What Is A Casting Defect?
Casting involves pouring of liquid in a mold with a hollow shape. It is then cooled until it becomes hard. This part is then taken out of the mold.
High pressure is applied in casting to the molten metal. This can lead to issues. It may also lower the quality of the final product. These issues are referred to as casting defects. It can create a big challenge for manufacturers.
Castings can have different types of defects, each with its own causes and solutions. By identifying these defects, you can fix issues early and produce high-quality products.
Casting Defect Types
Following are the main types of casting defects:
- Mismatch
- Misruns
- Swell
- Shrinkage defect
- Warpage
- Pinholes
- Open holes
- Blow holes
- Dirt
- Slag inclusion
- Honeycombing or Sponginess
- Fusion
- Fins and burns
- Metal penetration
- Cut and washes
- Drops
- Run out
- Hot tear or crack
- Cold shut
- Hot or hard spot
#1 Mismatch
Also called “shift,” this casting defect happens when the top (cope) and bottom (drag) parts of the mold don’t line up correctly. It can also happen if the core is misaligned along the parting line.
There are two main types of mismatch: mold shift and core shift. Mold shift happens when the mold halves are misaligned, showing as a horizontal shift. Core shift happens when the core is misaligned, showing as a vertical shift.
| Causes | Remedies |
|---|---|
| Improper alignment of top and bottom mold parts | Ensure proper match plate pattern alignment and mounting |
| Loose box pins and incorrect dowel pins | Use the correct molding box and closing pins |
| Misalignment of the flask | Check and correct flask alignment |
#2 Misruns
Misruns happen when liquid metal cools too quickly. Also it happens when it doesn’t reach all areas of the mold. This issue is also related to “cold shut”. It occurs when the metal solidifies before filling the entire mold cavity.
| Causes | Remedies |
|---|---|
| Low fluidity of molten metal | Increase the fluidity of the molten metal |
| Poor mold and gating system design | Improve the mold and gating system design |
#3 Swell
Swell is when the mold cavity enlarges due to the pressure of molten metal, causing the casting to stretch locally or overall. This often appears as a small, smooth bump on the surface. It leads to increased feeding and machining needs, wasting metal.
| Causes | Remedies |
|---|---|
| Improper ramming of the sand mold | Use a strong, properly rammed mold |
| Low-strength mold | Design molds to withstand the pressure of molten metal and prevent mold wall movement |
#4 Shrinkage Cavity
Shrinkage cavities occur when holes form in the casting due to the metal shrinking as it cools and solidifies.
| Causes | Remedies |
|---|---|
| Uncontrolled solidification of molten metal | Apply directional solidification principles to mold design |
| Pouring temperature too high | Use chills and padding to control solidification in specific areas |
#5 Warpage
Warping in casting refers to an unwanted deformation that changes the shape and dimensions of the final product over time. It typically happens during or after solidification, causing the metal to bend or distort, especially in flat or large sections.
| Causes | Remedies |
|---|---|
| Different solidification rates between sections, creating tension in the walls | Use normalizing heat treatment to relieve residual stresses |
| Large and flat sections are more prone to warping | Prevent with proper casting design |
#6 Pinholes
Pinholes are small holes. They are about 2 mm. They appear on the top of a die-cast part. They are usually in areas with poor ventilation.
These holes are easy to see. They don’t need special equipment to spot. They usually appear in large numbers on the top of the casting or just below it.
#7 Open Holes
Air can get trapped when metal is casted. This cause open holes on the surface. These holes are usually round or oval. They have a smooth surface with a bright yellow or white color.
Through visual inspection, gas porosity defects can be spotted after casting. An X-ray device is needed to check for this defect in case of non-machined surfaces.
| Causes | Remedies |
|---|---|
| Insufficient venting of mold and cores | Lower metal temperature during casting to enhance solidification |
| Poor drying of mold and cores | Dry molds and cores before use and store properly |
| Excessive moisture in molding sand | Use proper fluxing and melting practices |
| Poor gas permeability of sand | Improve permeability of mold and cores |
#8 Blow holes
Blowholes are large cavities inside a cast workpiece caused by trapped gases during solidification. They are round or oval in shape. They are also invisible to the naked eye. They need X-ray, ultrasonic, harmonic, or magnetic analysis to detect.
| Causes | Remedies |
|---|---|
| Insufficient venting of mold and cores | Lower metal temperature during casting to improve solidification |
| Improper drying of mold and cores | Properly dry and store molds and cores before use |
| Excessive moisture in molding sand | Use correct fluxing and melting techniques |
| Low gas permeability of sand | Enhance permeability of molds and cores |
#9 Dirt
Dirt defects occur when dust or sand particles get embedded in the casting surface, leading to serious flaws.
| Causes | Remedies |
|---|---|
| Improper maintenance and sand washing | Operate molds properly to avoid crushing |
| Slag particles in molten metal | Perform adequate fluxing to remove slag impurities |
#10 Slag inclusion
This happen when slag particles in molten metal are poured into the mold and solidify. These defects, usually found on the casting surface, reduce the product’s mechanical properties.
They appear as ribbon-like pockets with sharp edges and irregular shapes, often just a few millimeters thick. Other inclusions, like sand, nails, dirt, or oxides, may also be present. These defects are visible without special tools.
| Causes | Remedies |
|---|---|
| Slag particles present during pouring and solidification | Perform regular die cleaning |
| Improper gating system design | Use a properly designed gating system |
| Formation of insoluble intermetallic compounds during solidification | Use clean molten metal to prevent separation |
| Using undissolved alloy in the melt | Test alloy changes with a small sample piece |
#11 Honeycombing or Sponginess
This casting defect involves a cluster of small cavities that form close to each other within the metal casting.
| Causes | Remedies |
|---|---|
| Mechanically frozen impurities in molten metal | Prevent dirt and scurf from entering the molten metal |
| Inadequate skimming in the ladle | Thoroughly skim slag material from the molten metal in the ladle |
#12 Fusion
This occurs when sand grains mix with the molten metal. It then forms a hard, glassy crust. This firmly gets attached to the casting. It happens due to the sand’s low refractoriness. This makes it unable to withstand the high temperatures of the molten metal.
| Causes | Remedies |
|---|---|
| Low refractoriness of sand | Use molding materials with higher refractoriness |
| Excessive metal pouring temperature | Reduce the pouring temperature of molten metal |
| Metal adhering to sand grains | Ensure proper sand and metal separation |
#13 Fins and burns
Fins, also called flash, are common casting defects where unwanted extra material attaches to the casting. They form as thin metal sheets at the edges of the parting line and are waste material that turns into slag after melting.
| Causes | Remedies |
|---|---|
| Incorrect combination of mold and core | Ensure accurate assembly of the mold and cores |
| Insufficient mold weight | Ensure the top part of the mold is heavy enough for a tight fit |
| Improper clamping of the flask | Use proper clamping techniques to secure the flask |
#14 Metal penetration
This occur when molten metal enters gaps in the molding sand. It causes a rough and uneven surface finish. This unevenness is clearly visible. This is mainly due to the coarse grain size. This disrupts the flow of liquid metal. It results in a rough, irregular surface. And in turn it reduces the casting’s appearance.
| Causes | Remedies |
|---|---|
| High permeability and low-strength sand | Use high-strength sand with a small grain size and low permeability |
| Soft or improper ramming of sand | Ensure the sand is rammed firmly and evenly |
| Lack of mold wash | Apply a mold wash to create a barrier against metal penetration |
#15 Cut and washes
Cuts and washes are casting defects where excess metal forms due to molten metal eroding the molding sand. Cuts are low projections that taper along the drag face. These defects occur when the sand lacks strength to resist erosion and typically appear on the side with higher liquid metal pressure.
| Causes | Remedies |
|---|---|
| High-velocity flow of molten metal causing excess metal to pass through the gate | Ensure proper design of the gating system |
| Excess metal in certain regions of the mold | Strengthen the core and mold |
| Inadequate binder in facing and core sand | Add more binder to improve sand strength |
#16 Drops
This happens when the casting is still liquid. Drop-casting defects occur when cracks form on the sand’s surface, causing sand to fall into the molten metal. This creates uneven bumps on the casting and makes the metal surface dirty and unusable.
| Causes | Remedies |
|---|---|
| Low sand strength | Use stronger sand. |
| Poor fluxing of molten metal | Clean contaminants with proper fluxing. |
| Soft ramming | Use high-strength sand and ensure proper hard ramming. |
| Weak reinforcement of sand projections in the cope | Support sand projections with gaggers or nails. |
#17 Run out
Run-out happens when liquid metal leaks from the mold, leaving the casting incomplete or partially missing.
| Causes | Remedies |
|---|---|
| Faulty mold | Replace defective molds before casting. |
| Incorrect flask design | Ensure precise casting mold design. |
#18 Hot tear or crack
A hot tear or crack occurs when residual tensile stress during cooling causes the casting to fail. These defects often appear during metal solidification. Some cracks are visible to the naked eye, while others need specialized equipment to detect. They usually show up as irregular or linear patterns on die-cast parts.
| Causes | Remedies |
|---|---|
| Thermal contraction on the metal surface | Eliminate residual stress in the casting metal. |
| Improper metal pouring temperature | Maintain uniform thickness throughout the material. |
| Incorrect gate placement or solidification process | Follow standard solidification methods and ensure proper gate placement. |
#19 Cold shut
A cold shut is a crack with rounded edges that forms when two streams of molten metal from different gates meet at low temperatures and fail to fuse. This defect is visible to the naked eye and creates a weak spot, often leading to the casting being rejected.
| Causes | Remedies |
|---|---|
| Faulty gating system design | Optimize the gating system for proper metal flow. |
| Low liquid metal temperature | Improve gas permeability of the mold. |
| Insufficient fluidity of molten metal | Increase the metal’s pouring temperature. |
#20 Hot or hard spot
Hot spots, or hard spots, occur when a specific area of the casting cools faster than the surrounding material, resulting in a harder section on the casting.
| Causes | Remedies |
|---|---|
| Rapid cooling of a specific area compared to the surrounding material | Use proper cooling methods. |
| Inappropriate chemical composition of the metal | Adjust the metal’s chemical composition. |
Conclusion
I hope I’ve covered everything about “Casting Defects” in this article. If I missed anything or if you have any questions, feel free to leave a comment. If you found this article helpful, please share it with your friends!
