Study On The Mould Heating And Cooling Method And Product Quality Control Technology Of RHCM Process

Rapid heat cycle molding (RHCM) is a newly emerging injection molding technology in recent years. It can enhance the surface quality of the molded plastic parts by dramatically reducing the part surface defects such as weld line, flow mark, jetting mark, silver mark, floating fibers, etc those nearly inevitable in conventional injection molding (CIM). By eliminating the following polishing and painting process, it thus shortens the entire production process and reduces the environmental pollution, energy consumption as well as the production cost. As a new and developing injection molding method, there are still many scientific and technical challenges need to be investigated, especially in the fields such as mold design, process control and product quality control, this paper is mainly focusing on the study of heating/cooling method and product quality control in RHCM.A dynamic mold temperature control device has been developed, which includes the electrical boiler and mold temperature control. By merging the electric boiler into a steam mold temperature control cabinet, it is possible to integrate the switching device between steam heat source and valve pipeline as well as the controlling and monitoring unit. A box shape part is used as an example to design and manufacture the RHCM mould, the thermal response is analyzed based on the mould thermal response process. The efficiency and characteristics of thermal response of RHCM is also analyzed. Furthermore, the influence model is established to evaluate the effect of heating and cooling time on cavity surface at highest and lowest temperature respectively. The thermal response test is analyzed using nonlinear regression analysis with least square method. A mathematical model of quadratic polynomial has been built to describe the highest and lowest temperature of the cavity surface during thermal cycling of the RHCM mould. According to the experimental results, it shows that the proposed model is very effective and accurate. Based on the developed quadratic polynomial model, it is very effective to predict the variation range of cavity surface temperature under different combination of heating and cooling time. If the variation range of cavity surface temperature of RHCM were known, the optimization of mould heating and cooling time can be obtained by solving the binary quadratic equation formed by two quadratic polynomials. Therefore, the model provides a theoretical guidance for setting reasonable parameters and simplifying the debugging process with better accuracy.A new rapid in-mold heating and cooling system has been developed based on electrical heating and water cooling method. The electrical heating elements are stuffed inside the mounting holes of a mold to heat the mold surface. Between the heating elements and walls, there are annular gaps which are used as the cooling channels to cool the mold surface. An analysis model is established to evaluate the thermal response efficiency of the developed system with numerical simulation. This paper also studied the influences of thermal response on the mold surface from the annular gap size, the power density of heater and the distribution of heaters. Based on the developed method, a RHCM mould for a LED TV panel is carried out, the practical effect is also verified based on the experimental system. The injection molding experiment shows that it is possible to obtain non-weld line products, reduce the energy and coolant consumption without extension of the injection molding cycle.The paper proposed a design method for rapid heating cycle mold targeting at the complex three-dimensional parts. The optimal design of heating and cooling pipeline, the effect of process parameters on the product quality has also been studied. For a complex three-dimensional plastics part, the rapid heating and cooling pipeline of mold has been developed based on the structure of the cooling well. The developed system can greatly enhance thermal response efficiency and uniformity of temperature distribution of the cavity surface. The mold time constant is reduced from the original5s to2.5s. The heating efficiency is increased27.3%. The temperature difference range of the whole cavity surface is decreased from40～50℃to20～30℃. A model is established to predict the temperature variation of the cavity surface using influence factors and regression analysis. It is provided to be effective to accurately predict the temperature change range and control the cavity surface temperature change. The optimized injection molding process parameters is successfully used to produce high gloss and non-weld line complex three-dimensional parts. The product surface gloss is higher than90while the difference of the part surface glossiness is less than2.4.Taking a LED TV panel plastic part as an example, the warpage of the molded part in RHCM is analyzed. The effect of the major parameters on the warpage is studied and the special attention is on the effect of packing pressure and packing time on warpage under different processing conditions. In accordance with the specialty of cooling and shrinkage of a high gloss model part, a two-steps packing pressure control method is proposed. The first step is to use lower pressure to bring relatively large shrinkage of low-temperature melt. The second step is to use higher pressure to reduce the shrinkage of cavity side, thus reducing the concave warpage of plastic part. The most effective parameter for warpage is the first-step packing time which contributes about21.63%, followed by cooling time16.61%, fist-step packing pressure14.92%, second-step packing pressure5.59%, second-step packing time3.27%, melt temperature2.93%and injection time1.77%. The actual experimental result shows that the concave warpage decreases gradually with the reduction of the first-step packing pressure with the same injection molding process parameters and condition. It means that the first packing pressure with lower pressure can ensure the large shrinkage of cavity core side. And the second packing pressure with higher pressure can fully compensate the shrinkage of cavity side plastic melt during solidification process. Eventually, it can effectively restrain the concave warpage of plastic part and improve the stability and qualification rate in RHCM process.With the same example above, this paper studied methods to effectively reduce the surface sink mark of high gloss plastic part, two approaches have been applied with the bench form screw stud and gas-aided injection molding respectively. It is the first time to introduce the gas-aided injection molding (GIM) into RHCM process and establish the rapid heating cycle gas-aided injection molding (RHCGIM) method. The internal gas-aided injection molding (IGIM) and external gas-aided injection molding (EGIM) are studied respectively with RHCGIM. The corresponding experiment verified the packing effect of gas-aided injection molding on the rapid heating cycle injection molding. Furthermore, the optimal design method of mould structure and gas assisted process parameters is studied. The experimental result shows that the IGIM is not suitable for RHCM, instead the EGIM can be applied in RHCM to effectively eliminate the sink mark and ensure the surface smoothness. To deal with the influence of mold core temperature and gas pressure on the EGIM performance, the paper further studied and found the key parameters which effectively solved the issue of surface sink mark around the rib and screw stud of RHCM parts.