Exothermic Riser Sleeves: Process Technology to Improve Casting Quality

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In the casting industry, the internal quality of castings directly affects their mechanical properties and service life. Shrinkage holes, shrinkage porosity and other defects are common quality problems in the casting process, and exothermic riser sleeves as an efficient shrinkage compensation technology can significantly improve the solidification control ability of molten metal and reduce casting defects.

This article will systematically introduce the working principle, material composition, advantages and characteristics, application scenarios and future development trends of exothermic riser sleeves to help foundry companies better understand and apply this key technology, improve casting quality and reduce production costs.

Definition and Function of Exothermic Riser Sleeves

Exothermic riser sleeves are a special functional material used in casting processes, usually composed of exothermic materials, thermal insulation materials and binders. Its core function is to release heat through chemical reactions, prolong the solidification time of molten metal in the riser, and ensure that the casting is fully compensated during the solidification process, thereby reducing defects such as shrinkage holes and shrinkage porosity.

Compared with ordinary insulation risers, exothermic riser sleeves can not only reduce heat loss, but also actively provide additional heat, so that the molten metal in the riser remains in liquid state for a longer time, improving the shrinkage compensation efficiency.

exothermic riser sleeves in metal casting

Working Principle of Exothermic Riser Sleeves

The core principle of exothermic riser sleeves is based on the dual functions of exothermic reaction and thermal insulation:

Exothermic Reaction

When high-temperature molten metal enters the riser, the heating material (such as aluminum powder, iron oxide powder, etc.) undergoes aluminothermic reaction at high temperature:

This reaction can release a lot of heat energy in a short time, so that the temperature of the molten metal in the riser rises or remains unsolidified for a long time.

riser sleeves for foundry

Insulation

The outer layer of the exothermic riser sleeve is usually made of low thermal conductivity materials (such as perlite, ceramic fiber, etc.) to reduce the heat loss to the sand mold or the environment, so that the heat is concentrated inside the riser and the shrinkage compensation efficiency is improved.。

Shrinkage Compensation Mechanism

During the solidification process of the casting, due to the liquid shrinkage and solidification shrinkage of the metal, additional molten metal is required for replenishment. The exothermic riser sleeve prolongs the liquid time of the molten metal in the riser to ensure that the casting can continue to obtain molten metal when feeding, thereby avoiding the formation of internal defects.

Material Composition of Exothermic Sleeves

The performance of the exothermic riser sleeve depends on its material formula, which mainly includes the following components:

Heating Agent
  • Aluminum powder (Al): reacts with iron oxide to release a large amount of heat.
  • Ferrosilicon (FeSi): assists in heating and increases the reaction temperature.
  • Magnesium powder (Mg): used in special casting scenarios such as high-alloy steel.
Oxidant
  • Iron oxide (Fe₂O₃): the main oxidant that reacts with aluminum powder.
  • Sodium nitrate (NaNO₃), potassium nitrate (KNO₃): promote reaction and increase combustion speed.
Insulation material
  • Expanded perlite: low thermal conductivity, reduce heat loss.
  • Vermiculite: high temperature resistance, improve thermal insulation performance.
  • Ceramic fiber: suitable for high-precision castings.
Binder
  • Water glass (Na₂SiO₃): provides initial strength.
  • Resin: improves high temperature stability.

Different formulas of exothermic riser sleeves are suitable for different casting scenarios, such as cast iron, cast steel, high alloy steel, etc.

Main Advantages of Riser Sleeves

Compared with traditional insulation risers, exothermic riser sleeves have the following significant advantages:

Improve shrinkage compensation efficiency：The exothermic reaction can keep the molten metal in the riser in liquid state for a longer time, and the shrinkage compensation efficiency is increased by 30%~50%, especially for thick-walled castings.

Reduce the size of the riser: Due to the improved shrinkage compensation efficiency, the volume of the riser can be reduced, the consumption of molten metal can be reduced, and the process yield can be improved.

Improve the quality of castings: Effectively reduce shrinkage cavities and shrinkage, improve the density of castings, and are suitable for high-requirement castings (such as automotive parts, hydraulic valve bodies, etc.).

Strong adaptability: It can be used in a variety of casting processes (sand casting, lost foam casting, V-method casting, etc.) and a variety of metals (cast iron, cast steel, aluminum alloy, etc.).

runner and riser in foundry

Common Types of Exothermic Riser Sleeves

According to different calorific value and application scenarios, exothermic riser sleeves can be divided into the following categories:

  • Ordinary Exothermic Riser Sleeves: Applicable to general cast steel and cast iron parts, with moderate calorific value and low cost.
  • High Exothermic Riser Sleeves: Higher heating agent content, suitable for large castings or high alloy steel (such as stainless steel, heat-resistant steel).
  • Composite heating and insulation riser sleeves: Combining the dual functions of heating and insulation, it is suitable for precision castings with extremely high shrinkage requirements.

Application and Installation of Riser Sleeves

Applicable scenarios
  • Thick-walled castings: such as machine tool bases, large gear boxes, etc.
  • High alloy castings: such as stainless steel, heat-resistant steel and other materials with large solidification shrinkage.
  • Precision castings: such as automobile engine cylinders, hydraulic valve bodies, etc.
Installation precautions
  1. Riser position: It should be placed above the hot section (thickest part) of the casting to ensure that the shrinkage channel is unobstructed.
  2. Drying treatment: Dry at 150~200℃ before use to avoid moisture causing pores.
  3. Matching size: Select the appropriate riser size according to the casting modulus (solidification time).
  4. Process verification: Small-scale tests should be carried out before mass production to optimize process parameters.

Conclusion

Exothermic riser sleeves are a key technology to improve casting quality and reduce production costs. Through reasonable selection and use, foundry companies can significantly reduce the defect rate and improve product competitiveness. In the future, with the development of new materials and intelligent technologies, the application of exothermic riser sleeves will be more extensive and efficient.

If you need more technical details or customized solutions, please contact us, we will provide you with professional casting process support.

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