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.
To determine how much force the platen must hold, the process engineer will calculate using the following parameters:
The area of the finished part’s footprint
The flow length, which is the distance from the gate of the mold to the furthest edge of the part.
The average wall thickness of the part
The number of cavities in the mold
These values are put into a formula that also takes into account a few other variables to yield the correct clamping force. These calculations used to be done by hand, with the aid of simple conversion charts. Now many molding programs can do them automatically.
What Else Do I Need To Know About Clamping Pressure?
There are other factors that the molding engineer needs to consider to set the clamping force. For example, small gates are more restrictive and therefore will require more pressure, as will thin-walled parts. And there is the question of the melt flow index.
After taking all this into consideration, the smart engineer will add 20% to the total. This gives the process technician the freedom to make adjustments to the injection pressure to achieve optimal quality. i.e. where a faster injection speed is needed or a higher packing pressure to minimize sink marks.
