Correlation between runner pressure and cavity pressure within injection mold Abstract Cavity pressure in the injection molding process is closely related to the quality of the molded products and is used for process monitoring and control to upgrade the quality of molded products。 However, the sensor installed inside the cavity makes the molded product of the cavity defective, reducing productivity。 In view of this, this study investigates the correlation between melt pressure and cavity pressure in different runner positions and determines the appropriate run- ner position, where the runner pressure can represent the cavity pressure, in order to increase productivity。 First, the Taguchi method is used to obtain the optimal parameter com- bination of injection molding, and then, the experiment is conducted based on this condition in order to discuss the difference between runner and cavity pressure history profiles。 According to the experimental results, the quality of molded products can be monitored and controlled by installing sensors at different runner positions, and the maximum value of the cavity pressure profile varies with the runner position。 When the distance between the top of the pressure sensor mounted in the secondary runner and the outside diameter of the runner is greater than the maximum thickness of the molded product, the obtained pressure history profile approximates to that inside the cavity。 In other words, the runner pressure can represent the cavity pressure。 Mold temperature significantly influences the runner pressure history profile and form accu- racy (contour precision) of the lens during the injection mold- ing process, which can be used as a process monitoring and control parameter。 In addition, the cavity pressure profile at the cooling stage is closely related to the form accuracy of the lens and is the key to determining process monitoring and control parameter。75756

Keywords Injection molding 。 Cavity pressure 。 Form accuracy 。 Runner design

1 Introduction

Injection molding is extensively used for manufacturing dif- ferent kinds of plastic products and is characterized by low cost, high precision, high productivity, and production of complex products。 The injection molding process is pided into several stages according to the operating cycles, including plastication, filling, packing, cooling, and ejection。 First, the plastic pellets are plasticized into the melt after screw shearing and feed pipe heating。 Then, the melt is injected into the runner system of the mold to fill the cavities。 Finally, the finished product is ejected after cooling。 In recent years, with the rapid developments of the computer, communication, and consumer (3C) electronic industry, the products are required to have a light weight, thinness, small size, and high mechanical strength, accuracy, and surface appearance。 In order to meet these requirements, the plastic injection molding process will be considerably challenging。

The plastic melt during the injection molding process is a kind of thermo-visco elastoplastic fluid, with a flow that is highly nonlinear when filling molds and is not only complex but also difficult to predict。 Its mechanism is not completely understood。 Therefore, in order to obtain good-quality prod- ucts, a sensor is often installed inside the cavity for process monitoring and control。 For example, the process parameters are corrected by monitoring the cavity pressure in order to reduce shrinkage and warpage resulting from residual stress inside the plastic parts [1–3]。 There are many methods of monitoring the physical quantities inside a cavity during the process, including measuring the melt flow front [4–9], mold heat flux [10, 11], mold temperature [11–15], cavity pressure/

temperature [1, 2, 14–23], and dimensions and properties of

the molded part [3, 24–27]。 The sensor types include the frequently used contact type, noncontact type [28–31], and even a wireless type of sensor [32–36]。 The above studies used different types of sensors, mostly flat specimens in the same thickness for research, while seldom discussing the molded part in nonuniform thicknesses。 The injection molding process often uses direct cavity pressure for real-time moni- toring and control, as the cavity pressure has higher accuracy than the pressure in other positions, e。g。, pressure of hydraulic line or injection nozzle [37]。 Installing the cavity pressure/ temperature sensor in the cavity can obtain an accurate history