Study Of The Morohology And Mechanical Properties Of Fluid-assistant Co-injection Molding Pure Resin Parts

Fluid-assisted co-injection molding (FACIM), an innovative injection molding processwhich combines fluid-assisted injection molding (FAIM) technology and co-injection molding(CIM) technology can be used to produce hollow part with dual-coating structure. It can bedivided into two categories according to the fluid used in it: water-assisted co-injectionmolding (WACIM) and gas-assisted co-injection molding (GACIM). The mechanism researchon the FACIM has rarely reported. While the process needs to be developed further for itsgood application prospects.. Therefore, a deep investigation on the relationships among theproceeding process, microstructure morphology and mechanical properties of the parts wascarried outto provide some theoretical guidance and technical support to the application anddevelopment of FACIM technology.The main contents of this research were summarized as follows:(1) The forming principle, characteristics, the research situation and practical applicationof FACIM process were stated briefly. The main subjects of this research thesis were derivedfrom the reviews.(2) An experimental study of the penetration interfaces in overflow FACIM (O-FACIM)was carried out based on a lab-developed FACIM system. The injection sequence of the outerlayer and inner layer plastics can be reversible. Several cross-section cavities wereinvestigated in the experiments. The penetration behaviors of the water and the gas indifferent sequences and cavities were compared and analyzed. The penetration interfaces werecharacterized by the residual wall thickness (RWT). The experimental results showed that theRWT of the inner layer in WACIM fluctuated along the flow direction, while that in GACIMwas more even. The difference of viscosity between the outer and inner layer melts affectedthe stability of the interface between them. The penetration sections of the inner layer and thegas were closer to the cavity sections in GACIM, while the penetration section of the innerlayer and the water were closer to the circular forms in WACIM.(3) Morphologies of pure resin parts molded by FACIM and FAIM were investigated bymeans of POM and2D-WAXD. POM observations showed that both the WAIM and WACIMparts exhibit a―skin-core-water channel‖structure, i.e. a large number of very smallspherulites developed in the skin and water channel zone and fewer but larger spherulitesappeared in the intermediate zone. While both the GAIM and GACIM parts have a―skin-gaschannel‖structure which the size of spherulites increase gradually from the skin to gaschannel.1D-WAXD curves obtained from circularly integrated intensities of2D-WAXDpatterns and the crustallinity were calculated and compared. The results showed that the crustallinity for the WAIM and WACIM parts first increase and then decrease with theelevating distance from the skin surface, while those for GAIM and GACIM parts tend toincrease monotonously. Higher cooling rate appears in the skin zone of both the part as wellas the water channel zone of the WAIM and WACIM parts, resulting in a lower degree ofoverall crystallinity.(4) The simulation of the filling, cooling and crystallization process of WACIM wasachieved by means of fluent, a fluid simulation programe, and user-defined-functions (UDFs).Nakamura crystallization model was adoped for crystallization calculation. The simulationresults were verified by experimentations. Then the effects of processing parameters on thecrystallization were investigated vis simulations. It was found that the main dominated factorof crystallization were water temperature, the mold temperature and the melt temperature.(5) The mechanical properties of FAIM and FACIM products were studied by means oftensile experiments.The results showed that the tensile strength was mainly determined by theresidual wall thickness of sample, while the tensile elongation was influenced by thecrystallinity of products. The GAIM parts had low tensile elongation for their large grainsize near gas channel, while the tensile elongation of the WAIM products were higher due totheir small spherulites developed in the skin and water channel zone. The tensile of FACIMproducts were divided into two stages for the poor adhesion between the two layers. The skinand inner layers would be separated during the tensile process.(6) Air bubble and second radial penetration occurred in WACIM products were studiesby a combination of experimental and theoretical analysis methods. It was found byexperimentaltions that air bubble lead to poor tensile strength and tensile elongation of theproducts.The cause of the generation of air bubbles in the residual wall thickness of WAIMparts was revealed. Its reason is not water gasification, but the air existing in the waterinjection channels as a result of the defects of water injection device design or process. Theinjection of air and water during the water injection stage leads to air bubbles. To avoid the airbubbles defect, water injection device need to be improved and ensure only water be injected.The sharp transition of cavity cross section in flow direction formed the flow dead zone whichresults in the second radial penetration. To avoid the second radial penetration defect, thetransition of cavity cross section should be smooth.