Enhanced Binders for Improving Performance of Backup Coats in Precision Investment Casting Shells

Transcription

1 Enhanced Binders for Improving Performance of Backup Coats in Precision Investment Casting Shells Originally published 2013 Remet UK Limited Robert J Brown Remet Corporation Joe Stanco

2 1.Abstract Investment casting shells are subjected to various mechanical and thermal stresses during the different stages of the investment casting process. The backup portion of the shell system is critical in providing support to the facecoat while maintaining overall shell strength during dewaxing, firing, and metal pour without distortion or loss of dimensional stability. Colloidal silica binders often provide minimal green strength to allow the shell to survive dewaxing but do not have adequate permeability to promote gas transfer through the shell wall. They also typically produce shells with higher than needed hot strength. Enhanced binders are engineered to provide a balance between increased green strength needed for dewaxing with sufficient hot strength for metal pouring and good gas flow. Studies of enhanced binder system, specifically, REMET s Remasol ADBOND 3301, were conducted and characterized for green, hot, and fired strengths along with dry times between dips and permeability. Results show that backups dipped in Remasol ADBOND 3301 retained higher green strength to minimize distortion during shell build with improved permeability. Benefits of Remasol ADBOND 3301 also include reduced dry times between dips to allow rapid building of shells with complex geometries that include cores, blind holes, and difficult to dry areas. 2.Introduction Foundry productivity is impacted by multiple factors: part shape; presence or absence of cores, blind holes; part loading on sprues, which can create bridging and / or drying problems; temperature and humidity levels and the ability of the foundry to control them; the properties of the binder and refractory products used; processing conditions; air movement; operator experience (unless robots are used); robot programming capability; etc. There are many more factors, which are very familiar to experienced foundry men. Part configuration and the way the parts are mounted on the sprue impact shell drying and the green strength that is obtained. The presence of cores and / or blind holes presents obvious drying issues. Shell bridging between parts as coats are applied can also create drying problems. Green strength development progresses only until the next coat is applied1. The more water that is retained, (i.e., the less dry the shell is), the lower the green strength. This produces a shell that will be more prone to cracking during autoclave and run out during pouring. The foundry man must recognize the impact of restricted air flow on shell drying and strength development2. Surfaces that are exposed to air movement will dry without issue in most instances, unless relative humidity (RH) is very high. It is the hard to dry areas that limit the ability of the foundry to achieve productivity levels required to be cost competitive and profitable in their markets. Remet s primary goal in developing the Remasol ADBOND Enhanced Binders and EZ-CAST Engineered Refractory products for the investment caster is to provide robust systems that can perform under varying process and environmental (temperature and humidity) conditions found within the foundry. We want to broaden the range within which a foundry controls its process so they can be cost competitive in today s global market. 2

3 3.Experimental Remet worked closely with two Japanese foundries in developing the data presented in this paper. All experimental work was conducted at Remet s Developmental Laboratory prior to conducting a limited production trial at one of the foundries. Each foundry first wanted to see the impact of Remet s Remasol ADBOND Enhanced Binder on their shell properties. Slurry samples were obtained from both foundries. Samples of the locally available Chamotte, which they use as a stucco, were also obtained. All primary and secondary coats consisted of the same slurries and conditions as followed by each foundry. Table 1and Table 2 summarize the three different backup coat system applied for each foundry using: 1 Foundry s current back-up slurry and viscosity, back-up stucco and application method, drying conditions and inter-coat drying times; 2.A)-Remasol ADBOND 3301 run at Remet s recommended viscosity, with the foundry s flour mixture and back-up stucco, and dry times of 1-hr. or 3-hr.; and 2.B - Remasol ADBOND BV run at Remet s recommended viscosity, with the foundry s flour mixture and backup stucco, and a dry time of 2-hr. In all instances, the drying room temperature was maintained at 24 C and the relative humidity at 60% with final dry in both cases for 24-hr. at 24 C. Table 1. Foundry #1 Slurry Backup Formulations Foundry # 1 Slurry System Viscosity (s) Zahn Cup Flour Room T and % RH Fused Silica Blend of 150 and 300 Mesh 14 to 16 # 4 Zahn Signature RP-1 Fused Silica 1 Standard 21 # 5 2.A Remasol ADBOND B Remasol ADOND BV 14 to 16 # 4 Zahn Signature RP-1 Fused Silica Chamotte mm Applied by Fluid Bed in All Cases 24 C 60% RH 3

4 Table 2. Foundry #2 Slurry Backup Formulations Foundry # 2 Slurry System Viscosity (sec) Zahn Cup Flour Room T and % RH 1 Standard 9.3 # 4 Zahn Signature RP-1 Fused Silica 2.A Remasol ADBOND B Remasol ADOND BV 14 to 16 # 4 Zahn Signature RP-1 Fused Silica 14 to 16 # 4 Zahn Signature RP-1 Fused Silica Chamotte mm Applied by Fluid Bed in All Cases 24 C 60% RH MOR values were measured under Green, Hot and Fired conditions. Green MOR values were determined at room temperature, which is representative shell strength at time of de-waxing. Hot and Fired samples were both fired to 1000 C and held at this temperature for 1-hr. Hot MOR samples were broken at about 1000 C, which represents the shell s strength when metal is being poured into it. Fired MOR samples are allowed to cool to room temperature before being broken which represents the relative ease of shell removal. Permeability was tested on samples fired to 1000 C and held at this temperature for 1-hr. before the test is run at this temperature. Nitrogen gas was used as the carrier gas. 4

5 4.Results Table 3 and Table 4 present test results for both Foundry # 1 and # 2, respectively. The MOR data along with the breaking loads are shown graphically in Figures 1 and 2. Figures 3 and 4 show the same data expressed as the percent change from Foundry # 1 s Standard System. Table 3. MOR and Breaking Load under Green, Hot and Fired Conditions and Permeability of REMET Backup System Compared to Foundry #1 Standard 5

6 Figure 1. Comparison of green, hot and fired MOR strengths between REMET and Foundry # 1 shell systems. Figure 2. Comparison of green, hot and fired breaking loads between REMET and Foundry shell systems. 6

7 Figure 3. The REMET shell system delivers improvement in green shell MOR strength with adequate hot and fired strength compared to Foundry # 1 system. Figure 4. The REMET shell system delivers improvement in green shell breaking load with adequate hot and fired strength compared to Foundry # 1 system. 7

8 Table 4. MOR and Breaking Load under Green, Hot and Fired Conditions and Permeability of REMET Backup System Compared to Foundry #2 Standard The MOR data along with the breaking loads are shown graphically in Figures 5 and 6. Figures 7 and 8 show the same data expressed as the percent change from Foundry # 2 s Standard System. 8

9 Figure 5. Comparison of green, hot and fired MOR strengths between REMET and Foundry # 2 shell systems. Figure 6. Comparison of green, hot and fired breaking loads between REMET and Foundry # 2 shell systems. 9

10 Figure 7. Comparison of green, hot and fired MOR strengths between REMET and Foundry # 2 shell systems. Figure 8. Comparison of green, hot and fired breaking loads between REMET and Foundry # 2 shell systems. 10

11 5.Discussion We ll review the individual Foundry results prior to summarizing the overall conclusions from the tests. Foundry # 1 Discussion The Remasol ADBOND 3301 system is Remet s fastest system in terms of developing strength. Because of this, we tested it at 1-hr. drying time, 1/3rd of the normal dry time for the Foundry s standard shell system. Under these conditions, it generated 33% higher Green MOR values than the Foundry s standard shell system. At the same 3-hr. drying time, the increase in Green MOR value was 54%. Breaking Loads increased by 16% and 21%, respectively. The Remasol ADBOND BV system does not develop Green Strength as quickly as the Remasol ADBOND 3301 system but it still generated 35% higher MOR and 19% higher Breaking loads at 1-hr. less drying time. Hot MOR values were reduced relative to the Foundry s standard shell system. Strengths are still adequate to hold metal. Standard colloidal silica bonded systems have low green strength and high hot strength characteristics. Because of this, foundries normally apply the current number of coats to allow the shell to survive de-waxing. This typically produces shells which have more hot strength than is required to hold metal. Likewise, Fired MOR values are lower than the Foundry s standard shell system, which should improve shell removal. Shell permeability was higher for both Remet systems compared to the Foundry s current system. Foundry # 2 Discussion Foundry # 2 uses an 8-hr. dry time between coats. Remasol ADBOND 3301 generated Green MOR values that were 41% and 62% higher than that of the standard shell system at 1-hr. and 3-hr. intercoat dry times respectively. Breaking loads were 38% and 50% higher, respectively. Remasol ADBOND BV generated Green MOR values that were 40% and 60% higher at 5-hr. and 8-hr. intercoat dry times respectively. Breaking loads were 17% and 46% higher, respectively. Hot MOR values for both Remet systems ranged from 7% lower to 30% higher compared to the Foundry s standard shell. Breaking loads ranged from 9% lower to 21% higher. Fired MOR values ranged from 15% lower to 19% higher than the foundry s standard shell. Breaking loads were 12% lower to 13% higher. Shell permeability was lower for both Remet systems compared to the Foundry s current system. Production Trial Foundry # 1 ran a small scale production test using Remasol ADBOND 3301 in conjunction with Remet s EZ- CAST CPS Fused Silica flour. This test used parts that were subject to cracking and mis-run. Comments from the report are excerpted and reported below. 11

12 Product A (Figure 9) Shell molds after dewax process (picture A). No crack was found on the shell molds of normal slurry, but small crack was found on shell mold of REMET (picture C). Picture D and E shows small fin appeared in same location of crack on runner. Product B (Figure 10) No crack was found on shell molds in both cases. But small crack was found on corner of axle. Product C (Figure 11) A small crack was found on REMET shell, but no fin was found after casting. Figure 9. (A) Product A Shell Molds. (B) Locates area where (C) small crack in shell developed along edge of runner resulting in finning along edge of runner (D and E) with REMET shell system. Figure 10. (A and B) Product B Shell Molds (C) Finning on corner of axle in with REMET system. 12

13 Figure 11. (A) Product C Shell Mold. (B) indicates location of small crack (C) but no finning in casting. Permeability Test pieces for permeability test were prepared by using the slurries and two kinds of alumina sand with three kinds of coating patterns. Using the Chromatography column permeability set-up shown in Figure 12, the permeability of each slurry mixture was measured. Permeability was evaluated as time (sec) required for fixed volume air to pass through the shell mold test piece. Shorter time means higher permeability. The graph in Figure 13 shows that shell molds prepared with REMET system had consistently higher permeability than the Normal system. Figure 12. Permeability Test Setup using Chromatography. 13

14 Figure 13. REMET systems resulted in higher permeability than the Normal system. 14

15 6. Summary of Foundry Trial The following is the foundry s summary of their results and future plans: 1. Permeability of shell mold using REMET s system was superior to one using normal back-up slurry. 2. Minimal cracks are found on shell mold using REMET s system. Small fin appeared on shell mold in same location the cracks. 3. Misrun appeared in both cases. 4. The sizes of products using the both back-up slurry system are almost same. Overall the properties of both back-up slurry systems are same. However, it is recommended to conduct further application of REMET slurry system to actual products, considering quantity of trees used for the trial. 7. General Discussion Achieving optimum drying conditions of 40% to 50% RH for back-up coats can be expensive for a foundry to implement. The use of 60% RH demonstrates the ability of the Remasol ADBOND 3301 and Remasol ADBOND BV systems under less than optimum conditions. Even if the external surfaces are exposed to low Relative Humidity conditions, the areas within the cores and blind holes are much higher in RH due to the trapping of the moisture within these cavities. The production trial was run under the direction of local plant personnel. The results were somewhat mixed, which is not unexpected for an initial trial. Optimization will occur during the next testing phase. The lab results from these tests confirm previous results obtained from Remet laboratories. The Remasol ADBOND binders will work with any refractory flour that is compatible with colloidal silica to enhance the rate of green strength development and improve the rate of shell survivability during de-waxing. Combining the Remasol ADBOND binder system with an engineered refractory, such as EZ-CAST CPS Fused Silica, Remasil 48 EZ-CAST or Remasil 60 EZ-CAST, will impact the hot strength of the shell through improved density of the shell as well as a thicker shell build. The use of coarser stucco in conjunction with one of these flour products will also accelerate the building of shell thickness. The use of coarser stucco must be tempered by recognizing the casting configuration and avoiding the use of stucco that is too coarse and which would bridge over details and allow metal break through to occur. 15

16 8.Conclusions 1. Remasol ADBOND 3301 accelerates the rate of Green Strength development relative to straight colloidal silica binders. 2. Remasol ADBOND 3301 is more effective in accelerating the rate of Green Strength development than Remasol ADBOND BV. 3. Remasol ADBOND 3301 and Remasol ADBOND BV are both superior to standard colloidal silica binders in terms of developing higher green strength levels. 4. Depending upon the configuration being cast, Remasol ADBOND enhanced binders can allow the foundry to reduce intercoat drying times or reduce the occurrence of shell failure in cores, blind holes and other areas which are difficult to dry. 5. Production implementation requires working with the foundry to optimize system performance under actual use conditions with the part mix being cast. 16

17 Acknowledgements The assistance, cooperation and contributions of the management and people at Castem are gratefully acknowledged as are those of Freeman (Japan). References 1. The Effect of Dry Time on Shell Strength, Manuel Guerra, Effect of Binder Selection on Drying of Recessed Areas in a Shell Mold, Manuel Guerra, Presented to 57th Annual ICI Technical Conference, October 12,

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