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Designing for economical coring. (Casting Design Notebook).

Part II

Cored holes should be designed as simply as possible while satisfying functional requirements. Several basic principles exist that should be observed when designing cored holes:

* assure the strength of the core during coremaking, handling and pouring;

* provide sufficient mass to the core sections to allow proper cooling during solidification;

* avoid thin casting sections formed by cores;

* avoid or minimize the need for complex and/or stacked cores;

* minimize the number of required cores.

These issues are demonstrated in Fig. 1, where the casting has core elements that should be redesigned. The core required to make this casting has thin sections that can be easily broken, does not allow proper cooling and venting, and results in a thin wall casting. A redesigned casting is shown in Fig. 2, which eliminates thin groove sections at the bottom of the casting that could potentially cause problems.

Core Fragility

Cores are typically made from sand and can be fragile, depending on the cross sectional area of the particular feature on the core and the core manufacturing method. Thin and long sections on the core can easily be broken during core handling. This will result in a scrapped core or a defective casting if the core breaks during pouring due to the high pressure exerted by the metal stream.

In some cases, thin core sections can cause significant rework on the casting and/or a scrap rate of more than 50%. The casting in Fig. 1 has a very narrow and tall groove on the bottom that requires a thin core section. This feature can be redesigned, eliminated (Fig.2) and later machined.

The feature already is on the side of the casting that is going to be machined so an additional machining operation would not significantly increase the cost of the casting. On the other hand, high scrap due to core failure can reduce the number of good castings and thus significantly increase casting cost. When designing narrow holes in a casting, designers must allow for the possibility that the particular feature should be machined instead of cast to allow for additional elements to strengthen the core.

Thin core sections also pose a problem for gating and feeding the casting. The ingate should be placed in such a way that the incoming stream does not impinge directly onto the thin core section, otherwise the core could fracture. Having too many thin core features will limit the possible location for ingates and thus proper gating of the casting. In addition, too many thin core sections protruding to the surface of the casting will affect the location of a riser (if required to feed the casting during solidification).

The casting design shown in Fig. 1 allows only side risers to be used to feed the casting. The holes in the flange part of the casting will not allow for easy location of a top riser.

Thin Core Sections

Thin core sections surrounded by heavy metal sections should be avoided due to the excessive amount of heat to which they are exposed without the ability to dissipate it properly. This can result in hot spots and shrinkage adjacent to the core, as shown in the simulation results in Figs. 3 and 4. In this design, hot tears (cracks) can be expected at these locations. These cracks represent significant stress risers in the casting, reducing its fatigue strength.

Gas related defects also are common in these configurations due to the inability to properly vent thin cores. The gas formed by a sand binder will not be able to dissipate through the mold material and could end up in the mold cavity and the metal itself. As a result, gas porosity could be expected adjacent to thin and wide core sections.

Another factor is metal penetration, which can be a common problem if there is premature decomposition of the core binder.

Last, cleaning of narrow cored sections can be a very difficult and labor intensive job. For example, the casting in Fig. 1 has several thin sections that could potentially cause casting related problems. Therefore, designers must allow for an increase in the cored sections according to the specification provided by the foundry engineer and according to the casting process and type of alloy.

Thin Wall Sections Formed by Cores

There is a minimum casting wall section that can be physically or economically achieved. The minimum casting wall thickness is governed by a number of different variables including casting process, type of alloy and pouring temperature.

Potential problems in the production of thin-wall castings are misruns and cold shuts. The cores that form thin-wall sections on the casting should be avoided or redesigned to provide uniform fill of the entire mold cavity.

The casting in Fig. 1 has a very thin vertical wall that can cause a misrun or cold shut. In order to avoid this problem, foundries may have to increase pouring temperature, which can cause other types of problems including metal penetration and runouts. The best solution would be to redesign the casting by increasing the wall thickness to match the capability of the metalcasting process and foundry expertise, or to provide features on the casting that allow delivery of molten metal into the entire mold cavity.
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Title Annotation:principles to observe when designing cored holes
Author:Tomovic, Mileta
Publication:Modern Casting
Geographic Code:1USA
Date:Jan 1, 2003
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