Study On Several Key Problems Of Rotational Molding Process And Equipments For Producing Large-Scale Plastic Products

It is possible to produce high-precision large plastic products by one-time processing and manufacturing with the advent of rotational molding technology. The process of rotational molding mainly includes four steps:charging, heating, cooling and demoudling.This article regards all-plastic car bodies’precision manufacturing of new energy vehicles as an important entry point to state and study several key issues of rotational molding process and equipment from the theoretical models,equipment optimization, mold innovation,intelligent control in rotational molding process, etc. This research has important scientific and practical significance for continuously improving rotational molding theories and promoting the development of rotational molding technology.Firstly, according to the temperature distribution and changing with time characteristics of large rotational molding machines and molds, the paper creatively proposes multi-stage and multi-mode heat transfer models of complex molds and establishes the parametric relationship between heat (q) and temperature (T) among the four key stages of rotational molding. Moreover, the influencing factors on heat transfer characteristics are discussed and obtained in rotational molding process. This research has filled in gaps in the theoretical calculation of heat transfer and temperature rising time in rotational molding process based on oven-type rotational molding machine.Based on the multi-stage heat transfer models, using the Finite Element Method, this article analizes and optimizes the oven temperature field of rotational molding machines as well as the stress fields of mechanical arms and mold racks. Meanwhile, this article puts forward a series of design schemes including distributing fins in the air outlet, integrating oven and bake, improving stiffness of mechanical arms and mold racks and so on. Furthermore, the optimization of existing equipment is made. And experimental study is also made. Study shows that the thermal efficiency has been greatly improved, mechanical parts have less deformation and higher stability. Last but not least, compared with traditional open fire system, the oven institutions can shorten the molding time by more than 20% and reduce energy consumption by 30% or more; compared with the original structure (the oven-tunnel separated), time can be shorten by more than 50% and energy consumption can be reduced by 40% or more.From the aspect of mold design and manufacture, considerting the problems in large hollow rotational molding products like difficult manufacture of over sized and over weight aluminum block molds, high requirement of machine tools, hard guarantee of accuracy. With the calculus design idea, the mold is divided into several units and then units are combined, which not only significantly reduces manufacturing difficulty and cost of the mold, but also effectively guarantees the mold’s manufacturing precision. Besides, for large thin-walled metal mold, mumerical control incremental forming and the dimensionality reduction method of multi-stage three-dimensional figures are put forward, which effectively solves technical problems, such as large deformation and easy cracking of the large thin-walled metal mold in field of rotational molding manufacturing process.In terms of process control, wireless temperature measuring method in high-temperature environments and the corresponding devices are invented innovatively in order to solve the technical problem that the temperature inside the rotational mold in high speed rotation can not be detected in rotational molding process. With the wireless temperature measuring machine, multi-point and real-time online monitoring and effectively controlling of the temperature can be achieved inside and outside surfaces of the oven and the mold in rotational machine. Besides, based on this method, PLC intelligent control module is designed in the rotational molding, which realizes intelligent manufacturing process, avoids large energy consumption, low precision and other drawbacks by control of molding time,proportion control of burner and automation control of the traditional system.Through the implementation of the above research, rotational molding and intelligent manufacturing of large plastic products have been relatively perfect and mature. Industrial production is currently achieved in Wenling Rising Sun Rotomolding Technology Co., Ltd. And the products have been exported to Europe, America and other industrialized countries and highly recognized by the customers.