Wood flour is a finely ground form of wood that is used in various applications, including as an additive in core sand for casting processes. When added to core sand, wood flour can improve the collapsibility of the core.

Collapsibility refers to the ability of the core to collapse and be easily removed from the casting after it has been solidified. This is an important characteristic in casting processes because if the core does not collapse properly, it can cause defects in the casting or make it difficult to remove the core without damaging the casting.

There are several reasons why wood flour improves the collapsibility of core sand:

1. Absorption of moisture: Wood flour has hygroscopic properties, meaning it can absorb and release moisture depending on the surrounding conditions. When wood flour is added to core sand, it absorbs moisture from the surrounding environment, which helps to reduce the bond strength of the sand. This makes it easier for the core to collapse when the casting is solidified and the sand is removed.

2. Thermal decomposition: When the casting is heated during the solidification process, the wood flour in the core sand undergoes thermal decomposition. This decomposition releases gases, such as carbon dioxide and water vapor, which create pressure within the core. This pressure helps to break the bond between the sand particles, further facilitating the collapse of the core.

3. Combustion: In some casting processes, such as shell molding, the core is heated to a high temperature to remove any residual moisture and improve the surface finish of the casting. Wood flour, being a combustible material, can burn off during this heating process, leaving behind voids in the core. These voids help to reduce the bond strength of the sand and enhance the collapsibility of the core.

In summary, the addition of wood flour to core sand improves the collapsibility of the core by absorbing moisture, undergoing thermal decomposition, and facilitating combustion. This allows the core to collapse easily and be removed from the casting without causing defects or damage.