Foundry and casting process illustration (2)
See also: Hydrogen gas porosity
Degassing is a process that may be required to reduce the amount of hydrogen present in a batch of molten metal. Gases can form in metal castings in one of two ways:
1. by physical entrapment during the casting process or
2. by chemical reaction in the cast material.
Hydrogen is a common contaminant for most cast metals. It forms as a result of material reactions or from water vapor or machine lubricants. If the hydrogen concentration in the melt is too high, the resulting casting will be porous; the hydrogen will exit the molten solution, leaving minuscule air pockets, as the metal cools and solidifies. Porosity often seriously deteriorates the mechanical properties of the metal.
An efficient way of removing hydrogen from the melt is to bubble a dry, insoluble gas through the melt by purging or agitation. When the bubbles go up in the melt, they catch the dissolved hydrogen and bring it to the surface. Chlorine, nitrogen, helium and argon are often used to degas non-ferrous metals. Carbon monoxide is typically used for iron and steel.
There are various types of equipment that can measure the presence of hydrogen. Alternatively, the presence of hydrogen can be measured by determining the density of a metal sample.
In cases where porosity still remains present after the degassing process, porosity sealing can be accomplished through a process called metal impregnating.
In the casting process, a pattern is made in the shape of the desired part. Simple designs can be made in a single piece or solid pattern. More complex designs are made in two parts, called split patterns. A split pattern has a top or upper section, called a cope, and a bottom or lower section called a drag. Both solid and split patterns can have cores inserted to complete the final part shape. Cores are used to create hollow areas in the mold that would otherwise be impossible to achieve. Where the cope and drag separates is called the parting line.
When making a pattern it is best to taper the edges so that the pattern can be removed without breaking the mold. This is called draft. The opposite of draft is an undercut where there is part of the pattern under the mold material, making it impossible to remove the pattern without damaging the mold.
The pattern is made out of wax, wood, plastic, or metal. The molds are constructed by several different processes dependent upon the type of foundry, metal to be poured, quantity of parts to be produced, size of the casting, and complexity of the casting. These mold processes include:
· Sand casting — Green or resin bonded sand mold.
· Lost-foam casting — Polystyrene pattern with a mixture of ceramic and sand mold.
· Investment casting — Wax or similar sacrificial pattern with a ceramic mold.
· Ceramic mold casting — Plaster mold.
· V-process casting — Vacuum with thermoformed plastic to form sand molds. No moisture, clay or resin required.
· Die casting — Metal mold.
· Billet (ingot) casting — Simple mold for producing ingots of metal, normally for use in other foundries.http://www.pump-casting.com/