HMR board is made by adding a special resin to the board which gives it extra moisture resistance. This resin can be added to numerous types of board, and so not all HMR boards are the same (but their moisture resistant properties are).  Common materials for HMR board are engineered woods such as chipboard and MDF, which stands for 'medium density fibreboard'.

Chipboard & MDF

Chipboard is made from wood chips and sawmill shavings, which are combined together with a resin. The final product is strong yet lightweight.  MDF is similar to chipboard but it uses wood fibres combined with a glue to create a denser composite wood, which makes it very strong and heavy like solid wood. 

Hmr Mdf Test Report