Study On The Key Technique Of The Gas-assisted Injection Molding

A new molding technique, gas-assisted injection molding (GAIM) which iscapable of producing parts having both thick and thin sections, with structuredrigidity and not sacrificing surface quality, has received extensive attention inrecent years. In this process, molten plastics are injected into a mold followed byan injection of pressurized nitrogen gas into the core of the melt to produce hollowparts. Despite some pitfalls in the practical application of GAIM process, it offers anumber of advantages over conventional injection molding (CIM). â‘ GAIM cansubstantially reduce cycle time. The melt injection time and cool time aredecreased, the pressure packing time is eliminated. It is propitious to reduce partscost. â‘¡GAIM can reduce the clamp tonnage greatly. During the CIM, clamptonnage must large enough to meet the needs of packing stage, especially to solvethe problem of sink marks. However, the low gas pressure is needed for packing inGAIM, thus, the clamp tonnage is reduced. â‘¢GAIM can save material. The firststage of this process is shot short, so the material is saved. â‘£GAIM can eliminatethe pitfalls of parts greatly. Part quality can be improved by reducing residual thestress, warpage, sink marks and shrinkage that are normally encountered in CIM.Focused on the key technology of gas-assisted injection molding, the mainstudy results are summarized as follows:1. Study on the "gas fingering"of the gas-assisted injection molding."Gas fingering"is a common problem of the gas-assisted injection molding.In this paper, it takes plate part as an example, studies the effects of melt / gas delaytime on the gas finger through the experience and the CAE simulation. It putforward a new way, that is largest gas fingering range and largest gas fingeringlength, to weight the extent of gas finger. From the result of experiment andsimulation, Some conclusions can be drew: melt / gas delay time is an importantfactor which forming gas finger. If the other parameters don't change, the largestgas fingering range and largest gas fingering length will reduce with the delay timeincreasing. 2. Analyzed numerical simulation of the gas-assisted injection molding. Based on the Moldflow, the FM truck roof handle is analyzed in ConventionalInjection Molding and Gas-Assisted Injection Molding. The influences of injectionshot, gas/melt delay time, melt temperature and gas pressure are observed and theinfluence of the single parameter to the result of process is also observed inGas-Assisted Injection Molding. Finally, the simulation result of ConventionalInjection Molding and Gas-Assisted Injection Molding is compared. Injection shot is a very important parameter. If the value of injection shot istoo low, the shot short and the gas penetration is existed, if it is too high, thevolume of gas injection is so small that the result of molding is affected. Thevolume of gas injection will decrease with the delay time increasing. However,ifthe value of delay time is too long, the temperature in the melt front will reducegreatly, the resistance of gas penetration is added, and the gas penetration is notenough, it will result in the shot short. The percentage of gas injection will increasewith the melt temperature increasing. When the temperature increase, the meltviscosity will decrease, the fluidity is so good that it is propitious to the molding.But, if the melt temperature is too high, the capability of gas entering the thin wallwill increase, and it can result in the gas penetrating the thin wall. The percentageof gas injection will increase with the gas pressure increasing. If the high gaspressure and injection velocity are used, it will reduce the melt viscosity, add thegas penetration and the percentage of gas in the gas channel, and it is propitious tothe molding. From the comparison of the simulation result of Conventional InjectionMolding and Gas-Assisted Injection Molding, we can draw a conclusion thatduring the Gas-Assisted Injection Molding , melt injection pressure is decreased by48.8%, clamp force is decreased by 54.4%. 3. Mold design of gas-assisted injection molding.