Journal of Mechanical Science and Technology 27 (10) (2013) 2967~2971

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-013-0811-6

Model study on flow behavior for investigating coating conditions

in the in-mold coating process† Phuong NguyenThi1,2, Arim Kwon1, Yeong-Eun Yoo1,2,* and Jae Sung Yoon1,2

1Korea Institute of Machinery and Materials (KIMM), 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon, 305-343, Korea 2University of Science and Technology (UST), 217 Gajeong-Ro, Yuseong-Gu, Daejeon, Korea

(Manuscript Received June 10, 2013; Revised July 3, 2013; Accepted July 29, 2013)

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Abstract Flow behavior for different coating conditions in in-mold coating (IMC) process was investigated. Silicon oil with viscosities of 100,

350 and 500 cps was used as model coating materials for the process and was injected into the mold cavity by using a syringe pumping machine. Flow patterns were recorded through a transparent poly (methyl methacrylate) (PMMA) window mounted in front of the mold. Flow shape index k, which is defined as the ratio of the downward flow length to the horizontal average flow length, was obtained for each testing condition. The flow characteristics, such as shape, stability, and uniformity, can be expressed in terms of the ratio k. This flow shape index analysis was used to investigate how the flow inside the mold is affected by resin viscosity, coating thickness, and in-jection rate, which are major parameters in the IMC process.

Keywords: Flow characteristics; In-mold coating process; Injection rate; Viscosity; Flow pattern index k ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Several plastic parts produced by injection molding, which is one of the typical manufacturing processes for plastic items, have to be coated or painted on the surface with various mate-rials for decoration, protection, or functionality, such as anti-reflection, anti-fingerprint, and better grip [1-3]. Coating or painting on the surface of the injection molded article is con-ventionally performed as an independent process after the injection molded part is released from the mold [4].

This conventional coating or painting process is not environmentally friendly because a number of chemical sol-vents are used in the process, and the process usually has higher costs. Moreover, a structured surface with texture or pattern is not available for this process.

Thus, in-mold coating (IMC) process is recently being de-veloped as an alternative process to reduce the drawbacks of the conventional coating process [5-11]. In the IMC process, the surface of the injection-molded plastic article in the mold is coated prior to being released from the mold. The overall IMC process is shown in Fig. 1.

Considering that the entire process is performed in the closed cavity of the mold with the thermoset resins as the

coating materials, possible volatiles from the material can be minimized during the IMC process.

The main advantage of this process is that the thickness and the mechanical properties of the coated layer can be controlled more precisely, and a structured surface can be achieved. Coating costs are reduced because additional coating or paint-ing processes are eliminated. Moreover, the IMC process is environmentally friendly because only a small amount of sol-vent is used.

As previously mentioned, the IMC process is a sub-process of injection molding. The IMC process mainly consists of four stages, namely, filling, packing, curing, and releasing. In the filling stage, the mold is slightly opened after injection mold-ing of the plastic part to create a small cavity for the coating materials, which are to be injected through a separate gate . Then, in the packing stage, the mold is closed again while more resin is injected into the mold cavity until the cavity is completely filled. In the curing stage, thermoset resin is solidi-fied by chemical reaction, that is, cross linking between monomers. Finally, the mold is opened, and the thermoplastic part with the coated surface is released from the mold.

The quality of the coated layer is determined by the flow pattern and the curing of the coating resin during the IMC process. Specifically, the flow pattern directly affects thick-ness uniformity, surface appearance, and bubble formation in the coated layer. Therefore, the flow characteristics of the coating resin during the IMC process must be controlled to

*Corresponding author. Tel.: +82 42 868 7883, Fax.: +82 42 868 7149 E-mail address: yeyoo@kimm.re.kr

† This paper was presented at the ICMDT 2013, Busan, Korea, May 2013. Recommended by Guest Editor Haedo Jeong © KSME & Springer 2013

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obtain a high-quality surface. The resin flow is affected by various parameters, such as the thickness of the coating layer, the viscosity and the surface tension of the coating resin, the flow direction with respect to gravity (gate location), and the injection rate. A number of studies have investigated flow characteristics in the IMC process. A study by the Ohio State University proposed a 2D or 3D mathematical model based on the Hele-Shaw approximation to simulate the flow pattern successfully and to verify the results experimentally [6-11]. A number of other researchers reported results on IMC process to obtain a high-performance surface [12, 13].

Although several results were reported by previous studies on the IMC process, data are quite limited, which means that they cannot be used for designing the mold or for optimizing the process. Thus, this study on the IMC process focused on determining the flow pattern based on major process parame-ters, such as injection rate, coating thickness, temperature, flow direction with respect to gravity (gate location), and vis-cosity of coating materials [14, 15].

In this paper, a series of experiments was performed to ana-lyze the flow pattern based on injection rate, viscosity of coat-ing materials, and coating thickness. The flow patterns were recorded and analyzed by introducing a flow shape index, which provided a quantitative analysis method.

2. Experiments

2.1 Experimental setup

A special mold was designed, which has a transparent front plate made of PMMA, to observe the flow pattern. The overall size of the front plate window was 150 mm × 120 mm (Fig. 2). To visualize the flow pattern in the IMC process, silicon oil with different viscosities were selected as model coating mate-rials to simulate the UV curable resin that remains uncured during filling. Silicon oil is an adequate model material be-cause it is inert to chemicals and heat, and is commercially available in numerous grades. Given that the typical viscosity of the coating material in the IMC process ranges from 100 cps to 500 cps, silicon oil with 100, 350, and 500 cps of viscosity (Shin-Etsu Silicone Korea Co., Ltd) were used as

model coating materials. The experimental flow visualization apparatus consists of a test mold with a transparent window, a syringe pump, a number of valves to deliver and control the silicon oil, and a pressure transmitter to monitor flow pressure, as shown in Fig. 2. Silicon oil was injected into the mold cav-ity through a gate by using the syringe pump (Legato 200), which can control the injection rate.

A pressure transmitter was connected to the system just be-fore the injection gate to record the pressure of the flow at the gate. Two valves were used to control the flow more precisely and to eliminate bubbles from the delivery line at start-up.

2.2 Experimental procedure

First, the delivery tube was filled with silicon oil to elimi-nate bubbles that may be generated inside the tube during the early stage of filling [16-20] and cause flow instability, which is one of the main problems in the IMC process. Then, the tube filled with silicon oil was connected to the gate of the mold to conduct the filling experiment with silicon oil. Once the silicon oil was injected into the mold cavity, the images of the flow front in the mold and the time were recorded through the transparent window.

Table 1 shows the experimental setup conditions for the test. All experiments were performed at the upper gate location at injection speeds of 10, 20, 40, and 60 ml/min. Silicon oil with viscosities of 100, 350, 500 cps were used as model coating materials. A series of coating experiments was performed for two different coating thicknesses of 0.15 mm and 0.3 mm.

3. Results and discussions

Fig. 3 shows a flow domain of silicon oil during the filling process that was recorded through the transparent front win-dow. In this study, a flow front shape index k was defined as Eq. (1) from the image of the flow domain recorded during the filling step for the quantitative analysis of flow pattern.

Fig. 1. Schematic of IMC process.

Table 1. Experimental conditions.

Parameters Setup conditions

Injection rate (ml/min) 10, 20, 40, 60

Viscosity of resin (silicon oil) (cps) 100, 350, 500

Coating thickness (mm) 0.15, 0.3

Fig. 2. Experimental apparatus for flow visualization of silicon oil for IMC process.

P. NguyenThi et al. / Journal of Mechanical Science and Technology 27 (10) (2013) 2967~2971 2969

3 .( 1 2) / 2

RkR R

=+

(1)

As shown in Fig. 3, R1 and R2 represent the distance of the

flow front to the left and the right directions, respectively, from the gate, which are perpendicular to gravity. R3 shows the distance to the flow front from the gate to the direction of gravity. The non-dimensional value k indicates how the flow pattern is anisotropic because of gravity in the IMC process, thereby adversely affecting the coating layer. Consequently, this index can quantify the process conditions for the coating layer quality.

Two main forces act on the flow of the coating materials, namely, the gravity applied on the coating materials inside the mold and the momentum caused by the syringe pump. The momentum results in isotropic flow regardless of the viscosity of the coating material or the thickness of the coating layer. However, compared with this momentum caused by the pump, gravity is anisotropic, which consequently induces an anisot-ropic flow pattern.

Figs. 4 and 5 show that the flow pattern index k changes significantly with the thickness of the coating layer, the injec-tion rate, and viscosity of the silicon oil. As the injection rate increases by the pump causing the increase in momentum, the k value decreases, which implies that the influence of gravity decreases. The k value also decreases as the viscosity of the silicon oil increases, which indicates that the influence of gravity is decreased because the viscosity of silicon oil is iso-tropic. Increase in the thickness of coating layer increases the k value, because the volume of the fluid increases and be-comes more influenced by gravity, thereby increasing anisot-ropic body force as a result. The k value varies more signifi-cantly with the viscosity of the silicon oil for low injection rates than for high injection rates. This phenomenon can be justified qualitatively by considering two extreme cases, namely, zero injection rate and infinite injection rate. For zero injection rate, the k value should increase infinitely, because no flow would exist in the orthogonal direction to gravity. For infinite injection rate, the k value should converge to 1 as the injection rate approaches infinity. These two extreme cases imply that the k value exhibits similar tendencies as those presented in Figs. 4 and 5. This non-linearity to injection rate

can be moderated by increasing the viscosity of silicon oil or decreasing the thickness of the coating layer, because the in-fluence of gravity shows similar tendency in these extreme cases for the viscosity and the thickness of the coating layer.

To analyze the variation of k depending on the filling time (Fig. 6), the flow front was also recorded for every 1 s or 2 s as the silicon oil fills the mold cavity. In addition, the flow pattern index was obtained accordingly (Fig. 7). Table 2 shows the process conditions for the analysis of the flow shape in four cases.

Figs. 6 and 7 show that if the injection rate decreases, the viscosity decreases, or if the coating thickness increases, the flow tends to move downward during the entire filling process, which is the same condition as in Figs. 4 and 5. The flow pat-tern index k changes more significantly with time for cases 3 and 4 compared with cases 1 and 2. At the beginning of the filling step, the volume of the fluid in the mold was not suffi-ciently large to be influenced by gravity. Thus, the body force exerted by gravity remained small. Therefore, the flow pattern index k does not change significantly from 1, which indicates a circular flow. As the filling proceeds, the volume of fluid in the mold increases, and the influence of the gravity increases to the point where it can extend the flow front shape downward.

Fig. 3. Flow pattern in the IMC process.

Fig. 4. Change in flow pattern index k with injection rate (0.1 mm coating thickness).

Fig. 5. Change in flow pattern index k with injection rate (0.3 mmcoating thickness).

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4. Conclusions

The injection speed, the viscosity of the coating material, and the coating thickness were found to be influential parame-ters for flow characteristics under gravity. Flow characteristics

were also analyzed quantitatively by defining the non-dimensional value k as the flow pattern. A good understanding of the relationship between these parameters in this study can be used to enhance the quality of the coating layer for the IMC process. For a more precise optimization of the process, a series of systematic experiments and an analysis of more pa-rameters by using DOE are currently being performed.

Acknowledgment

This study was supported by the Ministry of Trade, Industry & Energy of the Korean government (project no. 10040061).

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Table 2. Process conditions for the analysis of flow pattern of the flow in four cases.

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Coating thickness (mm) 0.1 0.3 0.3 0.1

Viscosity of coating material (cps) 100 350 100 100

Injection rate (ml/min) 60 40 10 10

Fig. 6. Flow pattern of silicon oil for each flowing step of 1 s or 2 s in four cases.

Fig. 7. Changing of flow pattern index k in four cases.

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Yeong-Eun Yoo has worked on poly-mer processing such as injection mold-ing. His main research includes injection molding of nano/micro structures, com-posites material injection molding, and coating process. He joined the Korea Institute of Machinery and Materials in 2003. He is also affiliated with the Uni-

versity of Science and Technology. He worked at LG Chem. in Korea from 2000 to 2003 and at the UIUC in U.S.A. as a post-doc. from 1997 to 2000 after completing his degrees in mechanical engineering at the Seoul National University in Korea: S.B. in 1990, S.M. in 1992, and Ph.D. in 1997.

Phuong NguyenThi has worked on polymer processing including injection molding/in-mold coating process, and mixing test. She is expecting to finish her Master’s studies at University of Science and Technology in Daejeon-Korea, where she is majoring in Nano-Mechatronics. She joined the Korea

Institute of Machinery and Materials in 2011 under the de-partment of nano-manufacturing technology, nano conver-gence and manufacturing systems research division.