Casting defects and it's types

CASTING DEFECTS AND TYPES:

•        Casting process involves a number of variables and a loss of control in any of these variables can cause defects under certain circumstances

•        Some of the common casting defects, their features and remedies to prevent such defects are discussed below.

1.    Shrinkage defect

Shrinkage is a void on the surface of the castings resulting from concentrated contraction or shrinkage of metal during solidification

Although a riser is used to overcome the shrinkage effect, in some cases it fails to feed the molten metal efficiently to the casting as it solidifies

Remedies                                                          

•Use large sprue and riser to promote directional solidification

•Locate risers and gating systems in correct positions

•Gates to be cut as wide as possible

2. Porosity defect (Blow hole and pin hole)

Molten metal absorb gases from various sources such as fluxes, moisture in sand, binders, additives and normal atmospheric gases like oxygen and nitrogen

If these gases are not allowed to escape, they get entrapped in the mould cavity forming small balloon shaped voids or cavities leading to porosity defect in castings

Two types of gas related defects occur in castings. They are:

•        blow hole

•        pin hole defect

Blow holes occur below the surface of the castings and are not visible from        the outside surface.

Remedies

•        Avoid excess ramming of mould

•        Provide proper vent holes

•        Avoid use of excess carbonaceous or other organic material in the sand/core binders, because these materials react with the molten metal producing large amount of gases.

3. Misrun

•        Misrun occur when the mould cavity is not completely filled with molten metal

•        It is a defect wherein a casting solidifies before the molten metal completely fills the cavity.

Remedies

•        Fluidity of metal should be high

•        Pouring rate and time should be controlled

•        Thin sections should be suitable designed

4. Penetration

•        When fluidity of liquid metal is high, it may penetrate into the sand mould/core (into the voids between the sand particles) causing a fused aggregate of metal and sand on the surface of the casting leading to defect.

        Remedies

•        Sand should be properly rammed

•        Moulding sand/core sand should not be too coarse to promote metal penetration

•        Control proper metal temperature.

5. Mould Shift

•        It is a step in the cast product at the parting line caused by sidewise relative displacement of cope and drag box.

Remedies

•        Proper alignment of cope and drag box

•         Proper handling of assembled cope and drag box during operations.

6. Cold Shut

Two portions of metal flow together, but lack of fusion due to premature freezing results in a defect known as cold shut .

Remedies

•        Place gates and risers at proper locations

•        Metal fluidity should be high.

7. Pin holes

Formation of many small gas cavities at or slightly below surface of casting

Caused by release of gas during pouring of molten metal

To avoid, improve permeability & venting in mold.

8. Scabs

Scabs are rough areas on the surface of casting due to un-necessary deposit of sand and metal

It is caused by portions of the mold surface flaking off during solidification and becoming embedded in the casting surface

Improve mold strength by reducing grain size and changing binders.

9. Sand Blow

Balloon‑shaped gas cavity caused by release of mold gases during pouring 

Low permeability of mold, poor venting, high moisture content in sand are major reasons.

10. Hot Tears

•        A hot tear is an internal or external ragged discontinuity formed in the casting due to the pulling action of the metal just after it has solidified.

Remedies

•        Provide adequate fillets at sharp corners

•        Proper metallurgical and pouring temperature

•        Place gates and risers at proper locations