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Unearthing secrets: new sand core expansion control method revealed.

The expansion of silica sand used in cores has plagued foundries for years. The phenomenon is caused by the rapid thermal expansion of the sand during the casting process, which results in core cracking. The molten metal then enters these cracks, creating a thin fin of metal--a veining detect (Fig. 1).

[FIGURE 1 OMITTED]

Many methods are used to offset this problem with some being more successful than others. Whether it is the use of anti-veining agents of lake sand instead of silica sand, the methods add cost to casting production. In an attempt to have silica sand perform as well as lake sand for a high-production engine block and head casting facility, Indianapolis Casting Corp., Indianapolis, discovered a new twist to vein reduction. This article presents this new twist on an old method for veining reduction that not only reduces expansion but also reduces cost.

Veining Defects

Sand cores are produced primarily to make internal cavities in a finished casting. When the sand cores are placed in a mold and molten metal is introduced, a rapid thermal expansion of the sand in the sand core takes place. This rapid expansion results in core failure, causing the core to crack. If the metal is still molten, it runs into the core's cracks creating a vein after solidification. A severe case of these phenomena results in a sponge iron defect that resembles metal penetration of burn in (Fig. 2).

[FIGURE 2 OMITTED]

In this case, the integrity of the coating and the core skin has been compromised, allowing migration of metal back into the core. This chunk of sponge iron is often misidentified as burn-in. Close examination will reveal that there is a vein going through or under the chunk of sponge iron. The expansion vein came first and then the more severe failure of the core next. This defect is noticed most often in an area of heavy metal sections of hot spots.

The solution to veining defects is to reduce/eliminate core expansion. In today's operations, veining defects are controlled by the addition of an anti-vein or expansion control agent (synonymous terms) to the sand in the mixing process to produce cores. The addition of an anti-vein agent changes the thermal coefficient of expansion of the core sand to allow the veining defect to improve or not exist.

Selecting the proper expansion control agent is critical as trade-offs exist when adding another material to the core sand. Most of the additives available today are finer than the sand; therefore the surface area of the core aggregate is increased, resulting in the need for more resin to maintain the desired core strength. In addition, many of these additives are combustible and may significantly add to the amount of gas generated from the core.

Expansion Evaluation Testing

The testing method used to evaluate core expansion was to use 2-in. x 2-in. test cores (lake and silica sand) made with a phenolic urethane coldbox binder. After a series of six cores were made (one of which is always the standard) a test casting was then poured, allowed to cool and shaken out.

This was a severe test that allowed good evaluation of expansion veining. The veining was evaluated by measuring the length and height of the veins and calculating the total surface (Table 1). The results in the table relate directly to the figures to follow. The results that offered some improvement were then verified on subsequent test castings. The results from the test castings were compared to results from production castings (such as motor blocks and heads) and showed strong correlation.

In total, 15 silica sands were evaluated using the veining test casting. The best two performers (those that exhibited the least veining) were further evaluated and modified to achieve similar results (in expansion control) compared to the lake sand.

One of the best expansion control methods uses a commercially available anti-vein agent that consist of a blend of Lithia-containing materials, alpha spodumene, lithium carbonate and black iron oxide among other elements. The proven level of this anti-vein agent required in the core to achieve benefits with expansion is 5% or more based on sand weight.

However, this amount of commercially available anti-vein agent can be reduced without sacrificing expansion control by adding commercially available red iron oxide ([Fe.sub.2] [O.sub.3]), also known as Hematite. Testing shows that there is a performance improvement of the commercially available anti-vein agent with the red iron oxide present.

In Fig. 3, lake sand is evaluated with and without 5% of the commercial anti-vein agent as well as with a combination of the commercial anti-vein agent and red iron oxide. The veining is eliminated with the commercial anti-vein agent and also with the blend of commercial anti-vein agent and the red iron oxide. Observe the 70% reduction in the expensive commercial anti-vein agent with the addition of red iron oxide.

[FIGURE 3 OMITTED]

In Fig. 4, Illinois silica sand is evaluated with and without 5% of the commercial anti-vein agent, and the results show heavy veining. Figure 5 shows the use of Illinois silica with a commercial anti-vein agent at 4% and the addition of 1% red iron oxide. In this test, the veining has been eliminated.

To further these results, the commercial anti vein agent was reduced to 3% with the addition of 1% red iron oxide. Again, the veining was eliminated. This process allowed comparable results to lake sand with a reduction of 40% of the commercial anti-vein agent normally used.

[FIGURE 4 OMITTED]

Wisconsin silica sand also was evaluated without an anti-vein agent, which resulted in considerable veining With a 5% addition of the commercial anti-vein agent, most of the veining was eliminated. When the anti-vein agent the veining starts to appear. But, when the commercial anti-vein agent was reduced to 4% with the addition of 1% red iron oxide, veining was eliminated. The same positive results for Wisconsin silica sand were shown with 3 and 2.5% anti-veining agent with 1% red iron oxide. This is a 50% reduction in anti veining agent.

Pushing to Production

Compared to the current sand casting process, adding red iron oxide to the commercial anti-vein agent allows for a reduction of the commercial anti-vein agent material by up to 70% and a reduction in resin of up to 0.1%. The addition of the red iron oxide is an extra step in the process, but the performance obtained and the cost savings offset the inconvenience. The red iron oxide is one-third the cost of the commercial anti-vein agent. This method equates to about $15 per ton savings on prepared core sand cost.

In expansion control, silica sands can be made to work as well as lake sand by using the above-described method. Lake sand expansion can be better controlled using this method as well, resulting in considerable savings and improved performance.

The result may not be exact for all applications, but the fact remains that the addition of red iron oxide allows for the reduction in the amount of expansion control agent used.

Inside This Story:

* Reducing veining defects through the use of anti-veining agents can be a high production cost for sand castings.

* Explored within this article are the results of using red iron oxide in combination with reduced levels of anti-veining agents and how this new method of defect prevention can improve casting production costs and results.
Table 1. Veining Test Measurement Results

Recipe Surface Area (sq. in)

Lake 1.2%+ CW 2.60
Lake 1.35%+ 5% AV+CW 0
Lake 1.4% + 1.5% AV + 1% Fe203 + CW
IL Silica 50 1.1% + CW 3.75
IL Silica 50 1.25%+ 5% AV + CW 0.6
IL Silica 50 1.20% +4% AV+ 1% Fe203+CW 0
IL Silica 50 1.10% +3% AV+ 1% Fe203+CW 0
IL Silica 55 1.0%+CW 2.33
WI Silica 55 1.15%+5% AV+CW 0
WI Silica 55 1.10% +4% AV+CW 0.1
WI Silica 55 1.3%+4% AV+1% Fe203+CW 0
WI Silica 55 1.2%+3% AV+1% Fe203iCW 0
WI Silica 55 1.2%+2.5% AV+1% Fe203+CW 0


For More Information

Visit www.moderncasting.com to view the full paper, "Expansion Control Method for Sand Cores," 2003 AFS Transactions Paper #03-023, American Foundry Society, Inc., Des Plaines, II.

"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

Stephen G. Baker is the senior materials engineer for Indianapolis Casting Corp. Joshua Werling is senior manufacturing engineer for the firm. A patent application on this new core expansion control method is pending and licenses will be granted for a royalty
COPYRIGHT 2004 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2004, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Werling, Joshua M.
Publication:Modern Casting
Date:Apr 1, 2004
Words:1468
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