Establishing new rules for coldbox coremaking: when establishing a method for vent and blow tube placement in coreboxes, foundries must maximize productivity and minimize amine consumption.
The established formulas caused problems when a large core was to be produced. Instances existed when the vent open area needed was, according to the formula, greater than could be obtained due to insufficient surface area of the cope or drag portions of the corebox. Also, on some occasions, it was not possible to install enough blow tubes due to space constraints in the corebox lid.
This article discusses the venting and blow tube placement parameters for coreboxes used to produce coldbox cores, focusing on horizontal tooling rather than vertically parted coreboxes. An analysis of the parameters is performed on a production core that was experiencing problems. Although this article addresses the amine-cured phenolic urethane resin system directly, the theory supporting the outcome would be similar in other coldbox systems.
Revised Tooling Parameter
In coldbox tooling design, it is necessary to create an atmosphere that allows the liquid amine to be converted to a vapor state for efficient curing of the core sand mass. The maintaining of amine in the vapor state is critical to ensure that cure times and minimal amine consumption are to be attained. The air/amine temperature must remain above 170F (77C) in order to assure that the vaporized amine is available to cure the sand mass.
Both pressure and amine concentration affects the tendency for the vaporized amine to revert back to a liquid. Higher concentrations of amine vapor in the curing air will tend to drop out of the vapor state and revert back to liquid at somewhat higher temperatures. A lower percent concentration is more stable at lower temperatures.
A decrease in pressure also makes the amine vapor more stable. However, during the curing cycle, the amine/air pressure is reduced as it flows into the gassing manifold. This decrease results in a reduction in temperature as the amine/air mixture expands. If the amine reverts back to a liquid, it is necessary to increase amine consumption during the curing cycle and to increase the purge time to assure that no residual amine odor is retained in the core after it is ejected from the corebox.
Before revising the established tooling guidelines, several goals were established, including:
* a target of 10 lb of sand cured/sec of purge time;
* a target of no more than 0.3 cc of amine/lb of cured sand;
* to see if the system would negate the variances in curing speed regardless of the type of vent used;
* the acceptability of the new system to various core configurations;
* a plausible explanation for why the new system is efficient and should be established.
What are the purpose and critical elements of venting relative to curing speed? Cope venting allows air and vaporized amine to enter the sand mass in the corebox, while drag venting allows air and vaporized amine to exit the core cavity after passing through the sand mass. There also is greater back-pressure generated at the gassing manifold when there is sand in the core cavity compared to when the air and amine is allowed to pass through the core cavity with no sand in the corebox.
The increased back-pressure can be explained by the fact that the addition of the sand is the only change made to the system. The back-pressure increased at the gas manifold with no change in pressure at the amine generator. This indicates that the sand mass reduced the air flow and controlled how quickly the air and vaporized amine can pass through the core sand mass.
The standard formula that is used for tooling design states that the percent of vent open area for the drag portion of the corebox should be approximately 70-80% of the vent open area of the cope portion of the corebox. The reduced amount of vent open area in the drag portion of the corebox is thought to be necessary in order to create a slight pressure in the sand mass during the curing cycle. This pressure forces the vaporized amine to move "sideways" in the sand mass to ensure that all areas of the sand mass comes into contact with the vaporized amine to enable curing of the core.
Since amine is a true catalyst, it is only necessary for the mixed sand to come into contact with the amine vapors in order to harden. The amine only catalyzes the curing mechanism and is not consumed in the chemical reaction. If any mixed sand does not come into contact with the alkaline amine, the chemical reaction will not occur, and soft sand will be left in the corebox when the core is ejected.
The fact that the vaporized amine must pass into all areas of the sand mass during the curing cycle is the initial reasoning for using fewer vents in the drag portion of the corebox compared to the cope portion of the corebox. This venting arrangement creates a slight back-pressure, forcing the amine to go sideways in the sand mass and cure all areas of the core.
A hypothesis proposed that the vents might control the air and vaporized amine pathways in the core, and may not necessarily be the pressure generator as the air and amine passes through the sand mass. The equal spacing of the vents becomes critical to assuring that all areas of the core are cured at the same time. In this situation, very little amine passes through the core and exits into the exhaust manifold in some areas while the rest of the core continues to have vaporized amine moving toward uncured areas.
Experimental Corebox Design
The corebox used for evaluating curing efficiency was a flat core with two coreprints. The core was approximately 6 in. thick. The two coreprints opposed each other and extended in. into both the cope and drag portions of the corebox. The total core weight was 136 lbs.
This corebox was chosen because it created a particularly difficult core to make. Problems had been encountered with "cope stickers" around the cope vents as well as depressions under the cope vents. It was necessary to stop production every 10-15 blows and clean the sand from the cope vents. In order to correct this problem, the amine vapors and air were allowed to enter the core sand mass only around the outside of the tamper pins and through vents in the tamper pins at the blow tube openings. The amine and air that passed through the cope vents were allowed to pass through channels that directed them to the exhaust manifold areas.
This solution had eliminated the problem with cope stickers and depressions under the cope vents. However, the purging time was more than 30 sec and amine consumption was 50 cc to cure the 136 lb of sand in the core cavity. This core did not meet the desired criteria. The 30 sec purge time meant that only 4.5 lb of sand was being cured/sec of purge time and amine consumption was 0.37 cc/lb of cured sand.
The 0.5 in. diameter mesh vents were evenly distributed at every 1.5 in. interval on the cope portion of the corebox. This pattern was begun at the outer perimeter of the cope portion. Then a grid-work design for vent placement was laid out for the rest of the venting pattern. No more than 1.5 in. between centers was allowed for all vents in the cope portion of the corebox. All vents were the same diameter and equally spaced across the surface of the cope.
The layout of the vent grid in the drag portion of the corebox was similar to the grid-work design that was followed in the cope portion of the corebox. However, the spacing of the drag vents was changed to 2.0 in. intervals to minimize the possibility of the drag vents being directly below the cope vents. This pattern would minimize the "vent over vent" possibilities. If the cope vents were located directly over any drag vents a "bypass" of the amine vapors can occur and reduce amine consumption efficiency.
After all the vents were installed in the corebox, it was scheduled for production. The results for cure time and amine consumption were good. Cores were produced utilizing a 9 sec purge time. All areas of the core were cured and several cores were produced. The curing rate was 15.1 lb of core sand cured/sec of purge time.
Amine consumption was reduced from 50 to 30 cc. This meant that the amine consumption rate was 0.22 cc/lb of cured core sand. The corebox had met and exceeded the initial two targets relative to purge time cure rates and amine consumption.
However, the original problem relative to the cope stickers had not been solved. After reviewing the problem, a decision was made that the blow tube spacing was the potential problem. The blow tubes were randomly spaced and there appeared to be an excessive distance between the blow tubes in the problem areas. The sand compaction in these areas was possibly low, allowing the sand to "settle" away from the cope surface of the corebox during machine movement prior to the start of the curing cycle.
Low compaction of the core sand mass also allowed the sand under the cope vents to be depressed by the air during the initial curing sequence, prior to the time amine vapors come into contact with the sand mass. Blow tubes were added to the corebox at approximately 5 in. between centers to ensure that sand compaction was similar in all areas.
This change in number of blow tubes and the spacing of the blow tubes eliminated the problem with cope stickers as well as the depressions under the cope vents. Sand compaction had been the major cause for the cope stickers and depressions under the cope vents in this corebox.
The third target objective related to differences in the style of vents to be used. All mesh vents were removed form the corebox and replaced with 0.5 in. slotted vents. The slotted style vent has approximately 30% of the open area of mesh vents.
After all mesh vents had been replaced with the same diameter slotted vents, it was found that the different style vent did not negatively affect the amine consumption. However, the best purge time possible was found to be 12 sec. This purge time was longer than the total purge time that had been attained with the mesh style vents (9 sec).
In order to test the grid-work pattern for venting, several coreboxes were modified since the initial investigation was undertaken. Some examples of the cores and venting patterns are shown in Fig. 2.
In all cases, the target of 10 lb of cured sand/sec of purge time has been met or exceeded. The general rule relative to a maximum of 5.0 in. between the blow tube centers also is used wherever possible.
In most instances, it is possible to see where the core does not come into contact with the curing vapor, indicating that there is a consistent flow of the curing vapors and air throughout the sand mass. An uneven flow would manifest itself by uneven areas of where the core has hardened.
The most logical explanation for why the system is efficient would be that the venting area is evenly distributed, allowing the curing vapors and air to pass evenly into and through the sand mass. Without this even flow through the sand mass, there would be areas of uncured sand along with areas of the sand mass that hardened before the other areas.
This method of venting design has been successful. No major problems have been encountered using the grid pattern design for coreboxes. In most instances, the blow tube spacing of no more than 5.0 in. between blow tube centers has been successful in producing good compaction of the sand in all areas of the core. For most cores, the blow tube diameters are as large as possible. This reduces wipe-off and excessive corebox wear under the blow tubes.
In instances where it is necessary to force sand into thin core areas, smaller diameter blow tubes are used to increase velocity to force the sand through the thin areas. However, the smaller blow tubes should only be used when necessary due to increased resin wipe-off and wear under these smaller diameter blow tubes.
The configuration does appear to have an effect on the ability to reduce the amine consumption levels to the target areas. More complex core configurations tend to require amine consumption levels higher than the target of 0.3 cc per lb of cured sand. This consumption level usually is no greater than 0.5 cc per lb of cured sand.
The grid venting system has been successful on a wide range of core sizes and weights. All new coreboxes have been vented with this method of vent placement with good results and no major problems encountered. The new system improves both productivity and amine consumption through better airflow and increased effectiveness.
This article was adapted from a paper in the 2001 AFS Transactions (01-085). For more information, contact the AFS Special Publications Dept. at 800/537-4237 x247.
For More Information
"In Search of a Cure: Optimizing Coldbox Core Systems," M. Adamovits, B. Thomas and W. Tinker, MODERN CASTING, May 2002, p.30-34.
"Examining Key Variables of a Coldbox Coremaking Operation," AFS Cured Sand Committee (4-1), J. Cavanaugh and D. Gilson, MODERN CASTING, June 2000, p.32-34.
About the Authors
Ken B. Horton retired as director of technical services of Ashland Chemical Co., Dublin, Ohio, in 2001. Todd Lewis is a coreroom supervisor at Waupaca Foundry Plant 2, Waupaca, Wisconisn.
|Printer friendly Cite/link Email Feedback|
|Comment:||Establishing new rules for coldbox coremaking: when establishing a method for vent and blow tube placement in coreboxes, foundries must maximize productivity and minimize amine consumption.|
|Author:||Horton, Ken B.|
|Date:||Jan 1, 2003|
|Previous Article:||What causes electromagnetic pump clogging problems?|
|Next Article:||Chilled sand casting process delivers higher strength components. (Technology in Progress).|