This article is an introduction to defects in casting, molding defects, casting defects and remedies. It is mainly introduced the main defects in casting, the main causes of defects in molding, and overcome of the above problem. In many situations, there are remedies that can be applied to the casting process, which if properly applied, can result in a good casting.
What is Sand Casting
Casting is the process of creating a mold from which a metal object can be formed. This mold is usually created by filling sand into a container with an exact replica of the objects shape, and then letting it cool and harden. The sand casting process has been used for thousands of years to create everyday items such as bowls, plates, cups, and even jewelry.
Defects in Casting:
• Production of castings involves a large number of steps including casting design, pattern making, moulding, melting, pouring, shake out, fettling, inspection and finishing.
• It is not uncommon for one or more of these steps to be performed unsatisfactorily due to use of defective material or equipment, carelessness of the operator or lack of skill.
• Such unsatisfactory operations result in a defective casting which may be rejected at the final stage.
• Since reclamation of defective castings is often costly and sometimes outright impossible, care should be taken to avoid the occurrence of the defects in the first instance.
• It is therefore necessary to understand the various defects that occur in sand castings and the main factors that are responsible for their occurrence.
Types of Defects In Casting:
Casting defects can cause a lot of problems in manufacturing. Even the most experienced mold maker might not notice such issues with the naked eye. Therefore, it is important to use an effective inspection method which enables you to check your product for quality and accuracy. Today we will discuss some of the most common casting defects and how they can be identified. Some of the common defects in casting are described below.
1. Open Blows and Blow Holes
2. Pin Hole Porosity
3. Entrapped Air and other gases
4. Cracked Casting
5. Bent or Twisted Casting
6. Dropped Mould
9. Run out
11. Mis-run and Cold Shut
13. Rat Tail and Buckles
14. Core Shift
15. Slag Inclusions
16. Cuts and Washes
17. Metal penetration
18. Hard Spots
20. Hot tears
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These are undesired abnormalities that occur during the metal casting process. There are several reasons or causes for cast metal flaws. We shall address all of the primary forms of Molding defects in this article. While some of the flaws generated may be overlooked or ignored, others are not acceptable and must be addressed to ensure the components perform properly.
Misrun and Cold Shut | Defects in Casting
- A misrun is caused when the section thickness of a casting is so small or the pouring temperature so low that the entire section is not filled before the metal solidifies. When the molten metal hardens before entirely filling the mold chamber, leaving a gap in the mold cavity.
- Cold shut is caused when two streams of metal which are too cold meet but do not fuse together. It is a sort of surface imperfection that is visible as a line on the surface. It seems to be a round-edged crack.
- Misrun and cold shut can be minimized by proper design of casting, providing suitable gating and risering and using correct temperature of the melt.
- An excessively thin part is avoided.
Shrinkage Faults | Molding Defects
- Shrinkage faults are faults caused by improper directional solidifications, poor gating and risering design and inadequate feeding.
- Solidification leads to volumetric contraction which must be compensated by feeding. If this compensation is inadequate either surface shrinkage or internal shrinkage defects are produced making the casting weaker.
Casting Defects and Remedies
- Shrinkage faults can be reduced by providing proper gating system, pouring at correct temperature and taking care of directional solidification.
- Prudent use of chills and padding (a chill is an item used to encourage solidification in a particular area of a metal casting).
Rat Tail and Buckles | Defects in Casting
- Rat tails and buckles are caused by the expansion of a thin outer layer of moulding sand on the surface of the mould cavity due to metal heat.
- A rat tail is caused by depression of a part of the mould under compression which appears as an irregular line on the surface of the casting.
- A buckle is a more severe failure of the sand surface under compression.
- The mould must provide for proper expansion instead of forming compressed layers to avoid this defect.
Core Shift | Molding Defects
Shift or Mismatch is a defect induced by the misalignment of the casting’s upper and lower halves, as well as the misplacement of the core at the parting line.
Reason of this defects in casting
- A core shift results from improper support or location of a core.
- It results in a faulty cavity or hole in the casting.
- It can be reduced by providing proper support for cores and correct alignment with the mould.
- Imperfect matching of the upper and lower mold components during the mold preparation process.
- Mismatches of flask i.e., Flask is made in different structures such as rectangle, square, circular etc., and these are used to retain the mold.
Casting defects and Remedies:
- Molding boxes must be properly aligned with the pattern or die component.
- Proper pattern mounting on pattern plates.
- Inspect the flask for alignment.
Slag Inclusions | Molding Defects
- Inclusions are any foreign materials present in the cast metal.
- These may be in the form of oxides, slag, dirt, sand or nails.
- Common sources of these inclusions are impurities with the molten metal, sand and dirt from the mould not properly cleaned, break away sand from mould, core or gating system, gas from the metal and foreign items picked on the mould cavity while handling.
- Inclusions are reduced by using correct grade of moulding sand and proper skimming to remove impurities.
Cuts and Washes | Molding Defects
- Cuts and washes are caused by erosion of mould and core surfaces by the metal flowing in the mould cavity.
- These defects are avoided by proper ramming, having sand of required strength and controlling the turbulence during pouring.
Metal penetration | Defects in Casting
- If the sand grains used are very coarse or the metal poured has very high temperature the metal is able to enter the spaces between sand grains to some distance. Such sand becomes tightly wedged in the metal and is difficult to remove.
Reason for this type of Molding Defects:
- Metal penetration occurs as a result of the Green sand’s poor strength, large grain size, high permeability, and soft ramming. As a result, the molten metal enters into the molding sand, resulting in an uneven or rough casting surface.
- The remedy is to remove the causes mentioned above.
Hard Spots | Molding Defects
- When a region of the casting cools more quickly than the surrounding components, hot spot faults emerge. The term “hot spot” refers to parts of a casting that are tougher than the surrounding area. Additionally, it is referred to as a “hard spot.”
- Hard spots are caused by chilling action of moulding sands in some metals like gray cast iron with insufficient silicon.
- These spots are extremely hard and often lead to machining difficulties.
- Hard spots are avoided by providing uniform cooling and pouring at the right temperature.
- By optimizing the metal’s chemical constituents.
Scabs | Molding Defects
- Scabs are rough, irregular projections on surface of castings containing embedded sand.
- Scabs occur when a portion on the face of mould or core lifts and metal flows underneath in a thin layer.
- They are caused by using too fine sand grains or using sand of low permeability or moisture content.
- They may also be caused by uneven mould ramming or by intense local overheating.
- Scabs can be reduced by mixing additives like sea coal, wood flour or dextrin in the sand, providing uniform ramming and pouring with correct velocity.
Hot tears | Molding Defects
- Hot tears are ragged irregular internal or external cracks occurring immediately after the metal have solidified.
- Hot tears occur on poorly designed castings having abrupt section changes or having no proper fillets or corner radii. Wrongly placed chills.
- Improper placement of gates and risers or incorrect pouring temperatures can also produce hot tears.
- When metal is hot, it is brittle, and when the molten metal cools, residual stress in the material causes the casting to fail. In this situation, the casting failure appears as cracks and is referred to as hot tears or hot cracking.
- Hot tears are also caused by poor collapsibility of cores.
- If the core does not collapse when the casting is contracting over it stresses will be set up in the casting leading to its failure.
Casting Defects and Remedies
- Hot tears can be eliminated by improved design, proper directional solidification, and uniform rate of cooling, correct pouring temperature and control of mould hardness.
- With proper mold design, these sorts of casting problems may be readily eliminated.
- Elimination of residual stress in the casting material.
It is an unintended distortion that occurs during or after solidification of the casting. As a result of this fault, the final product’s dimensions vary.
- As a result of the varied rates of solidification of the various portions. This creates tension in adjacent walls, resulting in warpage.
- These casting faults are particularly prevalent in large, flat portions or crossing parts such as ribs.
- It may be avoided by fabricating extensive regions of wavy, corrugated construction or by adding sufficient rib-like form to ensure that cooling rates are equal throughout the structure.
- Proper casting design may significantly decrease these defects.
Fins are a thin protrusion of metal that is not regarded to be a part of the casting process. It often happens during the mold or core section’s splitting.
- Improper assembly of the mold and cores.
- Inadequate weight in the mold or poor clamping of the flask might result in the formation of fins.
- Proper mold and core assembly.
- The top portion of the mold should be sufficiently weighted so that the two halves fit snugly together.
Casting defects are the most common failure mechanism in casting or molding. In this article, we will discuss the various defects that can occur during casting and what you can do to avoid them.
In the foundry business, there are a number of key mistakes that can lead to defects in casting. Understanding these errors and how to avoid them will increase your product quality and reduce costs.
In conclusion, it is always good to be aware of the different types of defects that can occur in casting. If you are interested in learning more about these defects or if you are looking for someone to help with your casting production, visit the website today.