Implementing a water-based shell mold system.
The Environmental Protection Act (1990) required U.K. foundries to reduce or phase out by April 1997 all volatile organic compound and ammonia emissions to the atmosphere. This signified the end of the use of alcohol-based binders and their rapid drying characteristics for investment casters, and the implementation of the less volatile water-based colloidal silica systems.
To comply with this act, Rolls-Royce Plc., a 600-employee, non-ferrous plant in Derby, U.K., made a strategic decision in 1996 to replace its current alcohol-based binders that have served the industry since the 1960s with a water-based shell system. The change had to be carried out without detrimental effect on production, while using the existing shellroom environment, minimizing the amount of new capital equipment required and optimizing the current working practices where possible.
This article presents the implementation of a polymer enhanced colloidal silica binder at Rolls-Royce and outlines the formulation and use of primary and secondary slurries, including the results from volume testing.
The main aim of the water-based mixing technique is to emulate current alcohol-based mixing methods to minimize disruption and equipment change. Drying behavior depends on the colloidal silica characteristics and the additives present in the mixtures. Different water-based binders contain varying proportions of additives and subsequently have different water contents in order to produce the ideal coating make-up for investment casting shell molds. The relative proportions of the additives and water content are generally selected to suit the method of dipping and specific geometry of the components being produced. Therefore, it is not possible to formulate an ideal slurry-water content and additive proportions for optimum drying rates for every foundry.
Rolls-Royce tested a series of polymer-modified primary coats and compared results with the existing coatings. The de-wax cracking increased when different types of primary and secondary coat binder systems were used to produce shell molds. Some of the polymers used in the available binder systems created an impervious layer that prevented the melted wax and wax vapors from escaping the internal cavities, resulting in localized build up in pressure. The pressure build-up may be sufficient to crack the shell molds at corner sections during de-waxing.
The percentage silica content of the binder must be controlled at 24-26%. Silica content above 30% may become unstable in some binder systems, resulting in rapid gelling and lower usable life. The fired strength of the slurry system increases as the percentage silica in the binder increases, which may cause knockout problems with delicate castings. Silica content below 24% may have insufficient strength to withstand de-waxing pressures and also result in mold distortion due to lower fired strength.
The move toward the use of water-based binders requires a complete reassessment of shell production techniques. The removal of moisture from a water-based mold is slower than that of equivalent alcohol-based systems due to the relative evaporation rates.
The main environmental parameters that most impact the drying characteristic of a water-based shell mold are air velocity, humidity, temperature and inter-coat drying. A series of trials was conducted to investigate the effect of these four parameters in order to establish the optimum conditions for the Rolls-Royce process.
Air velocity across the mold surface is the main parameter that can be most readily controlled and varied to alter the drying characteristics. Trials were set-up to compare the drying efficiency of a rotating fan system that produced a pulsed air flow across the mold surface vs. a constant air flow system. Drying rate and wet bulb temperature profiles produced under similar environmental conditions were compared between the two systems, as illustrated in Fig. 1.
Trials indicated that both systems effectively removed water from the shell molds, but cracking was significantly higher with the stationary drying fans than the pulsed air system. The rotary fans uniformly removed water from the shell mold surface, reducing the tendency for drying/shrinkage stresses to form and thus the incidence of the autoclave cracking problem. The constant airflow of the stationary system preferentially dried the surfaces at the position where the airflow was highest, leaving some areas wet. Regular rotation of the molds in the stationary system improved the drying rate across the whole surface and reduced the incidence of autoclave cracking.
The results show that air velocities below 3 m per sec substantially extend the inter-coat drying times due to the lack of water removal from the surface. Further, air velocities above 6 m per sec improve the drying rate, but any increase above this has little effect on the drying rate.
Dry Bulb Conditions
Relative humidity has a large influence on the drying process due to its effect on the partial pressure of water vapor at the mold surface and the ability of moisture to evaporate out of the shell mold. To promote drying, water vapor must be removed from the region immediately above the mold surface and enhanced by increasing the air velocity across the mold surface. Accurately controlling the dry bulb temperature according to the individual mold requirements may prove difficult in an industrial environment.
The wax/ceramic interface temperature was measured by embedding a thermocouple into the wax pattern surface of the shell mold being coated. This thermocouple provided a temperature profile that was transmitted to a data-logging instrument. The shell molds were produced by a 20 sec immersion in slurry, 50 sec drainage and 30 sec stucco application with inter-coat drying times from 30 min to 2 hr.
The slurry to stucco ratio was measured and used to calculate weight of water per unit mass of coating applied. Weight loss due to moisture evaporation was monitored as a function of time by weighing the mold immediately after coating and then at intervals of 15, 30, 60 and 120 min where applicable. Dipping continued until 8 coats (plus a seal) coat had been applied, with the weight loss measured for each coat. Weight loss was then monitored for 2 days prior to de-waxing. Final drying curves and temperature profiles at 40%, 50%, 60% and 80% relative humidity are illustrated in Fig. 2.
All 8 coats applied to a shell mold in a dry bulb environment of 6 m per sec air velocity, 40% relative humidity (RH) and 73F (23C) were approximately 100% dry within 60 min, with a final dry time of 12 hr. Increasing the relative humidity from 40% to 50%RH reduced the drying rate of each coating so that each coat had not fully dried before the next coating was applied. This had the effect of extending the final drying period to 26 hr. Increasing the relative humidity to 60% and 80%RH further reduced the rate of water loss for each coat so that none of the coats were fully dry prior to the application of the next coating. The 60% and 80%RH drying environments did not produce 100% dryness during the 48 hr final drying period, reaching a maximum of 98% and 72% water loss, respectively.
Inter-coat Drying Time
A series of experiments was conducted in order to investigate the effect of the inter-coat drying time on the wax/ceramic interface temperature and the drying rate. The drying parameters selected - 6 m per sec air velocity, 50%RH and a dry bulb temperature of 73F (23C) - were established from the dry bulb investigation. Mold coating techniques were conducted as described above.
Figure 3 illustrates the drying rate curves and temperature profiles for molds dried with inter-coat drying times of 30 min, 1 hr and 2 hr between coating cycles. Increasing the drying time prior to re-dipping the shell mold from 30 min to 1 hr significantly increased the drying rate. The drying rate did not significantly increase, however, when extending the drying time from 1 hr to 2 hr.
The final drying time of the molds coated every 30 min was 40 hr; for molds coated every 1 hr, it was 28 hr;, and for ones coated every 2 hr, it was 26 hr. For many foundries, robot coating output is a major problem so the coating cycle is to be kept to a minimum to increase throughput. It may be advantageous to reduce the inter-coat cycle time in order to increase the throughput and remove any extra water remaining in the shell mold by extending the final drying period.
The main limitation to efficient drying is due to soak back of water from the slurry re-wetting the shell mold during dipping. A fully dry shell mold can be re-wetted during dipping and may also cause the inner coats to absorb water, extending the total drying time required to produce the shell molds. A conductivity cell was used to measure the degree of soak back of water into the shell mold during and after dipping.
Electrodes were attached to the surface of a wax pattern at a distance of 10 mm apart. The mold was successively dipped and the rate of water loss logged as a function of the conductivity. The investigation showed that after 4 coats had been applied, the soak back into the primary coat was minimal.
A low viscosity slurry contains more water than a higher viscosity slurry and thus, more water soaks into the previously dry coats and to a greater depth. As the time that the mold is immersed into a slurry increases, so does the amount of soak back that occurs. An immersion time of less than 10 sec may result in delamination due to insufficient binder soaking into the previous coats. Excessive wetting occurs when the molds are immersed for longer than 40 sec, resulting in softening of the previously dried coats and an increase in the amount of water to be removed during drying.
Further drying of the shell molds after the final coat has been applied is generally required to remove any remaining moisture and any soak-back that has occurred during the seal coat application. Moisture contained in the inner coats also has greater distances to migrate as coats are applied and thus, extends the final drying period.
Failure to remove the water from the inner coats results in damage to the primary and inner secondary coats due to a rapid build-up in pressure that occurs when the water turns to steam during de-waxing. The blowout damage is generally not detectable until the mold has been cast. Each assembly configuration should be tested to verify that the drying cycle is sufficient to remove all the water contained within the shell mold structure prior to de-waxing.
Water System Implementation
The production implementation program commenced at Rolls-Royce with the installation of a pilot production cell within the existing shellroom environment. The validation studies comprised of producing numerous molds on a wide variety of parts, across three slurry systems.
The parts chosen were: eight directionally solidified/single crystal (DSX cast) parts, eight equiax nozzle guide vane/turbine blade parts, and four land-based industrial gas turbine (IGT cast) parts. The validation package for each part involved a statistical analysis of all features on at least 25 castings, a metallurgical evaluation, 100% non-destructive testing (NDT) and 100% grain structure analysis.
Most parts were dispatched with no detrimental effect on scrap rates, grain structure, dimensions and metallurgical integrity. The statistical analysis for both systems showed a shortening of the annulus and a smaller overall aerofoil shape. Also, the water-based shell is much more resistant to shell bulge on the larger molds than the existing alcohol-based molds.
The only problems experienced during this phase were with "blow out" on a part number in an isolated pocket shielded from the air flow and a higher incidence of "hot tearing" on a casting with a history of this defect. The former was resolved by slightly extending the intercoat drying time (re-alignment of the patterns on the assembly was not necessary) and the latter was resolved by reducing the number of secondary coats by 30%.
Volume Production Trials
The next phase of the implementation program was to produce volume trials across a representative number of parts, with statistical data being collected on at least 250 castings per part number.
Zircon Slurry System
To validate all the parts made by this slurry system, about 900 molds (5000 castings) were manufactured.
The results suggest that the initial tears over major tooling modifications due to annulus and aerofoil shape variations were unfounded. The majority of casting yields obtained from the volume trials were comparable to the yields experienced from the ethyl silicate shell system, and no detrimental effects were experienced with regard to surface finish and NDT. The main concern, however, was a higher susceptibility to recrystallization on some single crystal parts with particular blade geometries/alloy/cast temperature combinations. This problem again seems due to higher rigidity of the shell after cast. Modifications to the method of manufacture in conjunction with a reduction in the number of secondary coats has resolved the problem.
Molochite Slurry System
There was concern that the binder system chosen may not be totally suitable to the plant's molochite slurry, and that a great deal more problems would be experienced with respect to non-conformance and dimensional stability due to the varied nature of the assemblies. Apart from initial problems with "burn-on," which were addressed by adjustments to the primary slurry formulation, the volume trials have been successful to date and the molochite slurry has lasted more than 6 months without signs of aging.
High Strength Zircon Slurry System
* Again, no problems were encountered in terms of slurry life, as a laboratory sample ran for about 4 months with no deterioration or creep results. Some of the larger castings made from the alcohol-based version of this slurry-exhibited slight shell bulge in the concave aerofoil region. The water-based shell eliminated this problem, as it is stronger at elevated temperatures.
* Two other problems were the monitoring and control of the silica solids content of the slurries, and a higher incidence of mold "cup" and "skirt" breakage.
* In terms of the silica content control, a test procedure was devised so that a result can be obtained within 5 hr, and regular monitoring allowed the slurry to be kept within the optimum range.
* The mold breakage problem was split into two distinct problems. First, the "skirt" weakness was due to using a robot draining program that was designed for alcohol-based slurries, and only came to light when molds were manufactured with the least number of coats. The make-up of the water-based shell slurry meant that the draining characteristics were different from the alcohol-based slurry, and therefore the correct coating thickness was not achieved around the "skirt." Small modifications to the draining program solved this problem.
* Second, the "cup" breakage problem was a result of the current method employed to remove the surplus shell on the top and bottom of the mold. As the "green strength" of the water-based shell molds is less than the alcohol-based shell molds at this stage, more shell damage was occurring. An alternative method to remove the pouring cup and chill plate skirt has since been devised to eliminate this problem.
Since conversion to a full-fledged production system, a number of lessons have been learned on how to optimize the system to produce the best quality molds and castings.
Slurry control and testing is of prime importance, with the following checks (in addition to viscosity and temperature) recommended at regular intervals:
* refractory solids content;
* silica solids content;
* plate weight.
Assessments of coating thickness and stucco penetration on primary coatings are also recommended.
The silica solids content is not only of importance on secondary coatings as it affects the final strength of the mold; it is also important on primary coatings as levels around 28% tend to lead to a thixotropic nature.
Due to the weaker green strength of the water-based shell system compared with its predecessor, a number of minor wax assembly and shelling method changes have been necessary to enable all the blade geometries made at the foundry to be manufactured successfully.
RELATED ARTICLE: Monitoring Polymer Levels in Aqueous Slurries
The switch to water-based colloidal silica binders in compliance with the EPA guidelines in the U.S. has left investment casters with shell molds of weak green strengths. To address this problem, many foundries have employed polymer-enhanced binders to improve the strength, stucco pickup and drying behavior of the aqueous slurries. But, a concern evolved that newly developed aqueous slurries lose their polymer content with use over time. And, until now, investment casters didn't have a way to analyze the variability of polymer content during slurry usage.
The test developed to analyze polymer content is a comparison of the decomposition weight loss (DWL) of a dried sample of the slurry to slurry standards prepared with varying known amounts of polymer. It is shown that a linear relationship exists between the percent polymer and DWL for the prepared standards. This relationship can then be used to calculate the actual percent polymer of the slurry in question.
In the month that it took to develop the test, weekly samples from a production slurry were collected. By the 19th week of slurry age, the amount of polymer had dropped to 76% of the beginning level.
A small build-up slurry for week 19 was modified to a higher than normal level of polymer in an attempt to bring the % polymer up in the production slurry. This addition was calculated to yield a value of 90% of the normal polymer in the production slurry. The corresponding measured value of the production slurry for week 20 was 89.92%, using this test.
A similar small build-up slurry was then made for week 21 to bring the production slurry to 98% of the normal polymer level. The result was a 97.89% measured value for week 21. These results confirmed that the procedure can accurately measure the true % polymer of a slurry and that the % polymer of the slurry does decrease over time.
- Mike Sorbel, PCC Structurals, Inc., Portland Oregon
This was adapted from a presentation at the 45th ICI Technical Conference and Meeting in 1997.
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|Title Annotation:||polymer enhanced colloidal silica binder|
|Date:||Jan 1, 1998|
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