H 2 O is required. In general, the ideal range for the mole ratio of MgO/MgCl 2 is 4 6. When the contents of the active MgO and MgCl 2 are known, the quantified MgO and MgCl 2 can be calculated. Q1 Active MgO The content of active MgO must be controlled carefully so that hydration reaction can be successfully completed with more 5MgðOHÞ 2 ðmgcl 2 Þ8H 2 O: If the content of active MgO is too high, the hydration reaction time will be too short with a large reaction heat, which increases too quickly. The concentrations of the thermal stress can cause generation of cracks in the grinding wheel. On the contrary, if the content of active MgO is too low, the reaction does not go to completion and the strength of the grinding wheel is decreased. Fills and Additives The fills and additives play an important role in grinding wheels. Some porous fills must be added to 2# and 3# grinding wheels in order to improve the capacity to contain the grinding chips, and hold sufficient cutting grit. Waterproof additives such as sulfates can ensure the strength of grinding wheels in processing under water condition. Some fills are very effective in increasing the surface quality of ceramic tile, but the principle is not clear. Manufacturing of Polishing Wheels Fine Al 2 O 3 and some soft polishing materials, such as Fe 2 O 3,Cr 2 O 3, etc., are mixed together with fills. Unsaturated resin is used to bond these powders, where a chemical reaction takes place between the resin and the hardener by means of an activator. The performance of polishing wheels depends on the properties of resin and the composition of the polishing wheel. In order to contain the fine chips, which are generated by micro-cutting, some cheap soluble salt can be fed into the coolant. On the surface of the polishing wheel, the salt will leave uniform pores, which not only increase the capacity to contain chips and self-sharpening of the polishing wheel, but also improves the contact situation between polishing wheel and ceramic tiles.

5 POLISHING OF CERAMIC TILES Experimental Procedure Tests were carried out in a special manual grinding machine for ceramic tiles. Two grinding wheels were fixed in the grinding disc that was equipped to the grinding machine. The diameter of grinding disc was 255 mm. The rotating speed of the grinding disc was 580 rpm. The grinding and polishing wheels are isosceles trapezoid with surface area 31.5 cm 2 (the upper edge: 2 cm, base edge: 5 cm, height: 9 cm). The pressure was adjusted by means of the load on the handle for different grinding procedures. A zigzag path was used as the moving trace for the grinding disc. To maintain flatness and edge of the ceramic tiles, at least one third of the tile must be under the grinding disc. During the grinding process, sufficient water was poured to both cool and wash the grinding wheels and the tiles. Four kinds of vitreous ceramic tiles were examined, as shown in Table 1. T1 Two different sizes of ceramic A, A400 (size: mm 3 Þ and A500 (size: mm 3 Þ were tested to understand the effect of the tile size. For ceramic tile B or C, the size was mm 3 : The phase composition of the tiles was determined by x-ray diffraction technique. Surface reflection glossiness and surface roughness of the ceramic tiles and the wear of grinding wheels were measured. The grinding and polishing wheels were made in-house. The 2# grinding wheels with abrasives of mesh #150 and 3# grinding wheels with mesh #320 were used during rough grinding. Using the ceramic tiles with different surface toughness ground by the 2# grinding wheel for 180 sec, the action of the 3# grinding wheels were tested. The ceramic tile was marked as A500-1 (or B500-1, C500-1, A400-1) with higher initial surface toughness or A500-2 (or B500-2, C500-2, A400-2) with lower initial surface toughness. Two kinds of finishing wheels, 4#A and 4#B were made with the same structure, abrasivity, and process, but different composition of fills and additives. Only in 4#B, a few Al 2 O 3, barium sulfate, and magnesium stearate were added for higher surface glossiness. The composition of the polishing wheels 0#A and 0#B were different as well. In 0#B, a few white alundum (average diameter 1 mm), barium sulfate, and chrome oxide were used as polishing additives, specially. After ground by 4#A (or 4#B) grinding wheel, the ceramic tiles were polished with 0#A (or 0#B). The processing combinations with 4# grinding wheels and 0# Ceramic Tiles HV0.1 Table 1. Crystalline Grain Size (mm) Properties of Ceramic Tiles Mullite (vol.%) Quartz (vol.%) Vitreous Mass (vol.%) Porosity (vol.%) Pore Size (mm) A400, A B C

6 406 WANG, WEI, AND YUAN polishing wheels were marked as 4#A 0#A, 4#A 0#B, 4#B 0#A, 4#B 0#B for each ceramic tile. Surface Quality RESULTS AND DISCUSSIONS Effects of 2# and 3# Grinding Wheels In rough grinding with a 2# grinding wheel, the surface roughness for all the tiles asymptotically decreases as the grinding time increases, see Fig. 1. The initial F1 asymptote point of this curve represents the optimized rough grinding time, as continued grinding essentially has no effect on the surface roughness. In these tests, the surface roughness curves decrease with grinding time and become smooth at,120 sec. The final surface quality for different kinds of ceramic tiles is slightly different. In terms of the initial size of the tile, the surface roughness of ceramic tile A400 ð mm 3 Þ is lower than that of A500 ð mm 3 Þ: The surface roughness of ceramic tile B500 rapidly drops as the grinding time increases. Thus, it is easier to remove surface material from the hardest of the three kinds of the ceramic tiles (Table 1). However, as the final surface roughness of ceramic tile A500 is the same as that of ceramic tile C500, the hardness of the ceramic tile does not have a direct relationship with the final surface quality. In the 3# grinding wheel step, all craters and cracks on the surface of ceramic tiles caused by the 2# grinding wheel must be removed. If residual cracks and craters exist, it will be impossible to get a high surface quality in the next step. The surface roughness obtained by the 2# grinding wheel will also affect the surface F2 Figure 1. wheel. Surface roughness of several ceramic tiles as a function of grinding time for 2# grinding

7 POLISHING OF CERAMIC TILES F2 quality of next grinding step by the 3# grinding wheel. In Fig. 2, the actions of the 3# grinding wheels are given using the ceramic tiles with different initial Ra, which were ground by the 2# grinding wheel for 180 sec. The curves of surface vs. grinding time rapidly decrease in 60 sec. Asymptotic behavior essentially becomes constant after 60 sec. In general, the larger the initial surface roughness, the worse the final surface roughness. For example, for ceramic tile B500-1, the initial R a was 1.53 mm, the finial R a was 0.59 mm after being ground by the 3# grinding wheel. When the initial R a was 2.06 mm for ceramic tile B500-2, the finial R a was 0.67 mm. In Ref. [8], we studied the relations between abrasive mesh size and evaluation indices of surface quality, such as surface roughness and surface glossiness. In rough grinding, the ground surface of ceramic tile shows fracture craters. These craters scatter the light, so that the surface glossiness values are almost constant at a low level. It is difficult to improve the surface glossiness after F3 these steps. Figure 3 shows the slow increase in surface glossiness with time by means of the 3# grinding wheel. It can be seen that the glossiness of ceramic tile B500-1 is the highest. The surface glossiness of ceramic tile A400-1 is better than that of A500-1 because the effective grinding times per unit area for former is longer than for latter. These trends are similar to those for surface roughness in Fig. 2. Wear of Grinding Wheels The wear of grinding wheels is one of the factors controlling the machining cost. As shown in Fig. 4, the wear of grinding wheels is proportional to grinding F4 Figure 2. wheel. Surface roughness of several ceramic tiles as a function of grinding time for 3# grinding

8 408 WANG, WEI, AND YUAN Figure 3. wheel. Surface glossiness of several ceramic tiles as a function of grinding time by 3# grinding time for both the grinding wheels and the three types of ceramic tiles. The wear rate of the 3# grinding wheel is larger than the 2# grinding wheel. It implies that the wear resistance of the 3# grinding wheel is not as good as 2# for constant grinding time of 180 sec. When the slope of the curve is smaller, life of the grinding wheels will be longer. Comparison of the ceramic tiles hardness (Table 1) with the wear resistance behavior in Fig. 4 does not reveal a strong dependency. Therefore, the hardness of the ceramic tile cannot be used to distinguish the machinability. The difference of Figure 4. Wear of grinding wheels of several ceramic tiles as a function of grinding time for 2# and 3# grinding wheels.

9 POLISHING OF CERAMIC TILES initial surface roughness of ceramic tile will affect the wear of grinding wheel. In Fig. 4, the wear of the 3# grinding wheel for ceramic tile B500-1 is smaller than that for ceramic tile B The initial surface roughness of the latter is higher than that of the former so that additional grinding time is required to remove the deeper residual craters on the surface. Improvement of the initial surface roughness can be the principal method for obtaining better grinding quality and grinding wheel life during rough grinding. Effects of 4# Grinding Wheels and 0# Polishing Wheels Surface Quality The combination and the performance of 4# grinding and 0# polishing wheels show different results for each ceramic tile. The grinding quality vs. grinding (polishing) time curves are presented in Fig. 5, where all the ceramic tiles F5 were previously ground by 2# and 3# grinding wheels to the same surface quality. The surface glossiness is used to assess surface quality because the surface roughness is nearly constant as finishing or polishing time increases [8]. In this test, the ceramic tile A400 were fast ground by 4#A and 4#B grinding wheels [Fig. 5(a)]. The surface glossiness increased rapidly during the initial 90 sec and then slowly increased. The surface glossiness by grinding wheel 4#B is higher than by 4#A. Afterwards, polishing was done by four different combinations of finishing wheel and polishing wheel. By means of polishing wheels 0#A and 0#B, we processed the surface finished by 4#A grinding wheel (described as 4#A 0#A and 4#A 0#B in Fig. 5), and the surface finished by 4#B grinding wheel (described as 4#B 0#A and 4#B 0#B in Fig. 5). The curves of surface glossiness vs. polishing time show parabolic behavior. After 60 sec of polishing, the surface glossiness reaches to,508, then slowly increases. The polishing wheel 0#B gives a better surface quality than 0#A. In Fig. 5(a), the maximum surface glossiness of ceramic tile A400 is about,75 by 4#B 0#B. The relation between initial surface glossiness and the final surface quality is not strong. The effect of pre-polishing surface glossiness can be observed by 0#B polishing wheel as polishing ceramic tile A500 [Fig. 5(b)]. The maximum surface glossiness that can be achieved is 748 in 240 sec by 4#A 0#B or 4#B 0#B. This value is lower than that of ceramic tile A400 [Fig. 5(a)]. The final surface glossiness by 4#A grinding wheel is highly different from that by 4#B grinding wheel for ceramic tile B500, as shown in Fig. 5(c), but the final polishing roughness is the same when 0#A polishing wheel is used. The better performance of 0#B polishing wheel is shown because the surface glossiness can increase from 17 to 228 in 30 sec. The maximum surface glossiness is 658 by 4#B 0#B. The curves of polishing time vs. surface glossiness in Fig. 5(d) present the same results as polishing of ceramic tile B500 [Fig. 5(c)]. With 0#A polishing

10 410 WANG, WEI, AND YUAN Figure 5. Surface glossiness for ceramic tiles (a) A400, (b) A500, (c) B500, and (d) C500 as a function of grinding (polishing) time for 4# grinding wheels and 0# polishing wheels. wheel, the action of pre-polishing surface glossiness is significant. The best value of surface glossiness in 240 sec is 708 by 4#B 0#B as polishing ceramic tile C500. The results discussed earlier describe that the surface glossiness by 0# polishing wheel will depend not only on the pre-polishing surface glossiness formed by 4# grinding wheel, but also on the characteristics of the ceramic tiles and the performance of 0# polishing wheel. The differences of initial surface glossiness and final surface glossiness are larger for 4#A and 4#B. If the prepolishing surface roughness is lower, the final surface glossiness will be higher.

11 POLISHING OF CERAMIC TILES Figure 5. Continued. The polishing time taken to achieve the maximum surface glossiness will be also shorter. The initial surface quality will limit the maximum value of polishing surface glossiness that can be obtained. To reach a final surface glossiness of above 608, the minimum pre-polishing surface glossiness must be above 208. The performance of the polishing wheel is the key to good surface quality. The polishing ability of the polishing wheels depends on the properties of the ceramic tiles as well. Even if the same grinding and polishing wheels are used, on all four ceramic tiles, the maximum surface glossiness values of ceramic tiles are

12 412 WANG, WEI, AND YUAN different. The ceramic tile A500 shows the best surface glossiness, and ceramic tile B500 shows the worst, although it is easier to roughly grind ceramic tile B500. The peak value of the surface glossiness is also limited by the properties of ceramic tiles. Wear of Grinding and Polishing Wheels The life of 4# grinding wheels and 0# polishing wheels (Fig. 6) are longer F6 than those of the rough grinding wheels (Fig. 4). For finer grinding (Fig. 6), it is impossible to distinguish the relation between grinding wheels and ceramic tiles. Polishing wheels have longer life because they produce more plastic deformation than removal. SUMMARY OF RESULTS (1) The performance of grinding and polishing wheels will affect its life and the surface quality of ceramic tiles. (2) In ceramic tile machining, the surface quality gained in the previous step will limit the final surface quality in the next step. The surface glossiness of pre-polishing must be higher than 208 in order to get the highest polishing quality. The optimization of the combination of grinding wheels and polishing wheels for all the steps will shorten machining time and improve surface quality. Optimization must be determined for each ceramics tiles. Figure 6. Wear of grinding wheels 4# and polishing wheels 0# for several ceramic tiles as a function of grinding time.

13 POLISHING OF CERAMIC TILES (3) The effect of hardness of ceramic tiles is not direct, thus the hardness of ceramic tiles cannot be used for evaluating the machinability of ceramic tiles. ACKNOWLEDGMENT The authors thank Nature Science Foundation of Guangdong Province and Science Foundation of Guangdong High Education for their financial support. REFERENCES 1. Wang, C.Y.; Liu, P.D.; Chen, P.Y. Grinding Mechanism of Marble. Abrasives Grinding 1987, 2 (38), 6 10, (in Chinese). Q2 2. Inasaki, I. Grinding of Hard and Brittle Materials. Ann. CIRP 1987, 36 (2), 436B. 3. Zhang, B.; Howes, D. Material Removal Mechanisms in Grinding Ceramics. Ann. CIRP 1994, 45 (1), Malkin, S.; Hwang, T.W. Grinding Mechanism for Ceramics. Ann. CIRP 1996, 46 (2), Q Black, I. Laser Cutting Decorative Glass, Ceramic Tile. Am. Ceram. Soc. Bull. 1998, 77 (9), Black, I.; Livingstone, S.A.J.; Chua, K.L. A Laser Beam Machining (LBM) Database for the Cutting of Ceramic Tile. J. Mater. Process. Technol. 1998, 84 (1 3), Jiang, D.F. Mirror Surface Polishing of Ceramic Tile. New Building Mater. 1994, 20 (11), 27 30, (in Chinese). 8. Ma, J.F. Analysis on Man-Made Floor Brick and Manufacture of Grinding Segment Used for Floor Brick. Diamond Abrasive Eng. 1996, 6 (95), 35 46, (in Chinese). 9. Wang, C.Y.; Wei, X.; Yuan, H. Grinding Mechanism of Vitreous Ceramic Tile. Chin. J. Mech. Eng. 1998, 9 (8), 9 11, 46 (in Chinese).