A Study on Computer Simulation of Plastic Lens Molding Process

In this paper, some molding process parameters such as injection time, packing time, packing pressure and process temperature etc. were optimized by the Computer Aided Engineering (CAE) simulation (Moldex 3D) for injection molding of a plastic lens. Some experimental trials were carried out for verifying of the CAE simulation results with checking of the lens shrinkage and birefringence etc. as well. The results showed that, the recommended molding process parameters from CAE simulation and the actual experiments were almost the same, hence the CAE is a established tool based on the scientific approach to reduce experimental works, to identify critical parameters and to save substantial costs. Lately, a perfect plastic lens was gained by the Injection–Compression Molding process with the optimized process parameters by a CAE simulation. Introduction The Plastic lens made by the injection molding process is one of the most demanding plastic fabrication process, which requires some stringent conditions in terms of stressfree and accurate profile of final products to meet the optical design requirements such as accurate transmission of light rays, narrow focus length tolerance, etc.[1]. Using Computer Aided Engineering (CAE) simulation tools, alternative design and process parameters can be explored before embarking on expensive molding trials. The lead-time for mold manufacturing and process cycle time can hence be significantly reduced, resulting in substantial operational cost savings. Furthermore, potential-molding problems can be detected early in the mold design stage prior to the mold making. This can shorten product development cycles and speed up the introduction of new products or alteration of existing product designs to meet constantly changing demands in the marketplace [2]. In this simulation, the product tested is an Optic Lens that is used for the data transmission. Thus, the amount of the mold-in stress, shrinkage (Fig.1), etc., become extremely important, and any deviations in profile or moldin-stress would deviate the accurate transmission of data. CAE for injection molding will be done to test and check for process feasibility, optimize process parameters as well as to determine the most critical parameters, which will influence lens quality. The result will then be compared with the actual mold trail parameters to further investigate the effects of injection molding process on the final product. CAE Simulation and Experiments Injection molding process is a complicated process, which consists of many process controls parameters and most of the parameters are interrelated to each other. Any variation of process parameters or reliance on trials and errors could result in product quality problems. Therefore, identifying the most critical process parameters is extremely important. Experimental Conditions . Moldex CAE software is used for the simulations of Injection Molding. The resin selected for this simulation is Styrol Polystyrene. The processing conditions Materials Science Forum Vols. 471-472 (2004) pp 490-493 online at http://www.scientific.net © (2004) Trans T ch P blications, Switzerland Online available since 2004/Dec/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-15/04/08,02:43:58) Materials Science Forum Vols. *** 491 recommended by the resin manufacturer and the default parameters setting from Moldex CAE are tabulated in Table1. The experiments were done in a Sodick injection molding machine TR 40/55 EH. Table 1 Initial CAE simulation parameter Melt temperature [ C ] 230 Mold temperature [ C ] 50 Injection pressure [Mpa] 120 Filling time [Sec ] 1 Packing pressure [ Mpa] 120 Packing time [Sec ] 1 Cooling time [ Sec ] 30 Optimization of the Critical Molding Process Parameters Injection Time vs Injection Pressure . Injection time denotes the velocity at which the screw advances during the injection phase. The experiment showed that short injection time requires high pressure due to the high flow rate. This item defines the time needed for the melt to fill the cavity during the filling stage. Fig.2 shows that the lowest point is at 0.6 second. Hence, the optimum injection time is 0.6 second since that required lowest injection pressure. Volumetric Shrinkage vs Packing Time. A packing pressure is exerted on the part according to the specified packing pressure profile over this time interval. If the specified fill time is too short, part of the pack time is used to push the melt until it fills the entire cavity. Fig.3 showed that the pack time has the excellent effect in determining volume shrinkage. In this experiment, the pack time of 2.5 second is selected because of the relatively low volumetric shrinkage of near to zero percent. Volumetric Shrinkage vs Packing Pressure. Fig.4 shows that pack pressure has the excellent effect in determining volume shrinkage. Too high pack pressure and Too low pack pressure imposes may cause flashes formation or high shrinkage and heavy sink mark on the molded part. It also shows that pack pressure of 50 Mpa is an ideal setting, because its results in the low volumetric shrinkage of near to zero percent. Melt Temperature . Fig.5 shows that the melt temperature has a good effect in determining filling stress and volumetric shrinkage. However, the melt temperature must not be higher than a critical point such that high shrinkage and plastic degradation occur. It also shows that the melt temperature of 230 °C is an ideal setting, because this resulted in relatively and low volumetric shrinkage. Fig.1 Volumetric shrinkage Fig.2 Injection time vs. injection pressure 0.0 0.2 0.4 0.6 0.8 1.0 10 20 30 40 50 In je ct io n p re ss ur e (M pa ) Injection Time (second) CAE Experiment Materials Science Forum Vols. 471-472 491 Advances in Materials Manufacturing Science and Technology 492 Mold Temperature . The main reasons for developing the optimal mold temperature are to improve the lens optical properties (lower birefringence) and to obtain better lens spherical contour which affect the focus length. Higher mold temperature corresponds to a rise in the volumetric shrinkage. Fig.6 shows that the mold temperature has a good effect in determining volume shrinkage, and the mold temperature of 50 °C is an ideal setting, which is due to low volumetric shrinkage of near to zero. Compare the Results with CAE Table 2 shows the recommended parameter from the CAE simulation and the actual optimized parameter from the injection molding experiment. Hence, the results between the simulation and experimental trials are almost the same. The differences are that the Injection pressure in CAE has a lower injection pressure and slightly shorter packing times than those the experiment. These differences could be due to the long nozzle of orifice diameter of 2.5 mm with the length of 100mm. This additional passage was not considered in the CAE simulation and it caused significant pressure drop due to such a narrow and long passage. 1.0 1.5 2.0 2.5 3.0 3.5 -1 0 1 2 3 4 V ol um et ric S hr in ka ge (% ) Packing Time (sec) CAE Experiment Fig.3 Packing time vs. volumetric shrinkage Fig.4 Packing pressure vs. volumetric shrinkage 25 30 35 40 45 50 55 60 65 70 75 -1 0 1 2 3 V ol um et ric S hr in ka ge ( % ) Packing Pressure (Mpa) CAE Experiment 210 220 230 240 250 -1 0 1 2 3 V ol um et ric S hr in ka ge (% ) melt Temperature (C) Fig.5 melt Temperature vs volumetric shrinkage CAE Experiment 2 0 30 40 50 60 70 -1 0 1 2 3 V ol um et ric S hr in ka ge (% ) mold Temperature (C) Fig.6 mold Temperature vs volumetric shrinkage CAE Experiment Fig.5 Melt temperature vs. volumetric shrinkage Fig.6 Mold temperature vs. volumetric shrinkage 492 Advances in Materials Manufacturing Science and Technology Materials Science Forum Vols. *** 493 Experime nt Trials on Injection Compression Molding The basic controlled parameters of Injection Compression Molding (ICM) are similar to the Injection Molding (IM), with additional parameters to control the second motor to activate the compression motions. It is obvious from Table 3 that the lenses produced by CIM had shrunk more and this will affect the contour (focus length) of the lenses. In comparison lenses produced by ICM have almost perfect lens thickness, hence, ICM is more suitable for production of high quality lenses. Table 2 Compare molding parameters of the CAE with experimental optimized data S/N Parameter CAE Recommended data Experimental Molding Optimized data 1 Process temperature [ °C ] 230 230 2 Mold temperature [ °C ] 50 50 3 Injection pressure [ Mpa ] 20 36 4 Filling time [ Sec] 0.6 0.6 5 Packing pressure [ Mpa ] 50 50 6 Packing time [ Sec ] 2.5 3.0 Table 3 Compare the results of IM lenses with ICM lenses after optimization Molding Method