expansion process for exhibiting greater bulk the sheet may be heated without drying for expansion of the heat expanding particles and then drying performed in a separate drying step since no bonding force is be produced between fibers in the expansion step for the heat expanding particles in this method the bulk of the sheet is not inhibited by expansion of the heat expanding particles and sufficient bulk can be exhibited if the sheet is placed on a support and suction is applied from the bottom of the support while spraying moist hot air or water vapor from the top side the entire sheet will be heated rapidly and evenly thereby increasing the thermal expansion effect and therefore this method may be considered to be most efficient the support may be but is not limited to a net or other type of conveyor belt when steam is sprayed onto the sheet from a nozzle hole positioned at a prescribed spacing from it in a method that involves spraying moist hot air or water vapor an excessively high moisture content of the sheet about 80 by mass or greater will produce uneven expansion due to the pitch of the nozzle hole regardless of whether the sheet surface is at uniform temperature for this reason a lower moisture content of the sheet is preferred when steam is evenly sprayed.