Casting Terms and Definitions Related to Foundry Production

Casting - Casting is a manufacturing process for making complex shapes that is difficult or costly to make through other methods. Liquid metal is poured into a mold of the desired shape, and then allowed to solidify. The solid part is also called a casting and it is ejected from the mold once solidified.

Foundry - A facility engaged in the manufacture of numerous types of castings using various casting processes.

Metals

Alloy - A metallic solid created by combining two or more metals or adding a non-metal to a metal for the purpose of enhancing its properties for the end use.

Ferrous Alloys - Alloys with iron as its main constituent.

Non-ferrous Alloys - Non-iron-based metals.

Cast iron - A hard, brittle alloy of iron and carbon that can be readily cast in a mold and contains a higher proportion of carbon than steel does. It is firm and unchangeable.

Ductile iron - A cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites. This imparts a measurable degree of ductility (easily manipulated) to the cast metal. The ductile iron family offers the design engineer a unique combination of strength, wear resistance, fatigue resistance, and toughness, as well as excellent ductility characteristics.

Aluminum Castings - Aluminum is a silvery-white, ductile metallic element. As the most abundant metal in nature, it is extensively used for castings and foundry patterns, and is also utilized as a deoxidizer in iron and steel making.

Steel Casting - A specialized form of casting that involves various types of carbon steel and alloy steels.

Steel Alloys - Steel that has been alloyed with other metals from 1.0% to 50% by weight to improve the mechanical properties of the steel. Alloy steels are broken down into two groups: low-alloy steels and high-alloy steels

Carbon Steel Casting - Cast, low carbon steels contain mainly carbon as the principal alloying element. Other elements are present in small quantities, including those added for de-oxidation. They are produced to a great variety of properties because composition and heat treatment can be selected to achieve specific combinations of properties, including hardness, strength, ductility, fatigue resistance, and toughness.

Stainless Steel Casting - A casting made from stainless steel. Stainless steel is a steel alloy with a minimum of 10.5 or 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it stains less, but it is not stain-proof). There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime.

CASTING PROCESSES

Investment Casting - Often thought of as the ultimate technique for low weight, precision steel casting production; investment casting is used to make parts that cannot be produced by normal manufacturing techniques. It is done by casting metal into a mold produced by surrounding, or investing, an expendable pattern with a refractory slurry coating that sets at room temperature, after which the wax or plastic pattern is removed through the use of heat prior to filling the mold with liquid metal. It is also called precision casting or lost wax process.

Centrifugal Casting - A casting process performed by rotating the mold so as to produce a centrifugal force in the molten metal. The end product is often more reliable -- free from gas and shrinkage porosity.

Die Casting - A rapid, water-cooled permanent molding casting process that is quite limited to non-ferrous metals. The metal is melted, and then injected into a die. It is cooled rapidly to form the part. The die-cast part can then be taken through a number of finishing steps for a wide variety of end products.

No-Bake Molding / Sand Casting - A versatile process for metal casting designers in which sand is mixed with a chemical binder/catalyst system and then molded around a pattern. It is an ideal option for production runs from 1 - 5,000 castings / year.

PATTERN AND PRODUCTION TERMS

Casting Drawing - Contains all of the information for the foundry operations involved including machining allowances, tolerances and other specifications.

Machining - Process such as abrading, cutting, drilling, forming, grinding, and/or shaping of a piece of metal or other material performed by machine tools such as lathes, power saws, and presses. In casting, the supplier foundry is responsible for giving the designer a cast product that is capable of being transformed by machining to meet the specific requirements intended for the function of the part.

Degassing - A step in the casting process, usually necessary to reduce the amount of hydrogen dissolved in the liquid metal. If the hydrogen concentration in the melt is too high, the resulting casting will be porous as the hydrogen comes out of solution as the aluminum cools and solidifies. Porosity often seriously deteriorates the mechanical properties of the metal.

Machining Allowance - The amount by which dimensions of a casting are made oversize to provide excess stock for machining.

Pattern - A replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process.

Mounted Pattern - A pattern that is fastened permanently to a board or match plate. Mounted patterns cost more than loose patterns - but when many castings are to be made from a pattern, the time saved in operation warrants the cost of mounting it. Mounted patterns have the shape of the casting often with forms for sprues, risers, etc. attached.

Loose Pattern - The cheapest pattern to make, has the shape of the casting without forms for sprues, risers, etc. attached. May be made in one or more pieces.

Core Box - A type of pattern into which sand is rammed or packed to form a core.

Core - A separate piece (often made from molding sand) placed inside the mold to create openings and cavities, which cannot be made by the pattern alone. Every attempt should be made to eliminate or reduce the number of cores needed for a particular design, which proportionately reduces the cost of the casting.

Tolerance - An allowance, given as a permissible range, in the nominal dimensions of a finished product. The acceptable dimensional tolerances must be indicated when a drawing is provided. Tolerances are normally decided by agreement between the supplier foundry and customer. Close cooperation between the customer's design engineers and the supplier foundry is essential to optimize the casting design.

Soundness - In metal components, refers to the level of freedom from impurities and/or discontinuities such as sand inclusions, slag inclusions, macro porosity (pores greater than 50 nm in diameter), and shrinkage.

Shrinkage - Volumetric contraction that occurs as a steel casting begins to solidify. Testing should be done to ensure the end product will meet specifications in castings when the design is likely to result in shrinkage.

Minimum Section Thickness - The minimum thickness to which a section can be designed. In custom cast work, the specifications in the bid should include this information. A minimum thickness of 0.25 in (6 mm) is suggested for design use when conventional steel casting techniques are employed. Wall thicknesses of 0.060 in (1.5 mm) are common for investment castings and sections tapering down to 0.030 in (0.76 mm) can readily be achieved.

Draft - The amount of taper (or the angle) which must be allowed on all vertical faces of a pattern to permit its removal from the sand mold without tearing the mold walls. Draft should be added to the design dimensions while maintaining minimum metal thickness.

Parting Line - The line where parting surface meets with the casting surface of the mold. Patterns with straight parting lines (that is, with parting lines in one plane) can be produced more easily and at lower cost than those with irregular parting lines. Casting shapes, which are symmetrical about one centerline or plane readily suggest the parting line. Such casting design simplifies molding and coring, and should be used wherever possible. They should always be made as "split patterns" (separate cope and drag) which require a minimum of handwork in the mold, improve casting finish, and reduce costs.