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Forming by casting

By Udo O.J. Huff, M.ED.


Innovative technique provides effective process for forming complex components

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September 2012 - From forging steel to melting ore in large furnaces, primary casting using different types of molds is an engineering practice that also offers fabricators an innovative development process. The metalworking industry usually focuses on mainstream production technologies and processes. For some parts–such as large, hollow profiles or complex machine components difficult to make from solid or structural steel–the usual methods are not applicable. The technique of forming by casting offers an effective approach for these challenging applications.

Casting with “dead molds”

During casting with dead molds, the mold cavity is filled with molten material. When the workpiece solidifies it is considered to be in its primary form. Different mold and casting processes can involve the following:

• Dead molds and permanent patterns require a flask mold, template mold and permanent pattern
• Lost patterns dictate investment castings and male form castings
• Permanent molds are accompanied by ingot molding, centrifugal casting and compound casting

Making the pattern

The casting model used to produce the mold is known as the pattern. Made of wood, metal or plastic, this pattern follows a technical drawing. When making a pattern, it is important to consider the contraction rule. This rule requires the pattern be made longer than actual scale by an amount that is equal to the degree of shrinkage of the cast material between solidification and room temperature. Examples include cast iron at 1 percent, zinc casting alloys at 1.5 percent and cast steel at 2 percent. If the pattern is made from wood, it is then varnished to protect it from moisture using a color code that represents the casting material (i.e. cast iron is color-coded red, malleable cast iron is color-coded grey, steel is color-coded blue).

Molding

Molding is carried out in sand or clay, processed mechanically or manually, then prepared for the furnace in molding boxes. Molding sand, used with casting blocks, consists of small grains of quartz and a binding agent. Typically, the binding agent is a fine layer of clay surrounding each particle of quartz that is activated when the molding sand is moistened. Today’s molding processes use synthetic resin adhesives instead of clay. The molding sand’s critical properties include plasticity, stability of form, heat resistance and permeability to gases.

Hollow spaces are created in cast pieces by allowing molten metal to flow around a core that is produced with a core box. The core must be stronger than the core box. For that reason, special sand (core sand) is used. The dried core is then inserted into the drag (the bottom of the box).

Finishing the mold for casting

Once the pattern has been molded in the bottom box it is turned over and the cope, or top box, is placed on it. The pattern is penetrated with an ingate funnel and a riser funnel. After piercing an air hole and extracting the wood pieces, the cope is lifted off. The pattern is removed and the mold fitted with a founding gate. The cope is then replaced and clamped to the drag, or weighed down, so that it does not lift off during casting.

The casting process is performed at the foundry. Once the casting has attained the shape of the pattern, the cavity is formed by the core. The ingate and riser also are still attached to the cast. At this stage of the process, the cast is considered rough and still requires finishing. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.

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