Mould making is a fundamental aspect of foundry work where sand moulds are prepared to cast metal components. For Mechanical Production Technicians in Kenya, mastering various moulding processes ensures production of high-quality castings that meet technical specifications and client demands. Sand moulding remains widely used due to its adaptability and cost-effectiveness, especially in workshops serving industries such as automotive parts manufacturing, agricultural machinery repairs, and hardware fabrication. This chapter explores key moulding processes, detailing their methods and applications relevant to Kenyan foundries and mechanical workshops.

5.1 Moulding Process

Moulding processes in foundry work involve shaping sand around a pattern to create a cavity for molten metal casting. Different moulding techniques suit various casting sizes, shapes, and production volumes. Kenyan mechanical workshops often select moulding methods based on resource availability, production scale, and complexity of components needed in sectors like construction equipment repair or small-scale manufacturing.

5.1.1 Bench Moulding

Bench moulding is a manual sand moulding process where moulds are prepared on a workbench, suitable for small to medium-sized castings. It is common in workshops producing replacement parts for agricultural machinery or small engine components.

Definition and Scope

Bench moulding involves forming the mould on a flat surface or bench using hand tools. It is ideal for casting items with relatively simple shapes and moderate sizes, allowing precise control over sand compaction and mould detail.

Tools and Materials Used

• Pattern: Made from wood, metal or plastic, representing the shape of the casting.
• Moulding sand: A mixture of silica sand, clay, and water that holds shape when compacted.
• Rammers and spoons: Hand tools used to pack and shape the sand around the pattern.
• Flasks: Metal or wood frames holding the sand in shape during moulding.
• Sprue and riser pins: Used to form channels for molten metal flow.

Procedure Steps

1. Place the drag flask on the bench and secure the pattern inside it.
2. Fill the flask with moist moulding sand and ram it tightly around the pattern.
3. Level the sand surface and remove excess sand.
4. Turn the flask over and prepare the cope (top half) similarly.
5. Insert gating system components such as sprues and risers.
6. Separate the cope and drag, remove the pattern carefully, and assemble the mould ready for pouring.

Applications in Kenyan Workshops

Small workshops repairing diesel engine parts in Eldoret or producing custom farm equipment components in Nakuru utilize bench moulding for its flexibility and accuracy. It facilitates casting limited quantities without expensive machinery.

5.1.2 Floor Moulding

Floor moulding is a manual moulding technique where the mould is prepared directly on the floor, suitable for large castings or bulky components.

Characteristics and Suitability

This method is preferred for large-scale castings that cannot fit on benches, such as heavy machine bases or pump housings. The floor provides a stable and spacious surface for mould preparation.

Preparation and Equipment

• Large pattern: Usually wooden or metal, placed upside down on the floor.
• Moulding sand: Coarser sand with good permeability to withstand large molten metal volumes.
• Rammers and shovels: For packing sand by hand.
• Flasks or wooden frames: Sometimes used but often the mould is free-standing.

Stepwise Moulding Process

1. Place the drag pattern upside down on the floor.
2. Fill and ram sand around the pattern evenly to form the drag.
3. Remove the pattern and prepare the cope pattern placed over the drag.
4. Fill the cope with sand and ram firmly.
5. Insert gating and riser systems.
6. Separate the mould halves, remove patterns, assemble the mould, and prepare for pouring.

Practical Use Cases

County government workshops fabricating large water pump casings or stair railing components in Mombasa use floor moulding due to the size and weight of the castings. The method is cost-effective for low-volume large parts.

5.1.3 Pit Moulding

Pit moulding involves making sand moulds inside a pit dug into the ground, suitable for very large or heavy castings.

Definition and Context

A pit is excavated to act as the drag, with the pattern positioned inside. The pit provides natural support and containment for the sand mould, especially for oversized castings.

Advantages and Limitations

• Advantages: Can handle very large castings without requiring large flasks; low cost as the ground acts as a mould frame.
• Limitations: Labour-intensive setup and difficult to control sand quality; limited to outdoor or suitably prepared areas.

Procedure Overview

1. Excavate a pit of suitable dimensions in firm soil.
2. Place the pattern at the bottom of the pit.
3. Fill the pit with sand and ram to form the drag.
4. Remove the pattern and prepare the cope separately on a bench or floor.
5. Assemble the mould halves and prepare for metal pouring.

Application in Kenyan Foundries

Rural foundries producing large agricultural implements or water storage tank fittings in Kisumu use pit moulding due to resource constraints and the need for large castings without complex equipment.

5.1.4 Plate Moulding

Plate moulding is a method where moulds are prepared on a flat plate, combining features of bench and floor moulding for medium to large castings.

Equipment and Materials

• Moulding plate: Usually a steel or cast iron flat plate.
• Patterns and sand: As in other moulding methods.
• Flasks: To hold the sand mould halves.

Stepwise Procedure

1. Place the drag pattern on the moulding plate.
2. Pack and ram sand around the pattern to form the drag.
3. Remove the pattern and prepare the cope pattern on the plate.
4. Fill and ram sand to form the cope.
5. Insert gating and risers.
6. Separate mould halves, remove patterns, assemble and prepare for casting.

Usage in Kenyan Mechanical Workshops

Manufacturers of medium-sized hardware components in Nairobi or Kisii use plate moulding to produce consistent moulds with easier handling and better surface finish control.

5.1.5 Flaskless Moulding

Flaskless moulding eliminates the use of flasks, relying on the sand's natural cohesiveness or mechanical support to hold the mould shape.

Concept and Innovation

This method reduces equipment costs and increases moulding speed for simple castings. It is gaining traction in small foundries aiming to minimize capital expenditure.

Techniques Employed

• Using special sand mixtures with higher bonding properties.
• Mechanical clamping or external supports to keep mould integrity.
• Vacuum or pressure systems to compact sand without flasks.

Advantages and Challenges

• Advantages: Lower equipment cost, faster mould preparation.
• Challenges: Limited to simple shapes, less dimensional accuracy, and higher risk of mould collapse.

Kenyan Applications

Small-scale foundries in Nakuru producing simple castings like valve bodies or pump parts experiment with flaskless moulding to reduce costs and improve turnaround times.

5.1.6 Vacuum Moulding

Vacuum moulding uses a vacuum to compact sand around the pattern, resulting in precise and strong moulds.

Principle and Equipment

A pattern is placed on a perforated plate connected to a vacuum pump. When vacuum is applied, sand is compacted tightly around the pattern, creating a strong mould without the need for heavy ramming.

Process Steps

1. Place the pattern on the perforated plate.
2. Spread sand over the pattern.
3. Activate vacuum pump to compact the sand tightly.
4. Remove the pattern carefully.
5. Assemble mould halves and prepare for casting.

Advantages for Kenyan Foundries

Vacuum moulding improves surface finish and dimensional accuracy, useful for precision components like pump impellers produced by Nairobi-based engineering firms.

5.1.7 Machine Moulding

Machine moulding automates the sand moulding process using specialised equipment, increasing production speed and consistency.

Types of Moulding Machines

• Jolt-squeeze machines: Use vertical and horizontal compaction.
• Sand slinger machines: Spray sand into flasks with high velocity.
• Roller compactors: Compress sand through rollers.

Advantages for Production

Machine moulding is ideal for mass production of standard castings, reducing labour costs and ensuring uniform mould quality.

Typical Procedure

1. Place pattern in moulding machine flask.
2. Feed sand into the machine.
3. Machine compacts sand around the pattern by vibration, squeezing, or sand slinging.
4. Remove pattern automatically or manually.
5. Assemble mould halves for casting.

Kenyan Industry Application

Large casting workshops supplying parts to the automotive and manufacturing industries in Thika and Athi River employ machine moulding to meet high demand efficiently.

Chapter Summary

The chapter on making sand moulds explores various moulding processes essential in foundry works, beginning with bench moulding, floor moulding, and pit moulding, each suited to different casting requirements and workspace conditions. It further examines plate moulding, flask less moulding, and vacuum moulding, highlighting their unique techniques and applications. Machine moulding is also covered, emphasizing mechanization's role in improving mould production efficiency. The moulding procedures section details the critical steps starting with flask setup, ensuring the moulding frame is properly prepared. Compacting and setting sand is discussed next, focusing on achieving the correct mould density and shape. Mould parting explains how to separate the cope and drag halves without damaging the mould cavity, and the chapter concludes with closing and sealing the mould to prepare it for pouring molten metal. Together, these topics provide a comprehensive understanding of the sand mould making process in foundry operations.