- 上一篇：太阳能除湿型空调系统英文文献和中文翻译
- 下一篇：侧凹特征在注塑模具设计英文文献和中文翻译
3.2 Boundary conditions for the heating transfer
In conventional injection molding process, heating transfer among different components of the mold during heating and cooling stage is very complex, especially for the RHCM mold due to its structure complexity. Therefore, it is necessary to do some appropriate simplifications based on the following hypotheses:
1. Thermal properties of the polymer melt, cooling water,heating medium, and the mold are constant and are not relative to temperature. The gap resistance between the mold and the polymer melt is neglected. The temperature of the cooling water and the heating medium is unchangeable.
2.The plastic part is thin, and it contacts the mold wall completely. The surface temperature of the polymer melt is equal to that of the mold cavity wall.
3.Heat conduction, as well as heat convection between the mold and coolant or heating medium, is considered.In the RHCM mold, the stationary mold insert is surrounded by thermal insulations, so the outer surface of the stationary mold insert could be considered adiabatic from the surroundings.
3.2.1 Surface of the mold cavity
For the RHCM mold, the boundary conditions of the cavity surface during the heating and cooling stages are different.When it is heated, the cavity surface is exposed to the air,and heat convection and radiation must be considered. Itbelongs to the third boundary conditions and can be expressed as:
where, q is heat flux, λ is the thermal conductivity of the mold, t is the temperature of the cavity surface, t1is the air temperature, and α is the heat transfer coefficient between the mold and the air.
During the cooling stage, the mold cavity is full of melted polymer, so only heat conduction between the melted polymer and the mold cavity is considered.
3.2.2 Surfaces of the heating/cooling channels and outer boundary of the mold insert
The shared channels for heating and cooling are used in the RHCM mold for the panel of LCD TV. In the heating stage,around 180 saturated steam is poured into the channels. In the course of the cooling stage, the cooling water in the channels is about 20–30 . So, heat transfer between the
channels and different temperature media must be considered differently. As there are heat insulations between the stationary mold insert and the fixed plate of the RHCM mold, the external surface of the mold insert can be regarded as adiabatic surface.
Based on the analysis mentioned above, the heat transfer coefficients used in this paper are shown in Table 1.
3.3 Material properties and processing conditions
According to the requirements of the RHCM mold, a sort of hot work tool steel (VIDAR SUPERIOR) produced by Sweden is selected as the mold material, and the polymer material is high rigidity ABS. The relationship between
material properties and temperature is not considered in the analysis process. Table 2 gives the main properties of the mold and polymer materials. For RHCM, when the temperature of the cavity surface reaches at about 120 , the high rigidity ABS polymer is allowed to inject in order to ensure the quality of the products. When the average temperature of the mold cavity surface is cooled down to around 80 , the mold is opened and the molded part is ejected. The processing parameters used in this paper are shown in Table 3.
Based on the research above, the finite element model for the mold of the panel is established in MSC.MARC software. Analysis of the heating and cooling processes is carried out.
3.4 Heat transfer analysis of RHCM mold
In RHCM injection process of the panels, the polymer is allowed to inject when the cavity temperature approaches 120 . Otherwise, the quality of the products will be badly affected by the cavity temperature and its distribution uniformity. In this paper, the diameter of the channels is set to 7 mm while the distance between them is 16 mm. P1–P8 are the temperature tracking points along the cavity surface which are shown in Fig. 7. When the mold is heated about 25 s, each temperature of the tracking points on the cavity surface is shown in Fig. 9, and the average temperature of the tracking points is also shown in the figure.