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The rapid heating cycle has the advantage of improving product quality in injection molding. In this study, steam heating was combined with cool water on the same mold design to achieve dynamic mold surface temperature to establish control. By applying the steam system on a TV housing mold, the advantage of using steam heating for injection molding was then evaluated and compared with water heating by experiment and simulation. The effect of steam on the quality of the part was also studied. Results showed that as steam was used, the heating time of the simple mold plate can be reduced from 18 s to 8 s with the heating rate of 9 °C/s, and the cooling time is reduced over water heating. When the target temperature is changed from 70 °C to 110 °C, the heating time of the TV housing mold plate varies from 7 s to 19 s. For the product quality, steam heating showed an improvement in both the gloss and hardness of the TV housing.23987
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1. Introduction
Injection molding is one of the most widely used processing technologies in the plastic industry. The mold surface temperature is critical in the plastic injection molding process. With a high mold surface temperature, the surface quality of the part will be better, although the cooling time will increase and, accordingly, the cycle time will rise as well. The decreasing of the mold surface temperature will reduce the cooling time, but there is no benefit for the surface quality of the part. A critical requirement for the current studies is to increase the mold surface temperature and still maintain a cycle time which is not too long.
In recent years, the requirement for a much thinner, lighter and better mechanical performing product is more and more important for the company. There are many challenges and difficulties in producing this type of traditional injection molding. Problems such as warpage, flow mark and weld line often appear. Consequently, a new injection molding technology, rapid heat cycle molding (RHCM) is proposed. Contrasted with traditional injection molding, in RHCM, the mold temperature is increased to the target value, then, after the filling step is finished, the mold will be cooled with cold water. With the help of the heating process, the melt can easily fill into the cavity under a low injection pressure. Besides that, the surface defect such as weld line, flow mark, and floating fibers can be eliminated.
For the heating process in RHCM, there are two main types of heating systems in use, surface heating and volume heating. In the previous group, several techniques have been researched. An insulation layer is
☆ Communicated by W.J. Minkowycz.
* Corresponding author.
E-mail address: shiachun@cycu.edu.tw (S.-C. Chen).
coated onto the mold base then a heating layer is applied to the insulation layer as the cavity surface. The heating layer can be quickly heated with a pair of electrodes and the insulation layer is used to enhance heating efficiency and decrease consumption [1,2]. On the other hand, for increasing the mold surface temperature in the filling process, a coating on the cavity surface with TiN and Teflon has reduced the heat transfer from the melt to the mold material, which increased the temperature on the cavity surface to 25 °C [3,4]. In another research project, on the heating surface, an electromagnetic induction coil with a different configuration was used to heat the cavity surface to reduce the weld line, shrinkage and other defects of the part surface [5,6]. Furthermore, an infrared heating system was also applied for heating the mold surface. This system can heat the surface of one or two mold halves using a suitable design [7,8]. For the newest application of surface heating, the hot air flowed into the cavity and heat convection from the hot air can directly heat the surface of the cavity [9]. The advantage of surface heating is the high rate in heating, so, the cycle time can be reduced. However, the user must have a special mold design when the mold is complex, and more equipment is needed to calculate the parameters for a high quality product. For volume heating, the most inexpensive way to achieve high mold temperature is to use hot water at a temperature as high as 90 °C or 100 °C for both heating and cooling. If the mold temperature needs to be higher than 100 °C, either a high pressure water supply system or a hot oil may be used [10]. The former may damage the channel connection and safety may be an issue after long-term use. Also, the latter may not be energy-efficient due to the low heat transfer coefficient of the oil. Local mold heating using an electric heating element is sometimes used to assist in high mold temperature control, especially for a thin-wall product. However, this still requires extra design and tool costs [11]. Further, a heater is usually used for