Sand Casting Technology

Sand casting is the process of filling molten metal into a mold made of sand.

1. Patterns and Core Boxes

• Include: patterns, pattern plates, patterns for the gating system, vents, and risers.
• Pattern: Used to create the mold cavity; the pattern usually has a shape similar to the external surface of the casting.
• Pattern plate: Used to clamp and position the pattern during mold making.
• Patterns for the gating system, vents, and risers: Used to form these elements in the mold.
• Core box: Used to manufacture cores. Cores are used to create internal shapes of the casting.

a) Materials for Patterns and Core Boxes

Materials for patterns and core boxes must meet the following requirements:

• Ensure good surface finish and dimensional accuracy;
• Be strong, rigid, and durable;
• Not subject to shrinkage, swelling, or cracking;
• Resistant to mechanical and chemical effects, corrosion, and rust;
• Low cost and easy to machine.

Commonly used materials: wood, metal, plaster, cement, plastics.

b) Technology for Manufacturing Patterns and Core Boxes

Detail drawing → Casting drawing → Pattern and core box drawing → Manufacturing of patterns and core boxes.

– Casting drawing: Must reflect casting manufacturability, including the parting line, draft angles, fillet radii, shrinkage allowance, and machining allowance.

– Pattern and core box drawing: Must clearly indicate manufacturing technology and materials used.

– If patterns and core boxes are made of wood, to prevent warping due to shrinkage, cracking, and to increase durability, the following should be noted:

• In the cross-section, wood growth rings should not be oriented in the same direction;
• Along the longitudinal direction, wood grains should not be arranged parallel;
• Large surfaces should be divided into multiple pieces;
• To increase joint strength, joints should be made in stepped form;
• Use sandpaper for grinding and polishing, then paint with color codes: Blue – steel casting; Red – cast iron; Yellow – non-ferrous metal casting.

2. Mold and Core Making Technology

A. Materials for Molds and Cores

Requirements:

• Plasticity: To facilitate mold and core making and obtain clear mold and core cavities;
• Strength: To prevent breakage during handling, assembly, and pouring of molten metal;
• Collapsibility: The ability of the molding mixture to reduce its volume under external forces, allowing the casting to shrink easily;
• Permeability: To allow gases to escape, preventing gas porosity in castings;
• Thermal resistance: To prevent mold and core burning during pouring;
• Moisture content: The amount of water contained in the mixture (approximately 8%).

Materials for molds and cores:

• The mixture includes: sand, clay, binders, and additives.
• Sand (main component is SiO₂): The primary constituent of mold and core mixtures.
• Clay: Increases plasticity and strength of the mixture.
• Binders: Vegetable oil, sugar, molasses, rosin, cement, water glass, etc.
• Additives: Improve collapsibility, permeability, surface finish, and heat resistance. Common additives include sawdust, straw, and mold coatings.

B. Manual Mold and Core Making Technology

• Low mold and core accuracy; low productivity;
• Requires high worker skill and involves heavy labor conditions;
• Capable of producing complex molds and cores with arbitrary sizes and weights.

Manual mold and core making methods:

• Mold and core making using two molding boxes (two-part molds);
• Mold and core making using strike-off boards;
• Mold and core making using patterns (core boxes) with loose pieces.

C. Machine Mold and Core Making Technology

• Overcomes the disadvantages of manual mold making: achieves better quality and higher productivity;
• However, machine operation is economical only when machine utilization exceeds 40%, therefore suitable for mass or large-scale production.

Methods:

• Mold and core making on pressing machines;
• Mold and core making on jolting machines.

Types of mold and core making machines:

• Combined jolting-and-pressing machines.