The measurement control strategy and its effect play a very important role in stabilizing the process conditions and obtaining precision and high-quality products in the injection molding process. Back pressure is one of the most important process parameters that affect the metering action. A suitable and stable back pressure value is a necessary condition to ensure product quality.
     Electric injection molding machines are significantly different from traditional hydraulic injection molding machines in back pressure control: traditional hydraulic injection molding machines use a throttle valve to control the back pressure value, while electric injection molding machines need to pass the rotation of the metering motor and the injection motor. Withdraw the cooperation of the two actions to control the back pressure value. Calculate the retreat speed of the injection motor during the metering process by establishing a physical model to meet the set back pressure curve. However, due to the uncertainty and time variation of each damping force, external electromagnetic interference, and the complexity of the model, etc. , The algorithm proposed in the above-mentioned literature has certain difficulty in realization. This paper proposes an easy-to-implement control method, which has been verified to meet the technical requirements for back pressure in the metering process.
1 Formation of back pressure and its influencing factors
1.1 Back pressure formation mechanism
     During the metering process of the injection molding machine, the rotation of the screw completes the forward accumulation of the resin pellets in the barrel, and at the same time achieves the effect of shearing the resin raw material. When the amount of melt increases, the resin raw material will produce a reverse force on the screw, which will form back pressure. The proper increase of the back pressure is beneficial to the discharge of air in the melt, increase the uniformity of the plasticization of the melt, and control the consistency of the product weight.
1.2 Influencing factors of back pressure
     In the control of an electric injection molding machine, there are three main factors that affect the back pressure: screw speed, screw structure, and screw retreat speed. Under the same process parameters, the higher the screw speed, the greater the back pressure; when the screw speed is constant, the plasticizing ability of different functional sections of the screw is different, and the back pressure produced is also different; the slower the screw retreat speed, the greater the back pressure of the melt. The greater the pressure.
2 Back pressure control algorithm
     In order to ensure the uniform melting of the resin, the screw rotates at the set speed, that is, the screw speed is not adjusted. The control target is achieved by adjusting the screw''s retreat speed in real time.
2.1 The basic principle of the algorithm
     The basic principle of back pressure control is: when the set back pressure value is constant, the screw retreat speed is inversely related to the current actual back pressure value. When the actual pressure value is less than the set back pressure value, the screw will only rotate and not retreat. When the actual pressure value is greater than the set back pressure value, the screw will start to rotate while retreating, and the difference between the actual pressure value and the set back pressure value will be greater. The greater the backward speed, the faster; when the actual back pressure value is close to the set back pressure value, the speed jitter should be smaller to avoid the jitter of the actual back pressure value.
2.2 Realization of control algorithm
     The slope of the quadratic curve is greater where it is farther from the symmetry axis. This geometric relationship is consistent with the above-mentioned control principle: the set command back pressure value is taken as the symmetry axis of the quadratic curve, when the actual back pressure value is much larger than the command back pressure value. At the time of pressure value, the screw’s retreat speed increases rapidly and non-linearly, making the actual pressure drop rapidly; when the actual back pressure value is close to the command back pressure value, its flatness near the axis of symmetry is used to ensure that the retreat speed is small. Fluctuation; in addition, we can use the offset brought by the symmetry axis to cleverly avoid the offset of the actual back pressure value caused by external interference.
     In order to meet the control requirements, the proposed control algorithm: when the actual back pressure value is greater than the command back pressure value, the difference between the two is amplified to obtain the required screw command retreat speed v, and then the command displacement increment is calculated. Output to the servo drive.
3 Experiment and analysis
3.1 Implementation of the injection motor control module
     We use the state machine design method to develop the injection motor control module. The process of the entire injection motor is divided into seven different states, which are: pause state (pause), injection speed state (inj_fwd_vel), injection speed-pressure conversion state (inj_fwd_v2p), injection holding pressure state (inj_fwd_prs), pre-measurement state Decompression state (inj_bak_unload1), measurement state (inj_bak_pre-load) and anti-salivation 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 plasticizing motor starts to rotate, the injection motor retreats according to the algorithm proposed in this article, and the two cooperate to complete the back pressure control.
3.2 Experiment and analysis
     The mechanical, technological and control parameters in the experiment are as follows: the diameter of the universal screw is 21mm, the raw material is pp pellets, the temperature of the heater is 185℃, 210℃, 200℃, 170℃, and the control period is 1ms; the control parameter is poffset =0.28mpa, a0=9.5mm/min, a1=85mm/(min·mpa), a2=95mm/(min·mpa2).
     In order to observe and compare the control effect under different back pressure values, the experiment set four groups of back pressure values as 6mpa, 8mpa, 10mpa, 12mpa, and set two different speed values of 80r/min, 120r/min. The actual back pressure control effect, it can be seen that the actual control output error of the back pressure is within 1mpa. Taking into account the influence of external interference (about ±0.35mpa), the actual effect meets the control requirements.
     In order to observe the control effect of multi-stage back pressure, set the two-stage back pressure 6mpa and 10mpa, and the corresponding speed is 75r/min and 110r/min respectively. Set the three-stage back pressure 6mpa, 12mpa, 9mpa, and the corresponding speed is 75r/min, 110r/min, 95r/min, respectively. It can be seen that for multi-stage back pressure, this algorithm can also control the output error within 1 mpa, and the response speed is also satisfactory.
     Sometimes there will be a pressure peak at the end of the metering process, which is mainly caused by the deceleration of the injection motor at the end. After the decompression process after metering, the actual back pressure value will be significantly reduced. Excluding the actual pressure peak value at the control end, the average value of the corresponding actual back pressure value and the iae (integral absolute error) value are calculated as shown in Table 1. It can be seen from the table that the error between the actual back pressure output average value and the set back pressure value is within 1 mpa, and when the speed increases, the actual back pressure output value will be slightly greater than the set back pressure value. The algorithm has good repeatability and stability.

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