Strength Tests on Core and Mould Sand

1. Compression Strength Test for Moulding Sand

The compression strength test is conducted to determine the bonding strength of moulding sand in its green state (moist condition). The moulding sand consists of silica sand, clay, water, and special additives, with clay providing the necessary binding strength.

Purpose:

· To ensure proper bonding of moulding sand. · To assess the load-bearing capacity before metal pouring. · To identify the optimum mixture composition for different foundry applications.

Test Procedure:

1. Prepare a cylindrical specimen of standard dimensions using a sand rammer.
2. Place the specimen in the universal sand testing machine.
3. Apply a gradual axial compressive force until the specimen fails.
4. Record the maximum load at which failure occurs.

Factors Affecting Green Compressive Strength:

· Clay Content: Higher clay content increases strength. · Moisture Content: Optimum water percentage is necessary for proper bonding. · Degree of Compaction: A well-rammed specimen provides higher strength.

2. Tensile Strength Test for Core Sand

Cores are compacted sand masses used to create hollow sections in castings. Since molten metal contracts upon cooling, it exerts tensile forces on the core sand. Thus, understanding the tensile strength of core sand is crucial to prevent core breakage during casting.

Purpose:

· To determine the load-bearing capacity of core sand under tensile forces. · To prevent core damage during metal solidification.

Test Procedure:

1. Prepare a dog-bone-shaped core specimen using a split core box.
2. Place the specimen in the universal sand testing machine.
3. Apply a gradual tensile load until the specimen breaks.
4. Record the maximum tensile strength before failure.

Factors Affecting Tensile Strength of Core Sand:

· Binder Content: More binder increases cohesiveness. · Compaction Pressure: Proper compaction prevents weak spots. · Core Drying Conditions: Proper oven drying enhances core strength.

3. Shear Strength Test for Moulding Sand

Shear strength is the resistance of sand particles to shear stress. It ensures that moulds and cores remain intact during handling, metal pouring, and solidification. If shear strength is too low, the mould may collapse or deform, leading to casting defects.

Purpose:

· To evaluate the ability of sand particles to resist shear forces. · To prevent mould failure and defects due to low shear strength.

Test Procedure:

1. Prepare a cylindrical specimen using a sand rammer.
2. Place the specimen in the universal sand testing machine.
3. Apply a gradual lateral (shearing) force until the specimen breaks.
4. Record the maximum shear force before failure.

Factors Affecting Shear Strength:

· Grain Size & Shape: Fine, angular grains increase strength. · Moisture Content: Excess water weakens shear resistance. · Binder Type & Quantity: Binders improve cohesion and shear resistance.