- Email: [email protected]
Warping evolution of injection-molded ceramics
Journal of Materials Processing Technology 102 (2000) 14±18
Warping evolution of injection-molded ceramics Wenjea J. Tseng* Department of Materials Science and Engineering, I-Shou University, Kaohsiung 84008, Taiwan Received 15 August 1998
Abstract This study is focused on the shape retention of an injection-molded ZrO2 ceramics from their molded state to their sintered state. Varying mold temperatures (38±558C) were used for the molding process, and the molded samples were solidi®ed either at a relatively uniform temperature or under an inhomogeneous temperature distribution. Higher mold temperatures, as well as the application of non-uniform temperature distribution across the molding thickness, resulted in a signi®cant increase in the magnitude of the curvature of the molded samples. For all the moldings fabricated, the curvature changed prominently when the moldings were subjected to elevated temperatures; yet, insigni®cant binder loss was detected at such temperatures. The subsequent curvature evolution after thermal debinding was moderate, but the ®nal sintered deformation was found closely related to the green deformation. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Ceramic injection molding; Warpage; Deformation; Process
1. Introduction Dimensional control is an important aspect in ceramic injection molding (CIM). In order to obtain the full bene®ts of near-net-shape forming of the CIM process, the molded sample needs to be free from distortion and its warping deformation and shrinkage behavior be precisely predictable [1,2]. Mastery of the dimensions of sintered CIM parts would ultimately lead to the cost-effective manufacturing of complex-shaped ceramic products. The literature regarding the underlying mechanisms of the deformation behavior in CIM is not extensive [3±7]. An asymmetrical distribution of residual stress within ceramic moldings has been reported to be the main cause for the occurrence of distortion in the `green' state [3,6]. However, a molding with insigni®cant distortion in the green state does not necessarily mean a reduced level in its internal stress and hence a predictable shape retention after sintering may not be ensured. For example, relaxation of the residual stress occurs when CIM moldings are subjected to elevated temperatures [3]. This has been reported to lead to deformation of the moldings, presumably due to the rearrangement of the molded microstructure as the temperature of the molding
* Present address: Institute of Materials Science and Manufacturing, Chinese Culture University Yang Ming Shan, Taipei, Taiwan 11114.
was raised. The residual stress may have originated from non-uniform packing of suspensions during molding, and/or from cooling stresses, since solidi®cation of ceramic moldings within a geometrically constrained mold cavity is highly inhomogeneous [7,8]. Zhang et al. [4] further indicated that interactions between non-spherical particles (which tend to orient themselves along the direction of the molding pressure) and polymeric binder vehicles may also induce residual stresses in the CIM process. Distortion results when moldings are subjected to high temperatures, yet no signi®cant binder loss has been found in such situations. These previous studies all seem to suggest that the residual stress introduced during the forming process would become more prominent as the temperature of the ceramic moldings was raised for subsequent debinding purposes, and at a certain critical point (which has never been reported so far to the author's knowledge) that the stress would affect the bulk properties, such as the shape retention, of the moldings. As the temperature was raised above Tg of the binder species, one would suspect that moldings would soften and the shape retention of the moldings would become more susceptible to any stress remaining in the moldings. A study has shown that signi®cant distortion of ceramic moldings was accompanied by the major weight loss of binder vehicles during thermal debinding [5]. This suggests that the dimensions of molded CIM parts, in fact, changes in the process. How the warping deformation of the molded cera-
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 4 5 5 - 0
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
mics evolves during the course of CIM fabrication is not known, and this is the objective of the present study. 2. Experimental procedure A commercially available zirconia powder containing 3 mol% yttria (HSY3.0, Daiichi Kigenso Kagaku, Japan) with an average particle size of 0.25 mm and a speci®c surface area of 6.9 m2 gÿ1 was used for the study. The powder presented a relatively spherical shape and was extensively agglomerated. Low molecular weight organic binder composed of paraf®n wax and vinyl acetate polymer in a volumetric ratio of 6:4 was used as the major and minor ingredients [9]. The powders were ®rst ball-mixed with stearic acid (3 wt% on the basis of the powder loading) in a toluene medium for 24 h before the addition of the binders for another overnight mixing. The overall solid loading was held constant at 50 vol%. The mixtures were stirred and dried in a moderate temperature of about 408C, then kneaded through an extruder with a maximum temperature of 1008C (Model 70-20vex-6, KCK Industrial Co., Japan) to form pellets, followed by injection molding (Battenfeld BA 250/50 CDC, Austria) to obtain parallelepiped moldings of dimensions 4 mm5 mm 60 mm. A barrel temperature series of 90±140±150± 1508C from feed to nozzle was used. The molding pressure and cooling time were 80 MPa and 20 s, respectively. The mold temperature varied from 38 to 558C. The molded parts were ejected by pin ejectors with a nominal load of 2.5 kN, which corresponds to a pressure of 125 MPa on the molded dimensions. It has been shown that the warping deformation of injection-molded ceramics is closely related to the asymmetrical residual stresses induced from solidi®cation in the molding process [6]. A differential cooling rate was introduced deliberately by placing a thermal barrier onto the ejector side of the mold cavity prior to molding, with the aim of comparing the warping deformation with that for the noninsulated situation. This barrier material was paper with a controlled thickness of 300 mm, which can be removed from the molded parts after ejection. This was anticipated to introduce an augmented non-uniform temperature distribution across the molding thickness. In order to minimize the distortion resulting from premature ejection of the ceramic moldings, a theoretical estimation of the minimum cooling time for the injection molding has been conducted prior to the molding process. For rectangular-shaped moldings, the minimum cooling time can be expressed as [7]: s2 8 T M ÿ TW tc 2 ln 2 p T D ÿ TW p a
(1)
where tc is the cooling time; s is the wall thickness of the moldings; a is the thermal diffusivity; TM is the melt
15
temperature; TW is the molding wall temperature; and T D is the average temperature across the sample thickness during demolding. The thermal conductivity of the ceramic/binder mixture varied from 0.5±0.7 Wmÿ1 Kÿ1 at the working temperature employed, and the corresponding thermal diffusivity, a, was calculated from ak/rcp, where r is the density and cp is the heat capacity. By assuming that T D is equal to 658C (Tg of the binders is at about that temperature, determined from differential scanning calorimetry), the shortest cooling time was obtained for demolding ceramic moldings with the given thickness to be 8.4, 10.8 and 13.9 s for the varying mold temperatures of 38, 48 and 558C, respectively. For the moldings with the insulated layer of 300 mm thickness and a thermal conductivity of about 0.18 Wmÿ1 Kÿ1 [6], the `composite' conductivity by assuming that heat ¯ows mainly along the direction of the molding thickness is approximated by: k
k1 k2 n1 k2 n2 k1
(2)
where k1 and k2 represent the conductivities of two individual layers and n is the volume fraction of the respective layer. The calculated conductivity for the thermal insulating case was 0.44±0.58 Wmÿ1 Kÿ1. This yielded a minimum cooling time of up to about 17.2 s with a mold temperature of 558C. For the given molding geometry (4 mm5 mm 60 mm), some of the heat would dissipate from the samples in directions other than the molding-thickness direction assumed in previous calculations (Eq. (2)). Therefore, the calculated value was thought to serve as an upper-bound estimate for the process. Note that all of the calculated values were less than the cooling time, i.e. 20 s, used in this study. The warping deformation along the long-axis direction of the moldings was determined by a linear variable differential transformer (LVDT). Arbitrarily chosen positive and negative deformations were de®ned to distinguish the warping direction [6]. The curvature of the moldings was calculated from [4]: c
8y l2
(3)
where c is the curvature; y is the deformation measured by the LVDT; and l is the length of the moldings. It has nevertheless been assumed implicitly that the warping deformation along the axial direction of the moldings is symmetrical about the center of the molding. The forces applied to the moldings by the LVDT is considered negligible in respect of introducing any extra distortion of the moldings. The characterization of the warping deformation of the moldings included warping curvature in the green state, after annealing at 1508C for 30 min under ambient atmosphere, and after calcination at 10008C with 30 min holding (which would then minimize potential errors arising from the LVDT's contacting forces when the measurement was conducted), as well as measurement after sintering at 15008C for 1 h. Measurement from 10 to 20 samples was
16
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
Fig. 1. The curvature deformation of injection-molded `green' moldings.
Fig. 2. The curvature deformation of injection-molded moldings after annealing at 1508C for 30 min.
conducted to determine an average value for each processing parameter.
trolled cooling rate on both sides of the mold cavity. This argument is implicitly vindicated by the augmented curvature observed in the green moldings of the insulated case (Fig. 1), where it is seen that the curvature increases as a greater residual stress (than that of the non-insulted ones) is introduced via enlarged differential cooling being applied over the molding thickness. As shown in Fig. 2, the deformation recovers substantially after annealing at 1508C with 30 min holding time. Less than 0.05% weight loss was recorded for the moldings after annealing, this indicates that the resultant curvature change was not caused by the stresses involved during binder thermolysis [10,11]. The curvature change was particularly pronounced for the insulated case; therefore, the recovery is believed to be proportional to the existing difference in the magnitude of the residual stress across the molding thickness. It is interesting to note that when the curvature change in Figs. 1 and 2, are compared, the recovery increases with the mold temperature for both cases, i.e. fabricated with and without the thermal insulation. It was anticipated initially that the moldings which were the most stressed at the green state should recover the most towards a less deformed state when subjected to elevated temperature. If this is true, the insulated moldings formed at the lowest mold temperature, possessing a relatively greater magnitude in the difference of the `frozen-in' stresses, should become less warped after annealing. Then, the magnitude of curvature recovery should be greater than that of specimen molded at the higher mold temperatures (since some stresses of the moldings produced at the higher mold temperature were relieved after ejection). In terms of the percentage curvature change, the curvature recovered was 59.8, 55.5 and 51.2% compared to their green state for the moldings fabricated at mold temperatures of 38, 48 and 558C, respectively. However, in terms of the absolute values of the curvature recovered, the experimental result in Fig. 2 was just the opposite to what had been anticipated. Possible reasons for such discrepancy may include that an extra deformation was introduced from the pressure applied
3. Results and discussion Fig. 1 shows the warping deformation of green moldings fabricated at varying mold temperatures. The magnitude of the curvatures increased with the mold temperature, and became much more pronounced when the thermal barrier was applied. The mechanisms for this temperature dependence are believed to be the residual stresses in the green moldings formed during the solidi®cation stage of the molding process [6]. When the molding was ejected from the constrained mold cavity, distortion of the moldings occurred spontaneously in order to accommodate the stress, as long as the demolding temperature is higher than the Tg of the binder phases. At a higher de-molding temperature, a less stiff molding integrity was produced, which would facilitate the occurrence of the deformation. Note that the non-insulated moldings also showed a gradual increase in curvature with mold temperature. This indicates that even without the insulation, the temperature of the moldings on the nozzle side was different from that of the ejector side, regardless that both the mold halves were controlled by the same temperature controller at nominally identical temperature settings. The mold half on the nozzle side consisted of sprues which transported hot melt into the mold cavity, and must thus be maintained at a suf®ciently higher temperature than that of the ejector-side. Warping curvature of the molded ceramics seems to be quite sensitive to minute differences in the mold temperatures (even though not speci®cally quanti®ed presently) with the given mold design used in this study. This also suggests that precision shape control in the green state of ceramic injection moldings may be attainable if the temperatures of the mold halves are controlled separately by different temperature controllers, from which a truly equalized temperature (or a desired temperature difference) can be obtained to ensure a con-
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
onto the molding at the part-ejection stage, in addition to the deformation resulting from the residual stress during cooling. This ejection-induced deformation would be particularly pronounced when the moldings were ejected before reaching full solidi®cation or when a too-large ejecting pressure was applied onto the moldings. The cooling time used in the study was 20 s for all of the moldings in this study and this is only slightly greater than the minimum cooling time required for the insulated moldings, especially when the molding was fabricated at the higher mold temperatures (see Section 2). The present calculation of the minimum cooling time was based on the assumption that half of the molding thickness was solidi®ed under the molding conditions provided (i.e., for the melt and the mold temperatures used). It is hence possible that the ejection pressure may well exceed the `allowable' stiffness of the `half-solidi®ed' ceramic moldings when ejected. This insuf®cient cooling together with the relatively high ejection pressure (about 125 MPa in this study) is considered the prime cause for the discrepancy found. The ejector pins were applied onto the insulated side of the moldings; therefore, the pressure would make the curvature of the moldings to be further more negative (i.e., to be warped toward the nozzle side for the insulated case) and would produce a residual stress opposite to that of the buckling direction when the moldings were subjected to the elevated temperatures, when the whole moldings would gain more freedom for shape recovery. Therefore, the moldings that were fabricated using a higher mold temperature would be more susceptible to the buckling caused by the ejection pressure and would exhibit a greater recovery after the annealing. This argument seems to be consistent with the results found in Fig. 2. However, other mechanisms may still be operative, so that a further analysis is required to elucidate such mechanisms. In Fig. 3, the curvature of the moldings changed only slightly following the calcination process (holding at 10008C for 30 min). The change is signi®cantly smaller than that after the annealing, indicating that the warping
17
Fig. 4. The curvature deformation of injection-molded moldings after sintering at 15008C for 1 h.
curvature change induced after the debinding is only moderate. The curvature continues to develop more towards a negative value for the insulated moldings after sintering (Fig. 4). A more seriously deformed molding at the green state is seen to result in a greater curvature (in magnitude) after sintering. This indicates that the sintered distortion of ceramic moldings is closely related to the deformation at the green state, which is consistent with the observations made in the authors previous study [6]. 4. Conclusions Warping evolution of an injection-molded ceramics has been investigated from its green state to its sintered state at varying process variables. The deformation of the moldings changed markedly as the moldings were heated to temperatures above the glass transition temperature of the binders; but at the temperature, most of the binders remained and had not yet been removed. The residual stress arising from the solidi®cation process and the ejection pressure were considered to be the main cause. As the temperature of the moldings increased, the bulk stiffness of the moldings decreased and the effect of the residual stress on the shape retention of the moldings eventually prevailed as the dominant factor. A model experiment by deliberately placing thermal insulation on one side of the mold cavity to produce an asymmetrical stress distribution partially con®rmed this hypothesis. The curvature change after thermal debinding was only moderate. The moldings showed a substantial warping deformation after sintering, the sintered warpage being proportional to that of the green deformation. Acknowledgement
Fig. 3. The curvature deformation of injection-molded moldings after calcination at 10008C for 30 min.
This work is sponsored by the National Science Council through no. 88-2216-E-214-017.
18
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
References [1] R.M. German, Powder Injection Molding, Metal Powder Industries Federation, Princeton, New Jersey, 1990. [2] B.C. Mutsuddy, R.G. Ford, Ceramic Injection Molding, Chapman & Hall, Princeton, London, 1995. [3] T. Zhang, J.R.G. Evans, Relaxation effects in large injection moulded ceramic bodies, J. Eur. Ceram. Soc. 12 (1993) 51±59. [4] T. Zhang, S. Blackburn, J. Bridgewater, The orientation of binders and particles during ceramic injection moulding, J. Eur. Ceram. Soc. 17 (1997) 101±108. [5] C.A. Sunback, M.A. Costantini, W.H. Robbins, Part distortion during binder removal, in: Tennery, V.J. (Ed.), Ceramic Materials and Components for Engines, Proc. of Third Int. Symp., The American Ceramic Society, Westerville, OH, 1989, pp. 191±200. [6] W.J. Tseng, D.-M. Liu, Effect of processing variables on warping
[7] [8] [9] [10] [11]
behaviors of injection-moulded ceramics, Ceram. Int. 24 (1998) 125± 133. G. PoÈtsch, W. Michaeli, Injection Molding: An Introduction, Hanser/ Gardner Publications, Inc., Cincinnati, Ohio, 1995. K.N. Hunt, J.R.G. Evans, N.J. Mills, J. Woodthorpe, Computer modeling of the origin of defects in ceramic injection moulding IV. Residual stresses, J. Mater. Sci. 26 (1991) 5229±5238. D.-M. Liu, W.J. Tseng, In¯uence of solids loading on the green microstructure and sintering behaviour of ceramic injection moulding, J. Mater. Sci. 32 (1997) 6475±6481. M.J. Cima, J.A. Lewis, A.D. Devoe, Binder distribution in ceramic greenware during thermolysis, J. Am. Ceram. Soc. 72 (1989) 1192± 1199. G. Badyopadhyay, K.W. French, Injection molded ceramics: critical aspects of the binder removal process and component fabrication, J. Eur. Ceram. Soc. 11 (1993) 23±34.
Warping evolution of injection-molded ceramics Wenjea J. Tseng* Department of Materials Science and Engineering, I-Shou University, Kaohsiung 84008, Taiwan Received 15 August 1998
Abstract This study is focused on the shape retention of an injection-molded ZrO2 ceramics from their molded state to their sintered state. Varying mold temperatures (38±558C) were used for the molding process, and the molded samples were solidi®ed either at a relatively uniform temperature or under an inhomogeneous temperature distribution. Higher mold temperatures, as well as the application of non-uniform temperature distribution across the molding thickness, resulted in a signi®cant increase in the magnitude of the curvature of the molded samples. For all the moldings fabricated, the curvature changed prominently when the moldings were subjected to elevated temperatures; yet, insigni®cant binder loss was detected at such temperatures. The subsequent curvature evolution after thermal debinding was moderate, but the ®nal sintered deformation was found closely related to the green deformation. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Ceramic injection molding; Warpage; Deformation; Process
1. Introduction Dimensional control is an important aspect in ceramic injection molding (CIM). In order to obtain the full bene®ts of near-net-shape forming of the CIM process, the molded sample needs to be free from distortion and its warping deformation and shrinkage behavior be precisely predictable [1,2]. Mastery of the dimensions of sintered CIM parts would ultimately lead to the cost-effective manufacturing of complex-shaped ceramic products. The literature regarding the underlying mechanisms of the deformation behavior in CIM is not extensive [3±7]. An asymmetrical distribution of residual stress within ceramic moldings has been reported to be the main cause for the occurrence of distortion in the `green' state [3,6]. However, a molding with insigni®cant distortion in the green state does not necessarily mean a reduced level in its internal stress and hence a predictable shape retention after sintering may not be ensured. For example, relaxation of the residual stress occurs when CIM moldings are subjected to elevated temperatures [3]. This has been reported to lead to deformation of the moldings, presumably due to the rearrangement of the molded microstructure as the temperature of the molding
* Present address: Institute of Materials Science and Manufacturing, Chinese Culture University Yang Ming Shan, Taipei, Taiwan 11114.
was raised. The residual stress may have originated from non-uniform packing of suspensions during molding, and/or from cooling stresses, since solidi®cation of ceramic moldings within a geometrically constrained mold cavity is highly inhomogeneous [7,8]. Zhang et al. [4] further indicated that interactions between non-spherical particles (which tend to orient themselves along the direction of the molding pressure) and polymeric binder vehicles may also induce residual stresses in the CIM process. Distortion results when moldings are subjected to high temperatures, yet no signi®cant binder loss has been found in such situations. These previous studies all seem to suggest that the residual stress introduced during the forming process would become more prominent as the temperature of the ceramic moldings was raised for subsequent debinding purposes, and at a certain critical point (which has never been reported so far to the author's knowledge) that the stress would affect the bulk properties, such as the shape retention, of the moldings. As the temperature was raised above Tg of the binder species, one would suspect that moldings would soften and the shape retention of the moldings would become more susceptible to any stress remaining in the moldings. A study has shown that signi®cant distortion of ceramic moldings was accompanied by the major weight loss of binder vehicles during thermal debinding [5]. This suggests that the dimensions of molded CIM parts, in fact, changes in the process. How the warping deformation of the molded cera-
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 4 5 5 - 0
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
mics evolves during the course of CIM fabrication is not known, and this is the objective of the present study. 2. Experimental procedure A commercially available zirconia powder containing 3 mol% yttria (HSY3.0, Daiichi Kigenso Kagaku, Japan) with an average particle size of 0.25 mm and a speci®c surface area of 6.9 m2 gÿ1 was used for the study. The powder presented a relatively spherical shape and was extensively agglomerated. Low molecular weight organic binder composed of paraf®n wax and vinyl acetate polymer in a volumetric ratio of 6:4 was used as the major and minor ingredients [9]. The powders were ®rst ball-mixed with stearic acid (3 wt% on the basis of the powder loading) in a toluene medium for 24 h before the addition of the binders for another overnight mixing. The overall solid loading was held constant at 50 vol%. The mixtures were stirred and dried in a moderate temperature of about 408C, then kneaded through an extruder with a maximum temperature of 1008C (Model 70-20vex-6, KCK Industrial Co., Japan) to form pellets, followed by injection molding (Battenfeld BA 250/50 CDC, Austria) to obtain parallelepiped moldings of dimensions 4 mm5 mm 60 mm. A barrel temperature series of 90±140±150± 1508C from feed to nozzle was used. The molding pressure and cooling time were 80 MPa and 20 s, respectively. The mold temperature varied from 38 to 558C. The molded parts were ejected by pin ejectors with a nominal load of 2.5 kN, which corresponds to a pressure of 125 MPa on the molded dimensions. It has been shown that the warping deformation of injection-molded ceramics is closely related to the asymmetrical residual stresses induced from solidi®cation in the molding process [6]. A differential cooling rate was introduced deliberately by placing a thermal barrier onto the ejector side of the mold cavity prior to molding, with the aim of comparing the warping deformation with that for the noninsulated situation. This barrier material was paper with a controlled thickness of 300 mm, which can be removed from the molded parts after ejection. This was anticipated to introduce an augmented non-uniform temperature distribution across the molding thickness. In order to minimize the distortion resulting from premature ejection of the ceramic moldings, a theoretical estimation of the minimum cooling time for the injection molding has been conducted prior to the molding process. For rectangular-shaped moldings, the minimum cooling time can be expressed as [7]: s2 8 T M ÿ TW tc 2 ln 2 p T D ÿ TW p a
(1)
where tc is the cooling time; s is the wall thickness of the moldings; a is the thermal diffusivity; TM is the melt
15
temperature; TW is the molding wall temperature; and T D is the average temperature across the sample thickness during demolding. The thermal conductivity of the ceramic/binder mixture varied from 0.5±0.7 Wmÿ1 Kÿ1 at the working temperature employed, and the corresponding thermal diffusivity, a, was calculated from ak/rcp, where r is the density and cp is the heat capacity. By assuming that T D is equal to 658C (Tg of the binders is at about that temperature, determined from differential scanning calorimetry), the shortest cooling time was obtained for demolding ceramic moldings with the given thickness to be 8.4, 10.8 and 13.9 s for the varying mold temperatures of 38, 48 and 558C, respectively. For the moldings with the insulated layer of 300 mm thickness and a thermal conductivity of about 0.18 Wmÿ1 Kÿ1 [6], the `composite' conductivity by assuming that heat ¯ows mainly along the direction of the molding thickness is approximated by: k
k1 k2 n1 k2 n2 k1
(2)
where k1 and k2 represent the conductivities of two individual layers and n is the volume fraction of the respective layer. The calculated conductivity for the thermal insulating case was 0.44±0.58 Wmÿ1 Kÿ1. This yielded a minimum cooling time of up to about 17.2 s with a mold temperature of 558C. For the given molding geometry (4 mm5 mm 60 mm), some of the heat would dissipate from the samples in directions other than the molding-thickness direction assumed in previous calculations (Eq. (2)). Therefore, the calculated value was thought to serve as an upper-bound estimate for the process. Note that all of the calculated values were less than the cooling time, i.e. 20 s, used in this study. The warping deformation along the long-axis direction of the moldings was determined by a linear variable differential transformer (LVDT). Arbitrarily chosen positive and negative deformations were de®ned to distinguish the warping direction [6]. The curvature of the moldings was calculated from [4]: c
8y l2
(3)
where c is the curvature; y is the deformation measured by the LVDT; and l is the length of the moldings. It has nevertheless been assumed implicitly that the warping deformation along the axial direction of the moldings is symmetrical about the center of the molding. The forces applied to the moldings by the LVDT is considered negligible in respect of introducing any extra distortion of the moldings. The characterization of the warping deformation of the moldings included warping curvature in the green state, after annealing at 1508C for 30 min under ambient atmosphere, and after calcination at 10008C with 30 min holding (which would then minimize potential errors arising from the LVDT's contacting forces when the measurement was conducted), as well as measurement after sintering at 15008C for 1 h. Measurement from 10 to 20 samples was
16
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
Fig. 1. The curvature deformation of injection-molded `green' moldings.
Fig. 2. The curvature deformation of injection-molded moldings after annealing at 1508C for 30 min.
conducted to determine an average value for each processing parameter.
trolled cooling rate on both sides of the mold cavity. This argument is implicitly vindicated by the augmented curvature observed in the green moldings of the insulated case (Fig. 1), where it is seen that the curvature increases as a greater residual stress (than that of the non-insulted ones) is introduced via enlarged differential cooling being applied over the molding thickness. As shown in Fig. 2, the deformation recovers substantially after annealing at 1508C with 30 min holding time. Less than 0.05% weight loss was recorded for the moldings after annealing, this indicates that the resultant curvature change was not caused by the stresses involved during binder thermolysis [10,11]. The curvature change was particularly pronounced for the insulated case; therefore, the recovery is believed to be proportional to the existing difference in the magnitude of the residual stress across the molding thickness. It is interesting to note that when the curvature change in Figs. 1 and 2, are compared, the recovery increases with the mold temperature for both cases, i.e. fabricated with and without the thermal insulation. It was anticipated initially that the moldings which were the most stressed at the green state should recover the most towards a less deformed state when subjected to elevated temperature. If this is true, the insulated moldings formed at the lowest mold temperature, possessing a relatively greater magnitude in the difference of the `frozen-in' stresses, should become less warped after annealing. Then, the magnitude of curvature recovery should be greater than that of specimen molded at the higher mold temperatures (since some stresses of the moldings produced at the higher mold temperature were relieved after ejection). In terms of the percentage curvature change, the curvature recovered was 59.8, 55.5 and 51.2% compared to their green state for the moldings fabricated at mold temperatures of 38, 48 and 558C, respectively. However, in terms of the absolute values of the curvature recovered, the experimental result in Fig. 2 was just the opposite to what had been anticipated. Possible reasons for such discrepancy may include that an extra deformation was introduced from the pressure applied
3. Results and discussion Fig. 1 shows the warping deformation of green moldings fabricated at varying mold temperatures. The magnitude of the curvatures increased with the mold temperature, and became much more pronounced when the thermal barrier was applied. The mechanisms for this temperature dependence are believed to be the residual stresses in the green moldings formed during the solidi®cation stage of the molding process [6]. When the molding was ejected from the constrained mold cavity, distortion of the moldings occurred spontaneously in order to accommodate the stress, as long as the demolding temperature is higher than the Tg of the binder phases. At a higher de-molding temperature, a less stiff molding integrity was produced, which would facilitate the occurrence of the deformation. Note that the non-insulated moldings also showed a gradual increase in curvature with mold temperature. This indicates that even without the insulation, the temperature of the moldings on the nozzle side was different from that of the ejector side, regardless that both the mold halves were controlled by the same temperature controller at nominally identical temperature settings. The mold half on the nozzle side consisted of sprues which transported hot melt into the mold cavity, and must thus be maintained at a suf®ciently higher temperature than that of the ejector-side. Warping curvature of the molded ceramics seems to be quite sensitive to minute differences in the mold temperatures (even though not speci®cally quanti®ed presently) with the given mold design used in this study. This also suggests that precision shape control in the green state of ceramic injection moldings may be attainable if the temperatures of the mold halves are controlled separately by different temperature controllers, from which a truly equalized temperature (or a desired temperature difference) can be obtained to ensure a con-
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
onto the molding at the part-ejection stage, in addition to the deformation resulting from the residual stress during cooling. This ejection-induced deformation would be particularly pronounced when the moldings were ejected before reaching full solidi®cation or when a too-large ejecting pressure was applied onto the moldings. The cooling time used in the study was 20 s for all of the moldings in this study and this is only slightly greater than the minimum cooling time required for the insulated moldings, especially when the molding was fabricated at the higher mold temperatures (see Section 2). The present calculation of the minimum cooling time was based on the assumption that half of the molding thickness was solidi®ed under the molding conditions provided (i.e., for the melt and the mold temperatures used). It is hence possible that the ejection pressure may well exceed the `allowable' stiffness of the `half-solidi®ed' ceramic moldings when ejected. This insuf®cient cooling together with the relatively high ejection pressure (about 125 MPa in this study) is considered the prime cause for the discrepancy found. The ejector pins were applied onto the insulated side of the moldings; therefore, the pressure would make the curvature of the moldings to be further more negative (i.e., to be warped toward the nozzle side for the insulated case) and would produce a residual stress opposite to that of the buckling direction when the moldings were subjected to the elevated temperatures, when the whole moldings would gain more freedom for shape recovery. Therefore, the moldings that were fabricated using a higher mold temperature would be more susceptible to the buckling caused by the ejection pressure and would exhibit a greater recovery after the annealing. This argument seems to be consistent with the results found in Fig. 2. However, other mechanisms may still be operative, so that a further analysis is required to elucidate such mechanisms. In Fig. 3, the curvature of the moldings changed only slightly following the calcination process (holding at 10008C for 30 min). The change is signi®cantly smaller than that after the annealing, indicating that the warping
17
Fig. 4. The curvature deformation of injection-molded moldings after sintering at 15008C for 1 h.
curvature change induced after the debinding is only moderate. The curvature continues to develop more towards a negative value for the insulated moldings after sintering (Fig. 4). A more seriously deformed molding at the green state is seen to result in a greater curvature (in magnitude) after sintering. This indicates that the sintered distortion of ceramic moldings is closely related to the deformation at the green state, which is consistent with the observations made in the authors previous study [6]. 4. Conclusions Warping evolution of an injection-molded ceramics has been investigated from its green state to its sintered state at varying process variables. The deformation of the moldings changed markedly as the moldings were heated to temperatures above the glass transition temperature of the binders; but at the temperature, most of the binders remained and had not yet been removed. The residual stress arising from the solidi®cation process and the ejection pressure were considered to be the main cause. As the temperature of the moldings increased, the bulk stiffness of the moldings decreased and the effect of the residual stress on the shape retention of the moldings eventually prevailed as the dominant factor. A model experiment by deliberately placing thermal insulation on one side of the mold cavity to produce an asymmetrical stress distribution partially con®rmed this hypothesis. The curvature change after thermal debinding was only moderate. The moldings showed a substantial warping deformation after sintering, the sintered warpage being proportional to that of the green deformation. Acknowledgement
Fig. 3. The curvature deformation of injection-molded moldings after calcination at 10008C for 30 min.
This work is sponsored by the National Science Council through no. 88-2216-E-214-017.
18
W.J. Tseng / Journal of Materials Processing Technology 102 (2000) 14±18
References [1] R.M. German, Powder Injection Molding, Metal Powder Industries Federation, Princeton, New Jersey, 1990. [2] B.C. Mutsuddy, R.G. Ford, Ceramic Injection Molding, Chapman & Hall, Princeton, London, 1995. [3] T. Zhang, J.R.G. Evans, Relaxation effects in large injection moulded ceramic bodies, J. Eur. Ceram. Soc. 12 (1993) 51±59. [4] T. Zhang, S. Blackburn, J. Bridgewater, The orientation of binders and particles during ceramic injection moulding, J. Eur. Ceram. Soc. 17 (1997) 101±108. [5] C.A. Sunback, M.A. Costantini, W.H. Robbins, Part distortion during binder removal, in: Tennery, V.J. (Ed.), Ceramic Materials and Components for Engines, Proc. of Third Int. Symp., The American Ceramic Society, Westerville, OH, 1989, pp. 191±200. [6] W.J. Tseng, D.-M. Liu, Effect of processing variables on warping
[7] [8] [9] [10] [11]
behaviors of injection-moulded ceramics, Ceram. Int. 24 (1998) 125± 133. G. PoÈtsch, W. Michaeli, Injection Molding: An Introduction, Hanser/ Gardner Publications, Inc., Cincinnati, Ohio, 1995. K.N. Hunt, J.R.G. Evans, N.J. Mills, J. Woodthorpe, Computer modeling of the origin of defects in ceramic injection moulding IV. Residual stresses, J. Mater. Sci. 26 (1991) 5229±5238. D.-M. Liu, W.J. Tseng, In¯uence of solids loading on the green microstructure and sintering behaviour of ceramic injection moulding, J. Mater. Sci. 32 (1997) 6475±6481. M.J. Cima, J.A. Lewis, A.D. Devoe, Binder distribution in ceramic greenware during thermolysis, J. Am. Ceram. Soc. 72 (1989) 1192± 1199. G. Badyopadhyay, K.W. French, Injection molded ceramics: critical aspects of the binder removal process and component fabrication, J. Eur. Ceram. Soc. 11 (1993) 23±34.