yvonne@nydcasting.com 
In casting production, hole defects are common defects and are also one of the defects that cause the greatest losses to foundries. Porosity defects are divided into pores and shrinkage pores. Porosity is mostly caused by the intrusion, inclusion, and entrainment of gas in the molten metal.
The positions of pores in castings are different and the causes of their occurrence are also different. This requires us to understand the principles of the occurrence of various types of pores and what characteristics they have when judging the causes of pore defects. Only in this way can we prescribe the right medicine and solve the pore defects.
Characteristics of pores generated by different reasons:
(1) Involved pores: Pores are formed in the casting due to the inclusion of gas in the molten metal during the filling process. They are mostly isolated round or elliptical large pores with unstable positions, generally biased towards the middle and upper parts of the casting.
(2) Intrusion pores: Pores generated by molds, cores, coatings, core supports, and chills invade the surface of castings and form pores. They are mostly pear-shaped or oval, large in size, with smooth pore walls and oxidized surfaces.
(3) Reaction pores: Grouped pores form due to chemical reactions between molten metal components or with the mold/core at the interface. Surface needle holes and subcutaneous pores—needle- or waist-shaped—result from reactions with mold or core coatings. These pores may appear scattered or clustered across the casting’s cross-section or in specific areas.
Causes of formation
(1) Dampness, rust, and oil pollution cause issues. Tools and ladles not dried affect quality. Improper molten metal composition leads to defects. Over- or under-refined alloys contain excess gas. This results in pore or reaction pore precipitation in castings.
(2) The mold and core are not fully dried, leading to poor air permeability. Poor ventilation, excessive moisture, or gas-generating substances contribute to defects. The coating is not fully dried or contains too many gas-generating components. Cold iron and core support have rust spots, oil stains, or are not dried. Poor venting of the metal mold leads to intrusive pores in the casting.
(3) An unreasonable pouring system, fast pouring speed, and poor mold venting cause turbulence and gas entrapment, forming entrained pores in the casting.
(4) The alloy liquid is easy to absorb air. Effective refining, protection and purification measures are not taken during the smelting and pouring process, so that the molten metal contains a large amount of gas, slag inclusions and gas inclusions, forming precipitation pores and reaction pores during the filling and solidification process.
(5) The molding sand, core and coating are improperly prepared, and the interface reaction occurs with the molten metal, forming surface pinholes and subcutaneous pores.
(6) Low pouring and mold temperatures, improper slag removal, and high viscosity prevent gas from escaping the mold or reaching the riser.
(7) When melting alloys that are easy to absorb air in humid seasons, the alloy liquid absorbs a large amount of air, causing batches of castings to be scrapped.
(8) Too much resin and curing agent increase gas emission. High nitrogen content worsens it. Angular and fine sand reduce permeability. High loss on ignition and excess micropowder further increase gas and lower permeability.
Prevention methods
(1) When melting the alloy, the charge, solvent, tools and ladle should be fully preheated and dried, rust and oil should be removed, and the amount of charge added for multiple remelting should be appropriately limited.
(2) Prevent excessive oxidation and gas absorption during smelting. Deoxidize, degas, remove slag, and cover the molten metal with solvent to prevent secondary oxidation and impurities. When using aluminum to deoxidize cast steel and cast iron, the residual aluminum content should be strictly controlled. For molten steel with serious gas absorption tendency, aluminum deoxidation should be avoided as much as possible. AVD, VOD, inert gas blowing, and powder spraying can refine molten steel outside the furnace to remove gas and impurities. For ductile iron, desulfurization should be enhanced, flow rate reduced, spheroidizing agent minimized while ensuring effectiveness, magnesium content lowered, and inoculation improved.
(3) The molten metal should not be interrupted during pouring, and the filling speed should not be too high. The casting position and the setting of the pouring system should ensure that the molten metal fills the mold cavity smoothly and facilitates the smooth discharge of the gas in the open cavity.
(4) During casting, the mold and core should be ensured to have smooth exhaust. The sand core should have an exhaust channel. When closing the mold, the core head gap should be filled to prevent the metal liquid from drilling into the exhaust channel.
(5) Increase the height of the sprue to increase the static pressure of the metal liquid filling the mold.
(6) Reduce resin and curing agent amounts, use low-nitrogen or nitrogen-free resin, and rounded materials. Control particle size, ignition loss, and micro-powder content to improve gas emission and air permeability.