The clamping pressure is a measurement of the force needed to hold the mold closed during the injection.  Clamping force refers to the force applied to mold by the clamping unit of an injection molding machine. In order to keep the mold closed, this force must oppose the separating force, caused by the injection of molten plastic into the mold.
Clamping pressure is the first variable that should be calculated by the injection molding engineer since it will determine the capacity of the machine to be used to get the best results. This is important for product developers because the choice of the machine affects cycle times, parts per hour and cost. If this pressure is not set high enough the mold will be forced open prematurely by injection pressure and will create flash on the related part. 
Getting the best results from plastic injection molding requires that the process control engineer carefully balances several related variables when setting up the machine. One of these variables is clamping pressure.