Intermediate Casting Technician

Author: Chief Editor: Liu Jirong
Publisher:
Publish Date: 2004-06-01
Features: This book is based on the "Workers' Technical Grade Standards" and "Occupational Skill Identification Specifications" jointly issued by the former Ministry of Machine Industry and the Ministry of Labor. The content of the book includes: mixing of mold (core) sand, molding and core making, gating system, chills, and casting fins, casting alloys and their melting, casting and casting finishing, casting defect analysis, casting process specifications and production technology management, molding machinery, special casting, etc. It has strong industry targeting and a focus on practicality, emphasizing the combination of theory and practice, organically integrating "professional knowledge" and "operational skills."
Excerpt:
3. Sawdust (powder) Sawdust is the waste material generated during wood processing, mainly fine particles from sawing, drilling, and milling operations. Sawdust is used in backing sand to increase the collapsibility and permeability of the mold.
4. Mold Release Agent Common mold release agents include kerosene, diesel, machine oil, artificial pine pitch powder, detergents, and resin sand-specific mold release agents.
Section 2: Properties and Testing of Mold (Core) Sand
I. Properties of Mold (Core) Sand
Mold (core) sand must possess certain properties to meet the needs of casting production. Some properties directly affect casting quality, while others directly impact working conditions and production efficiency.
1. Permeability
The permeability of mold sand refers to the porosity of a compacted sand sample. Specifically, it is the amount of air that passes through a 1 cm2 cross-sectional area and 1 cm height of the sample in 1 minute under standard temperature and 100 Pa pressure. When the mold is subjected to high-temperature molten metal, a large amount of gas is generated. The mold must have sufficient to prevent choking during pouring, which may also lead to casting defects such as gas porosity or misruns. Gas discharge occurs not only through the mold's vent holes and risers but also through the pores between the sand grains. Therefore, permeability is one of the important properties of mold sand. However, the permeability of the sand sample does not equal the permeability of the mold (core), as it can only indirectly reflect the permeability of the mold cast from this sand. Factors affecting permeability include the particle size and uniformity of the base sand, the content of clay, coal powder, and moisture, the compactness of the sand, and the mixing process of the sand.
2. Gas Evolution (Gas Volume), Gas Evolution Rate (Gas Evolution Speed)
Gas evolution refers to the ability of mold (core) sand to release gas when heated. It is expressed as the volume of gas evolved per unit mass (cm3/g). The gas evolution rate is the amount of gas produced per unit mass of the evolving material (e.g., core sand, binder) per unit time at a certain temperature, with the unit being cm3/(g·s). If the mold's gas evolution speed is too fast, gas is likely to penetrate during the early stages of casting solidification, increasing the likelihood of gas porosity. The gas volume of mold sand mainly depends on the content of moisture and additional substances such as coal powder. The moisture content should be minimized, and the organic binder and coal powder content should be controlled. Additionally, the gas volume is also related to the pouring temperature—the higher the pouring temperature, the greater the gas volume.
3. Mold (Core) Sand Strength
Mold (core) sand strength refers to the ability of the sand to resist external forces. It includes wet strength, dry strength, and hot strength. Only with high mold (core) sand strength can the mold withstand external forces during processes such as mold turning, demolding, handling, and mold closing, as well as the static and dynamic pressure of the molten metal during pouring. Otherwise, casting defects such as sand sticking or sand swelling may occur.
(1) Wet Strength
Wet strength refers to the strength of a wet sand sample at room temperature, including compressive, shear, tensile, and flexural strength. Wet strength directly relates to molding operations and has a certain relationship with dry and hot strength. The main factors affecting wet strength include the particle size, uniformity, and shape of the sand grains, the amount and type of clay added, moisture content, mixing quality, and compactness.
(2) Dry Strength
Dry strength is the compressive, shear, tensile, and flexural strength of a mold sand sample after being dried according to a certain specification and cooled to room temperature. The main factors affecting the dry strength of clay-based sand include the content of clay and moisture, as well as the type of clay and drying specifications.
(3) Hot Strength
Hot strength is the strength of a mold sand sample measured after being heated above room temperature. Hot strength reflects the change in the surface layer strength of the mold after pouring. If the hot strength is too low, it may cause casting deformation, sand swelling, or sand erosion. If the hot strength is too high, it may hinder casting shrinkage, increase casting stress, and even cause cracks. Hot strength is related to the type of binder, compactness, and additional substances.
4. Residual Strength, Collapsibility, and Sand Shedding
Residual strength refers to the compressive, shear, tensile, and flexural strength of a mold sand sample after simulating the casting process and undergoing one or more heating and cooling cycles. A higher residual strength indicates a greater tendency for the mold to form clumps after pouring. The property of mold (core) sand to easily disintegrate after pouring is called collapsibility, while sand shedding refers to the ease with which the mold disintegrates after pouring. Clay-based sand generally has high residual strength but poor collapsibility and sand shedding, which significantly affects sand removal and finishing efficiency. Adding materials such as sawdust can improve collapsibility.