# Injection Molding Process Control of Servo-Hydraulic System
## 1 Introduction  
Servo-hydraulic systems have many advantages for injection molding machines, including high precision, low noise, a low operating temperature, and good repeatability [1; 2; 3]. Furthermore, compared with traditional hydraulic systems, servo-hydraulic systems have the potential to achieve energy savings of up to 70%, or more [4; 5]. However, to achieve this energy-saving potential, the servo-hydraulic system must first be carefully designed and optimized.  
Of all the parameters involved in the injection molding process, the injection velocity and packing pressure are two of the most important ones in determining the quality of the molded part. The injection velocity refers to the speed at which the screw moves in the forward direction during the cavity filling stage, while the packing pressure refers to the pressure applied to the molten resin in the cavity as it cools and solidifies to form the final component. However, even when the injection velocity and packing pressure are properly controlled, the accuracy with which the injection cycle transits from the constant-velocity (V) filling the stage to the constant-pressure (P) packing stage plays a critical role in ensuring a correct and consistent quality of the molded component [6; 7]. Many approaches have been proposed for controlling the V-P switchover point, including pressure switch, time switch and position switch. In the position control method, V-P switchover is initiated automatically as soon as the screw displacement reaches a certain predetermined value [8; 9; 10]. Among all the methods available, the position control method is one of the most commonly used since, the same amount of resin is injected into the mold in each shot, and hence the consistency of the molded part quality is guaranteed [11].  
Previous studies have reported that moving the screw backward through a short distance when reaching the V-P transition point is beneficial in decreasing the pressure at the screw front and hencein preventing overflow or part warpage [12]. [PERSON] et al. [13] had illustrated how the pull-back motion work, yet the discussion including the distance and performances of pull-back motion have been omitted. Particularly, pulling the screw back too far runs the risk of trapping air bubbles in the mold, while moving the screw back too little risks not reaching the pressure reduction function. Hence, the pull-back distance must be carefully controlled to ensure that the molded components satisfy the required quality standard.  
The present study designs and constructs a servo-hydraulic system to simulate the filling and packing stages of an injection molding machine. A proportional-integral-derivative (PID) controller is additionally designed to control the V-P switchover point using the position control method. Experiments were performed to investigate the performance of the proposed system in terms of its speed control (filling process), pressure control (packing process) and V-P switchover point control (V-P transition). In general, the results confirm the feasibility of the proposed servo-hydraulic system for injection molding applications given an appropriate setting of the sampling frequency (1000 Hz).