Chapter 1: Introduction
Sand foundry defect, particularly gas porosity, represents one of the most prevalent and technically challenging defects encountered in metal casting processes. Through decades of production experience and systematic investigation, the foundry industry has come to recognize that gas porosity defects manifest in diverse forms with fundamentally different formation mechanisms. This complexity renders gas porosity a persistent technical challenge within the casting community, warranting extensive research attention and practical investigation.
My research focuses on the cylinder block produced on the static pressure molding line, with particular emphasis on understanding the formation mechanism of invasive gas porosity. Through comprehensive analysis of sand processing, core-making, melting, and tooling systems, I have identified the root causes of this sand foundry defect and implemented targeted countermeasures that successfully reduced the scrap rate from 23% to approximately 5%.
1.1 Research Progress and Classification of Gas Porosity in Iron Castings
Gas porosity in castings has been systematically studied for decades. The American Foundry Society classifies gas porosity formation as fundamentally resulting from localized pressure exceeding the metal’s static pressure during solidification. British foundry expert Beeley categorized gas defects into internal porosity, surface porosity, gas pockets, surface or subsurface pinholes, and interdendritic cavities, with formation mechanisms divided into two principal groups: gases actually entrapped during pouring, and gases precipitated during metal cooling.
Gas porosity can be classified according to formation mechanism into five distinct types: invasive gas porosity, entrained gas porosity, precipitated gas porosity, endogenous reaction gas porosity, and exogenous reaction gas porosity. Each type exhibits unique visual characteristics and requires specific preventive approaches.
1.2 Sources and Existing Forms of Gases in Castings
Gases within castings exist in three fundamental forms: solid solutions, compounds, and gas porosity. When gas atoms dissolve in the metal at an atomic level, they form solid solutions. When gas elements exhibit greater affinity for specific metal constituents, chemical compounds form. When gas content exceeds solubility limits or undissolved gases are entrapped, molecular-state bubbles form within the liquid metal, potentially creating gas defects if not expelled before solidification completes.
1.3 Main Factors Contributing to Sand Foundry Defect Formation
My analysis of the sand foundry defect formation reveals six principal contributing categories that require systematic examination:
- Sand system parameters – including grain fineness, clay content, moisture content, and additive concentrations that fundamentally affect permeability and gas evolution characteristics
- Melting parameters – encompassing metal fluidity, pouring temperature, fill rate, gas content, cleanliness, and trace element concentrations
- Core parameters – involving core gas evolution, permeability, and exhaust channel effectiveness in complex cored castings
- Gating system design – determining filling characteristics, turbulence, and gas entrainment potential
- Casting characteristics – including casting geometry, complexity, and material requirements that influence gas porosity susceptibility
- Environmental factors – primarily atmospheric humidity, which significantly affects sand foundry defect occurrence rates
1.4 Research Significance and Scope
The cylinder block represents a critical component of the diesel engine, and its quality directly determines engine performance characteristics. As a thin-walled casting with complex external geometry and internal cavity configurations requiring numerous sand cores, the cylinder block presents substantial technical complexity. Through years of production experience, our company reduced the overall casting scrap rate from initial production levels of approximately 40% to roughly 10%.
Analysis of current reject composition reveals that sand inclusion and gas porosity constitute the predominant defects, with gas porosity accounting for 45-55% of total rejects and contributing to approximately 70% of welding repairs. Effective control of gas porosity would significantly reduce overall casting losses and generate substantial economic benefits.
My research objectives encompass: determining the primary sand foundry defect mechanisms through visual characteristic analysis combined with theoretical investigation; implementing targeted experimental programs under existing production conditions; and reducing gas porosity scrap to below 3% through systematic process optimization.
| Experiment No. | Experiment Title | Experiment Content | Remarks |
|---|---|---|---|
| 1 | Sand system optimization | Moisture control; mulling parameter optimization; high-quality bentonite and coal dust substitutes | Production validation |
| 2 | Exhaust system improvement | Modified tooling; additional vent channels | Production validation |
| 3 | Core sand optimization | Optimal resin and hardener additions | Laboratory testing |
| 4 | Core-making optimization | Hardening time versus gas evolution relationship | Statistical analysis |
| 5 | Tempering parameter optimization | Tempering time versus gas evolution for core No. 5 | Process validation |
| 6 | Pouring temperature study | Temperature effects on gas porosity and sand adhesion | Comparative trials |
Chapter 2: Sand Foundry Defect Analysis of Cylinder Block Castings
2.1 Casting Process Description
The cylinder block serves as a vital component in the diesel engine series, requiring excellent mechanical properties and dimensional precision. With maximum external dimensions of 685 mm × 380 mm × 380 mm and weight of 95 kg, the casting is produced in HT250 gray cast iron with minimum wall thickness of only 4.5 mm, classifying it as a high-strength thin-walled complex casting.
Production utilizes the KW static pressure molding line employing two-box green sand high-pressure molding with one casting per mold. The push rod chamber and oil pump side are positioned in the drag, while the main water jacket side is in the cope. The casting requires eight sand cores, each individually coated with water-based paint and dried through a surface drying furnace. A semi-closed bottom gating system delivers the molten metal.
2.2 Sand Foundry Defect Status
Gas porosity defects have persistently affected cylinder block production since the KW line commissioning phase. Initially, scrap rates reached 35-40%, with gas porosity constituting 40-50% of all defects. Preliminary countermeasures included modifying to double-row gating systems and adding vent pins to ensure adequate exhaust, which brought gas porosity scrap under initial control.
From 2002, systematic improvement campaigns yielded gradually accumulating experience, though results remained inconsistent. The annual cumulative scrap rate decreased to approximately 9% by 2006. Monthly gas porosity data from 2006 indicates significant variability, demonstrating that sand foundry defect control remained relatively imprecise.
| Month | Gas Porosity Ratio (%) | Gas Porosity Scrap Rate (%) |
|---|---|---|
| January | 58.6 | 4.72 |
| February | 64.8 | 5.18 |
| March | 49.5 | 3.14 |
| April | 35.7 | 2.04 |
| May | 52.6 | 3.62 |
| June | 47.3 | 3.08 |
| July | 60.5 | 4.45 |
| August | 58.2 | 4.20 |
| September | 44.8 | 2.55 |
| October | 50.3 | 3.37 |
| November | 62.1 | 4.96 |
| December | 55.4 | 3.82 |
2.3 Theoretical Analysis of Gas Porosity Defects
My investigation of the sand foundry defect systematically integrates visual characteristic analysis, formation mechanism elucidation, and influencing factor identification. This comprehensive approach enables precise countermeasure development aligned with actual production conditions.
2.4 Visual Characteristic Analysis
Over 85% of cylinder block gas porosity defects concentrate on the main water jacket surface, with most becoming visible only after machining. Through extended observation, I identified two distinct categories based on visual characteristics:
Category I: Larger gas pores exhibiting circular, flattened, or pear-shaped morphology. These pores feature smooth walls showing metallic luster or oxide coloring. The pear-shaped pores’ narrow ends point toward the main water jacket core, indicating core-derived gas invasion. This category represents 30-40% of total gas porosity.
Category II: Deep irregular cavities located beneath and adjacent to the three 15 mm cavity exhaust pins on the water jacket surface. These are detectable only after machining, measuring 5-8 mm at the top but extending unusually deep, with irregular walls. The narrow bottom portions often connect directly to the main water jacket core. This category represents 50-60% of gas porosity defects.
| Classification | Quantity | Percentage |
|---|---|---|
| Total poured castings | 2,845 | — |
| Total gas porosity defects | 136 | 100% |
| Category I gas pores | 44 | 32.4% |
| Category II gas pores | 83 | 61.0% |
| Other gas porosity | 9 | 6.6% |
Both defect categories qualify as invasive gas porosity based on visual characteristics. Category I represents gas invasion during pouring or early solidification, with circular or flattened pores forming from gases that failed to float out before surface skin solidification. The pear-shaped pores result from water jacket core gas invasion during the liquid-state cooling period. Category II represents invasion at the exhaust pin locations, where thermal contact creates delayed solidification relative to surrounding areas.
2.5 Sand Foundry Defect Formation Mechanism
The concentration of the sand foundry defect on the main water jacket surface reflects the combined effects of gating system design and solidification sequence. The semi-closed bottom gating system delivers molten metal that reaches the water jacket surface at the lowest temperature and poorest fluidity. The extensive exhaust fins on this surface promote premature skin formation that hinders gas bubble floatation.
Gas invasion into the liquid metal occurs when the gas pressure at the mold surface exceeds the opposing pressure from the metal. This condition is expressed by:
$$P_{gas} > P_{metal} + P_{cavity} + P_{resistance}$$
where \(P_{gas}\) represents the residual gas pressure at the mold surface, \(P_{metal}\) represents the metal static pressure (\(P_{metal} = \rho g h\), with \(\rho\) as metal density and \(h\) as metal height), \(P_{resistance}\) represents the resistance to gas invasion depending on metal viscosity and surface tension, and \(P_{cavity}\) represents the gas pressure within the mold cavity.
Four principal conditions facilitate gas invasion into the cylinder block:
- High mold compaction: The KW line produces mold hardness reaching 90-95 units, creating high compaction that diminishes sand permeability and impedes gas escape
- Multiple sand cores: Eight cores create substantial gas-generating surface area while their exhaust channels may become blocked by core sand or molten metal
- Low metal temperature: The water jacket area receives the coolest metal with highest viscosity, making bubble rise difficult
- Premature skin formation: Exhaust fins and thin sections solidify quickly, limiting gas escape pathways
2.6 Main Influencing Factors
My systematic analysis identifies the following cause-and-effect relationships for invasive gas porosity:
Permeability factors: Mold sand clay content (higher clay reduces permeability), mold/core exhaust channel blockage, inadequate sand mulling uniformity, excessive mold compaction, and excessive sand fineness all contribute to poor gas escape.
Gas evolution factors: Core resin content represents the primary gas source, with incomplete core hardening and inadequate coating drying increasing gas generation. Mold sand moisture content, bentonite content, and effective coal dust content similarly influence total gas evolution.
Metal fluidity and static pressure factors: Carbon equivalent, sulfur content, pouring temperature, pouring rate, and effective static head all affect gas bubble buoyancy and escape capability.
Chapter 3: Sand System and Molding Process Optimization
3.1 Molding Line and Sand Processing System
The cylinder block is produced on a KW static pressure molding line employing airflow pre-compaction combined with high-pressure multi-piston squeezing and counter-pressure, achieving mold surface hardness exceeding 90 units with rational hardness distribution. The system includes four sand hoppers, specifically sized to accommodate the 900 mm × 600 mm × 300 mm molding boxes that cycle continuously during production.
The sand processing system comprises a comprehensive loop for system sand, recovery, and cooling. The sand mixture combines Hunan and local sands with 100% recycled material, using sodium-activated bentonite and coal dust as additives. Sand processing includes old sand recovery, mulling through an EIRICH mixer, electronic proportioning scales, pneumatic water supply, dual-disc coolers, bucket elevators, belt conveyors, and an automatic sand properties testing instrument.
3.2 Sand Quality Control Relationship to Sand Foundry Defect
Since the primary sand foundry defect was identified as invasive gas porosity, mold sand permeability and gas evolution characteristics became critical control parameters. The high compaction pressure of the molding line, combined with inadequate inherent permeability and excessive gas generation, creates residual gas pressure that drives invasion. Additionally, high gas evolution coupled with low permeability increases mold explosion risk during pouring.
During 2005-2006, the sand system exhibited significant instability with total clay content of 13-14%, permeability of only 70-90, and mold explosion rates exceeding 10% during cylinder block pouring. Gas porosity scrap correspondingly reached 4.75%, substantially above the annual average of 3.79%. Notably, these periods showed moderate green compression strength of approximately 0.09 MPa, confirming permeability as the limiting factor.
To optimize the sand system for adequate strength properties while achieving higher permeability and lower gas evolution, I implemented systematic improvements based on factor analysis:
3.2.1 Sand Moisture Control
Controlling moisture begins with stabilizing return sand moisture. The primary causes of return sand moisture variation include: un-poured sand retaining moisture, varying sand-to-metal ratios between different castings produced on the line, different pouring temperatures affecting heat input, and extended cooling periods during shift changes or equipment downtime.
Effective return sand cooling control is achieved by calibrating the dual-disc cooler automatic instrumentation. The humidity set-point matrix enables maintaining outlet moisture at 1.8-2.2%. Excessively wet return sand is diverted to waste disposal. The temperature-humidity calibration table follows:
| Temperature (°C) | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | 140 |
|---|---|---|---|---|---|---|---|---|---|---|
| Humidity set point | 2.8 | 2.6 | 2.4 | 2.2 | 2.0 | 1.8 | 1.6 | 1.4 | 1.2 | 1.0 |
3.2.2 Bentonite and Mulling Process Control
High-density molding requires elevated bentonite content and extended mulling for proper development. The previous dry-mulling process allowed powdered materials to segregate at mixer dead zones, forming bentonite agglomerates that increased moisture requirements without contributing to bond strength.
I converted to a wet-mulling process where water is added first, followed by dry additives, enabling better dispersion and significantly reducing the homogenization period. Through systematic trials, three wet-mulling variants were evaluated:
| Parameter | Variant 1 | Variant 2 | Variant 3 | Original dry mulling |
|---|---|---|---|---|
| Moisture (%) | 3.60 | 3.60 | 3.55 | 3.70 |
| Compactability (%) | 45 | 42 | 40 | 46 |
| Green compression strength (MPa) | 0.118 | 0.121 | 0.126 | 0.112 |
| Shear strength (MPa) | 0.040 | 0.041 | 0.044 | 0.035 |
| Permeability | 92 | 95 | 105 | 78 |
| Appearance | Few sand agglomerates | Few sand agglomerates | No visible agglomerates, smooth feel | Obvious agglomerates, rough |
Additional water addition experiments established three fundamental principles: wet mulling substantially outperforms dry mulling; pre-water should exceed two-thirds of total water; and post-coarse-water mixing time must be at least 10 seconds. The optimized parameters achieved significant sand property improvements with reduced bentonite and coal dust consumption.
| Parameter | Before optimization | After optimization |
|---|---|---|
| Clay content (%) | 13.5 | 11.8 |
| Bentonite addition (kg/batch) | 68-72 | 58-62 |
| Coal dust addition (kg/batch) | 20-24 | 18-20 |
| Green compression strength (MPa) | 0.09-0.13 | 0.10-0.14 |
| Moisture (%) | 3.8-4.2 | 3.2-3.6 |
| Permeability | 70-90 | 90-110 |
| Mold explosion rate (%) | 3-5 | 1-2 |
3.2.3 Quality Bentonite Selection
Bentonite quality directly influences sand strength, clay content, and moisture requirements. Evaluation of locally sourced Yunnan bentonite compared to the previous Sichuan bentonite revealed significant differences in critical properties. The Yunnan bentonite exhibits superior methylene blue adsorption and wet compression strength at lower addition rates, permitting reduced clay content and improved permeability while maintaining adequate strength development.
| Material | Wet compression strength (MPa) | Methylene blue adsorption (g/100g) |
|---|---|---|
| Sichuan bentonite | 0.52-0.64 | 30-35 |
| Yunnan bentonite | 0.70-0.85 | 38-45 |
| Return sand property | With Sichuan bentonite and coal dust | With Yunnan bentonite and coal dust |
|---|---|---|
| Clay content (%) | 13.2 | 12.5 |
| Effective bentonite (%) | 7.8 | 8.2 |
| Gas evolution (mL/g) | 25.4 | 23.1 |
3.2.4 Powder Application as Coal Dust Substitute
Despite successful optimization, total clay content remained near 12%, and mold explosion rates persisted at approximately 2.5%. To further reduce clay loading and gas evolution while maintaining anti-sand-adhesion properties, I evaluated Powder as a coal dust substitute. Powder combines coal dust, asphalt, and starch-based anti-sand-adhesion components, with notably higher lustrous carbon formation capability (12-15%) compared to natural coal dust (2-4%).
The starch component contributes several beneficial effects: improved permeability, enhanced hot-wet tensile strength and reduced thermal compressive stress, decreased moisture sensitivity, reduced moisture requirements with improved moldability, increased resistance to air drying, and improved shakeout characteristics through reduced high-temperature strength.
Following initial trials, I adjusted the Powder addition rate from 3.0% to 2.5% to resolve minor sand adhesion issues. After one month of optimization, the following improvements were confirmed:
| Parameter | With coal dust | With Powder |
|---|---|---|
| Bentonite addition (kg/batch) | 58-62 | 48-52 |
| Green compression strength (MPa) | 0.10-0.14 | 0.10-0.13 |
| Moisture (%) | 3.2-3.6 | 2.8-3.2 |
| Permeability | 90-110 | 100-120 |
| Mold explosion rate (%) | 1-2 | Below 0.5 |
| Scrap rate from mold damage (%) | 1.2 | 0.5 |
| Gas porosity scrap rate (%) | 3.5 | 1.8 |
3.3 Molding Process Improvements for Sand Foundry Defect Prevention
The KW molding line produces mold hardness of 90-95 units on the face and 70-80 units on the sides. Initial attempts to reduce molding pressure were unsuccessful because lower hardness caused mold wall movement defects. I therefore focused on enhancing ventilation capability through process modifications:
3.3.1 Exhaust Pin Layout Modification
Analysis of the second category of gas porosity identified the coarse 15 mm vent pins on the water jacket surface as contact thermal centers that delayed solidification while the surrounding metal had already formed a solid shell, allowing core gas to invade the still-liquid region. I eliminated three 15 mm vent pins near the sand outlet holes and added three exhaust fins. To compensate for the removed vents, I added three 6 mm exhaust pins on the cylinder head surface connected through bridges. This modification reduced category II gas porosity from 50-60% to below 20% of total gas defects.
3.3.2 Non-penetrating Vents in the Cope Back
To compensate for high mold compaction limiting permeability, I drilled 12 non-penetrating 8 mm vents in the mold back using a multi-spindle drill. These vents penetrated to 10-15 mm from the mold working surface. The critical distance ensures vent effectiveness: vents must penetrate the moisture condensation zone that forms beneath the mold surface during pouring. Insufficient depth cannot breach this condensation barrier, rendering vents ineffectual for gas escape before metal skin formation.
3.3.3 Improved Exhaust System on the Water Jacket Surface
Further optimization of the water jacket surface exhaust system was implemented through modification of a secondary tooling set. This involved machining additional exhaust slots on the water jacket surface covering the main water jacket core region. The improvement demonstrated effective reduction in sand foundry defect incidence, validating the systematic approach to mold ventilation enhancement.
3.3.4 Backup Mold Vent Drilling for Exhaust Management
In addressing the sand foundry defect, I discovered that during pouring, core gas from the core-mold exhaust system becomes blocked when the core print fills with metal. This occurs when the 8 mm core print exhaust channels become sealed by molten metal invasion, forcing the core gas to pass through the liquid metal, creating invasive gas porosity. To mitigate this, I added semicircular flame-arresting grooves on both cope and drag tooling at the core print locations. This modification significantly prevented molten metal penetration into the exhaust channels, reducing the core print metal penetration rate from 30% to below 5%.
Chapter 4: Core Sand and Core-Making Process Optimization
4.1 Furan Resin Sand and Hot Box Core-Making
The cylinder block requires eight sand cores designated as follows: core No. 1 for push rod chamber, core No. 2 for cylinder bore, cores No. 3 and No. 4 for end closures, core No. 5 for main water jacket, core No. 6 for oil filter, core No. 7 for oil pump, and core No. 8 for core sand removal. All cores are produced using furan resin sand with the hot box method. Cores receive individual water-based coating application followed by surface drying furnace treatment. After drying, cores are transferred to storage or directly to the molding line via conveyor systems.
The core manufacturing process flow follows: sand drying → sand mixing → core shooting → core finishing → coating application → tempering → final cleaning. The raw sand is a blend of Hunan and local sands at 50/140 or 40/70 AFS grain fineness with angular particle shape, requiring 1.2% resin and 30-50% hardener relative to resin content.
4.2 Core Sand Formulation Optimization
Core gas evolution is primarily determined by resin content. My testing established the relationship between resin addition and gas evolution at 850°C for 3 minutes:
| Resin addition (%) | Gas evolution (mL/g) | Core damage rate during stripping (%) |
|---|---|---|
| 1.5 | 265 | <1 |
| 1.3 | 240 | 1-2 |
| 1.2 | 230 | 1-2 |
| 1.0 | 205 | 3-5 |
| 0.8 | 180 | >10 |
Based on these results, I optimized the resin addition from 1.5% to 1.0-1.2% (by sand weight), establishing the lower limit as the standard operating condition. To maintain adequate core strip strength with reduced resin, I increased hardener to the middle-to-upper process range and implemented improved handling practices: mandated careful core handling, prohibited core stacking to prevent damage, converted resin addition from volumetric to gravimetric measurement to improve accuracy, and established strict muller maintenance procedures for consistent mixing quality. Core scrap rate decreased from 5% to approximately 2%.
4.3 Core-Making Process Optimization
For a given sand formulation, core hardening temperature and time directly affect core gas evolution. The cylinder bore core and the main water jacket core require specific process windows to achieve proper curing. I conducted experiments on the main water jacket core at a constant core box temperature of 240°C, air pressure of 0.5 MPa, varying hardening times:
| Hardening time (s) | Gas evolution (mL/g) |
|---|---|
| 25 | 19.8 |
| 35 | 26.5 |
| 45 | 27.8 |
| 60 | 28.5 |
The data demonstrate that hardening times below 35 seconds produce markedly higher gas evolution, while times above 35 seconds yield acceptable performance. With productivity constraints, I set 35 seconds as the lower limit. However, operator deviations during evening and night shifts led to episodes of elevated gas porosity. To eliminate this variability, I locked the hardening time in the programmable logic controller to a minimum of 35 seconds.
For the main water jacket core, the long mandrel creating the main gas channel occasionally removed prematurely, causing sand collapse that blocked the exhaust passage. Inspection revealed blockage rates reaching 25%. I established a practice requiring operators to delay mandrel extraction until at least 45 seconds of hardening, verified by checking each core’s exhaust channel. This corrective approach substantially reduced core-related sand foundry defect contributions.
4.4 Coating Application and Drying Optimization
Core coatings serve multiple functions including sand adhesion prevention; however, coating composition and drying state significantly influence gas evolution during pouring. The coating gas sources include organic binders and suspending agents, residual moisture from incomplete drying, and re-absorbed moisture during storage. If these volatile components are not eliminated, they generate gas that may invade the casting surface, creating invasive gas porosity.
For the main water jacket core, the original drying process specified 150-170°C for 30-40 minutes. I evaluated extended drying periods:
| Drying time (min) | Temperature (°C) | Coating gas evolution (mL/g) | Core surface color |
|---|---|---|---|
| 30 | 160 | 18 | White |
| 60 | 160 | 13 | Light yellow |
| 90 | 160 | 12 | Light yellow |
Extending drying time to 60 minutes reduced coating gas evolution by 5 mL/g while maintaining practical productivity. I implemented several additional standards: coated core storage limited to 3 days maximum with re-drying required for extended storage, routine inspection of coating and drying operations across all shifts, specification of light yellow surface color as the acceptance criterion for the main water jacket core, and procedural requirements mandating finger coverage of core exhaust channel openings during dip coating to prevent coating ingress into gas passages.
Chapter 5: Melting and Pouring Process Optimization
5.1 Iron Melting and Pouring System
The foundry employs a water-cooled hot-blast cupola (acid-lined) paired with coreless induction holding furnaces in duplex melting operation. The cupola features continuous melting capability over extended campaigns, while the induction furnaces provide superheating, holding, homogenization, and composition adjustment. This configuration ensures consistent iron temperature and chemistry suitable for high-strength cylinder block production.
The melting process flow follows: ingredient preparation → cupola melting → electric furnace homogenization → spectrographic analysis → inoculation → pouring. Process controls require chemical analysis each shift, temperature measurement every 2 hours, and temperature measurement for each ladle after extended idle periods. Pouring temperature is measured at a depth exceeding 100 mm following inoculation treatment.
5.2 Melting and Pouring Optimization for Sand Foundry Defect Reduction
Since over 85% of the sand foundry defect is invasive gas porosity, optimization efforts focus on increasing metal static pressure during pouring and reducing liquid metal viscosity to enhance gas bubble escape:
5.2.1 Increased Pouring Rate and Static Pressure
The original gating system was classified as open bottom-pour with the following cross-sectional relationships: sprue area of 1.54 cm², runner area of 2.85 cm², and in-gate total area of 4.20 cm². The area ratio \( \sum A_{sprue} : \sum A_{runner} : \sum A_{ingate} = 1 : 1.85 : 2.73 \) characterized an open system with a restrictive sprue limiting pouring rate. The pour time of approximately 55 seconds was excessive, increasing gas entrainment potential.
I enlarged the minimum sprue diameter from 14 mm to 16 mm, resulting in a modified area ratio of \(1 : 0.71 : 1.05\), creating a semi-closed bottom-pour system. This modification reduced pouring time to 42-46 seconds, a 9-second improvement. Additionally, I established pouring standards requiring the sprue to remain filled throughout the pour, maintaining maximum static pressure while avoiding overflow.
| Condition | Area ratios | Pour time (s) | System type |
|---|---|---|---|
| Original | 1 : 1.85 : 2.73 | 55 | Open bottom-pour |
| Modified | 1 : 0.71 : 1.05 | 42-46 | Semi-closed bottom-pour |
5.2.2 Pouring Temperature Optimization
I conducted controlled trials correlating pouring temperature with gas porosity and sand adhesion:
| Pouring temperature (°C) | Castings poured (units) | Gas porosity (units) | Gas porosity rate (%) | Severe sand adhesion (units) | Severe sand adhesion rate (%) |
|---|---|---|---|---|---|
| 1360-1380 | 860 | 26 | 3.0 | 2 | 0.2 |
| 1380-1400 | 920 | 14 | 1.5 | 4 | 0.4 |
| 1400-1420 | 890 | 7 | 0.8 | 18 | 2.0 |
Higher pouring temperatures improve fluidity and facilitate gas bubble escape, reducing the sand foundry defect. However, excessive temperatures increase sand adhesion. The optimal range is 1380-1400°C. To maintain this range despite the increased sand adhesion tendency, I supplemented the upper mold with local alcohol-based coating application.
5.2.3 Chemical Composition Control
High-carbon-equivalent iron exhibits lower viscosity and surface tension, promoting better fluidity. The cylinder block specification for HT250 requires the following chemical composition:
| Element | C | Si | Mn | P | S |
|---|---|---|---|---|---|
| Range (%) | 3.10-3.30 | 1.90-2.20 | 0.70-0.90 | ≤0.08 | ≤0.10 |
The carbon equivalent calculation follows:
$$CE = C + \frac{Si}{3} + \frac{P}{3} = 3.20 + \frac{2.05}{3} + \frac{0.06}{3} = 3.90$$
To enhance metal fluidity while preserving mechanical properties, I increased carbon and silicon targets within the specification range, to 3.25% and 2.00% respectively. Sulfur significantly impairs fluidity by forming high-melting manganese sulfide inclusions. The cupola operation inherently introduces sulfur from coke, with typical values of 0.08% S. Without desulfurization capability in the acid cupola, I implemented two primary controls: optimized coke-to-metal ratio at 1:8 minimizing charge coke while maintaining melt temperature, and controlled bed height at 1500-1600 mm to prevent excessive sulfur pickup. These measures collectively improved metal fluidity and gas bubble escape, directly addressing the invasive sand foundry defect mechanism.
Chapter 6: Conclusions
Through systematic theoretical analysis and practical production implementation, this research has established comprehensive understanding of the sand foundry defect affecting cylinder block castings produced on the KW static pressure molding line and developed effective countermeasures to substantially reduced gas porosity scrap rates. The key conclusions from my work are:
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The gas porosity defects affecting cylinder block castings are predominantly invasive-type, concentrated on the main water jacket surface, appearing in two visual forms: smooth-walled circular, flattened, or pear-shaped pores from gas invasion during pouring and early solidification; and deep irregular cavities beneath exhaust pin locations from later gas invasion through still-liquid regions adjacent to thermal contact centers.
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The primary sand system factors influencing the sand foundry defect are gas evolution and actual permeability. Through systematic control of return sand moisture, optimized wet mulling process, adoption of high-quality local bentonite, implementation of Powder as a coal dust substitute, addition of non-penetrating back vents, and refinement of exhaust pin layout, effective control of sand gas evolution and assured cavity exhaust integrity were achieved.
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Core-related factors proving decisive include core gas evolution and exhaust channel integrity. Optimized core sand formulations with minimal practical resin content, controlled hardening parameters, assured mandrel extraction timing, extended coating drying duration, and strict storage limitations effectively reduced core gas generation while maintaining adequate core quality for production requirements.
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Melting and pouring parameters requiring optimization are metal static pressure during pouring and liquid metal fluidity. Improvements to the gating system converting from open to semi-closed configuration, adherence to full-sprue pouring practice, optimized pouring temperature control at 1380-1400°C, and careful compositional management including carbon equivalent adjustment and sulfur control all contributed positively to minimizing sand foundry defect occurrence.
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The combined optimization program achieved significant progress over approximately two years of production validation:
| Year | Gas porosity scrap rate (%) | Gas porosity repair rate (%) |
|---|---|---|
| 2005 average | 3.24 | 9.5 |
| 2006 average | 3.61 | 8.8 |
| 2007 first half | 2.4 | 5.2 |
The theoretical foundation established in this work, combined with practical implementation experience, has effectively addressed the invasive gas porosity problem in cylinder block castings. The systematic methodology presented provides a framework applicable to similar sand foundry defect challenges. While complete elimination of all gas porosity defects has not been universally achieved, the significant improvements documented in this research have successfully demonstrated that understanding and controlling the key process parameters within the statistical framework presented enables effective management of invasive gas porosity in high-volume cylinder block production environments.