Mechanical Engineering - Workshop Technology - Discussion

Scabs are a thin layer of metal that sits proud of the casting.

A swell occurs when the mold wall gives way across a whole face and is caused by an improperly rammed mold.

It's scab.

The correct answer is scab.

A scab is a type of casting defect that occurs near the ingates (where the molten metal enters the mould) as rough lumps on the surface of a casting. Scabs are caused by:

- Metal penetrating the sand mould and solidifying under the surface.
- Sand grains or mould material becoming embedded in the metal surface.

Scabs can be caused by:

- Poor mould design or construction
- Insufficient sand compaction
- Incorrect pouring techniques

Here's why the other options are incorrect:

- Shift: A shift is a casting defect that occurs when the mould is not properly aligned, resulting in a mismatch between the cope and drag.
- Sand wash: Sand wash is a casting defect that occurs when the molten metal erodes the sand mould, creating a rough surface.
- Swell: Swell is a casting defect that occurs when the molten metal expands and solidifies, creating a bulge or protrusion on the surface of the casting.

Note: Casting defects can be caused by a variety of factors, including mould design, material selection, and pouring techniques. Identifying and correcting the root cause of a defect is crucial to producing high-quality castings.