There are 2 reasons, one is the pulp density is changing, when the paper pulp is released from pulp tank of pulp molding machine, the density of pulp start to change and it is controlled during 2.56%-3.6%. The other reason is vacuum stress is always changing. In paper tray molding process, vacuum stress created by vacuum tank is not consistent. So both the variables makes the paper tray weight vary between ±10 percent.