Types of Defects

Cast metal parts are prone to certain defects based on the type of process and material used. Some of the common defects are:

Misrun: A misrun is a defect where the metal does not fill the mold cavity completely, resulting in a thin or incomplete casting. It is caused by low pouring temperature, high viscosity of the molten metal, insufficient fluidity, or improper design of the gating system.

Cold Shut: A cold shut is a defect where two streams of metal do not fuse properly in the mold cavity, creating a discontinuity or a crack in the casting. It is caused by low pouring temperature, high pouring speed, insufficient fluidity, or turbulence in the molten metal.

Porosity: Porosity is a defect where there are small holes, voids, or pockets of air in the casting. It is caused by trapped gas in the molten metal, improper venting of the mold or core, excessive moisture in the mold or core, or low permeability of the sand.

Shrinkage Void: A shrinkage void is a defect where there is a cavity or a depression in the casting due to insufficient feeding of the molten metal during solidification. It is caused by high pouring temperature, large temperature gradient, improper design of the riser or feeder, or insufficient contraction allowance.

Hot tear: A hot tear is a defect where there is a crack or a fracture in the casting due to thermal stress during solidification. It is caused by high pouring temperature, high cooling rate, low ductility of the metal, or improper design of the mold or core.

Core Shift: A core shift is a defect where there is a misalignment of the core in the mold cavity, resulting in a deviation from the desired shape or dimension of the casting. It is caused by improper alignment of the core, loose core prints, insufficient support for the core, or excessive pressure of the molten metal.

Inclusion: An inclusion is a defect where there are foreign materials such as slag, sand, oxide, or metal particles embedded in the casting. It is caused by improper cleaning of the furnace or ladle, improper fluxing of the molten metal, improper gating system, or formation of insoluble compounds during solidification.

DFM for Metal Casting

The design of casting parts should consider the material properties, the casting process, and the part requirements to ensure optimal performance, quality, and cost-effectiveness.

Some of the design for manufacturing (DFM) principles for casting parts are:

• Choose the appropriate material and process for the part function, specification, and quality level. Different materials have different melting points, fluidity, shrinkage, and mechanical properties that affect the casting process and the final part characteristics. Different casting processes have different advantages and disadvantages in terms of accuracy, surface finish, production rate, and complexity.
• Design the part geometry to facilitate smooth and complete filling of the mold cavity. The part should have uniform wall thickness to avoid hot spots and cold spots that can cause warping, cracking, or porosity. The part should also have smooth transitions and rounded corners to reduce turbulence and resistance in the liquid metal flow. The part should avoid undercuts or features that require side cores or slides that can increase the mold complexity and cost.
• Design the part orientation and ejection mechanism to minimize defects and damage. The part should be oriented in the mold such that the direction of mold closure is perpendicular to the largest cross-section of the part. This can reduce the number of parting lines and improve the dimensional accuracy and surface quality of the part. The part should also have adequate draft angles and ejection pins or sleeves to facilitate easy removal from the mold without causing deformation or scratches. Additionally, the simpler the ejection system the cheaper the dies and molds will be.

More detailed DFM for Casting and Powder Metallurgy can be found in the ASM Metals Handbook.

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