What are the common defects in casting?
They are broken down into five main categories: gas porosity, shrinkage defects, mould material defects, pouring metal defects, and metallurgical defects.
How do you identify defects in casting?
Non-destructive testing (NDT) is a common method for inspecting castings without having to damage the product under assessment. NDT can help identify defects that adversely affect the material strength. Cracks appear after a melt has solidified in the form of thin fissures on the casting surface.
What are the defects in sand casting?
Possible Defects
| Defect | Causes |
|---|---|
| Unfilled sections | Insufficient material Low pouring temperature |
| Porosity | Melt temperature is too high Non-uniform cooling rate Sand has low permeability |
| Hot tearing | Non-uniform cooling rate |
| Surface projections | Erosion of sand mold interior A crack in the sand mold Mold halves shift |
What are the usual defects in green sand casting?
Usually, the causes of defects include shrinkage, gas porosity, sand expansion, sand adherence, and weak sand strengths. Shrinkage is a depression in the cured metal due to uneven solidification.
What is mold material defect in casting?
It occurs when loose sand washes into the mold cavity and fuses into the interior of the casting or rapid pouring of the molten metal. Causes: (i) Loose ramming of the sand. (ii) Rapid pouring of the molten metal into the mold results in wash away of sand from the mold and a hole is created.
What causes cracks in casting?
Cracks are often associated with shrinkage porosity, slag inclusion and other defects, which often occur in the inner side of the casting sharp corner, the junction of thick and thin sections, and the hot spot area where the riser is connected with the casting.
What causes porosity in castings?
Casting porosity can be caused by gas formation or solidification shrinkage while the metal is a liquid. If a casting needs to be pressure tight, then the porosity can allow gas and fluids to seep from the part. In addition, the porosity can weaken the casting.
What causes hot tears in castings?
The cause of hot tearing is generally attributed to the development of thermally induced tensile stresses and strains in a casting as the molten metal contracts during solidification and solid state shrinkage.
What is fin defect in casting?
Flash, fin and burrs
Flash is one of the most frequently occurring casting defects and also a common injection molding defect. Flash, also known as casting fin or burrs, is any unwanted and excess material attached to a cast. It’s typically a thin sheet of metal that forms at the parting faces.
What is cold shut defect?
A cold shut occurs when the molten material has begun to solidify before fully filling the mould. When the molten metal begins solidifying before the mould has been completely filled, it can form a seam that hasn’t merged correctly. This results in severe weak spots in the casting.
What is blowhole defect?
They are also called as endogenous gas holes or blowholes. These holes are caused due to excessive gas content in the metal bath and rejection of dissolved gases during solidification. The gases involved in this defect are hydrogen and nitrogen.
What causes shrinkage in casting?
Solidification shrinkage occurs because metals are less dense as liquids than solids. Risers and chills are used to address this problem. Risers feed a continuous supply of molten metal to the casting as it solidifies.
How can casting defects be reduced?
To prevent these sand casting defects, solving the above causes by: Use the small sand size. Increase sand mold and core strength. Provide enough hard ramming to the sand mold.
How much porosity is acceptable in casting?
Pits not larger than 1.25mm ( 0.05″) in any direction are acceptable. Exposed chain porosity must not exceed 3mm (1/8 “) in length. Unless otherwise specified, exposed porosity on machined diecast parts will conform to this level.
How do you get rid of porosity in casting?
Gas porosity can be eliminated through good mould design or by introducing nitrogen into the aluminium metal before the liquid pour. HiPping can also be used to eliminate small amounts of porosity inside the casting.
How do you fix hot tears?
It is a well known practical observation that hot tearing can be reduced or eliminated in controlled casting conditions that prevent formation of large temperature and stress gradients. In line with that, preheating permanent molds to a high enough temperature is done to alleviate or eliminate hot tearing.
What is scab defect?
What is it? A scab defect occurs by splash or boiling from teeming, casting, or conditioning. Scabs are usually round or oval shaped, have irregular surfaces beneath them, and can be seen prior to the rolling process. They are often mistaken as slivers, shearing, or rolled in scale.
What is Misrun defect in casting?
A misrun occurs when the molten metal freezes before it reaches all parts of the mould cavity – which leaves a completely unfilled part of the mould. Both problems can lead to weak spots in castings, so it is important that we prevent these issues from occurring during production.
What is rat tail defect in casting?
Rat tails, or veins, appear as an irregular line or crack on the casting, when the surface of the molding sand buckles up. Rat tails usually occur on the surface of the mold bottom, an area covered with molten material.
What are the 3 main causes of shrink?
There are four main causes of shrinkage: shoplifting, employee theft, administrative errors, and fraud.
What causes porosity in cast iron?
Trapped air and gasses can also cause porosity, either on the surface or inside the casting. Gas porosity occurs because the molten metal can hold a large amount of dissolved gas, unlike the solid form.
How do you stop mold from shifting?
The most common method to prevent a mould shift is making use of some type of alignment to ensure the cope and drag is aligned perfectly. Alignment pins are used on all our mold patterns to save time and money.
How do you fix porosity in casting?
How do you stop porosity in casting?
The most economical and successful approach to stop casting porosity is through vacuum impregnation. Vacuum impregnation is a method that seals the casting resulting from porosity. The impregnating sealant is introduced into the voids within the wall thickness of the casting through vacuum and/or pressure methods.
What is hot tear defect in casting?
A hot tear is a defect that occurs during solidification of the casting. Hot tears are internal or external ragged discontinuities or crack on the casting surface, caused by rapid contraction occurring immediately after the metal solidified.