Lesson Notes By Weeks and Term v4 - SHS 2

Lesson Video

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Performance objectives

• Describe the sand casting process and its materials.
• Identify the key characteristics of sand casting.
• Design and create a simple article using sand casting.
• Understand the importance of sand casting in local industries.

Lesson summary

Welcome, future engineers and innovators! Today, we are exploring one of the oldest and most versatile methods of shaping metal: Sand Casting. This process is not just something you read about in books; it is happening all around us in Ghana. From the local craftsman making cooking pots (*dadesen*) and coal pots, to the skilled mechanics at Suame Magazine in Kumasi or Abossey Okai in Accra who create replacement parts for vehicles and machines. Sand casting is a powerful skill that allows us to turn scrap metal into useful and valuable products. By understanding its principles, you gain the ability to create, repair, and innovate.

Lesson notes

2.1 What is Sand Casting? Sand casting is a metal casting process that uses sand as the mould material. In simple terms: A model of the object you want to make (called a pattern) is pressed into a special type of sand to create a cavity or impression. The pattern is removed, leaving a hollow shape in the sand (the mould). Molten (liquid) metal is poured into this mould. The metal is allowed to cool and solidify. The sand mould is broken away to reveal the finished metal object, called a casting. 2.2 The Essential Components of Sand Casting

A. Moulding Sand This is not ordinary beach sand. It is a special mixture with specific properties. It typically consists of: Silica Sand (SiO₂): The main component that can withstand high temperatures. Binder: An agent that helps the sand particles stick together. The most common is Bentonite Clay. Water: Activates the clay binder.

For a successful casting, the moulding sand must have these properties: Permeability: The ability of the sand to allow gases and steam (from the hot metal reacting with moisture) to escape. If gases are trapped, they cause defects like holes (porosity) in the casting. Cohesiveness (or Green Strength): The ability of the sand to hold its shape after the pattern is removed and to resist erosion from the flowing molten metal. Refractoriness: The ability to withstand the high temperature of the molten metal without melting or fusing to the casting. Flowability: The ability of the sand to flow into all the corners and details of the pattern when it is rammed.

B. The Pattern A pattern is a replica or model of the object to be cast. It is used to create the impression in the sand. Materials: Usually made of wood, but can also be metal or plastic for mass production. Pattern Allowances: A good pattern is made slightly different from the final desired object to account for the physics of metal casting. Shrinkage Allowance: Metals shrink when they cool and solidify. The pattern must be made slightly *larger* than the final product to compensate for this shrinkage. For example, aluminium shrinks by about 1.5%. Draft or Taper Angle: The vertical sides of the pattern are slightly angled (1-3 degrees). This allows the pattern to be removed from the sand mould easily without breaking the mould's edges. Think of trying to pull a straight-sided bucket out of wet sand versus a sloped-sided one. Machining Allowance: An extra layer of material is added to surfaces that need to be machined (e.g., drilled or smoothed) later to get a precise finish.

Evaluation guide

• Apply the principles of sand casting to produce articles
• Group work:
• Group students to describe accurately and honestly the processes used in sand
• reasoning