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Melt front surface asperity and welding-defect generation in ceramic injection molding
Journal of Materials Processing Technology 111 (2001) 219±224
Melt front surface asperity and welding-defect generation in ceramic injection molding Y.H. Chunga,*, K. Katob, N. Otakeb a b
Development Team, Daegu Ceramic Plant of Ssangyong Materials Corporation, Wolan-dong 1-85, Dalseo-gu Daegu 704-320, South Korea Department of Mechanical Engineering and Science, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8552, Japan
Abstract Melt front roughness was examined ®rstly, and it was found that an injected part is separated into two regions: a rough surface region and a ®ne surface region. The rough surface region results from melt front asperities generated by the blowout of voids at the front surface which are mixed into the material by jetting ¯ow in the sprue. It was also found that voids in the sprue can be eliminated by applying counter-pressure. Secondly, the generation property of welding defects, weld line and voids, has been examined by counter-¯ow joining and their elimination criteria are obtained. It was shown that both criteria approximately coincide with each other. The welding-defect elimination criterion obtained is applied to the evaluation of welding-defect generation under various injection conditions and it was found that the criterion does not depend greatly on the asperities of the ¯ow front. Furthermore, the criterion is also effective for estimating welding defects in injection molding using a mold cavity with inserts. Finally, it was con®rmed that the above criterion approximately coincides with that in a previous paper which was obtained by the use of simple void-reducing conditions in the joining of green ceramic blocks with arti®cially grooved surfaces. # 2001 Published by Elsevier Science B.V. Keywords: Ceramic injection molding; Jetting in sprue; Melt front asperity; Counter-¯ow joining; Welding defects; Welding-defect elimination criterion
1. Introduction
2. Melt front surface asperity and void generation
In ceramic injection molding, a welding defect generated by the joining of melt fronts is a typical molding defect. The surface of the ¯ow front becomes rough and sometimes voids are generated in it [1±3]. These defects may remain in a product as a void and/or a crack at the welded part after dewaxing and sintering [4], so their occurrence must be avoided in the injection molding stage. In this paper, ®rstly, injection molding experiments using a circular disk have been carried out to examine the properties of ¯ow front asperities and the mechanism of void generation in it, and a method to reduce these defects is proposed. Then, the joining behavior of ¯ow fronts and the properties of welding-defect generation have been examined by welding experiments using various types of mold cavities. The effect of injection conditions and front status on welding-defect generation is also clari®ed, and the welding-defect elimination criterion is obtained.
2.1. Experimental The experimental apparatus is shown in Fig. 1. The cavity thickness is 2 mm and its diameter is 130 mm. The pressure distribution in the mold cavity is measured at radial positions of 20, 35 and 50 mm in the upper mold plate. The injection molding machine (made by Sanjo Seiki, Japan) has the speci®cation of an injection pressure of 170 MPa and injection rate of 27 cm3/s. The material compound consists of 57% alumina powder, 32.3% polystyrene, 6.5% paraf®n wax and 4.2% stearic acid in volume percent. These materials are kneaded and pelletized. The standard injection conditions employed are as follows: material temperature, TM 180 C, mold temperature, TD 50 C, and ¯ow velocity, V 285 mm=s, where the injection rate Q is represented by the corresponding material velocity Vat a radius of r 35 mm. 2.2. Observation of surface asperity and void
*
Corresponding author. Tel.: 82-53-580-4345; fax: 82-53-580-4329. E-mail address: [email protected] (Y.H. Chung). 0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 5 4 3 - X
Fig. 2 shows the surface of a part molded by a short-shot experiment. The surface is divided into two regions: an inner region with a rough surface (white region) and an outer
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Fig. 1. Experimental apparatus and details of the mold cavity.
region with a ®ne surface (gray region): these are called ``region I'' and ``region II'', respectively. Here, the radius of the inter-boundary of regions I and II is designated rS and the radius of a part (or the radius of the ®nal melt front) is designated rF. The value of rS does not change after it reaches certain length, even if the value of rF changes. When rF is smaller than rS, the melt front becomes very rough (part (i) of Fig. 3(a)), whereas it becomes ®ne when rF is larger than rS (Fig. 3(b)). From more detailed observation of Fig. 3(a), it was found that the material of the front surface moves to the surface contacting mold wall, and that material ¯ow slips at the mold wall considerably [5,6]. A photograph of section A±A0 is shown in part (ii) of Fig. 3(a), in which numerous voids are observed. 2.3. Discussions on the mechanism of molding-defect generation In the above discussion, it was suggested that the voids in a molded part have a relationship to the asperities of the melt front. This has been con®rmed by molding experiments under various values of V and TM, the results being shown in Fig. 4. Here, marks correspond to the levels of melt front asperity. The number of voids, written in parentheses, coincides with the front asperity level quite well, although the asperity level is determined rather qualitatively. It seems that voids are generated in the sprue during the ®lling stage.
Fig. 2. Sample of an injected part.
Fig. 3. Asperities of the ¯ow front and surface.
Fig. 4. Voids and front asperities TD 50 C; h 2 mm; rF 31:5 mm. Values in parenthesis express the number of voids.
The longitudinal sections of sprues that are attached to the molded parts are machined out to verify it (Fig. 5). Voids are shown over the full ®eld of the sprue in early stage (Fig. 5(a)), but they move away through gate (Fig. 5(b)). Finally, no voids remain after some quantity of material has been injected (Fig. 5(c)). Consequently, it was concluded that voids are generated by jetting ¯ow in the sprue, and that
Fig. 5. Voids in the sprue in various short-shot stages TM 190 C; TD 50 C; V 285 mm=s; h 3 mm.
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Fig. 7. Details of the mold cavity (thickness h of runner, gate and cavity 3 mm).
Fig. 6. Remaining voids in the sprue under the conditions of various counter-pressures at the exit of the sprue TM 190 C; TD 50 C.
they move into the cavity through the gate. Then the voids reach the melt front and blow out. This makes the surface of ¯ow front and the molded part (region I) rough. The application of counter-pressure to the material in the sprue may possibly be effective for reducing voids. This has been examined by the choking of the exit of the sprue, the results being shown in Fig. 6. The voids decrease rapidly with increasing counter-pressure. It was suggested that the application of a thin ®lm gate is useful for reducing voids and molding a part with a ®ne surface. 3. The occurrence and elimination properties of a welding defect 3.1. Welding-defect generation property in joining of counter-¯ows Welding experiments applying counter-¯ow joining were performed using a mold cavity with two gates, to explore the basic welding properties. The outline of the standard mold cavity is shown in Fig. 7, which is 3 mm thick, 68 mm long and 15 m wide. Gates are connected with both ends of the cavity. For injection conditions, TM 160 200 C; TD 40 60 C. Here ¯ow velocity is consistent as V 350 mm=s. Fig. 8(a) illustrates the typical welding pattern of counter-¯ows. It was observed that a weld line of groove
type on the surface and voids near the vertical section of the joining part remain. These are shown magni®ed in Figs. 8(b) and (c), respectively. The width at the center of the weld line d and the diameter of the spherical voids dV which remain to within 5 mm of both sides from the vertical section of the joining part are adopted for the present values to represent these two types of defects. 3.2. Welding-defect elimination criterion under standard condition It was shown that the width of the weld line d and the void size dV decreases rapidly with increasing TM and PW. Thus, both defects were measured under a wide range of TM and PW, and are plotted on the plane of TM and PW. Fig. 9 shows the result for d. It was shown that the elimination condition for d is obtained as the straight line passing through the two points of TM ; PW 160 C; 2:9 MPa and (2008C, 1.1 MPa). From Fig. 10 showing the result for dV, the elimination condition becomes the straight line passing through the two points of TM ; PW 160 C; 2:7 MPa and (2008C, 1.2 MPa). From the above results, it was found that both criteria approximately coincide with each other under these standard conditions. These criteria are called the welding-defect elimination criterion. 3.3. Welding-defect elimination properties under various injection conditions In order to investigate the in¯uence of the front asperities on welding-defect generation properties, the ¯ow length L in Fig. 7 was changed to two levels: shorter L 27 mm and
Fig. 8. Defects generated at the welded part TM 160 C; TD 40 C; PW 0:6 MPa: (a) schematic illustration; (b) weld line (surface); (c) voids (cross section).
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Fig. 9. Weld-line elimination criterion obtained by joining of the counter¯ows.
of the shorter cavity, but dV decreases with increasing ¯ow length (Fig. 12). This indicates that the in¯uence of voids mixed into the material in the sprue during the ®lling stage on the remaining voids is greater than that of the ¯ow front asperities. The experimental results when reducing the cavity thickness to 1.5 mm are shown in Fig. 13. It is seen that the elimination condition of the weld line is expressed as a straight line, which is shifted up about 0.5 MPa from the standard condition of a 3 mm thick cavity. This is because the weldability is degraded by the rapid cooling of the ¯ow front when the cavity thickness is less. On the contrary, the elimination condition of the remaining voids is reduced by about 0.4 MPa from the standard condition. This is considered to be due to the restraint of void generation in the sprue with increasing of the ¯ow resistance into the cavity when the cavity thickness is less. The effect of ¯ow pattern was investigated by using an H-type cavity as illustrated in Fig. 14(a). Here, the width of a is changed to two levels. It was shown that the front asperity formed by a cavity with a 9 mm is almost the same with that of the standard cavity, but cracks are observed at the surface of the ¯ow front when a 5 mm (Figs. 14(b) and (c)). Here, since the weld line is con®rmed to be independent of the front asperity, only the result for remaining void is shown in Fig. 15. It was found that those two void elimination criteria approximately coincide with that for the standard cavity, i.e. the void elimination criterion does not depend on the front asperities. 3.4. Welding-defect generation properties in a cavity with inserts
Fig. 10. Void elimination criterion obtained by joining of the counter¯ows.
longer L 87 mm than the standard cavity in Fig. 7. It was observed that the front asperities become worse with decreasing L, as described in Section 2.2. Fig. 11 shows that the elimination condition of d changes little with ¯ow length. On the other hand, it was found that dV does not increase regardless of the rougher front asperities in the case
Fig. 16 shows the outline of a cavity with four inserts. The behavior of welding-defect generation was investigated by short-shot molding experiments using this cavity, and it was found that an un®lled groove defect at the downstream side of insert (Fig. 17) is present in addition to the two types of defects observed in the joining of counter-¯ows (Fig. 8). For these three types of defects, ®rst, the elimination conditions of the weld line and groove at the downstream side of the
Fig. 11. Effect of front asperities on the weld-line elimination condition: (a) L 27 mm; (b) L 87 mm.
Y.H. Chung et al. / Journal of Materials Processing Technology 111 (2001) 219±224
Fig. 12. Effect of front asperities on the void elimination condition: (a) L 27 mm; (b) L 87 mm.
Fig. 15. Effect of front asperities on void elimination condition. Fig. 13. Effect of the mold cavity thickness on welding-defect elimination condition.
insert on the welding parts A and B are shown in Figs. 18(a) and (b), respectively. The solid line represents the weld line elimination criterion obtained by the joining of the counter¯ows. It was found that the two elimination conditions for parts A and B approximately coincide with those in the counter-¯ows, although the front asperities become worse due to the effect of the inserts. Fig. 19 shows the result of the void elimination conditions. It was also found that the two limiting lines for parts A and B approximately coincide, but they are a little below the line for counter-¯ow joining. The reason is considered to be the pressing effect where voids are suppressed and eliminated partially when material ¯ow
Fig. 14. Photographs of the front for a mold cavity with a branched channel: (a) branched channel; (b) a 5 mm; (c) a 9 mm.
Fig. 16. Details of the mold cavity with inserts.
Fig. 17. Groove at the downstream side of an insert (500 mm).
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Fig. 18. Weld line and groove elimination conditions: (a) weld line; (b) groove at downstream side of insert.
Fig. 19. Void elimination condition.
passes between the inserts and blows out to the front surface by extensional ¯ow after the inserts. As shown above, the ¯ow patterns at various positions before and after the insert much differ from each other. However, it was shown that any welding defect can be controlled by TM and PW, and its elimination criterion can be determined with the standard welding-defect elimination criterion. Finally, the authors proposed a simple evaluation method for a welding defect using simple void-reducing conditions in the joining of green ceramic blocks with an arti®cially grooved surface in a previous paper [7]. The void elimination criterion obtained by this method is shown in Fig. 19 with a dotted line, and it approximately coincides with the other lines within experimental error. It was con®rmed that the model method proposed in the previous paper is effective for estimating the property of welding defects under real injection molding using a mold cavity with inserts. 4. Conclusion Various defects generated during ®lling and welding in ceramic injection molding have been examined, and the
following conclusions have been reached. Firstly, jetting occurs in the ¯ow from the nozzle into the sprue and it generates some voids in the material. The voids in the sprue move into the mold cavity, and they burst in the fountain ¯ow at the melt front. Thus makes the melt front surface rough and results in a rough asperity surface of a molded part. The application of counter-pressure at the exit of the sprue is effective for reducing voids. Secondly, a weld line at the surface and voids in the cross section occur at the welded part. The criteria needed for these two types of welding defects to be eliminated are obtained on the plane of TM and PW. Both criteria approximately coincide with each other under standard condition, and they do not greatly depend on the asperities of the ¯ow front under various injection conditions. Moreover, the above welding-defect elimination criterion is also effective for estimating the degree of welding defects in injection molding by the use of a mold cavity with inserts. Finally, the above criterion approximately coincides with that in a previous paper, which was obtained by the use of simple void-reducing conditions in the joining of green ceramic blocks with arti®cially grooved surfaces. References [1] T. Zhang, J.R.G. Evans, J. Br. Ceram. Trans. 92 (4) (1993) 146±151. [2] K. Kato, R.H. Chen, J. Jpn. Soc. Tech. Plast. 32 (371) (1991) 1497±1502. [3] S.C. Malguarneras, A.I. Manisali, D.C. Riggs, Polym. Eng. Sci. 21 (1981) 1149±1156. [4] K. Kato, R.H. Chen, N. Suzuki, N. Otake, J. Jpn. Soc. Tech. Plast. 36 (418) (1995) 1257±1262. [5] S.J. Stedman, J.R.G. Evans, J. Woodthorpe, J. Mater. Sci. 25 (1990) 1833±1838. [6] I. Tsao, S.C. Danforth, A.B. Metzner, J. Am. Ceram. Soc. 76 (12) (1993) 2977±2982. [7] K. Kato, Y.H. Chung, N. Otake, J. Jpn. Soc. Tech. Plast. 40 (461) (1999) 606±610.
Melt front surface asperity and welding-defect generation in ceramic injection molding Y.H. Chunga,*, K. Katob, N. Otakeb a b
Development Team, Daegu Ceramic Plant of Ssangyong Materials Corporation, Wolan-dong 1-85, Dalseo-gu Daegu 704-320, South Korea Department of Mechanical Engineering and Science, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8552, Japan
Abstract Melt front roughness was examined ®rstly, and it was found that an injected part is separated into two regions: a rough surface region and a ®ne surface region. The rough surface region results from melt front asperities generated by the blowout of voids at the front surface which are mixed into the material by jetting ¯ow in the sprue. It was also found that voids in the sprue can be eliminated by applying counter-pressure. Secondly, the generation property of welding defects, weld line and voids, has been examined by counter-¯ow joining and their elimination criteria are obtained. It was shown that both criteria approximately coincide with each other. The welding-defect elimination criterion obtained is applied to the evaluation of welding-defect generation under various injection conditions and it was found that the criterion does not depend greatly on the asperities of the ¯ow front. Furthermore, the criterion is also effective for estimating welding defects in injection molding using a mold cavity with inserts. Finally, it was con®rmed that the above criterion approximately coincides with that in a previous paper which was obtained by the use of simple void-reducing conditions in the joining of green ceramic blocks with arti®cially grooved surfaces. # 2001 Published by Elsevier Science B.V. Keywords: Ceramic injection molding; Jetting in sprue; Melt front asperity; Counter-¯ow joining; Welding defects; Welding-defect elimination criterion
1. Introduction
2. Melt front surface asperity and void generation
In ceramic injection molding, a welding defect generated by the joining of melt fronts is a typical molding defect. The surface of the ¯ow front becomes rough and sometimes voids are generated in it [1±3]. These defects may remain in a product as a void and/or a crack at the welded part after dewaxing and sintering [4], so their occurrence must be avoided in the injection molding stage. In this paper, ®rstly, injection molding experiments using a circular disk have been carried out to examine the properties of ¯ow front asperities and the mechanism of void generation in it, and a method to reduce these defects is proposed. Then, the joining behavior of ¯ow fronts and the properties of welding-defect generation have been examined by welding experiments using various types of mold cavities. The effect of injection conditions and front status on welding-defect generation is also clari®ed, and the welding-defect elimination criterion is obtained.
2.1. Experimental The experimental apparatus is shown in Fig. 1. The cavity thickness is 2 mm and its diameter is 130 mm. The pressure distribution in the mold cavity is measured at radial positions of 20, 35 and 50 mm in the upper mold plate. The injection molding machine (made by Sanjo Seiki, Japan) has the speci®cation of an injection pressure of 170 MPa and injection rate of 27 cm3/s. The material compound consists of 57% alumina powder, 32.3% polystyrene, 6.5% paraf®n wax and 4.2% stearic acid in volume percent. These materials are kneaded and pelletized. The standard injection conditions employed are as follows: material temperature, TM 180 C, mold temperature, TD 50 C, and ¯ow velocity, V 285 mm=s, where the injection rate Q is represented by the corresponding material velocity Vat a radius of r 35 mm. 2.2. Observation of surface asperity and void
*
Corresponding author. Tel.: 82-53-580-4345; fax: 82-53-580-4329. E-mail address: [email protected] (Y.H. Chung). 0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 5 4 3 - X
Fig. 2 shows the surface of a part molded by a short-shot experiment. The surface is divided into two regions: an inner region with a rough surface (white region) and an outer
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Y.H. Chung et al. / Journal of Materials Processing Technology 111 (2001) 219±224
Fig. 1. Experimental apparatus and details of the mold cavity.
region with a ®ne surface (gray region): these are called ``region I'' and ``region II'', respectively. Here, the radius of the inter-boundary of regions I and II is designated rS and the radius of a part (or the radius of the ®nal melt front) is designated rF. The value of rS does not change after it reaches certain length, even if the value of rF changes. When rF is smaller than rS, the melt front becomes very rough (part (i) of Fig. 3(a)), whereas it becomes ®ne when rF is larger than rS (Fig. 3(b)). From more detailed observation of Fig. 3(a), it was found that the material of the front surface moves to the surface contacting mold wall, and that material ¯ow slips at the mold wall considerably [5,6]. A photograph of section A±A0 is shown in part (ii) of Fig. 3(a), in which numerous voids are observed. 2.3. Discussions on the mechanism of molding-defect generation In the above discussion, it was suggested that the voids in a molded part have a relationship to the asperities of the melt front. This has been con®rmed by molding experiments under various values of V and TM, the results being shown in Fig. 4. Here, marks correspond to the levels of melt front asperity. The number of voids, written in parentheses, coincides with the front asperity level quite well, although the asperity level is determined rather qualitatively. It seems that voids are generated in the sprue during the ®lling stage.
Fig. 2. Sample of an injected part.
Fig. 3. Asperities of the ¯ow front and surface.
Fig. 4. Voids and front asperities TD 50 C; h 2 mm; rF 31:5 mm. Values in parenthesis express the number of voids.
The longitudinal sections of sprues that are attached to the molded parts are machined out to verify it (Fig. 5). Voids are shown over the full ®eld of the sprue in early stage (Fig. 5(a)), but they move away through gate (Fig. 5(b)). Finally, no voids remain after some quantity of material has been injected (Fig. 5(c)). Consequently, it was concluded that voids are generated by jetting ¯ow in the sprue, and that
Fig. 5. Voids in the sprue in various short-shot stages TM 190 C; TD 50 C; V 285 mm=s; h 3 mm.
Y.H. Chung et al. / Journal of Materials Processing Technology 111 (2001) 219±224
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Fig. 7. Details of the mold cavity (thickness h of runner, gate and cavity 3 mm).
Fig. 6. Remaining voids in the sprue under the conditions of various counter-pressures at the exit of the sprue TM 190 C; TD 50 C.
they move into the cavity through the gate. Then the voids reach the melt front and blow out. This makes the surface of ¯ow front and the molded part (region I) rough. The application of counter-pressure to the material in the sprue may possibly be effective for reducing voids. This has been examined by the choking of the exit of the sprue, the results being shown in Fig. 6. The voids decrease rapidly with increasing counter-pressure. It was suggested that the application of a thin ®lm gate is useful for reducing voids and molding a part with a ®ne surface. 3. The occurrence and elimination properties of a welding defect 3.1. Welding-defect generation property in joining of counter-¯ows Welding experiments applying counter-¯ow joining were performed using a mold cavity with two gates, to explore the basic welding properties. The outline of the standard mold cavity is shown in Fig. 7, which is 3 mm thick, 68 mm long and 15 m wide. Gates are connected with both ends of the cavity. For injection conditions, TM 160 200 C; TD 40 60 C. Here ¯ow velocity is consistent as V 350 mm=s. Fig. 8(a) illustrates the typical welding pattern of counter-¯ows. It was observed that a weld line of groove
type on the surface and voids near the vertical section of the joining part remain. These are shown magni®ed in Figs. 8(b) and (c), respectively. The width at the center of the weld line d and the diameter of the spherical voids dV which remain to within 5 mm of both sides from the vertical section of the joining part are adopted for the present values to represent these two types of defects. 3.2. Welding-defect elimination criterion under standard condition It was shown that the width of the weld line d and the void size dV decreases rapidly with increasing TM and PW. Thus, both defects were measured under a wide range of TM and PW, and are plotted on the plane of TM and PW. Fig. 9 shows the result for d. It was shown that the elimination condition for d is obtained as the straight line passing through the two points of TM ; PW 160 C; 2:9 MPa and (2008C, 1.1 MPa). From Fig. 10 showing the result for dV, the elimination condition becomes the straight line passing through the two points of TM ; PW 160 C; 2:7 MPa and (2008C, 1.2 MPa). From the above results, it was found that both criteria approximately coincide with each other under these standard conditions. These criteria are called the welding-defect elimination criterion. 3.3. Welding-defect elimination properties under various injection conditions In order to investigate the in¯uence of the front asperities on welding-defect generation properties, the ¯ow length L in Fig. 7 was changed to two levels: shorter L 27 mm and
Fig. 8. Defects generated at the welded part TM 160 C; TD 40 C; PW 0:6 MPa: (a) schematic illustration; (b) weld line (surface); (c) voids (cross section).
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Fig. 9. Weld-line elimination criterion obtained by joining of the counter¯ows.
of the shorter cavity, but dV decreases with increasing ¯ow length (Fig. 12). This indicates that the in¯uence of voids mixed into the material in the sprue during the ®lling stage on the remaining voids is greater than that of the ¯ow front asperities. The experimental results when reducing the cavity thickness to 1.5 mm are shown in Fig. 13. It is seen that the elimination condition of the weld line is expressed as a straight line, which is shifted up about 0.5 MPa from the standard condition of a 3 mm thick cavity. This is because the weldability is degraded by the rapid cooling of the ¯ow front when the cavity thickness is less. On the contrary, the elimination condition of the remaining voids is reduced by about 0.4 MPa from the standard condition. This is considered to be due to the restraint of void generation in the sprue with increasing of the ¯ow resistance into the cavity when the cavity thickness is less. The effect of ¯ow pattern was investigated by using an H-type cavity as illustrated in Fig. 14(a). Here, the width of a is changed to two levels. It was shown that the front asperity formed by a cavity with a 9 mm is almost the same with that of the standard cavity, but cracks are observed at the surface of the ¯ow front when a 5 mm (Figs. 14(b) and (c)). Here, since the weld line is con®rmed to be independent of the front asperity, only the result for remaining void is shown in Fig. 15. It was found that those two void elimination criteria approximately coincide with that for the standard cavity, i.e. the void elimination criterion does not depend on the front asperities. 3.4. Welding-defect generation properties in a cavity with inserts
Fig. 10. Void elimination criterion obtained by joining of the counter¯ows.
longer L 87 mm than the standard cavity in Fig. 7. It was observed that the front asperities become worse with decreasing L, as described in Section 2.2. Fig. 11 shows that the elimination condition of d changes little with ¯ow length. On the other hand, it was found that dV does not increase regardless of the rougher front asperities in the case
Fig. 16 shows the outline of a cavity with four inserts. The behavior of welding-defect generation was investigated by short-shot molding experiments using this cavity, and it was found that an un®lled groove defect at the downstream side of insert (Fig. 17) is present in addition to the two types of defects observed in the joining of counter-¯ows (Fig. 8). For these three types of defects, ®rst, the elimination conditions of the weld line and groove at the downstream side of the
Fig. 11. Effect of front asperities on the weld-line elimination condition: (a) L 27 mm; (b) L 87 mm.
Y.H. Chung et al. / Journal of Materials Processing Technology 111 (2001) 219±224
Fig. 12. Effect of front asperities on the void elimination condition: (a) L 27 mm; (b) L 87 mm.
Fig. 15. Effect of front asperities on void elimination condition. Fig. 13. Effect of the mold cavity thickness on welding-defect elimination condition.
insert on the welding parts A and B are shown in Figs. 18(a) and (b), respectively. The solid line represents the weld line elimination criterion obtained by the joining of the counter¯ows. It was found that the two elimination conditions for parts A and B approximately coincide with those in the counter-¯ows, although the front asperities become worse due to the effect of the inserts. Fig. 19 shows the result of the void elimination conditions. It was also found that the two limiting lines for parts A and B approximately coincide, but they are a little below the line for counter-¯ow joining. The reason is considered to be the pressing effect where voids are suppressed and eliminated partially when material ¯ow
Fig. 14. Photographs of the front for a mold cavity with a branched channel: (a) branched channel; (b) a 5 mm; (c) a 9 mm.
Fig. 16. Details of the mold cavity with inserts.
Fig. 17. Groove at the downstream side of an insert (500 mm).
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Fig. 18. Weld line and groove elimination conditions: (a) weld line; (b) groove at downstream side of insert.
Fig. 19. Void elimination condition.
passes between the inserts and blows out to the front surface by extensional ¯ow after the inserts. As shown above, the ¯ow patterns at various positions before and after the insert much differ from each other. However, it was shown that any welding defect can be controlled by TM and PW, and its elimination criterion can be determined with the standard welding-defect elimination criterion. Finally, the authors proposed a simple evaluation method for a welding defect using simple void-reducing conditions in the joining of green ceramic blocks with an arti®cially grooved surface in a previous paper [7]. The void elimination criterion obtained by this method is shown in Fig. 19 with a dotted line, and it approximately coincides with the other lines within experimental error. It was con®rmed that the model method proposed in the previous paper is effective for estimating the property of welding defects under real injection molding using a mold cavity with inserts. 4. Conclusion Various defects generated during ®lling and welding in ceramic injection molding have been examined, and the
following conclusions have been reached. Firstly, jetting occurs in the ¯ow from the nozzle into the sprue and it generates some voids in the material. The voids in the sprue move into the mold cavity, and they burst in the fountain ¯ow at the melt front. Thus makes the melt front surface rough and results in a rough asperity surface of a molded part. The application of counter-pressure at the exit of the sprue is effective for reducing voids. Secondly, a weld line at the surface and voids in the cross section occur at the welded part. The criteria needed for these two types of welding defects to be eliminated are obtained on the plane of TM and PW. Both criteria approximately coincide with each other under standard condition, and they do not greatly depend on the asperities of the ¯ow front under various injection conditions. Moreover, the above welding-defect elimination criterion is also effective for estimating the degree of welding defects in injection molding by the use of a mold cavity with inserts. Finally, the above criterion approximately coincides with that in a previous paper, which was obtained by the use of simple void-reducing conditions in the joining of green ceramic blocks with arti®cially grooved surfaces. References [1] T. Zhang, J.R.G. Evans, J. Br. Ceram. Trans. 92 (4) (1993) 146±151. [2] K. Kato, R.H. Chen, J. Jpn. Soc. Tech. Plast. 32 (371) (1991) 1497±1502. [3] S.C. Malguarneras, A.I. Manisali, D.C. Riggs, Polym. Eng. Sci. 21 (1981) 1149±1156. [4] K. Kato, R.H. Chen, N. Suzuki, N. Otake, J. Jpn. Soc. Tech. Plast. 36 (418) (1995) 1257±1262. [5] S.J. Stedman, J.R.G. Evans, J. Woodthorpe, J. Mater. Sci. 25 (1990) 1833±1838. [6] I. Tsao, S.C. Danforth, A.B. Metzner, J. Am. Ceram. Soc. 76 (12) (1993) 2977±2982. [7] K. Kato, Y.H. Chung, N. Otake, J. Jpn. Soc. Tech. Plast. 40 (461) (1999) 606±610.