What are the back pressure control methods for all-electric injection molding machines?
1. Formation of back pressure and its influencing factors
1.1 Back Pressure Training Mechanism
While measuring the injection molding machine, screw rotation completes the accumulation of resin pellets in the barrel and at the same time the resin raw material is cut. When melting is increasing, the resin feedstock generates a reverse force on the screw and back pressure is formed. Proper increase of back pressure is conducive to air discharge into the melt, increases melt plasticity uniformity and controls the consistency of product weight.
1.2 Factors Affecting Back Pressure
In controlling electric injection molding machines, there are three main factors affecting back pressure: screw speed, screw structure and screw speed. Under the same process parameters, the higher the screw speed, the higher the back pressure; Under the condition of a constant screw speed, the plasticizing ability of different functional sections of the screw is different and the back pressure is also different; The lower the screw's return speed, the lower the melt. The higher the pressure.
2. Rear Pressure Control Algorithm
In order to ensure uniform resin fusion, the screw rotates according to the set rotation speed, ie the screw rotation speed is not adjusted and the control objective is achieved by adjusting the screw retraction speed in time. real.
2.1 Algorithm Basics
The basic principle of backpressure control is that when the set backpressure value is constant, the setback speed of the screw is inversely related to the current value of the current backpressure. When the actual pressure value is less than the back pressure value, the screw rotates only and does not retract. When the actual pressure value is greater than the back pressure value, the screw begins to retract as it turns, and the difference between the actual pressure value and the back pressure value increases. The higher the reverse speed, the faster; When the actual value of the back pressure is close to the set value of the back pressure, the speed jitter should be small to avoid jittering the actual value of the back pressure.
2.2 Implementation of the control algorithm
The slope of the quadratic curve is greater as it moves away from the axis of symmetry. This geometric relationship is consistent with the control principle mentioned above: the set command back pressure value is used as the axis of symmetry of the quadratic curve. Actual back pressure is much higher than commanded back pressure At the pressure value, the screw's back speed increases rapidly nonlinearly, causing the actual pressure to fall rapidly. When the actual back pressure value is close to the command back pressure value, the flatness near the symmetry axis is used to ensure that the back speed is small. In addition, we can use the offset brought by the symmetry axis to intelligently prevent the offset of the actual back pressure value caused by external interference.
To meet the control requirements, the proposed control algorithm: when the actual value of the back pressure is greater than the value of the commanded back pressure, the difference between the two is amplified to obtain the required screw-driven inverse speed v, and the increment of commanded displacement is calculated. Servo driver output.
3. Experience and analysis
3.1 Implementation of the injection engine control module
We use the state machine design method to develop the injection engine control module. The entire injection engine process is divided into seven different states, which are: pause, injection speed state (inj_fwd_vel), injection pressure speed transition state (inj_fwd_v2p), injection pressure state (inj_fwd_prs) and before measurement Decompression state (inj_bak_unload1), measurement state (inj_bak_unload2) and anti-salting state after measurement (inj_bak_unload2). Describes the transitions between different states, the conditions for transitions between states, and the required trigger actions. In the inj_bak_preload state, the plasticization engine starts to spin and the injection engine reverts according to the algorithm proposed in this article. The two cooperate to complete back pressure control.
3.2 Experience and analysis
The various mechanical, technological and control parameters of the experiment are as follows: the universal screw diameter is 21mm, the raw material is pp pellets, the heater temperature is 185 ° C, 210 ° C, 200 ° C, 170 ° C and the control period is 1ms; the control parameter is poffset = 0.28 mpa, a0 = 9.5 mm / min, a1 = 85 mm / (min · mpa), a2 = 95 m