Illuminating core gas: gas defects have longed plagued the metalcasting industry, but the findings of a former research consortium can help limit them.
Winardi is a post-doctoral employee of the Univ. of Alabama at Birmingham (UAB), Birmingham, Ala., and the majority of the work that helped him achieve that degree has revolved around predicting and eliminating gas defects in metal castings.
"We feel that it's a big problem in the industry," Winardi said. "We had a lot of inquiries about the topic from companies in the U.S., Europe and Asia. It's a global casting problem."
Winardi began his thesis work at the university under the tutelage of Professor Charles Bates, now retired and working as an independent consultant for AlchemCast LLC. While at UAB, Bates organized a core gas research consortium with two of his colleagues, Harry Littleton and Preston Scarber, and a number of metalcasters. The research team also enlisted a graduate student to assist them--Winardi.
In the course of its work, Bates's research group developed several methods of precisely measuring the variables associated with core gas. Other measuring methods existed, but none offered the veracity of data that the group required, according to Winardi.
Once the measuring methods were developed, the research group began collecting data. Once the data was collected, the consortium was able to develop equations to predict the occurrence of gas defects.
While the consortium disbanded prior to Bates leaving UAB in December 2006, much of its work was as yet unpublished. Since then, more than a half dozen of the papers the group produced have become available for public consumption (see the "For More Information" section at the end of this article). Now, metalcasters can apply the knowledge to limit the gas defects in their castings.
What They Learned
On a basic level, figuring out whether you will get gas defects is a balancing act, according to the research group.
"The fundamental problem is that more gas is being produced by the core or mold than can get out, so you either have to increase the [venting] or reduce the gas evolution," Bates said. "The practical question is, can this gas exit through the sand at the permeability actually experienced in the molds? If not, it will blow into the metal to produce oxide trails, oxides, dross and bubbles."
While four factors ultimately affect the entrapment of gas in a metal casting, three of them have a significant, measurable effect, according to Winardi:
1. amount of gas created by binders and coatings;
2. size, permeability and length of vents;
3. sand density.
The fourth, gas viscosity, is fairly small compared to the rest, so it doesn't go into most equations that are used to predict the occurrence of gas defects.
By closely controlling the first three variables, and considering them with the time it takes for each of them to react with one another (the rate of evolution), metalcasters can limit their exposure to the risk of gas.
What You Can Learn
When David Weiss, Eck Industries, Manitowoc, Wis., decided to degas his castings, he turned to Bates. He contacted the engineering professor just prior to the gas defect consortium's coalescence, so Eck Industries became one of several metalcasters that were involved in the project. The first casting Weiss submitted for examination was a cylinder head that exhibited the telling trail of a gas defect evolving from one of its water jacket cores.
"In the process of working with the consortium, we learned that oftentimes core washing can contribute significantly to the gas coming off the core, and if it happened to occur at the wrong time, it would end up getting trapped in the casting," Weiss said.
Eck Industries eliminated the core wash, or coating, from the water jacket cores, and the defects stopped appearing. Weiss said that prior to looking at the available data, he and his colleagues didn't know that coatings produced gas. They had previously focused only on the gas created by sand binders.
It's a common mistake, according to Winardi. "Gas evolutions are actually higher from the coating, although people often think only about the binder," he said.
While the papers recently published by the group amount to a compendium of data, graphs and equations that can help predict whether a gas defect might occur, the average metalcaster knows when he has a build up of gas, because he gets defects. Fortunately, the laboratory work of Bates and his colleagues also has lead to a variety of practical strategies that can help metalcasters limit their gas defects.
Eliminating Your Gas
If you are producing defective castings because of excess core gas evolution, several strategies can be employed to alleviate the problem. Try one or more of the following techniques.
Minimize Binders and Coatings--Every binder and coating produces gas when they come in contact with hot metal. So, use just enough to gain the strength and penetration-resistance needed. The coating Eck Industries used on its water jacket cores was serving only to shave a few seconds off the company's shakeout time. Eliminating that wash sacrificed the time savings, but it more than made up for the loss by keeping scrap rates down. If you must maintain your core wash, use a file to remove it from the face of the core that will be used as a vent (typically at the core print).
Minimize Metal Temperature--The hotter the metal is when it comes in contact with binders and coatings, the more gas is going to be produced. "Higher pouring temperatures lead to more rapid solution of gas in metal, and higher temperatures always increase the volume of gas produced when metal hits a sand mold or core," Bates said. Make your pours no hotter than they need to be to avoid filling defects.
Shorten Your Cores--Cores that develop high internal pressure should be kept as close to a venting location as possible. The longer the core is, the further the gas will have to travel to be released, and the more likely it will be that it will instead escape through the metal front.
Loosen Your Sand--Tight sand does not transmit gas. "If you put sand into your coffee cup and pack it once, the sand level will drop," Winardi said. "If you pack it ten times, the level will continue to drop each time. It's just that simple." But sand compaction can get more complicated. Using a sand with fewer screens also can limit compaction. When the grain size and shape of a sand is uniform, it has a lessened ability to interlock with its neighbors, which leaves pores in the sand.
Change Core Blowing Geometry--When a chemically bonded core is molded in an automatic blowing machine, the sand density varies across the core box. The density is greatest at the sand entry point and least at the far reaches of the box. Eck Industries has found that they can enhance their core venting by blowing the core from the opposite side of the core print.
"You tend to put your blowholes exactly where you want the gas to come out, which is in the core prints," Weiss said. "Sometimes it's driven by geometry, but we always take a look at where we blow cores. We're a lot more careful than we used to be."
Time It Correctly--By changing section thicknesses, you can beat gas in a proverbial footrace. Gas defects occur when the pressure behind the gas evolution is greater than the pressure exhibited by the advancing metal front. If the gas pressure prevails, it will push through the molten metal and create a defect. But gas can't push through a solidified metal front. According to the data presented by Bates and his colleagues, core gas evolves in primary and secondary waves. Weiss was able to use that to his advantage. "If you're able to skin over the casting a little faster by doing external cooling or something else on your die, the casting will actually solidify before the gas comes out of the core," he said.
Change Section Sizes--If you are unable to vent your cores adequately, gas actually can be allowed to evolve through the metal in some cases. By thickening certain sections of your casting, the gas can find it's way through the metal front and out of the mold via the overflows or riser.
Gas Research Evolution
Until about four years ago, precise tests for determining how much gas your cores were producing did not exist. Today, metalcasters have the tools necessary to get those values, as well as the permeability of their sand. Once they know how much gas they're making and how much gas they can release, they can determine the break even points between the two.
However, current U.S. Environmental Protection Agency standards are pushing coatings to contain increasing amounts of water and decreasing amounts of alcohol. According to Winardi, water-based prodiacts emit more gas than alcohol-based and are more difficult to dry, as water has a higher evaporation point than alcohol. So, core gas defects can increase with these types of coatings. (However, when dried properly, water-based coatings can be used to produce defect-free castings.)
Bates's gas defect consortium has disbanded, but metalcasters can continue to draw from its data to find ways to beat these defects. Tim Hays of Harrison Steel Casting Co., Attica, Ind., has just begun using the data to tackle some of the gas defects he finds in his castings.
"We're basically at the early stages of using the information," he said. "We're confident it will help us eliminate our defects."
For More Information
"Effects of Mold and Binder Formulations on Gas Evolution When Pouring, Aluminum Castings," P. Scarber, C. Bates and J. Griffin, 2006 AFS Transactions.
"Simulation of Core Gas Production During Mold Fill," P. Scarber and C. Bates, 2006 AFS Transactions.
"Gas Pressures in Sand Ceres," L. Winardi, H. Littleton and C. Bates, 2007 AFS Transactions.
"Comparison of Gas Evolution Results from Chemically Bonded Cores In Contact with Magnesium and Aluminum Melts," L. Winardi, P. Scarber, R. Griffin, D. Weiss, 2008 AFS Transactions.
"Variables Affecting Gas Evolution Rates and Volumes from Ceres in Contact with Molten Metal," L. Winardi, R. Griffin, H. Littleton and J. Griffin, 2008 AFS Transactions.
"Gas Evolution and Permeability of Shell Cores in Contact with Aluminum," L. Winardi, R. Griffin, J. Griffin, H. Onda, S. Harada and A. Yoshida, 2008 AFS Transactions.
"Comparison of Gas Evolution and Permeability of Green Sand Molds and Chemically Bonded Sands," L. Winardi, R. Griffin and G. Wilkinson 2008 AFS Transactions.
"Gas from Green Sand Molds and Vapor Transport Zone," L. Winardi, H. Littleton, R. Griffin and J. Griffin 2008 AFS Transactions.
Shea Gibbs, Associate Editor
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|Date:||Oct 1, 2008|
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