Bulletin of the JSME Vol.14, No.3, 2020

Journal of Advanced Mechanical Design, Systems, and Manufacturing

Simultaneous optimization of variable injection velocity profile and process parameters in plastic injection molding for minimizing weldline and cycle time

Sara HASHIMOTO*, Satoshi KITAYAMA**, Masahiro TAKANO***,

Yoshikazu KUBO**** and Shuji AIBA****

*Graduate School of Natural Science and Technology, Kanazawa University

Kakuma-machi, Kanazawa-shi, Ishikawa 920-1192, Japan

**Advanced Manufacturing Technology Institute, Kanazawa University

E-mail: kitayama-s@se.kanazawa-u.ac.jp

Kakuma-machi, Kanazawa-shi, Ishikawa 920-1192, Japan

*** Industrial Research Institute of Ishikawa

2-1 Kuratsuki, Kanazawa-shi, Ishikawa 920-8203, Japan

****Sodick Co., Ltd.

Ka-1-1 Miya-machi, Kaga-shi, Ishikawa 922-0595, Japan

Received: 30 May 2019; Revised: 8 July 2019; Accepted: 1 August 2019

Abstract

Weldline that is formed when two or more melt fronts meet is one of the major defects in plastic injection molding (PIM), which has an influence on not only the appearance but also the strength of a plastic product. Consequently, it is preferable to reduce it as much as possible for high product quality. On the other hand, high productivity is always required in the PIM. In other words, cycle time should be minimized. To achieve the high product quality and productivity, process parameters such as melt temperature, cooling time and so on plays an important role. In this paper, the process parameters in PIM are optimized for minimizing the weldline and the cycle time simultaneously. In particular, variable injection velocity that the injection velocity varies during the PIM process is adopted and optimized. A novel weldline evaluation is also proposed. Numerical simulation in PIM is so intensive that a sequential approximate optimization (SAO) using radial basis function (RBF) network is adopted for the design optimization. It is found from the numerical result that the trade-off between the weldline reduction and the cycle time is observed. Based on the numerical result, the experiment using the PIM machine (GL100, Sodick) is carried out. Through the numerical and experimental result, the validity of the proposed approach is examined.

Keywords : Plastic injection molding, Engineering optimization, Multi-objective optimization, Weldline reduction, Computer aided engineering

1. Introduction

Plastic injection molding (PIM) is a major manufacturing technology to produce plastic products. The PIM consists of three processes: filling, packing and cooling process. In the filling process, the melt plastic is filled into the cavity. Conventionally, the melt plastic is packed with a constant pressure in the packing process. Finally, the melt plastic is cooled down for solidification in the cooling process. High product quality is required in the PIM, and major defects such as warpage, volume shrinkage, short shot and weldline should be minimized or avoided, whereas high productivity is always required. In other words, cycle time is minimized in the PIM. Trade-off between the product quality and the productivity is generally observed, and process parameters such as melt temperature, injection velocity, packing pressure, packing time, cooling temperature and cooling time should be adjusted and optimized for the high product quality and productivity. Recently, computer-aided engineering (CAE) coupled with design optimization technique has been recognized as one of the powerful tools available and many papers have been published (Kurtaran

Hashimoto, Kitayama, Takano, Kubo and Aiba,

Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol.14, No.3 (2020)

and Erzurumlu, 2006; Changyu et al., 2007; Yin et al., 2011). The PIM simulation is numerically so intensive that response surface approach is valid for determining the optimal process parameters. In particular, a sequential approximate optimization (SAO) that the response surface is repeatedly constructed and optimized is widely used for the design optimization (Cheng et al., 2013; Dang, 2013; Shi et al., 2013). We have conducted several researches on process parameters optimization in PIM (Kitayama and Natsume, 2014; Kitayama et al., 2017a; Kitayama, et al., 2017b; Kitayama et al., 2018a; Kitayama et al., 2018b), through which it has been found that (1) conformal cooling channel could drastically improve the productivity, (2) the process parameters optimization using the conformal cooling channel was effective for enhancing the warpage reduction as well as the productivity, and (3) the clamping force should be taken into account for improving the product quality. Unfortunately, the weldline reduction is still a major issue to resolve. In the following paragraph, let us briefly review several related papers on weldline.

Weldline that is formed when two or more melt fronts meet is one of the major defects, which has an influence on not only the gloss appearance but also the strength of product. Then, it is important to reduce it for the high product quality. Li et al. optimized the melt temperature and the injection velocity for welldine reduction using Taguchi method (Li et al., 2007). Kim et al. also optimized several process parameters (the melt temperature, the mold temperature and the injection velocity) for welldine reduction using Taguchi method (Kim et a

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