Question

A low projection on the drag surface of a casting commencing near the gate by erosion of sand due to the high velocity liquid metal in the bottom gating. This defect is known as:

• A. Cold shut
• B. Swell
• C. Blow holes
• D. Wash

Hint:

To eliminate wash defects: - Adopt an unpressurized gating system design to decrease the fluid velocity. - Elevate the sand's green/dry binding strength by adding appropriate clay and binder content.

Answer 1

The Correct Option is D

Concept: In sand casting, defects arise due to multi-variable parameters such as sand properties, gating system design, and pouring parameters.

• Wash (or Cut): This manifests as an excess metal projection or an irregular low protrusion on the drag surface of the casting shell, starting predominantly near the gating entry channel. It is driven mechanically by the erosive action of high-velocity molten streams scouring away weak or poorly rammed molding sand.

• Cold Shut: A structural discontinuity formed when two separate streams of molten metal meet from opposite directions but fail to fuse completely due to early solidification or insufficient superheat.

• Swell: A localized enlargement of the casting dimensions caused by the displacement of the sand mold wall under the heavy metallostatic pressure of the liquid metal, typically due to soft or non-uniform ramming.

• Blow holes: Internal or surface gas cavities that are spherical or smooth-walled, generated by entrapped air, core gases, or vaporized moisture from low-permeability molds.

Step 1: Analyze the specific operational conditions given. The problem describes an occurrence marked by:

• Location: On the drag (bottom) surface, originating adjacent to the ingate.

• Action: Erosion or washing away of the molding sand matrix.

• Cause: High velocity flow of the liquid metal stream when passing through a bottom gating layout.

Step 2: Connect the mechanism to the correct nomenclature. When molten metal enters the mold cavity at high velocity, it creates significant hydrodynamic shear stress along the sand boundary layer. If the sand lacks sufficient hot strength, dry strength, or proper compact ramming, the metal physically detaches sand grains. The carried-away sand leaves behind a void space inside the mold boundary. During filling, this empty space is filled by liquid metal, resulting in an irregular, low projection called a Wash.