Numerical Simulation And Experimental Study On Gas-assisted Co-injection Molding Process

Gas assisted co-injection molding(GACIM), an advanced polymer molding processwhich combines gas assisted injection molding(GAIM)technology and co-injection molding（CIM）technology, can be used to produce hollow pipes with dual-coating structure. Themolding mechanism of GACIM is extremely complex because of multiphase-multilayer flowof outer polymer, inner polymer and gas in the process. However, the mechanism researchand experimental study on the GACIM has rarely reported. Therefore, the mechanism ofGACIM was investigated by means of numerical simulation and experimentation to providethe theoretical guidance and technical support to the development of GACIM technology.The main contents and results of this paper were summarized as follows:(1) The principle, process, characteristics, research situation and practical application ofGACIM technology and its related technology were stated. The main subjects of this researchthesis were derived from the reviews.(2) Based on the rheology and fluid dynamics theory, the governing equations ofGACIM process were established with some reasonable assumptions according to thecharacteristics of the GACIM filling process. And the Cross-WLF viscosity model wasadopted. Finite volume method is applied to discrete the governing equations and thegoverning equations were solved via PISO algorithm which is suitable to deal withpressure-velocity coupling problem.(3) A lab-developed gas-assisted co-injection molding system, including high pressuregas generator and injection mould was built. The mould cavity was made in insert blocks inorder to investigate different cavity geometry.(4) Based on2D axisymmetric model, numerical simulation of GACIM process forround straight pipes is conducted via Fluent, a famous CFD fluid analysis software, to analyzethe effect of processing parameters (the gas injection delay time, the gas injection pressure,the inner melt injection temperature, the inner melt injection delay time, and the inner meltinjection pressure) on GACIM parts quality. The simulation results were verified byexperiments. The results showed that the total residual wall thickness of GACIM partsdecreased with the increasing of the gas injection pressure, the inner melt injectiontemperature, and the inner melt injection pressure, and increased with the increasing of thegas injection delay time and the inner melt injection delay time. The effects of these processing parameters on the residual wall thickness of the inner melt were contrary to that ofskin melt. The residual wall thickness of overflow GACIM parts was much more uniformthan that of short shot GACIM parts by method. Furthermore, it was found that the maininfluences on the total residual thickness and the inner melt thickness were gas injection delaytime and gas pressure by means of orthogonal experiment.(5) Numerical simulation and experimental research on the GACIM pipes with curvedsections of different bending angle was conducted. The simulation results showed that thepenetration of the gas was always closest to the inner concave side. And the residual wallthickness at the inner concave wall was thinner than that at the outer convex wall. Theexperimental results showed that both total residual wall thickness and its fluctuationdecreased with the increasing of the bending angle, the gas injection delay time and gasinjection pressure was mainly affected the residual wall thickness of the inner layer melt.(6)3D numerical simulation and experiment research on the GACIM pipes with irregularcross section were performed. The inner and skin melt temperature, the distribution of theinner and skin melt residual wall thickness, and material distribution of irregular cross sectionwere showed by simulation. The simulation showed that the temperature on the cavityconcave angle dropped a lot, and it made the viscosity and the total residual wall thicknessincrease. The experiment research showed that the penetration shape of the inner layer meltwas similar with the cavity section, and the penetration shape of the gas was different with thechange of cavity section, gas injection delay time and gas injection pressure.