Study on Flow Imbalance during Filling a Multi-Cavity Mold Using a H-type Runners

Transcription

1 Key Engineering Materials Vols (2008) pp Online available since 2007/Dec/06 at (2008) Trans Tech Publications, Switzerland doi: / Study on Flow Imbalance during Filling a Multi-Cavity Mold Using a H-type Runners Sen Yeu Yang *, Tzu Chien Huang, Po Hsun Huang, Tai Yu Ko Department of Mechanical Engineering, National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei, Taiwan (R.O.C.) * Corresponding author: syyang@ntu.edu.tw Keywords: Multi-cavity injection molding, Flow imbalance, Shear heating Abstract. This study reports the experimental investigation on the flow imbalance phenomena during filling symmetrical multi-cavity in injection molding of optic elements. There are two aspects in this study: First, the flow imbalance in the eight-cavity mold with normal H-type symmetric runners were observed and measured with aid of short-shot experiments. Second, the effects of three processing conditions including injection speed, melt temperature and mold temperature on flow imbalance were investigated. Flow imbalances are consistently observed and quantitatively measured. The results have shown that proper injection speed, high melt and high mold temperature can reduce the flow imbalance. Introduction The electro-optical industry is growing rapidly in recent years and to develop effective mass fabrication techniques to lower the cost is critical for the electro-optical industry. Optical-grade polymers such as PC, PMMA, PS and COP have been among the best materials for mass production of optic elements due to their light weight, optical properties and easy processing. The production of plastic optic elements is usually performed with injection molding. To increase the productivity, the mold with multi-cavity is used. However, the filling of symmetrical runner system with sub-runners in a multi-cavity mold is not symmetrical, as shown in Fig. 1 [1-5]. The filling imbalance problem has been solved by using one-runner-per-cavity scheme (Fig. 2), which wastes material and time. Devices such as Melt-Flipper has also been employed to solve the problem of flow imbalance by mixing the melt inside the runners [6]. However, it increases the cost and operational complication. To observe the flow imbalance phenomena and to seek the possibility of reducing it through optimal processing condition, an eight-cavity mold with normal H-type symmetric runners was designed. Three processing conditions were chosen, namely the injection speed (V inj ), mold temperature (T mold ) and melt temperature (T melt ), to investigate their effects on flow imbalance. 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 TTP, (ID: , Pennsylvania State University, University Park, USA-23/07/14,01:12:06)

2 Key Engineering Materials Vols Figure. 1. Typical filling imbalance in a 8-cavity with symmetrical H-type runner system. [1] Figure cavity lenses molding with one-runner-per-cavity runner system. Experimental Approach In this study, a 15-ton injection molding machine (FANUC, ROBOSHOT S-2000i 30A) was used to perform the experiments. Since the focus of this study is on the runner system, the geometry of the cavity in this study is a disk, which has the diameter of 1 mm and thickness of 1.36 mm. The core can later be replaced for molding any optical element. The multi-cavity mold in this study was designed as two plate mold, which consists of movable and fixed platens. Two cooling channels of 8 mm diameter were implemented on both fixed and movable platens. The drawing and photograph of the 8-cavity with normal H-type symmetric runners are shown in Fig. 3. All the primary, secondary and tertiary runners are circular in cross section. The diameter in both primary and secondary is 3 mm, while that in the tertiary runners is 2 mm. For mold temperature control, a mold temperature controller (OTC-03S, KJ Co., Ltd., Taiwan) were employed. The mold temperature controller has a heating power of 0.5KW and is capable of raising the fluid of kerosene to 150 C. The polymer used in this study is Polystyrene (PS-861B, Taita Chemical Co., Ltd., Taiwan). Flow imbalances were observed and measured from short-shots. For measuring the flow length in symmetrical sections of runners, a profile projector (PJ-311, Miltutoyo, Japan) with a accuracy of 0.01 mm was used. The flow length difference in the inner and outer tertiary runners is defined as L = L inside L outside.

3 1308 Optics Design and Precision Manufacturing Technologies Normal H-type runners (a) (b) Figure cavity mold with normal H-type runners: (a) drawing, (b) photoghaph Results and Discussion Observation of the flow imbalance. Fig. 4 shows a typical short-shot obtained during filling the symmetrical H-type runners. The flow length in the inner side of the tertiary runner leads that in the outer side of the tertiary runner. The reason can be explained with Fig. 5. In the primary runner, shear heating causes the melt temperature near the wall to be higher than that in the center. The lower-temperature melt flows into the outer laminate of the secondary runner, and then into the outer tertiary runner. At the same time, the higher-temperature melt flows into the inner laminate of the secondary runner, and then into the inner tertiary runner [1-5]. As a result, the flow in the inner tertiary runner leads that in the outer tertiary runner. The shear heating induces melt temperature difference in tertiary runner and results in flow imbalance in the inner and outer tertiary runners.

4 Key Engineering Materials Vols Figure. 4. Filling imbalance observed in a typical short-shots showing leading in the inner side runners. Notations indicating the labels of location are also shown. (a) (b) Figure. 5. (a) Distribution of melt flowing in the primary, secondary and tertiary runners; (b) Simulated temperature in section A-A. Note that the melt flowing in the outer tertiary runner is thelow-temperature laminate. The effects of processing conditions on flow imbalance (a) The effect of injection speed on flow imbalance Consistently, the flow length in inner runner is longer than that in outer runner. The average flow length difference L as function of the injection speed is plotted in Fig. 6. Lowest difference is obtained with an injection speed of 50 mm/s. (b) The effect of mold temperature on flow imbalance Consistently, the flow length in inner runner is longer than that in outer runner. The average flow length difference L as function of the mold temperature is plotted in Fig. 7. Lowest difference is obtained with a mold temperature of 80 C. High mold temperature reduces the temperature gradient in the primary runner, reduces the shear heating effect, and therefore reduces the leading of flow length in inner runner.

5 1310 Optics Design and Precision Manufacturing Technologies (c) The effect of melt temperature on flow imbalance Consistently, the flow length in inside runner is longer than that in outer runner. The average flow length difference L as function of the melt temperature is plotted in Fig. 8. Least difference is obtained with a melt temperature of 245 C. Raising the melt temperature decreases significance of shear heating effects, and reduces the flow imbalance. Flow length difference (mm) H-type runners T melt=237.5 o C, T mold=80 o C, P back=85kg/cm 2 25mm/s 50mm/s 100mm/s 150mm/s Injeciton Speed (mm/s) Figure. 6. Difference in flow length between inner and outer tertiary runners when filling the normal H-type runner system under various injection speed. Flow length difference (mm) H-type runners V inj=25mm/s, T melt=245 o C, P back=85kg/cm 2 60 J 70 J 80 J Mold Temperature ( o C) Figure. 7. Difference in flow length between inner and outer tertiary runners when filling the normal H-type runner system under various mold temperatures.

6 Key Engineering Materials Vols Flow length difference (mm) H-type runners V inj=25mm/s, T mold=60 o C, P back=85kg/cm J J 245 J Melt Temperature ( O C) Figure. 8. Difference in flow length between inner and outer tertiary runners when filling the normal H-type runner system under various melt temperatures. Conclusion The flow imbalance phenomena in a 8-cavity mold with normal H-type symmetric runner system as the result of the shear heating effect during injection molding process are observed. Their differnece are measured. The effects of processing conditions on flow imbalance: injection speed, mold temperature and melt temperature are investigted. Results have shown that high melt and mold temperature can decrease the intra-cavity flow imblance, while only proper injection speed can result in least flow length difference. References [1] J. Beaunomt, J. Ralston, A. Shuttleworth and M. Camovale, Troubleshooting cavity to cavity various in multi cavity injection molds, Journal of Injection Molding technology, v.3, issue 2, PP (1999) [2] J. W. Bozzelli, Some Reasons Not To Use Multi-Cavity(>4) Tools, ANTEC [3] H. E. Casaldi, T. Michel, Process Window as Effected by Shear Induced Flow Imbalance in Multicavity Molds, ANTEC [4] P. Auell and M. Bailey, Effect of Thermoplastic Elastomer Material Properties on Mold Filling Imbalances, ANTEC [5] G. Y. Su, H. Yokoi, W. M. Yang, Study on Filling Balance of Plastic Injection Molding in Multi-Cavity Mold with H Pattern Runner System, PPS [6] Information of Beaumont Runner Technologies, Inc. on

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