Casing Layer

The Casing Layer of a golf ball is a secondary layer, under the urethane cover that encapsulates the golf ball core. The casing layer is a resilient, stiff thermoplastic layer made from materials called ionomers. 

By blending different ionomers together to create the casing layer’s composition and varying its thickness, golf ball engineers can affect performance – including launch conditions (ball speed, launch angle, spin) as well as feel.

Another function of the casing layer is protection of the golf ball core.  The polybutadiene core is not only porous, but it contains materials that readily absorb moisture.  When moisture in the form of water vapor comes in contact with a solid golf ball core, the core becomes slower.  The casing layer protects the core by preventing/resisting moisture from entering the core.  

Casing layers are not necessary in the construction of golf ball models that feature ionomer covers, as the cover itself protects the golf ball core from moisture.