In sand casting, shrinkage cavities and shrinkage porosity are the most common internal defects in castings. Shrinkage cavities typically manifest as irregular large cavities in thick areas or heat center zones of the casting, characterized by rough walls and dendritic crystals. Shrinkage porosity appears as fine dispersed pore clusters, predominantly occurring along casting axes or at thickness junctions. These defects severely compromise mechanical properties and airtightness, potentially leading to cracks and premature failure. As core processes in the casting workflow, mold design and core-making operations play a decisive role in preventing shrinkage porosity and cavities. This article systematically outlines effective measures to reduce their occurrence rates by focusing on critical aspects of mold-making procedures.

I. Optimize the design of casting stiffness and filling pressure

1. Ensure the upper box height and enhance the static pressure of metal

Insufficient upper box height significantly reduces the metal's contraction pressure during the final solidification stage of castings, leading to diminished cooling capacity and increased susceptibility to shrinkage porosity or shrinkage cavities at thermal joints. For horizontally split-structure cast steel pumps (Cast Steel Split Case Pump Casing), special attention must be paid to weight balance between upper and lower boxes: An undersized upper box exacerbates insufficient contraction pressure, causing concentrated shrinkage cavities at the pump flange top. The upper box weight should be ≥1.5 times the static hydraulic lift of the molten metal, with additional weights added when necessary. Production practices must strictly prohibit compromising upper box height to save sand or reduce mold costs. For structures requiring shallow upper boxes, compensation solutions should include:

Use insulation riser sleeve or heating riser to prolong the solidification time of metal liquid in the riser;

Use the pouring cup to lift or continuously add high temperature metal liquid to the riser to maintain the smooth filling channel and filling pressure.

2. Improve the overall rigidity and compactness of the casting

Insufficient mold rigidity may lead to cavity expansion during pre-casting baking or static pressure from molten metal flow, resulting in inadequate liquid refilling. Split Case Pump Bodies require enhanced structural reinforcement due to their complex runner surfaces and numerous cores.

The thick area of the pump body flange is locally cast with high heat storage materials such as chromite sand to accelerate solidification and reduce the tendency of shrinkage and loosening;

The cold iron skeleton (such as Φ20mm steel rod) is embedded in the flow channel core to enhance the stiffness of the core to prevent floating, and accelerate the heat dissipation to avoid the occurrence of micro shrinkage in the wall thickness of the flow channel;

 Apply pre-tightening force to the parting surface bolt after box closing (≥0.2MPa).

2、Scientific design of sprue and cold iron system

1. Strengthen sequential solidification and directional shrinkage

Special Casting Design: The mid-casing pump body shall feature a visible riser at the top of inlet/outlet flanges, with an inverted conical neck design (larger at the top and narrower at the bottom) to ensure unobstructed shrinkage compensation channels. A concealed riser is installed behind the impeller passage, utilizing the internal cavity of the sand core to vent air and prevent axial shrinkage due to slow heat dissipation in this area.

Innovative Cold Iron Application: Implement conformal curved cold irons (approximately 0.8 times wall thickness) in the variable cross-section area of pump volute. The spacing between cold irons is reduced to 1.5 times wall thickness, enabling precise control of local cooling gradients. The gap between cold irons and molding sand remains ≤0.5mm to prevent molten metal penetration that could create "thermal bridges" causing shrinkage cavities.

 Pouring system: The bottom injection open pouring system should be preferred to avoid excessive erosion of the cavity by molten metal. The position of internal pouring channel should be conducive to sufficient cooling of hot spots, so as to avoid the formation of isolated hot areas and shrinkage porosity.

2. Avoid thermal joints and optimize the structure of castings/moldings

 Optimize the casting structure by processing allowance and correction: try to avoid drastic changes in section size (excessive wall thickness difference) and reduce isolated hot joints. The transition design is used at the thick-thin connection.

 Proper Corner Design: Insufficient radius (R) at concave corners reduces sand cooling efficiency, delays solidification, and increases air shrinkage porosity. Excessive radius (R) may create thick thermal shock zones that lead to shrinkage porosity. The ideal corner radius should be approximately one-third of the adjacent wall thickness.

 Maintenance of mold/house: Regularly check and repair the worn mold/house to ensure that the wall thickness of the casting meets the design requirements and avoid local thinning that hinders the recasting channel.

III. Strict control of sand performance and coating process

1. The performance of molding sand is the foundation

• Air permeability: Poor air permeability will hinder the discharge of water vapor and gas in the mold, and the gas pressure may infiltrate into the un-solidified metal liquid, aggravating the tendency of shrinkage and forming composite defects of gas shrinkage. Regular detection and adjustment are required.

• Moisture and gas emission: strictly control the moisture of sand and reduce gas emission. Too much moisture will not only reduce strength, increase gas emission, but also delay the local cooling speed and induce shrinkage and loosening.

• Strength and thermal stability: ensure sufficient wet/dry strength and thermal stability to prevent the mold wall from yielding too early or cracking, which will destroy the environment for supplementary cooling.

2. Optimization of coating process

• Apply a rapid cooling coating (e.g., zirconia powder coating) to thick areas or hot nodes to accelerate the solidification of the area.

• Strictly control the concentration and thickness of the coating to ensure uniformity. Over-thick or uneven coating will heat insulation and affect the ideal sequential solidification gradient.

IV. Focus on the details and process control of core drilling operation

1. Fine shaping and core making

• Standardize the operation of molding and trimming to avoid excessive water brushing in local areas, resulting in moisture accumulation of molding sand and uneven cooling.

• Ensure that the sand core is fully dry (especially at the core head), and the ventilation channels (wax wire, ventilation needle) are unimpeded to prevent the gas of the sand core from interfering with the solidification and shrinkage process of the metal, resulting in loose shrinkage or air shrinkage holes.

• Ensure the positioning and stability of the sand core, avoid uneven wall thickness caused by floating or displacement of the core, and produce unexpected hot joints.

2. Preparations for box sealing and pouring

• The box is positioned accurately to prevent the formation of hot joints caused by local thickening due to wrong type.

• The pouring cup and riser ring are placed stably and sealed well.

• For large and complex castings, resin sand core molding is preferred to improve the overall precision and stiffness of the casting.

V. Operation characteristics and coordination with other processes

Special operating points of medium opening pump

• Sand core combination positioning: 3D printing positioning card plate is used to assemble the flow channel core group, and the gap between the core head is less than 0.8mm to prevent the drift of the core from causing uneven wall thickness and accidental thermal shrinkage and loosening;

• Maintain the box closing pressure: compress the thickness of the sealing strip on the parting surface to 80% of the original, and use the box closing pressure to compact the seal, so as to prevent the lifting of the box from causing the interruption of the filling and the formation of shrink holes.

• Information sharing in smelting: By precisely matching metal characteristics, the smelting process can provide real-time feedback on molten steel composition (e.g., C and Si content) to the molding end. Since high-carbon steel exhibits greater shrinkage tendency, the mold design requires increased riser volume compensation for shrinkage, along with additional chillers to accelerate solidification in thick-walled sections and prevent shrinkage porosity.

• Sand fall coordination: the insulation time of more than 500℃ is more than 6h (2-3h for conventional parts), slow cooling to release stress and avoid derivative cracks in the shrinkage zone.

 epilogue

The practice of controlling shrinkage and shrinkage holes in the pump body of medium-opening pump shows that complex structure castings need to break through conventional riser layout (such as hidden riser behind the runner), the key to prevent shrinkage in thin-walled areas is to strengthen the core rigidity (built-in cold iron skeleton), and the differentiated cooling strategy (curved cold iron + regional coating) can accurately control the hot section.

The control of shrinkage cavities and porosity in sand casting fundamentally relies on establishing a three-pronged solidification environment through molding processes: constructing rigid molds, implementing precision chill irons, and maintaining continuous pressure for shrinkage compensation. Only by deeply integrating core-forming techniques with the structural characteristics of castings (such as inverted conical riser necks in pump body flanges and flow channel-matched chill irons) can the defect rate be reduced below 0.5%, thereby meeting the inherent safety requirements for pressure-bearing castings.