Die casting is an efficient process for manufacturing complex metal parts, but defects such as porosity, shrinkage, and cold shuts can lead to quality issues and increased costs. This article outlines common defect types, causes, and prevention strategies to help you optimize the die casting process.
Table of Contents
Common Die Casting Defect Types
Understanding common die casting defects is crucial for manufacturers to produce high-quality components. These defects can appear in various forms, each with unique causes and characteristics that affect the component’s quality, functionality, and appearance. Below are the 22 common defect types.
Pinholes
Pinholes are small circular voids formed in solidified metal due to trapped bubbles, which may appear on the surface or inside the metal. These pinholes not only affect the material’s strength but also have a negative impact on appearance.
Causes: The formation of pinholes is mainly due to dissolved gases (such as hydrogen) in the molten metal being released during cooling. High humidity or improper melting processes can exacerbate this phenomenon, especially in aluminum die casting.
Prevention and Solutions: To reduce the occurrence of pinholes, the following measures can be taken:
- Use rotary degassing technology to remove gases from the molten metal.
- Strictly control the melting temperature between 650-700°C.
- Ensure the mold is completely dry before use.
Subsurface Blowhole
Subsurface blowholes are gas pockets formed beneath the surface of the casting, which are not easily detected due to their hidden nature. This defect can lead to internal weaknesses in the casting, potentially causing fatigue failure and other issues.
Causes: Subsurface blowholes are typically formed because gases trapped in the molten metal during solidification fail to escape completely. In zinc die casting, rapid cooling exacerbates this phenomenon, and improper venting design is a primary cause of such defects.
Prevention and Solutions: To effectively reduce the occurrence of subsurface blowholes, the following measures can be taken:
- Optimize the venting system design to ensure smooth gas discharge.
- Use computational fluid dynamics (CFD) simulation to model metal flow, predicting and resolving gas entrapment issues in advance.
- Regularly inspect the mold’s ventilation to ensure unobstructed venting channels.
Open Holes
Open holes are visible pores on the casting surface, similar to blowholes. Although easy to detect, these defects can negatively impact the casting’s aesthetics and corrosion resistance.
Causes: Open holes are formed mainly due to gases (such as oxygen or hydrogen) released from the molten metal during cooling. Additionally, excessively high injection speeds or improper melting processes can cause this issue, particularly common in aluminum die casting.
Prevention and Solutions: To reduce the occurrence of open holes, the following measures can be taken:
- Perform metal degassing to remove gases from the molten metal.
- Control injection parameters and optimize speed profiles to ensure uniform mold filling with the metal.
- Avoid using excessive lubricants to reduce gas sources.
Open Shrinkage
Open shrinkage refers to surface depressions or voids formed during the solidification of the casting, directly exposed to the exterior. This defect not only affects the casting’s appearance but also adversely impacts dimensional accuracy.
Causes: The main cause of open shrinkage is insufficient compensation for the volume contraction of the metal liquid during solidification. Particularly in thick-walled aluminum parts, uneven cooling further exacerbates this issue.
Prevention and Solutions: To effectively reduce the occurrence of open shrinkage, the following measures can be taken:
- Optimize part wall thickness during the design phase to ensure uniformity and reduce cooling variations.
- Strategically place cooling channels to ensure sufficient compensation for shrinkage during the solidification of the metal liquid.
- Use simulation tools (such as casting simulation software) to predict potential issues in the solidification process and optimize the design in advance.
Closed Shrinkage
Closed shrinkage refers to internal voids or microscopic shrinkage pores formed inside the casting, which are invisible but significantly weaken structural integrity and strength.
Causes: The main cause of closed shrinkage is the volume reduction during metal solidification, where hot spots fail to receive timely metal replenishment. This phenomenon is particularly common in isolated thick sections in zinc die casting.
Prevention and Solutions: To effectively control the occurrence of closed shrinkage, the following measures can be taken:
- Apply intensification pressure (such as secondary pressurization) to ensure sufficient filling of hot spots during solidification.
- Use computational fluid dynamics (CFD) simulation tools to predict solidification patterns, identifying and resolving potential issues in advance.
- Avoid isolated thick sections in the design phase and optimize the part’s geometry to promote uniform cooling and feeding.
Cuts and Washes
Cuts and washes are excess metal areas on the casting surface, often appearing as low protrusions. This defect is formed due to the molten metal eroding the mold surface under high pressure.
Causes: The main causes of cuts and washes are excessively fast metal flow under high pressure, leading to mold surface erosion. Additionally, insufficient mold coating in zinc die casting thin-walled parts exacerbates this issue.
Prevention and Solutions: To effectively prevent cuts and washes, the following measures can be taken:
- Optimize mold coatings to enhance erosion resistance.
- Reduce injection speed to minimize the impact of metal flow on the mold.
- Select mold materials with stronger corrosion resistance to improve mold lifespan and stability.
Fusion
Fusion refers to thin brittle shells formed on the casting surface after sand particles or impurities fuse with the metal. This defect not only affects the casting’s surface finish but may also reduce its overall quality.
Causes: The main cause of fusion is mold contamination or chemical reactions at high temperatures. In die casting, residues from release agents can also trigger this issue, especially under high-temperature melting conditions.
Prevention and Solutions: To effectively reduce the occurrence of fusion defects, the following measures can be taken:
- Strictly clean the mold to ensure no contaminants remain on the surface.
- Use high-quality release agents to reduce the possibility of chemical reactions.
- Regularly inspect pollution sources in the production process and promptly eliminate potential hazards.
Run Out
Run out refers to liquid metal leaking from the mold, resulting in incomplete or missing shapes in the casting. This defect not only wastes material but also significantly affects production efficiency.
Causes: The main causes of run out are poor mold sealing or insufficient clamping force, particularly common in the high-pressure phase of aluminum die casting. Inaccurate mold alignment or issues with the sealing system can exacerbate this phenomenon.
Prevention and Solutions: To effectively prevent run out defects, the following measures can be taken:
- Enhance the mold’s clamping force to ensure secure closure during the high-pressure phase.
- Regularly check mold alignment to ensure precise matching of all mold parts.
- Use automated sealing systems to improve sealing reliability and consistency.
Swells
Swells refer to smooth bulges on the vertical surfaces of the casting, causing shape changes. This defect not only affects the casting’s appearance but may also adversely impact dimensional accuracy.
Causes: The main causes of swells are mold deformation under high pressure or uneven pressure distribution. In zinc die casting, rapid filling further exacerbates this issue by increasing local pressure on the mold.
Prevention and Solutions: To effectively prevent swell defects, the following measures can be taken:
- Strengthen mold design to enhance compressive strength and reduce deformation risks.
- Control filling speed to avoid excessive local pressure from rapid filling.
- Ensure even pressure distribution and optimize injection parameters to reduce stress concentration.
Drops
Drops refer to protrusions formed on the casting surface after sand particles or impurities fall into the molten metal, typically appearing on the upper surface. This defect affects the casting’s appearance and surface quality.
Causes: The main cause of drops is contamination from loose particles in the mold or particle detachment due to vibration during production. Although less common in die casting, it still requires special attention, especially in high-precision casting production.
Prevention and Solutions: To effectively prevent drop defects, the following measures can be taken:
- Pre-treat the mold to remove loose particles and ensure a clean mold surface.
- Use filtration systems during molten metal flow to remove possible impurities.
- Reduce vibration sources on the production line to lower the risk of particle detachment.
Rat Tails, Veins and Buckles
Rat tails, veins, and buckles are cracks or lines formed on the casting surface due to mold buckling (bending deformation), with severe cases showing obvious wrinkles. These defects not only affect the casting’s appearance but may also reduce its surface quality.
Causes: The main causes of these defects are mold deformation due to thermal stress at high temperatures, especially when the mold base is covered by molten metal. This phenomenon is further exacerbated in high-temperature environments.
Prevention and Solutions: To effectively prevent these defects, the following measures can be taken:
- Evenly heat the mold to avoid local overheating leading to thermal stress concentration.
- Optimize the cooling system to ensure uniform mold temperature distribution and reduce the possibility of thermal deformation.
- Select mold materials with higher thermal stability to improve resistance to deformation in high-temperature environments.
Metal Penetration
Metal penetration refers to liquid metal entering tiny gaps on the mold surface and solidifying, resulting in rough textures on the casting surface. This defect significantly affects the casting’s surface finish and appearance quality.
Causes: The main causes of metal penetration are mold surface damage or excessive pressure, causing liquid metal to seep into mold gaps. This phenomenon is particularly common in zinc die casting, especially under high-pressure conditions.
Prevention and Solutions: To effectively reduce the occurrence of metal penetration, the following measures can be taken:
- Regularly maintain the mold to repair surface damage and ensure a smooth mold surface without cracks.
- Control injection pressure to avoid excessive pressure leading to metal penetration into mold gaps.
- Apply high-quality surface coatings to enhance the mold surface’s resistance to penetration.
Hot Tear/Crack
Hot tear/crack refers to branched cracks formed in the casting during solidification due to shrinkage stress. This defect significantly weakens the casting’s strength and reliability, especially in high-stress applications.
Causes: The main causes of hot tears are internal tension from shrinkage stress and uneven cooling during metal solidification. In aluminum die casting, rapid cooling further exacerbates this issue, increasing the probability of cracks.
Prevention and Solutions: To effectively prevent hot tear defects, the following measures can be taken:
- Implement progressive cooling and optimize cooling curves to reduce stress concentration during cooling.
- Optimize alloy composition by selecting materials with better crack resistance to lower crack sensitivity.
- Use simulation tools to predict stress distribution, identifying and resolving potential crack risks in advance.
Hot/Hard Spots
Hot/hard spots refer to localized areas in the casting with higher hardness than surrounding regions. These areas interfere with subsequent processing, increasing tool wear and reducing processing efficiency.
Causes: The main causes of hot/hard spots are changes in metal structure due to localized rapid cooling or temperature gradients. This phenomenon is typically caused by uneven cooling system design or improper cooling channel layout.
Prevention and Solutions: To effectively prevent the formation of hot/hard spots, the following measures can be taken:
- Design a uniform cooling system to ensure consistent cooling rates across all parts of the casting.
- Strategically place cooling channels to optimize cooling paths and reduce local temperature gradients.
- Monitor the casting’s heat distribution and promptly adjust cooling parameters to avoid localized rapid cooling.
Burn On
Burn on refers to attachments formed after chemical reactions between the metal and the mold surface. This defect not only affects the casting’s surface quality but also adversely impacts the mold’s service life.
Causes: The main cause of burn on is incompatibility between the metal and mold materials under high temperatures, especially during prolonged contact between the metal and mold. This phenomenon is particularly common in high-temperature casting processes.
Prevention and Solutions: To effectively prevent burn on defects, the following measures can be taken:
- Use heat-resistant coatings to reduce direct contact between the metal and mold surface, lowering the possibility of chemical reactions.
- Control the contact time between the metal and mold to avoid prolonged high-temperature contact leading to burn on.
- Select mold materials with better compatibility with the metal to reduce the occurrence of chemical reactions.
Cold Shut/Lap
Cold shut/lap refers to line-like or crack defects formed in the casting where metal flows fail to fully fuse. This defect not only affects the casting’s appearance but may also weaken its structural integrity.
Causes: The main causes of cold shut/lap are insufficient metal temperature or turbulence during flow, preventing complete fusion of metal streams. Additionally, poor flow path design exacerbates this issue.
Prevention and Solutions: To effectively prevent cold shut/lap defects, the following measures can be taken:
- Maintain appropriate metal temperature to ensure good fluidity and avoid fusion issues due to low temperature.
- Design laminar flow paths to reduce turbulence and ensure smooth metal flow.
- Use CFD (computational fluid dynamics) simulation tools to optimize metal flow paths, identifying and resolving potential flow issues in advance.
Misruns
Misruns refer to voids formed when the mold cavity is not completely filled with liquid metal. This defect results in incomplete castings that fail to meet design requirements.
Causes: The main causes of misruns are low metal temperature or poor fluidity, causing solidification during filling. Additionally, improper gating system design restricts metal flow, increasing the risk of misruns.
Prevention and Solutions: To effectively prevent misrun defects, the following measures can be taken:
- Increase the pouring temperature of the metal to ensure sufficient fluidity for filling the mold cavity.
- Optimize gating system design to reduce flow resistance and ensure smooth metal filling of the mold.
- Use simulation tools to analyze and optimize metal filling paths, identifying and resolving potential filling issues in advance.
Cold Shots
Cold shots refer to solid particles formed due to splashing during pouring, which embed in the casting, leading to surface defects and structural unevenness.
Causes: The main cause of cold shots is metal splashing during pouring, where formed metal particles embed in the casting surface after solidification. High-speed injection exacerbates splashing, increasing the probability of cold shots.
Prevention and Solutions: To effectively prevent cold shot defects, the following measures can be taken:
- Control injection speed to avoid excessive speed leading to metal splashing.
- Introduce filtration systems in the gating system to intercept solid particles formed by splashing.
- Ensure a smooth pouring process to reduce turbulence and splashing in metal flow.
Slag Inclusion (Scab)
Slag inclusion (scab) refers to metal scabs formed on the casting surface due to entrapped slag or oxides, resulting in uneven surfaces. This defect not only affects the casting’s appearance but may also weaken its mechanical properties.
Causes: The main causes of slag inclusion/scab are unclean molten metal containing uncleared slag or oxides. During pouring, these impurities are entrapped in the casting, forming surface defects.
Prevention and Solutions: To effectively prevent slag inclusion/scab defects, the following measures can be taken:
- Use filtration systems during molten metal flow to remove slag and oxides, ensuring metal purity.
- Perform degassing to reduce gas and impurity content in the molten metal.
- Strictly control the cleanliness of the molten metal to prevent impurities from entering the gating system.
Shift/Mismatch
Shift/mismatch refers to casting displacement defects caused by improper mold alignment. This defect affects the casting’s dimensional accuracy and appearance consistency, especially in production with high precision requirements.
Causes: The main causes of shift/mismatch are mold displacement during installation or operation, or errors in mold installation. Additionally, vibration during production can lead to mold misalignment, triggering shift issues.
Prevention and Solutions: To effectively prevent shift/mismatch defects, the following measures can be taken:
- Ensure precise mold alignment using high-precision alignment tools and methods.
- Incorporate locking mechanisms in mold design to prevent displacement during production.
- Adopt automated installation systems to reduce human error and improve mold installation precision and consistency.
Flash, Fin and Burrs
Flash, fin, and burrs refer to excess metal protrusions on the casting’s parting line. These defects not only affect the casting’s appearance but also increase the workload for subsequent processing.
Causes: The main causes of flash, fin, and burrs are insufficient mold clamping force or metal escaping the parting line under high pressure. Additionally, wear or damage to the parting line exacerbates these defects.
Prevention and Solutions: To effectively prevent the formation of flash, fin, and burrs, the following measures can be taken:
- Optimize the mold’s clamping force to ensure tight fitting of the parting line and prevent metal escape.
- Regularly maintain and repair the parting line to avoid sealing failures due to wear or damage.
- Control injection pressure to avoid excessive pressure leading to metal overflow from the parting line.
Warping
Warping refers to shape changes in the casting after solidification due to uneven stress or thermal residuals. This defect can cause dimensional deviations in the casting, affecting its assembly and functionality.
Causes: The main causes of warping are uneven stress distribution during cooling or the influence of residual heat. Differences in cooling rates lead to inconsistent shrinkage in different areas of the casting, triggering shape changes.
Prevention and Solutions: To effectively prevent warping defects, the following measures can be taken:
- Achieve uniform cooling by optimizing cooling system design to reduce stress concentration during cooling.
- Add support structures in the casting design to enhance shape stability.
- Use post-processing annealing to release residual stress and restore the casting’s shape stability.
Conclusion
To minimize casting defects, an integrated approach combining materials science and process engineering is required. The key to producing high-quality die castings lies in comprehensive defect control strategies throughout the entire production cycle. This includes in-depth design reviews, optimized process parameters, and strict inspection processes to ensure consistent product quality.
As one of the leading die casting manufacturers, Yonglihao Machinery adopts a systematic approach to deeply analyze the interrelationships between casting defects, such as porosity and shrinkage. We understand that adjustments to prevent one defect may influence the formation of others. By partnering with us, you will benefit from our expertise in defect prevention, advanced manufacturing technologies, and our commitment to delivering components that meet the most stringent quality standards.
Contact us immediately for professional part processing services to help your products achieve higher quality standards!
FAQ
What are the most common defect types in die casting?
Common defects in die casting include porosity (such as pinholes and subsurface blowholes), shrinkage (such as open and closed shrinkage), cold shuts, flash, and hot tears. These defects often stem from improper process parameters, such as temperature or pressure control issues, leading to compromised product strength and appearance. Early identification and optimization can effectively reduce their occurrence rates.
How to effectively prevent porosity defects?
The key to preventing porosity is optimizing molten metal treatment, such as using degassing techniques and vacuum-assisted die casting to remove dissolved gases. At the same time, ensure efficient venting systems to reduce gas entrapment.
What is the root cause of shrinkage defects?
Shrinkage defects are mainly caused by volume changes during metal solidification and uneven cooling, often occurring in thick-walled areas. Avoiding isolated thick sections in design and using simulation software to predict solidification patterns can significantly alleviate the issue.
How to avoid cold shut defects?
Cold shuts stem from insufficient fusion of metal flows, usually due to low temperatures or excessive turbulence. Prevention strategies include maintaining molten temperature, optimizing gating systems for laminar flow, and adjusting injection speeds.
What is the solution for flash defects?
Flash is caused by insufficient clamping force or mold wear, leading to excess metal overflow. Solutions include regular mold maintenance, precise clamping force calculations, and injection pressure control.