Research On The Supercritical Fluid Injection System For Foaming Injection Molding

Supercritical fluid is a substance which is in a state beyond bothcritical pressure (Pc) and critical temperature (Tc), it has the propertiesthat is different from either gas state or liquid. Using supercritical fluids,especially those permanent gases which are in supercritical state,asphysical blowing agent,microcellular plastics with smaller cell size andsome kinds of the mechanical properties improved can be produced,meanwhile,saving the energy and material on foaming injection moldingprocess. In the microcellular foaming process, the permanent gas insupercritical fluid state is precisely controlled and measured by the SCFagent delivery system to inject into the melt through the barrel, and thenforms a single phase solution with the polymer melt. When a rapid andgreat pressure drop occurs, large numbers of nucleation sites are formed,and then the cells are expanded by the diffusion of the gas into thebubbles until the melt cools down. In the foaming process, the quality ofsupercritical fluid agent injected into the melt, as well as the injectionmethod, has a crucial impact on the final product performance. The purpose of this subject is to design the supercritical fluid agent injectionsystem that is suitable for micro-foam injection molding process based onthe requirements of microcellular foaming process and the characteristicsof the supercritical fluid under high pressure.Firstly, by analyzing the existing blowing agent injection system andthe requirement of microcellular injection molding process, SCF injectionsystem in two different injection methods were designed, one was thesegmented pulse injection mode, and the other was periodic continuousflow restricted injection mode. Secondly, siemens S7-200series PLC wasused as system controller, and the analog input and output expansionmodules EM235were selected to collect and dispose the analog data fromthe process, then the design of other hardware and the compiling of thecontrol program was completed. Meanwhile, siemens Smart700touchpad was selected as a platform for human-computer interaction, andconfigured the interface according to the requirements of the injectionsystem. Before setting the periodic continuous flow restricted injectionsystem, the flow restrictor, which was one of the key components of thissystem, should be simulated and do some verification experiments, onlyby this way can the flow restrictor designed meet the system requirements,meanwhile, sumarising the experience formula for the mass flow ratecalculation of flow restrictor which is suitable for this kind of studycondition. Finally, the SCF injection process was studied and two sets of the supercritical fluid injection systems were used on the injectionmolding machine to make products, at the same time, analyzing theapplication effects of the injection systems and comparing the differencebetween these two systems.