Casting Defects and Remedies

Casting defects and how Omnidex prevent them

What is Porosity?

Porosity refers to a hole or void in an otherwise solid cast metal part. The size of pores can range from microscopic (micropores) to larger voids, measured in cubic millimetres or more. A porous is not always circular in cross-section, but can also occur in the form of irregular linear cracks.

Main Causes of Porosity in Casting

There are two major causes of porosity: solidification shrinkage and gas porosity. In the following sections, we will explore the root causes of these two types of violence and suggest strategies to fight them.

Mold cavities solidify when the molten metal comes into contact with relatively cooler walls. As the metal solidifies, it shrinks; however, the rate of shrinkage will vary depending on the geometry of the mold tool and the type of alloy.

In this semi-liquid metal, pore formation is more likely to occur further away from the tool wall, which is called slush. The solidified metal might also block the flow of liquids to other areas in the mold once it becomes solidified. It is possible to create pores when preventing full liquefaction of all design features.

A gas pocket can form in a few different ways. With aluminum, hydrogen may fall out of suspension and fill voids with hydrogen gas.

It is possible that the mold tool may have been partially or completely vacated or vented as the cavity filled. Air that is trapped in this manner is called entrained air.

In addition to the liquids mixed with the molten metal, other liquids may be injected along with it. Among these might be mold release agents, hydraulic fluid leaks or even humidity. It is possible to vaporize and create gas bubbles from any of these substances. A liquid or oil that does not evaporate can become a contaminant which can lead to particle inclusions in the final product.

3 Forms of Casting Porosity

Porosity comes in three forms: enclosed, blind, and through:
Basic Types of Casting Porosity_defect free Products_International Manufacturing_OmnidexCastings

17 Common Types of Metal Casting Defects

The following section provides a better understanding of the types of casting defects, including the causes and remedies of these defects. 

• Pinholes

There are very small holes in the casting about 2 mm in size that appear on the surface. In molten metal, hydrogen gases dissolve, resulting in this defect. During the solidification process, as the solubility of hydrogen gas decreases in the molten metal, it escapes through tiny holes called pinholes. As the molten metal solidifies, the hydrogen gas can no longer dissolve as easily and escape.

 

Causes :

  1. Use of high moisture content sand.
  2. Absorption of hydrogen or carbon monoxide gas by molten metal.
  3. Steel pouring from wet ladles or insufficient gasification.

Remedies :

  1. By reducing the moisture content in molding sand.
  2.  Fluxing and melting must be done correctly.
  3. Increasing the permeability of the sand.
  4. Performing rapid solidification.

• Blowholes

Blowhole is formed when gases are trapped on the surface of casting because of solidifying metal. The defect always occurs in the cope part of the mold.

 

Causes :

  1. Sand with excessive moisture.
  2. The sand has a low permeability.
  3.  The sand grains are too fine.
  4. Too hard rammed sand.
  5. The ventilation is insufficient.

Remedies :

  1. Sand moisture content must be maintained at the desired level.
  2. Use high-permeability sand.
  3. Suitable grain size of sand must be used.
  4. Ramming should be performed adequately.
  5. Ventilation facilities ought to be available.

• Shrinkage Cavity

During volumetric contraction, a hollow spot forms in a cast. This is referred to as a shrinkage cavity.

 

Causes :

  1. Unstable or uncontrolled solidification of molten metal.
  2. The pouring temperature is too high.

Remedies :

  1. This defect can be eliminated by applying directional solidifications principles to tool design
  2. Use of chills (a chill is a device placed in certain non-solid areas of a metal casting to promote solidification) and padding.

• Swell

The molten metal pressure results in an enlargement of the mold cavity, which results in either localised or overall enlargement of the casting.

 

Causes :

  1. Failure to properly ram the mold before injection.

Remedies :

  1. It is important to ram the sand properly and evenly.

• Drop

Drop defects occur when sand pieces fall into molten metal due to cracks on the upper surface.

 

Causes :

  1. Soft ramming and low strength of sand.
  2. The molten metal is not properly fluxed. To remove impurities from molten metal, it is flushed with a substance. The impurities can be easily removed from the molten metal after fluxing.
  3. Inadequate reinforcement of sand projections.

Remedies :

  1. Proper ramming of high strength and is necessary (not too soft nor too hard).
  2. Molten metal should be fluxed properly so that impurities can be easily removed before pouring into molds..
  3. Proper reinforcement of the sand projections in the cope.

• Sand Holes

This refers to the holes made on the casting's exterior or internally. It occurs when loose sand is washed into the cavity of a mold and sets up or when molten metal is quickly poured into a mold cavity.

 

Causes :

  1. Loosely ramming the sand.
  2. As the metal molten is poured into the mold rapidly, sand is washed away from the mold and a hole is created.
  3. The mold cavity was not properly cleaned 

Remedies :

  1. Ramming the sand correctly.
  2. The molten metal should be poured carefully into the mold.
  3. In addition to (1), the molten cavity needs to be cleaned to prevent sand holes from occurring.

• Dirt

A dirt defect is caused when dust and sand embed themselves in the casting surfaces.

 

Causes :

  1. Mold damage caused by improper handling and Sand wash (sloping sand spread out by molten metal).
  2. Slag particles are present during metal melting.

Remedies :

  1. Proper mold handling to prevent crushing.
  2. Ensuring that molten metal is adequately fluxed to remove slag impurities.

• Honeycombing / Sponginess

It is an external defect characterized by many small cavities closely adjacent to one another present in metal casting.

 

Causes :

  1. It is caused by dirt and scurf trapped in the suspension of molten metal.
  2. Due to insufficient skimming in the ladle.

Remedies :

  1. Prevent dirt and scurf from entering molten metal.
  2. Prevent sand-washing.
  3. Carefully skim the molten metal to remove slag materials.

• Hot Tears / Hot Cracks

Castings fail when molten metal cools because of residual stress (tensile) left in the metal when it's hot. In this case, casting failure appears as cracks or hot tears.

 

Causes :

  1. Poor mold design.

Remedies :

  1. Mold design can significantly reduce the likelihood of these types of casting defects.
  2. Elimination of residual stresses in the cast material.

• Hot Spot / Hard Spot

The hot spot defect occurs when an area on the casting cools faster than the surrounding materials. The hot spots on a casting are areas which are harder than the nearby areas. Alternatively, it is referred to as a hard spot.

 

Causes :

  1. The area of the casting that is rapidly cooled has this defect because it is cooled faster than the surrounding materials.

Remedies :

  1. This defect can be prevented using proper cooling.
  2. By modifying the metal's chemical composition.

• Metal Penetration

A rough and uneven surface of the casting is indicative of these casting defects. If the size of the sand grains is large, the molten metal will fuse into the sand and solidify, causing a metal penetration defect.

 

Causes :

  1. Sand is rammed softly due to low strength, large grain size, high permeability and high grain size. This causes the molten metal to penetrate the sand during molding, which results in rough or uneven casting surfaces.

Remedies :

  1. To avoid this defect, sand should be rammed softly, have a small grain size, below permeability and have high strength.

• Cold Shut / lap

The defect appears as a line on the surface. A cold shut occurs when two streams of molten metal meet at the junction of the mold with low temperatures and do not fuse together (appear as lines on the casting). This appears to be a crack with rounded edges.

 

Causes :

  1. Poor gating system
  2. Low melting point
  3. Lack of fluidity

Remedies :

  1. A more sophisticated gate system.
  2. Pouring at the proper temperature.

• Misrun

This occurs when the molten metal solidifies before fully filling a mold cavity and leaves a space. when the molten metal solidifies before completely filling the mold cavity and leaves a space in the mold called misrun.

 

Causes :

  1. Molten metal has a low fluidity.
  2. A low temperature diminishes the fluidity of the molten metal.
  3. Too thin section and inappropriate gate system.

Remedies :

  1. The molten metal pours more fluidly as its pouring temperature increases.
  2. An effective gating system
  3. Avoid too thin sections.

• Shift or Malmatch 

A defect as a result of the upper and lower parts of the casting being misaligned and the core being misplaced at the parting line.

 

Causes :

  1. The upper and lower parts of the mold were not aligned properly during the molding process.
  2. Improper alignment of the flask (a flask is a tool used in metal casting for containing a mold. It may be round, square, rectangular or any other shape that is convenient.)

Remedies :

  1. Alignment of mold parts and patterns.
  2. Properly mounting the pattern on the pattern plate.
  3. Make sure the flask is aligned correctly.

• Slag Inclusion (scab)

The defect occurs when slag particles and molten metal are poured into the cavity of the mold and then solidified.his defect is caused when the molten metal containing slag particles is poured into the mold cavity and solidifies.

 

Causes :

  1. Slag is present in molten metal

Remedies :

  1. Clean the molten metal of slag particles before pouring it into the mold cavity.

• Warpage

During or after solidification, it causes an uncontrolled deformation in the casting. Consequently, the product dimension changes as a result of this defect.

 

Causes :

  1. Due to different rates of solidification among sections. Stress is exerted on adjoining walls, resulting in warpage.
  2. Large and flat sections or intersecting sections such as ribs are more prone to these casting defects.

Remedies :

It can be prevented by producing large areas with wavy, corrugated construction, or add sufficient rib-like shape, to provide equal cooling rates in all areas.

Proper casting designs can reduce these defects more efficiently.

• Fins

Fins or fins are thin projections of metal that are not considered as part of the casting process. This usually occurs when the mold or core section is being parted.

 

Causes :

  1. Molds and cores were assembled incorrectly.
  2. The mold may not contain enough weight or be clamped improperly, causing the fins to form.

Remedies :

  1. Correctly assembling the mold and core.
  2. The top part of the mold should be sufficiently heavy to ensure that the two parts fit tightly together.

A single casting tool simulates the entire metal casting process for defect free parts every time

ProCAST is an outstanding finite element solution for simulation of metal casting processes. Designing and developing processes with this tool will help improve yields and casting quality.

With comprehensive finite element solvers, accurate and efficient output, ProCAST meets the manufacturing industry’s needs. By reducing manufacturing costs, improving mold development time and improving casting quality, the software is the ideal alternative to traditional trial-and-error methods. 

Porosity-Stimulation Illustration_Metal Casting Quality Control_OmnidexCastings
Porosity Stimulation Illustration

In addition to predicting residual stress, ProCAST is an expert at predicting deformations and it can also be used for more specific processes, such as sand castinginvestment castingdie castinglost foam casting, etc.

Simulations help foundries analyse how metals expand and contract in the process of casting, so they can predict where shrinkage porosity will be located. There are several possible defects that can occur during a casting process, causing a final product to be imperfect. Therefore, an effective gating and feeding system is essential for reducing or eliminating these defects.

By seeing how castings fill and solidify within a mold, manufacturers can validate and improve the locations and sizes of the down sprue, runners, gates, vents, chills, and feeders. ProCAST is used as a tool to simulate casting processes and ensure that molds are made correctly the first time.

With Omnidex ProCAST Software, our customers will not recevie any scrap cast parts in the future. ProCAST has served as their go-to tool to predict porosity, filling, and solidification of castings for decades. It can also predict complex issues like deformations and residual stresses, and it is capable of dealing with most castable alloys, most casting processes such as sand castings, die castings, investment castings, and the variants that go with each process.

ProCAST Simulation Result (Before)_Metal Casting Quality Control_OmnidexCastings
Illustration 1. ProCAST Simulation Result (Before)
ProCAST Simulation Result (Before)_Metal Casting Quality Control_OmnidexCastings
Illustration 1. ProCAST Simulation Result (Before)
ProCAST Simulation Result (Before)_Metal Casting Quality Control_OmnidexCastings
Illustration 2. ProCAST Simulation Result (Before)
ProCAST Simulation Result (After)_Metal Casting Quality Control_OmnidexCastings
Illustration 2. ProCAST Simulation Result (After)

Benefits of ProCAST

Omnidex Metal Casting Services

- Over 18 years of excellence in casting simulation

There are no 100% perfect casting processes. Porosity in metals must be considered despite excellent properties.

It is almost always sand casting that is the most favorite and popular foundry process. The ability of sand casting to casting large volumes, high-quality and low-cost make it an exceptional machining process. Yet, examination of sand casting defects is vital to determining the customer’s tolerance as well as expectations.

We at Omnidex are transparent about casting defects that can occur during the process with our customers. We always strive to minimize the number and severity of casting defects so that we can supply the best quality products.

Depending on the defect, we approach casting defects differently. Whenever possible, we solve problems from the perspective of prevention. Therefore, we strictly inspect all of the manufacturing processes, such as raw materials input, melting metal, pouring system, cooling, and machining.

Furthermore, we monitor the gates and pouring process continuously in order to ensure that they are working properly in accordance with each casting requirement. In this way, we can ensure our customers receive the best quality of metal casting services and products.

The Omnidex Casting team is committed to providing quality machining and manufacturing services with no compromise. If you are interested in our metal casting services, feel free to contact us for a quick quote.

We are always ready to help with your latest Casting project.

Contact our Customer Service and let us know how we can help to catapult your next project to success.

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