These defects may be the result of:
(a) improper pattern design,
(b) improper mould and core construction,
(c) improper melting practice,
(d) improper pouring practice and
(e) Because of molding and core making materials.
(f) Improper gating system
(g) Improper metal composition
(h) Inadequate melting temp and rate of pouring
It creates a deficiency or imperfection.
Exceeding quality limits imposed by design and service casting defects are mainly 3 categories. These are:
(1) Major or most severe defects
(2) Intermediate defects
(3) Minor defects
Surface defects:Due to design and quality of sand molds and general cause is poor ramming.
Blow:Blow is relatively large cavity produced by gases which displace molten metal form.
Scar:Due to improper permeability or venting.A scare is a shallow blow. It generally occurs on flat surf; whereas a blow occurs on a convex casting surface. A blister is a shallow blow like a scar with thin layer of metal covering it,
Scab:This defect occurs when a portion of the face of a mould lifts or breaks down and the recess thus made is filled by metal. When the metal is poured into the cavity, gas may be disengaged with such violence as to break up the sand which is then washed away and the resulting cavity filled with metal. The reasons can be: - to fine sand, low permeability of sand, high moisture content of sand and uneven moulds ramming.
Drop:Drop or crush in a mould is an irregularly shaped projection on the cope surface of a casting. This defect is caused by the break-away of a part of mould sand as a result of weak packing of the mould, low strength of the molding sand, malfunctioning of molding equipment, strong jolts and strikes at the flask when assembling the mould.
The loose sand that falls into the cavity will also cause a dirty casting surface, either on the top or bottom surface of the casting, depending upon the relative densities of the sand and the liquid.
Misrun or cold sheet or short run:
Comparison of Casting processes
Advantages and Disadvantages of Centrifugal casting process
Type of molding sand in casting process
Natural and synthetic sand properties for molding process
Types of Patterns used in casting process