A chill is one of the most commonly used metal blocks for controlling the solidification of castings. Chills can be used for all types of casting alloys, but they are most widely applied in steel castings. The main functions of chills are as follows:
1. Used together with risers to enhance directional solidification of the casting, extend the effective feeding distance of the riser, and not only help prevent shrinkage cavities and porosity defects, but also reduce the number or size of risers, thereby improving the process yield.
2. Accelerate the cooling rate of hot spots in the casting so that it tends toward simultaneous solidification, helping to prevent deformation and cracks.
3. Increase the cooling rate of certain specific parts of the casting to improve the matrix structure and properties, and enhance surface hardness, wear resistance, etc.
4. In areas where risers are difficult to set or cannot effectively feed, placing chills can reduce or prevent shrinkage cavities and porosity.
Chills are divided into external chills and internal chills.
External Chills
An external chill forms part of the mold and does not fuse with the casting during pouring; it can be recovered and reused after shakeout.
The materials for external chills should have good thermal conductivity, high heat capacity, and a sufficiently high melting point. Common materials include rolled steel, and pre-formed chills made of cast iron or cast steel. The shape is generally determined according to the contour of the casting area to be chilled.
External chills are classified into direct external chills and indirect external chills. Direct external chills are in direct contact with the casting surface and have strong chilling effect; if direct chills cause excessive chilling and lead to cracks, indirect chills may be used instead. Indirect chills are separated from the chilled casting surface by a sand layer 10 ~ 15 mm thick, and are therefore also called sand-separated chills or blind chills. Because the chilling effect is weaker, this type avoids surface cracks and white iron formation in gray cast iron, and gives the casting a smooth surface without chill–casting fusion defects.
Precautions when placing external chills:
1. The working surface of chills must be free of holes, cracks, scale, and other defects. To extend service life and prevent fusion with the casting, the working surface should be coated with refractory paint and kept dry.
2. Avoid using external chills in areas where the casting thickness exceeds 150 mm, to prevent fusion with the casting.
3. Plate-shaped chills should not exceed 80 mm in thickness, and round bar chills should not exceed 40 ~ 50 mm in diameter, as the chilling effect does not continue to increase with greater thickness. For thick-walled castings, internal chills are preferred.
4. The edges of thick plate-shaped chills should be beveled at 45° so the interface between mold and chill transitions smoothly, preventing cracks caused by excessive temperature differences.
5. The length of external chills should not be too great, to avoid placement difficulties or expansion that damages the mold and causes cracks in the casting.
6. The position of chills must be chosen to ensure feeding channels remain open and to prevent shrinkage cavities at hot spots.
Internal Chills
Internal chills are metal inserts placed directly into the mold cavity at locations needing chilling; during pouring, they chill the molten metal and fuse with the casting, becoming part of it.
Internal chills are generally used only when the effect of external chills is insufficient, and mostly for large, thick castings with lower quality requirements. They are not recommended for castings that must withstand high temperature or high pressure.
Since internal chills are required to fuse with the casting metal, their material should be essentially the same as, or compatible with, the casting material.
The size, weight, and quantity of internal chills should be determined so that they provide sufficient chilling to control solidification and fuse with the casting without reducing its strength. The mass of internal chills mchill can be estimated from the empirical formula:
mchill =Kmunit
Where:
• mchill — mass of the casting or the hot spot section (kg)
• K — ratio coefficient, i.e., the percentage of the hot spot mass that the internal chill should weigh.
Precautions when using internal chills:
1. The surface of internal chills must be clean; remove rust, oil, and moisture before use.
2. In dry sand molds, insert internal chills only after mold baking; in sodium silicate sand molds and green sand molds, pour as soon as possible after placing the chills to prevent oxidation or moisture accumulation, which can cause gas porosity.
3. Internal chills to be stored should be tin-plated to prevent rust.
4. Provide vent holes or open risers above the position of the internal chill in the mold.
Post time: Aug-14-2025