University of Nevada Las Vegas

MEG426/626 Manufacturing Processes

Department of Mechanical Engineering

Fall Semester 2000

  

Fundamentals of Casting

 

Casting is a process in which molten metal flows by gravity or other force into s mold where it solidifies in the shape of the mold cavity.

 

Basic Requirements of Casting Processes:

    1. A mold cavity with desired shape and size.
    2. A melting process.
    3. A pouring technique
    4. The solidification process.
    5. It must be possible to remove the casting from the mold.
    6. Mold clearing, finishing, and inspection.

 

Casting Terminology:

Pattern: an approximate duplicate of the final casting.

Flask: is the box that contains the molding aggregate.

Cope: the top half of the molding aggregate.

Drag: the bottom half of the molding aggregate.

Core: is the sand shape that is inserted into the mold to produce the internal features of a casting.

Core Print: is the region added to the pattern, core, or mold that is used to locate and support the core within the mold.

Mold cavity: is the shaped hole formed by the mold material and core, which the molten metal is poured and solidified to produce the desired casting.

Riser: is an extra void created in the mold that will also fill with molten metal. It provides a reservoir of metal that can flow into the mold cavity to compensate for any shinkage that occurs during solidification.

Gating system: is the network of channels used to deliver the molten metal to the mold cavity.

Pouring cup: is the portion of the gating system that initially receives the molten metal from the pouring vessel and controls its delivery to the rest of the mold.

Sprue: the vertical portion of the gating system.

Parting line (parting surface): is the interface that separates the cope and drag halves of a mold, flask, or pattern and also the halves of a core in some core-making processes.

Draft: is the taper on a pattern or casting that permits it to be withdrawn from the mold.

Core box: the mold or die used to produce casting cores.

Pouring temperature: is the temperature of the molten metal as it is introduced into the mold.

Pouring rate: refers to the volumetric rate at which the molten metal is poured into the mold.

Fluidity: is a measure of the capability of a metal to flow into and fill the mold before freezing.

 

Solidification of Metals:

Pure Metals: solidifies at a constant temperature.

Most Alloys: freeze over a temperature range (mushy zone).

Eutectic Alloys: solidifies at a constant temperature, such as lead-tin alloys.

 

Shrinkage:

Three steps: 1). Liquid contraction during cooling prior to solidification.

2). Solidification shrinkage: contraction during the phase change from liquid to solid.

3). Thermal contraction of the solidified casting during cooling to room temperature.

Pattern shrinkage allowance: the amount by which the mold must be made larger relative to the final casting size.

Directional solidification: in which the solidfication interface sweeps continuously through the material, can be used to assure the production of a sound casting.

Chills: are metal parts that cause rapid freezing in certain regions of the casting.

Internal chills and external chills.

Chvorinovís rule:

 

ts = B (V/A)n where n = 1.5 to 2.0

ts = the time from pouring to the completion of solidification, min; V = the volume of the casting, in.3 (cm3); A = surface area of the casting, in2. (cm2); and B = mold constant.

 

Created by Dr. Wang