Prediction of the Film Thickness Distribution and Pattern Change During Film Insert Thermoforming Gugyong Kim, Kwango Lee, Sungsu Kang Department of Precision Mechanical Engineering, Pusan National University, Jangjeon-Dong, Geumjeong-Gu, Busan, South Korea Various surface process methods have been developed to decorate plastic or metallic products. Film insert molding (FIM) is one of the methods that enhance the functional and/or aesthetic qualities of a product’s surface. However, the drawbacks of FIM are that the thickness of the film can change, depending on the product configuration, and further, the pattern of the decorated film may change. Therefore, this article attempts to quantify the changes in the thickness and in the pattern of the decorated film during the FIM pro- cess. G’Sell’s viscoelastic constitutive law was adopted to describe the rheological behavior of polymer film. A constant-velocity uniaxial tensile test at high tempera- ture, which is a new method proposed in this research, was used to obtain the rheological parameters. We also suggested a visual method for predicting pattern change, which was validated by comparing analytical results with those of real products. POLYM. ENG. SCI., 49:2195–2203, 2009. ª 2009 Society of Plastics Engineers INTRODUCTION Traditional methods of decorating injection-molded parts have included painting, pad printing, and hot stamping. These methods are all post-molding process operations, which require additional processing steps and environmental controls. Recently, alternative decoration methods have been developed, including in-mold decora- tion and film insert molding (FIM). These processes are designed to enable increased flexibility in design, recycling, and reductions in the total cost compared with traditional decoration methods [1]. The in-mold process has several notable limitations such as wrinkles and the problem of indexing that can arise in large parts or in parts with complex or deeply contoured geometries. Also, because the decoration is on the part’s outer surface, it is vulnerable to abrasion, chemical attack, and UV degrada- tion. However, FIM differs from conventional in-mold decoration in that the decorated film, either flat or formed, becomes an integral part of the molded product during the molding process [2]. Typically, FIM begins by form- ing a preheated preprinted film, by means of vacuum or high-pressure forming [film insert thermoforming (FITF)], into the exact shape that is required for a tight fit in the mold. Then the formed film is cut (in the trimming pro- cess) and placed into the mold. During injection molding, plastic injects behind the film to form a molded part with an integral film layer (injection molding), as shown in Fig. 1. Therefore, the FITF process has a great influence on the shape and characteristics of film that has been decorated with a specific pattern. FITF differs from con- ventional thermoforming processes due to its use of multi-layered film. Numerous studies [3–14] have focused on the change in the sheet thickness and the effect of process conditions on formability during typical thermo- forming. Although research with regard to FIM [15–16] provides some information on the bonding strength between the inserted film and the plastic resin, little research has been conducted on FITF. The film thickness, following the FITF process, is an important parameter that determines the following factors: the pattern change on the decorated film; the clearance between the punch and the die in the trimming; and the filling of resin into the mold during injection molding. It is important to predict the film thickness distribution and the pattern change during the FITF process because a slight difference in the film thickness across cavities in a multi-cavity system, used for increased productivity, would result in different patterns. Therefore, in this arti- cle, an attempt is made to quantify pattern change during FITF using the finite element method. G’Sell’s viscoelastic constitutive law was adopted to describe the rheological behavior of polymer film. A con- stant-velocity uniaxial tensile test at high temperature, which is a new method proposed in this research, was used to obtain the rheological parameters. We also sug- gested a visual method for predicting pattern change, which was validated by comparing analytical results with those of real products. Correspondence to: K. Lee; e-mail: royallko@pusan.ac.kr Contract grant sponsors: The Ministry of Education, Science and Tech- nology (MEST); Ministry of Knowledge Economy—MKE (Industrial- Academic Cooperation Centered University). DOI 10.1002/pen.21467 Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2009 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2009