1. Introduction to Sand Casting Exhaust System Design

Sand casting is a widely used casting method, and the design of the exhaust system in sand casting is crucial for ensuring the quality of castings. This article focuses on the design of the exhaust system in sand casting, including its functions, classification, design principles, common defects, and preventive measures.

1.1 Functions of the Exhaust System

The exhaust system in sand casting serves multiple important functions:

Function	Description
Gas Removal	It expels various gases from the cavity, sand core, and those precipitated by the molten metal in the sand mold.
Pressure Reduction	Decreases the gas pressure inside the cavity during filling, enhancing the filling ability of the molten metal.
Removal of Impurities	Ejects the low-temperature molten metal and slag that initially fill the cavity.
Observation Aid	Facilitates observing the filling state and fullness of the molten metal in the cavity.

1.2 Classification of the Exhaust System

The exhaust system can be classified into two main categories:

Classification	Components
Mold Cavity Exhaust	Includes vents, exhaust needles, exhaust sheets, exhaust filter sheets, sand (mold) gaps, various top and side open and hidden risers, pressure edge risers, overflow risers, insulated and exothermic exhaust risers, exhaust plug exhaust, and ignition venting.
Sand Core Exhaust	Involves vents, exhaust needles, exhaust sheets, exhaust grooves, exhaust ropes, exhaust channels, filling of combustion media, materials to increase voids, sand (core) gaps, and exhaust plug exhaust.

1.3 Design Principles of the Exhaust System

When designing the exhaust system, several principles should be followed:

Principle	Details
Separate Exhaust	Cavity gas and sand core gas should be separately discharged, and direct exhaust from the casting body should be avoided.
Vent Location	Vent locations should not disrupt the feeding conditions of the casting and are usually not placed at hot spots or thick walls.
Material Selection	Appropriate molding and core-making methods and formulations with low gas evolution and good permeability should be chosen.
Optimal Placement	Vents are typically set at the highest point of the casting, where the molten metal reaches last, areas with more gas evolution and accumulation in the sand core, and ‘dead corners’ where gas is difficult to discharge.
Indirect Connection	Vents should preferably be indirectly connected to the cavity to prevent sand from falling into the cavity.
Complex Castings	For large and complex castings, exhaust channels should be designed in each sand core, and a main exhaust port should be set on the outer mold.
Vent Function	Different types of vents have specific functions, and their sizes and configurations should be designed accordingly.
Sufficient Area	The total exhaust area should be large enough to ensure smooth gas discharge, especially for certain complex castings.
Vent Size	The size of the vents should be appropriate to prevent gas holes and ensure smooth gas discharge.
Ignition Venting	Ignition venting during pouring is necessary to expel gas and reduce pressure.

2. Common Defects and Causes in Exhaust System Design

Inadequate attention to the exhaust system design in sand casting can lead to various defects in the castings.

Defect	Cause
Gas Pores, Sand Adhesion, and Burn-on	Unreasonable exhaust system layout, insufficient cavity and sand core exhaust, improper sand core structure design, and small total exhaust area.
Gas Pores and Sand Adhesion	Small effective vent area, unpenetrated exhaust needles, and high sand mold compactness with poor permeability.
Mold Lifting and Core Shooting Problems	‘Air trapping’ in the mold cavity during high-pressure wet mold and core box molding and core making, resulting in poor mold lifting and core shooting.
Gas Pores and Shrinkage Cavities	Insufficient venting in complex thin-walled parts with large gas evolution from the sand core, poor permeability, and improper drying.
Gas Pore Tendency	Failure to select appropriate sand molds and cores with low gas evolution before designing the exhaust system.
Gas Pores and Burn-on	Poor sealing between the sand core and core seat, and blocked exhaust channels by the molten metal.
Gas Pores, Burn-on, Incomplete Filling, and Cold Shuts	Improper gating system design, resulting in poor temperature gradient, unsteady pouring, low pouring temperature, and slow pouring speed.
Gas Pores, Slag Holes, and Shrinkage Cavities	Inadequate venting in resin sand molds with large gas evolution and small vents.
Gas Pores and Burn-on	Failure to ignite for venting during pouring, leading to increased pressure in the cavity.

3. Reasonable Exhaust System Design and Preventive Measures

To ensure the quality of castings, a reasonable exhaust system design should consider multiple factors.

3.1 Mold Cavity Exhaust System Design

The mold cavity exhaust system design includes the following aspects:

3.1.1 Gating and Riser Exhaust

The gating and riser system is a major pathway for cavity exhaust. Reasonable design can enhance exhaust and metal flow.

Measure	Description
Riser Selection	Preferentially use pressure edge risers, flash risers, side risers, and top necking open risers to increase the exhaust area and improve the cavity’s exhaust and overflow capabilities.
Gating System Design	Adopt a reasonable gating system, such as a middle or stepped gating process, to ensure smooth metal flow and create a favorable temperature gradient for gas escape.
New Exhaust Riser	The application of new exhaust risers with good exhaust and insulation properties can solve the problems of poor ventilation and gas blockage in traditional risers.

3.1.2 Vent, Exhaust Needle, Exhaust Sheet, and Exhaust Filter Sheet Exhaust

Proper design and application of vents and related components can improve the exhaust effect and prevent defects.

Measure	Description
Vent Application	Use vents, exhaust needles, and exhaust sheets in appropriate positions and ensure they are connected to the atmosphere for enhanced exhaust.
Cavity and Core Gas Separation	Separate the cavity gas from the sand core gas and avoid direct exhaust on the casting surface. Use exhaust connection sheets and exhaust filter sheets to prevent sand from entering the cavity.

3.1.3 Sand (Mold) Gap Exhaust

The permeability of the sand mold is crucial for gas discharge.

Measure	Description
Sand Selection and Control	Select appropriate sand with good permeability and control the sand mold’s performance and compactness.
Additional Measures	Increase the number of air holes in the sand mold and reduce the sand consumption and mold wall thickness to facilitate gas escape.

3.1.4 Exhaust Plug Application

In high-pressure molding and core making, exhaust plugs can solve the problem of ‘air trapping’.

Measure	Description
Exhaust Plug Usage	Reasonably select and install exhaust plugs to compensate for the deficiencies in the core box or sand mold structure and ensure good mold and core formation.

3.1.5 Ignition Venting

Ignition venting during pouring can effectively expel gas and reduce defects.

Measure	Description
Ignition Venting Method	Ignite the combustion medium during pouring to expel gas, reduce cavity pressure, and prevent defects. Ensure proper elevation for bottom exhaust molds.

3.1.6 Other Preventive Measures

Additional measures can be taken to prevent gas pores and improve casting quality.

Measure	Description
Metal Liquid Treatment	Increase the liquid surface temperature and pouring speed to delay the formation of the oxide film and allow sufficient time for gas in the metal liquid to escape.
Resin Sand Casting	For resin sand castings, design larger vents and use overflow riser cups to ensure smooth cavity exhaust.
Material Selection	Select reasonable molding and core-making methods and formulations with low gas evolution and good permeability according to the casting requirements.
Vent Area Calculation	Calculate the exhaust area of the cavity vents using the appropriate formula to ensure sufficient exhaust area.

3.2 Sand Core Exhaust System Design

The design of the sand core exhaust system should consider the following factors:

3.2.1 Production and Design of Sand Core Exhaust Holes and Exhaust Channels

Different methods are used to produce exhaust holes and channels in sand cores according to their complexity.

Sand Core Type	Exhaust Method
Simple Small Sand Core	Use a ventilation needle to pierce ventilation holes in the sand core.
Elongated Sand Core	Insert a thick wire into the core and remove it after sand ramming to form an exhaust hole.
Slightly Curved Sand Core	Bury a smooth rope in the core and remove it after ramming to create an exhaust hole.
Complex Thin-section Sand Core	Use wax or rosin wire, which melts away after baking, leaving an exhaust hole.
Thick and Complex Sand Core	Bury a straw or exhaust rope in the core, which burns away after baking, leaving an exhaust hole.
Tall Cylindrical Sand Core	Use a steel pipe with holes drilled in the wall as the core bone and the ventilation hole.
Two-piece Core Assembly	Mark the ventilation channel at the core head before assembly and bonding, and then dry the core.
Thick and Large Sand Core	Bury coke or slag near the core box wall to improve ventilation.
Sealed Sand Core Casting	Use exhaust core supports for positioning and exhaust.
Complex Thin-wall Sand Core	Use a manual or electric tool to hollow out the core head or drill exhaust holes in the core.
Hot Box Core Making	Design ventilation needles in the hot box core to form ventilation holes during shooting. For large cores, pierce holes when the sand is not fully solidified or use machine flipping to empty the core.
Large and Complex Sand Core	Open crisscross exhaust channels in the core, connect them to the core head end face, and use hollow nylon ropes or drilled steel pipes for exhaust.
Complex Casting with Inner Cavity	Bury exhaust media, such as foam plastic, in the sand core, which vaporizes during pouring to form an exhaust channel.
Complex Casting Flow and Water Channel Core	Use 3D printing technology to form exhaust holes and channels in resin sand and hot/cold box cores that are difficult to produce by traditional methods.

3.2.2 Good Exhaust through the Cooperation between the Sand Core and the Mold

Proper design and cooperation between the sand core and the mold are essential for smooth exhaust.

Measure	Description
Structure Design	Ensure reasonable structure design, firm core bonding, and sufficient core head size to prevent gas channel blockage by the molten metal.
Sealing and Exhaust Channel Protection	Ensure proper sealing between the core head and core seat, and use sealing strips or asbestos ropes to prevent the molten metal from entering the exhaust channels. Drill exhaust channels in the upper mold corresponding to the lower mold and core exhaust holes.

3.2.3 Good Permeability, Low Gas Evolution, and Good Exhaust Performance of the Sand Core

Selecting appropriate materials and improving the sand core’s properties can enhance its exhaust performance.

Measure	Description
Material Selection	Use low gas evolution and high-strength coated sand, and make the sand core hollow as much as possible to improve permeability, sintering, drying, and high-temperature resistance.

4. Conclusion

In sand casting, the design of the exhaust system is a key factor affecting the quality of castings. By following the design principles, considering various factors, and taking appropriate preventive measures, the exhaust system can be optimized to ensure smooth gas discharge, prevent defects such as gas pores and sand adhesion, and improve the overall quality of castings. Continuous research and improvement in exhaust system design are necessary to meet the increasingly high quality requirements in the casting industry.