2.1. Experimental Method

Considering the disadvantages of GMTC, in this study, Ex-GMTC was employed in injection molding for mold temperature control within a gas temperature range of 200–400 °C. The general process consisted of six steps, as presented in Figure 1. In Step 1, as soon as the molding cycle completed and the product was ejected, the hot gas source is transported to the heating area at which the cavity temperature is still low. In the next step, the hot gas sprayed directly into the heating area. Due to heat convection, the thermal energy was transferred to the cavity area, heating that area to the target temperature, after which the heating process stopped. At this time, the cavity remained at a high temperature. In Step 3, the gas drier was removed from the molding area and two half molds closed, before a new molding cycle began by melt filling into the cavity in Step 4. In this step, the hot cavity allowed the melt to easily flow into the thin-walled area. As soon as the entire cavity filled, the cooling process started with the heat transfer from the hot melt to the mold plate in Step 5. The process stopped when the part temperature reached the ejection temperature. In Step 6, the two half molds opened to eject the part, and the next molding cycle occurred.

General steps of external gas-assisted mold temperature control in the injection molding process.

Figure 2 shows the assembly of the heating system. This system includes a cooling system, a hot gas supporting with a 12-kW power, an Ex-GMTC controller. The controller will control the robot arm for moving the hot gas supporting. An air drier with outside dimensions of 240 × 100 × 60 mm was used to generate the hot air. Ambient air was allowed to flow into the air drier at 0.7 MPa, after which it flowed along the gas channel to absorb the thermal energy from the hot wall of air channel. The hot air then flowed out at the gas gate with a hole diameter of 10 mm. The high-power hot gas generator system supported the heat source, which provides a flow of hot air reaching 400 °C.

Schematic of the Ex-GMTC system.

A thin-walled product, presented in Figure 3, was examined to assess any improvement in the weld line appearance. This part had dimensions of 80 mm × 35 mm and a thickness of 0.4 mm; it also had a hole at the center. During injection molding, with the melt gate entrance presented in Figure 3, the hole was the main reason for the appearance of a weld line, with the two-flow molten plastics knitting together. The line creates issues in the product’s notch or gloss difference, discoloration, or glass fiber streaks in composite materials, which become more serious with thin-walled products. Figure 4 presents the injection mold design structure for the observation of the weld line. There were two cavities in the mold plate: one designed with a common structure and another with an insert for supporting the heating process at the weld line area, which is presented in Figure 5. This design is practical as it promotes heating efficiency and better control of the heated area.

Dimensions of product used in the weld line appearance experiment.

Mold design used in the experiment.

Insert plate used in the heating process.

After investigation of the weld line appearance, the weld line strength was measured using a tensile strength test model. In some studies, a flat product was used for weld line strength measurement. In this study, the heating efficiency of Ex-GMTC on the complex cavity surface was verified using the model on a weld line area with a mesh structure. This design is detailed in Figure 6. This part was filled by two gates at the side, allowing the weld line to appear in the mesh area. The thickness (t) of the mesh area used was set to 0.4, 0.6, or 0.8 mm. To improve the tensile strength of the mesh structure, Ex-GMTC was applied to increase the temperature of the meshing area before the melt filled into this position. In the experiment, the tensile strength was measured with the mold presented in Figure 7, designed with two cavities. The melt filled both cavities; however, only one was heated by Ex-GMTC. Thus, the tensile strength could be compared both with and without Ex-GMTC. Each cavity was also filled by the two gates at the side, which allowed the weld line to appear in the center of the molding part.

Dimensions of the specimen for a tensile strength test with a mesh structure at the center.

Cavity of the specimen used for the tensile strength test.

As with the mold in Figure 7, the insert structure and stamp in Figure 8a,b were also applied in the mesh area. The insert stamp was 10 mm × 25 mm with a thickness of 5 mm. Here, the hot gas was sprayed directly to the insert stamp to increase the insert temperature and improve the quality of the weld line during the molding process.

(a) block insert; (b) insert stamp.

In both experiments (weld line appearance and strength), the molds and the Ex-GMTC module were assembled using a SW-120B molding machine (Shine Well Machinery Co., Ltd., Tainan, Taiwan) and the other equipment shown in Figure 2. Figure 9 presents the heating step and the positions of the hot gas generator, hot gas gate, and heating area.

Heating step in the experiment.