Cavity pressure control of powder injection moulding machines

Abstract

In this study, cavity pressure control of a computer controlled powder injection moulding setup driven by an electro-hydraulic servovalve was investigated both theoretically and experimentally. For this purpose, a detailed system model of a powder injection moulding machine was developed and the important parameters that affect the system behaviour were determined by carrying out computer simulations. In light of the simulation results, a closed-loop control system was designed and manufactured. The control system was implemented to a specially designed powder injection moulding setup. Comparisons of the theoretical and experimental results demonstrate that the developed system model describes the dynamic behaviour of the system satisfactorily. The results of experiments show that accurate control of the cavity pressure can be achieved by the proposed control system. A 1 Area of the right-hand side of the hydraulic piston [m 2 ] A 2 Injection piston (screw piston) area [m 2 ] A 3 Area of the left-hand side of the hydraulic piston [m 2 ] A i Cross-sectional area of the ith passage [m 2 ] A c , b Coefficients which describe the feedstock properties b mh Damping coefficent [N.s/m] C ai Circumference of the ith passage [m] C d Discharge coefficent of the servovalve ports C lh Coefficient of leakage around the hydraulic piston [m 3 /s/Pa] C lm Coefficient of leakage around the injection piston [m 3 /s/Pa] d hi Hydraulic diameter of the ith passage at a given time [m] I v Servovalve input current [mA] K v Gain of the servovalve [m/mA] L ei Length of the ith passage at a given time [m] P 2 Injection pressure [Pa] P c Cavity pressure [Pa] P s Supply pressure [Pa] Q m Flow rate into the mould [m 3 /s] R Total resistance [N.s/m 5 ] T mi Feedstock temperature in the ith passage [ C] T 0 Initial temperature of the feedstock [ C] V 1 Oil volume in the right-hand side of the hydraulic cylinder [m 3 ] V 2 Feedstock volume in the injection cylinder [m 3 ] V 3 Oil volume in the left-hand side of the hydraulic cylinder [m 3 ] V fi Volume of the filled part of the ith passage [m 3 ] q h Density of the hydraulic oil [kg/m 3 ] b h Bulk modulus of the hydraulic oil [Pa] b m Bulk modulus of the feedstock [Pa] l i Absolute viscosity of the feedstock flowing through the ith passage [N.s/m 2 ] w Peripheral length of the servovalve ports [m] _ c c i Shear rate in the ith passage [s À1 ] x v Natural frequency of the valve [rad/s] n v Damping coefficient of the second-stage valve x v Servovalve spool position [m]

Development and Validation of a 1D Dynamic Model of an Injection Moulding Process and Design of a Model-Based Nozzle Pressure Controller

“…The model is seen as two volumes: the antechamber and the cavity. The approach with two cavities is widely used in, e.g., [34][35][36]. Rafizadeh et al [37,38] describes the filling phase and holding phase with two models, wherein the pressure drop along the flowpath is described by the power law model for non-Newtonian fluids.…”

Control-oriented modeling of servo-pump driven injection molding machines in the filling and packing phase

“…This two-cavity approach is widely used, see, e.g. [34][35][36]. Rafizadeh et al [37,38] proposed separate models for the filling and the packing phase with a power law for the pressure drop along the flow path.…”

“…For instance, Shankar and Paul [33] used a machine with a constant hydraulic oil volume flow and a pressure-relief valve. Alternatively, servo-valves were used as flow control valves and the systems were typically modeled with constant supply pressure [7,36,39,41,45,[48][49][50][51]. Some manufacturers use variable displacement pumps instead of servo-valves [52].…”