Displacement and Tablet weight

The image below represents the Top Pre-compression Roller with its Pneumatic Compensator. This Compensator is a cylinder wherein the pressure P is controlled and kept constant by a proportional valve. S_piston is the calculated surface of the piston and is a constant value. Therefore the maximum pre-compression force F=P x S_piston=constant.

During the transition to automatic weight control, the control system reduces the distance between the lower and the upper punches by lifting the lower roller and so increasing the Pre-compression force on the powder.

When the force applied to the top pre-compression roller through the top punch by the compacting powder equals the constant force applied in the Compensator, the pre-compression roller moves up. The roller reaches its maximum displacement (d) when the force needed to compact the powder to a height PCH + d equals the force applied by the compensator. PCH is the distance between the punches at pre-compressions with the same settings without powder in the die. See image below.

The maximum pre-compression force is attained as soon as the roller is pushed upwards away from the dead stop. The vertical travel of the roller is limited to a few tenths of a millimeter.

In between two tablets the piston sits on the cylinder dead stop.

As the powder is compressed by a constant force, the displacement (d) will vary with the amount of powder in the die.