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Venting cores and molds.

Discussing foundry practices recently at AFS with two foundry veterans, the subject of core and mold venting came up. One comment was that venting practices are not always followed, and another was that, when in doubt, vent, vent and vent some more. The consensus was that venting is such a simple, inexpensive yet effective quality control procedure that it shouldn't be overlooked.

The need to vent cores and molds has been recognized for centuries as a means of avoiding the adverse effects of gases entrapped or evolved from cores, molds, coatings, adhesives and fillers during pouring and solidification. Without adequate venting, gases become entrapped and result in gas-related defects such as blowholes or scabbing. Proper venting techniques can result in fewer rejects, decreased finishing times and increased production economies.

Venting is more necessary in cores than in molds because a major portion of a core is in direct contact with molten metal and the core area where gases may escape is usually limited. While venting molds to allow the gas generated at the mold face to escape is not generally of great concern, it is essential to provide mold vents to relieve the buildup of core gas.

Mold/Core Pormeability

The grading and shape of the sand and its degree of compaction in the mold or core can affect sand permeability and the subsequent ease with which gas flows through the sand. The gas evolved from naturally or fully bonded green sand during casting may be unable to pass through the sand quickly enough to prevent blowholes. Sand coatings can reduce permeability to near zero, leaving venting through coreprints as the lone option for coated cores. The fit of the coreprint into mold print should be tight enough to prevent metal penetration yet not so tight as to block the core venting system.

Forming Vents

Poor vent connections and partial or complete blockage of vents increase the risk of producing defective castings. Among the variety of techniques and materials available for venting cores successfully are

* rod-or plate-formed vents;

* textile tubing;

* wax;

* rope;

* coke, slag or cinders.

Rod-formed vents are simply round, cylindrical holes or passageways that extend through the core. There are several methods for making channels or open passages in cores using rods, wire or plates. A core is either formed around a metal rod or other stiff material, or the rod is inserted into the formed, but uncured, core. When the rod is withdrawn a channel remains in the core. It is one of the most commonly used methods for making core vents because of its simplicity, and it does not require the additional expense of consumable materials.

Textile tubing is an all-purpose venting material that is suitable for baked and nobake binder systems. Tubing is placed in the core during core production where it remains to provide a flexible internal passageway for gas to escape. The tubing usually consists of a three-fiber bundle, two being cellulose-based and the third a thermoplastic fiber material. They are woven together to produce an integral, porous tube. This construction assures flexibility to fit complex pathways within a core and mold. Gas flows easily through the fibrous structure, which is woven tightly enough to exclude sand and other foreign elements. The textile tubing is stiff enough to resist flattening under reasonable hand or machine ramming and is unaffected by extremes in temperature or moisture.

Wax vents are similar to lost wax molding in that wax preforms are positioned in the core material, and then the wax is melted out of the core, leaving a hollow passageway. Available in solid rods, hollow tubing or flat or oval ribbons, the wax has the disadvantages of requiring an extra heating operation, and it tends to diffuse into the core itself to contribute to gas formation during casting.

Rope vents are among the oldest methods for relieving gases from molds and perform much in the same manner as textile venting materials.

Coke, slag and cinder vents are used in large cores. One or all three permeable materials are placed in a hollow section of the core, which is then vented to the coreprint or mold by textile tubing or rope vents. Its use is limited due to the difficulties in the design of this type of vent. The effectiveness deviates from site to site due to variations in particle size and density of the vent materials.

Bibliography

Mattlin, J. E., Mobley, C. E., "Venting: A Continuing Need for an Old Art,' modem casting, pp. 54-56 (Dec. 1980).

"Venting of Molds and Cores," BCIRA Broadsheet 188 (1980).

Tom Bex Senior Editor
COPYRIGHT 1991 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Cast Facts
Author:Bex, Tom
Publication:Modern Casting
Date:Aug 1, 1991
Words:764
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