System for controlling clamp pressure in an automatic molding machine

A method for determining the velocity profile of the piston of a clamp assembly, wherein the piston velocity is controlled by the operation of a servo valve within the clamp assembly, and wherein the displacement of the servo valve is controllable, the velocity profile having a ramp-up interval, a constant speed interval, ramp-down interval;

• and a touchdown interval, said method comprising the following steps:
• determining a maximum displacement xs of the servo valve;
• selecting a final velocity of the piston (vf);
• choosing a ratio α of a piston ramp-down interval to the servo valve displacement “x” during the ramp-up interval, according to equation α = ( x s - x d ) x u
• where xu is a servo valve displacement and xd is a displacement where a ramp down interval begins;
• selecting a desired maximum piston velocity (vmax) during the ramp-up interval and constant speed interval;
• and
• determining the servo valve displacement xu from simulations or estimate from previous cycles according to a mathematical relationship x u ≤ x s ( α + 1 ) .

A control unit for controlling the force and velocity of a piston on a mold within the clamp assembly of a injection molding machine, based on a servo valve command signal which acts to operate a servo valve, said control unit comprising:

• a desired force limit, wherein said desired force limit is adapted to generate a desired force limit signal;
• a cascade compensator connected to the desired force limit, wherein said cascade compensator is adapted to generate the valve command signal, based on comparison of the desired force and an estimated actual force of the piston on the mold;
• an estimator coupled to the clamp assembly, wherein said estimator is adapted to estimate an actual force using cylinder pressure and a piston position signal;
• a position and velocity controller being coupled to the clamp assembly and coupled to said estimator, wherein said position and velocity controller is adapted to generate the valve command signal based on desired position and velocity profiles;
• an auto selector being coupled to said cascade compensator, wherein said auto selector is adapted to select between a velocity control function signal and a force control function signal so as to generate the servo valve command signal;
• and
• a feedback compensator being coupled to said estimator, wherein said feedback compensator is adapted to generate a feedback signal in correspondence with said estimated actual force signal.

A method for determining the velocity profile of the piston of a clamp assembly, wherein the piston velocity is controlled by the operation of a servo valve within the clamp assembly, and wherein the displacement of the servo valve is controllable, the velocity profile having a ramp-up interval, a constant speed interval, ramp-down interval;

• and a touchdown interval, said method comprising the steps of:
• determining a maximum displacement xs of the servo valve;
• selecting a final velocity of the piston (vf);
• choosing a ratio α of a piston ramp-down interval to the servo valve displacement “x” during the ramp-up interval, according to equation α=(xs−xd)/xu where xu is a servo valve displacement and xd is a displacement where a ramp down interval begins;
• selecting a desired maximum piston velocity (vmax) during the ramp-up interval and constant speed interval;
• determining the servo valve displacement xu from simulations or estimate from previous cycles according to a mathematical relationship x u ≤ x s ( α + 1 ) ;
• and
• selecting a piston deceleration profile during the ramp-down interval according to a linear velocity profile.