Like most materials, molten metals typically have a lower density than solid ones, so there is an expectation that the casting will be proportionally smaller (i.e., it will shrink) than the pattern from which it was cast. Shrinkage is result of the following factors:
•contraction of the liquid as it cools prior to its solidification
•contraction during phase change from a liquid to solid
•contraction of the solid as it continues to cool to ambient temperature.
These natural phenomena are manifest as either volumetric or linear shrinkage.
a) Volumetric Shrinkage
Volumetric, or liquid-to-solid shrinkage is the shrinkage of the metal as it goes from a state of disconnected atoms and molecules (liquid) to the formed crystals of atoms and chemical compounds, the building blocks of solid metal. The amount of solidification shrinkage varies a great deal from alloy to alloy. In the treatment of some alloys, disregard for this type of shrinkage may result in voids in the casting. Both the design engineer and the foundry engineer have the tools to combat this problem, but the designer has the most cost-effective tool, which is geometry. For some alloys finding that geometry can be very simple. For other alloys finding that geometry is the real essence of good casting design.
The shrinkage caused by solidification can leave cavities in a casting, weakening it. Risers provide additional material to the casting as it solidifies. The riser is designed to solidify later than the part of the casting to which it is attached. Thus, the liquid metal in the riser will flow into the solidifying casting and feed it until the casting is completely solid. In the riser itself, there will be a cavity showing where the metal was fed. Risers are often necessary to produce parts that are free of internal shrinkage voids.
Sometimes, to promote directional shrinking, chills must be used in the mold. A chill is any material that will conduct heat away from the casting more rapidly then the material used for molding. Thus, if silica sand is used for molding, a chill may be made of copper, aluminum, or graphite.
Table 1.2 presents some typical values of volumetric contraction during the solidification and cooling of various casting metals.
b) Linear Shrinkage
Shrinkage after solidification can be dealt with by using an oversized pattern designed for the relevant alloy. Pattern makers use special shrink rulers to make the patterns used by the foundry to make castings to the design size required. These rulers are 2–6% oversized, depending on the material to be cast. Using such a ruler during pattern-making will ensures an oversize pattern. Thus, the mold is larger also, so when the molten metal solidifies, it will shrink and the casting will be the size required by the design.
Table 1.2 Solidification and cooling/shrinking of various metals
Various defects can occur in manufacturing processes, depending on factors such as materials, part design, and processing techniques. While some defects affect only the appearance of parts, others can have major adverse effects on the structural integrity of the parts made. According to the International Committee of Foundry Technical Association, several basic categories of defects can develop in castings. For each basic category, only one typical defect is being presented here.
a) Metallic Projections
Consists of fins, flash, or massive projections.
Example: Joint flash or fins.
A flat projection of irregular thickness, often with lacy edges, perpendicular to one of the faces of the casting. It occurs along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect.
Possible causes:
•Clearance between two elements of the mold or between the mold and the core
•Poorly fitting mold joint
•Flasks not being held at low burnout temperature long enough
•Speed being set too high on centrifugal casting machine.
Remedies:
•Care in pattern making, molding and core-making
•Control of dimensions
•Care in core setting and mold assembly
•Sealing of joints where possible.
b) Cavities
Cavities consist of rounded or rough internal or exposed cavities, including blowholes, pinholes, and shrinkage cavities.
Example: Blowholes, pinholes.
These formations are smooth-walled cavities, essentially spherical, often not contacting the external casting surface (blowholes). The largest cavities are most often isolated; the smallest, pinholes, appear in groups of varying dimensions. In specific cases the casting section can be strewn with blowholes or pinholes. The interior walls of blowholes and pinholes can be shiny, more or less oxidized, or, in the case of cast iron, covered with a thin layer of graphite. The defect can appear in all regions of the casting.
Possible causes:
•Excessive gas content in metal bath (charge materials, melting method, atmosphere, etc.). Dissolved gases are released during solidification.
•In the case of steel and cast irons, formation of carbon monoxide by the reaction of carbon and oxygen present as a gas or in oxide form. Blowholes from carbon monoxide may be increased in size by the diffusion of hydrogen or, less often, nitrogen.
•Excessive moisture in molds or cores.
•High level of aluminum or titanium in the base iron.
•Core binders that liberate large amounts of gas.
•Excessive amounts of additives containing hydrocarbons.
•Blacking and washes, which tend to liberate too much gas.
•Insufficient evacuation of air and gas from the mold cavity.
•Insufficient mold and core permeability.
•Entrainment of air due to turbulence in the runner system.
•Investment not mixed properly or long enough.
•Invested flasks not having been vibrated during vacuum cycle.
•Vacuum extended past working time.
Remedies:
•Making adequate provision for evacuation of air and gas from the mold cavity.
•Increasing permeability of mold and cores.
•Avoiding improper gating systems.
•Assuring adequate baking of dry sand molds.
•Controlling moisture levels in green sand molding.
•Reducing the aluminum content of the base iron metal.
•Increasing metal pouring temperature.
•Reducing amounts of binders and additives used or changing to other types.
•Using blackings and washes, which