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Effect of decoration film on mold surface temperature during in-mold decoration injection molding process ☆

Shia-Chung Chena, b, , , Hai-Mei Lic,

Shih-Tsun Huanga, Yi-Chao Wanga

a Department of Mechanical Engineering, Chung Yuan Christian University, Chung-Li, Taiwan b R&D Center for Membrane Technology, Chung Yuan Christian University, Chung-Li, Taiwan c Advanced Polymer Processing Technologies of National Engineering Research Center, Zhengzhou University, Zhengzhou, Henan, China Available online 4 February 2010.

http://dx.doi.org/10.1016/j.icheatmasstransfer.2010.01.005, How to Cite or Link Using DOI Permissions & Reprints

Abstract

In-mold decoration (IMD) during injection molding is a relatively new injection molding technique and has been employed for plastic products to improve surface quality and achieving colorful surface design, etc. During IMD processing, the film is preformed as the shape of mold cavity and attached to one side of the mold wall (usually cavity surface), then molten polymer is filled into the cavity. Heat transfer toward the mold cavity side during molding IMD part is significantly retarded because the film is much less thermal conductive than metal mold. To investigate the effect of film on temperature field, polycarbonate (PC) was injection molded under various conditions including coolant temperature, melt temperature, film material and film thickness. Simulations were also conducted to evaluate the melt–film interface temperature and its influence from film initial temperature and film thermal properties. For PC film, it was found that the heat transfer retardation results in the mold temperature drop in cavity surface and the maximum temperature drop as compared to that of conventional injection molding without film may be as high as 17.7 °C. For PET film, this maximum mold temperature drop is about 13 °C. As PC film thickness increases, the retardation-induced mold temperature difference also increases. The initial film temperature (30 °C and 95 °C) may affect the melt–film interface temperature at the contact instant of melt and film by about 12 °C to 17 °C. When thermal conductivity of film increases from 0.1 W/(m–k) to 0.2 W/(m–k), melt–film interface temperature may vary by 22.9 °C. The simulated mold temperature field showed reasonable agreement with experimental results. Keywords

Mold surface temperature;

Film–mold and melt–film interface temperature; Heat transfer retardation; Film-insert;

In-mold decoration injection molding

1. Introduction

Injection molding is one of the most useful polymer processing methods because complex geometry products can be manufactured rapidly at relatively low cost. Injection molding is a cyclic process and can be divided into four stages, namely filling, packing, cooling and ejection, with pressure and temperature variations [1]. In-mold decoration injection molding (IMD), or film-insert injection molding, is a relatively new injection molding technique in which molten polymer is introduced into the cavity after a decorated film is attached to one side of the mold walls, usually in cavity surface. Recently, IMD has been employed for production of various molded products to improve surface quality such as automotive interior parts, cellular phone cases, and logo imprinted plastic products. IMD's primary advantage is the integration of several manufacturing steps (production and decoration of molded part) into one production operation [2]. Since surface quality of plastic parts can be improved with the inserted decorating film during the injection molding process, the process is designated as in-mold decoration (IMD). The relevant IMD molding also includes in-mold labeling (IML) and in-mold rolling (IMR) [3]. The surface quality of film-injection molded parts without any post-processing such as screen printing or spray painting, therefore IMD is a highly advanced method compared with the conventional injection molding [1], [2], [3] and [4]. Moreover, the injected hot melt resin may partially join with the film

resulting in enhancement of adhesion between the film and the molded part after cooled [5] and [6]. On the contrary, the heat from hot melt may soften film, e.g., PET film [4], cause winkles on the film surface, and damage printed color patterns result in unacceptable parts. The thermally induced residual stress/warpage and film–part interface, etc., are also challenging issues for IMD molding [6], [7] and [8]. The film-insert molding process was also applied for other purposes [9] and [10].

Heat transfer within the IMD part in the perpendicular direction to the cavity wall can be retarded similar to that of cavity surface coating [11] because the film is attached to the cavity wall. Furthermore, the heat transfer along the flow path causes different temperature boundaries for cavity surface (with film) and core surface (no film). The non-uniform heat transfer in the cavity introduces non-uniform temperature distribution across the gapwise direction during filling and cooling stage (Fig. 1). Unbalanced flow front advancement, severe warpage/stress and other influences on part's properties such as non-uniform crystallinity and orientation may occur due to asymmetric temperature distribution in the cavity wall [3], [4], [5], [6], [7] and [8]. All these studies were more focused on the processing characteristics. Although thermal effect from inserted film has been reported [3], [4], [9] and [10], detailed investigations on the characteristics of mold temperature field via simulations or measurements are not yet reported. It is essential to understand the heat transfer behavior at the interfaces of mold–polymer and melt–film–mold induced from the inserted film. Afterwards, one can continue to perform the relevant predictions regarding residual stresses and deformation, which are important plastic parts' properties for better practical applications.

Fig. 1. Schematic of (a) film-induced asymmetric melt flow front advancement and (b) asymmetric melt and temperature profile. View full-size images

In this study, the asymmetric mold temperature profile in the direction perpendicular to the cavity wall due to the one-side film attachment was the key concern for IMD processing. In order to investigate film effects on mold temperature field of IMD injection molding, polycarbonate (PC) was injection molded under various conditions including changes of coolant temperature, melt temperature, film material, film thickness, and film initial temperature. Interface temperatures of mold–polymer (side without film) and polymer–film and film–mold (film side) were measured by experiments. To understand the interface temperature of polymer–film variation during processing, thermal simulation and analysis of the IMD process were also performed. The experimental results were compared with numerical results. 2. Experiment and simulations

The material for film-injection molding experiments was polycarbonate, Lexan HF1130, with density of 1191.5 kg/m3, thermal conductivity of 0.22 W/m-°C, and specific heat of 2100 J/kg-°C, supplied by General Electric Plastics. The mold was made by P20 steel with density of 7850 kg/m3, thermal conductivity of 31.5 W/m-°C, and specific heat of 501.6 J/kg-°C.

Four square plates, with dimension 98.5 mm by 98.5 mm by 1.2 mm, were injection molded on a Sodick HSP100EH2 Japanese injection molding machine. The mold had a fan-shape gate of 20 mm length and an entrance of 27 mm by 1 mm and a size of 5.5 mm in beginning diameter and 7 mm in ending diameter for the sprue runner.

The IMD injection molding conditions were listed in Table 1. The injection speed, filling time, packing pressure and packing time were set to 100 mm/s, 0.306 s, 100 MPa, and 1 s, respectively. To investigate the relevant influence, coolant temperature was varied from 75 °C to 95 °C in increase of 10 °C by mold temperature controller (BYCW-021410FS, Taiwan), while melt temperature was varied from 280 °C to 310 °C with an increase of 15 °C.

Table 1. Film-injection processing parameters.

Mold temperature (°C) 75, 85, 95 Melt temperature 280, 295, 310 Injection speed (mm/s) 100 Packing time (s) 1 Packing pressure (Mpa) 100 Filling time (s) 0.306

Full-size table

The PC films of 0.175 mm thickness and PET film of 0.05 mm thick were used and attached to one side of the mold walls, respectively. The associated effects on mold temperature and relevant influence were investigated. The PET film used has density of 1405 kg/m3, thermal conductivity of 0.2745 W/m-°C, and specific heat of 1924 J/kg-°C whereas PC film having density of 1250 kg/m3, thermal conductivity of 0.207 W/m-°C and specific heat of 1224 J/kg-°C.

Temperatures at three locations as shown in Fig. 1(b) along the gapwise direction of the mold cavity in IMD process were selected as reference for evaluating the retarding effect from film on mold surface temperature [3], [11] and [12]. The first one is designated as TB, the temperature at the interface of mold and polymer melt. TB is approximately the same mold surface temperature as that of conventional injection molding without film; the second one is designated as Tc, the temperature at the polymer–film interface, a special location difficult to measure the interface temperature during IMD process; the other one is TD, the temperature film–mold interface, a special temperature boundary in IMD process, which is different from conventional injection molding. Tc here only could be specified by simulated methods while TB and TD could be obtained by both experimental measurement and numerical simulations.

Mold surface temperature or mold–polymer interface temperature, and film–mold interface temperature were measured by temperature sensors (Type 4003B from PRIAMUS). Two temperature sensors with 1 mm diameter were flash embedded in mold cavity surface and mold core surface at the center location to monitor the temperature variation during IMD molding process. Fig. 2 and Fig. 3 show the typical temperature profiles of cavity and core surface for conventional injection molding and IMD process, respectively. The sight mismatch of temperature profiles for mold core and cavity is due to that different mold components are composed. For example, the existence of runner system leads to a higher cavity surface temperature even though the cooling system is designed symmetrically on both mold bases. Despite of this mismatch, one is able to identify the influence for film by looking at the maximum temperature on the cavity surface with and without film. The situation is clearly shown in Fig. 4. For conventional injection molding, mold surface temperature will rise when melt contacts mold surface during the melt filling stage. In the case of IMD molding, the film retards heat flow into mold surface resulting in a slower rise in mold surface temperature. As a result, there exists a difference in the peak temperature value of mold cavity surface when molded with and without film. In Fig. 4, this temperature difference is designated as retardation-induced temperature drop, RTD.

Fig. 2. Experimental measurement on the temperature profiles of mold cavity and core surfaces during conventional injection molding process. View full-size images

Fig. 3. Experimental measurement on temperature profiles of mold cavity and core surfaces during IMD injection molding process. View full-size images

Fig. 4. Retardation-induced temperature drop (occurring at the end of melt filling) at mold cavity surface between film-insert IMD and conventional injection molding. View full-size images

Furthermore, the thermal analysis software COMSOL was used to simulate the film effects on the temperature variation during film-injection molding by setting similar boundary conditions and initial conditions according to processing conditions. Various film thicknesses (0.175 mm, 0.350 mm, and 0.525 mm), initial film temperatures (30 °C and 95 °C) and film thermal conductivities (0.1 W/m-°C, 0.15 W/m-°C and 0.2 W/m-°C) were also assumed for the simulations.

3. Results and discussions

Fig. 5 showed measured results of mold center temperature difference for cavity surface at the end of melt filling with and without PC film. This retardation-induced temperature drop, RTD, decreases with increased coolant temperature and it increases with increased melt temperature. Within the processing window, RTD is in the range of 13 °C to 18 °C. For PET film, Fig. 6 shows similar trend and RTD is in the range of 10 °C to 13 °C. Since PC film is thicker than PET film, its heat retardation is expected to be higher than PET film, therefore, RTD is higher.

Fig. 5. The experiment results of melt temperature effect on the retardation-induced temperature drop (PC film).

View full-size images

Fig. 6. The experiment results of melt temperature effect on the retardation-induced temperature drop (PET film).

View full-size images

For melt–film interface temperature, one has to resort to simulation. Fig. 7 shows the simulated profiles for TB, Tc and TD when melt temperature is 310 °C and coolant temperature is 75 °C. Fig. 8(a) shows the comparisons of simulated and measured retardation-induced temperature drop at various coolant temperatures and Fig. 8(b) depicts the case for various melt temperatures. The simulated results show reasonable agreement with the experimental measurements. The slighter higher value (～ 3 °C) of simulation is because it assumed that melt fills cavity completely initially. In reality, within the several tenths of seconds, the retardation-induced temperature difference decreases.

Fig. 7. Simulated temperature profiles of TB, Tc and TD. View full-size images

Fig. 8. Comparisons of simulated and measured retardation-induced temperature drop: (a) variation with coolant temperature and (b) variation with melt temperature. View full-size images

Fig. 9 shows the simulated contact temperatures (Tc) at the melt–film interface under various processing conditions assuming different thermal conductivities of PC film. When thermal conductivity of PC film increases from 0.1 W/m-°C to 0.2 W/m-°C, the contact temperatures (Tc) at the melt–film interface may decrease by 23 °C.

Fig. 9. Simulated contact temperatures (Tc) at the melt–film interface under various processing conditions.