Study On Cell Growth Mechanism In Microcellular Injection Molding

As a low-density material, the polymer foams attracts much attention and they have many excellent properties,like high specific strength,good thermal stability and heat insultance,and good insultance to sound. There are many kinds of techniques for producing microcelluar polymer foams,like polymer-solvent phase separation,microemulsion synthesis,solvent exchange and saturation by inert gases.Among these approaches,the preparation of polymer foams by inert gas saturation,has the potential advantages for the industrial production at a large scale.Microcelluar foaming(also called Mucell technique) is a processing technique adopting the inert gas saturation step in the production of polymer foams and can derive plastic foams in which the cell size and the related distribution are both uniform owning to the high nucleation rate.Several processing techniques have been developed,like microcellular batch foaming,injection molding foaming,continuous extrusion foaming,semi-continuous extrusion foaming and heat foaming.By the incorporation of microcellular foaming,the density of the material can be reduced 5-95% and the impact strength can be enhanced 1-2 times as well as the fatigue life,compared with the material manufactured by tranditional processing approach. Microcelluar foaming by extrusion method and batch method are widely used in the laboratory research.As the only processing approach for the plastic products with complicated structure,microcellular injection molding have many advantages over the tranditional injection molding,like processing products with lower injection pressure,shorter cycling time,less material consumption and better dimension stabiliy.When coming to plastic products with high precision surface, making use of microcellular injection molding is a meaningful selection,and the expoitation of this technique has been an important trend in the plastic injection molding industry.The process of microcellular injection molding can be divided into the following four steps:(1)The loading of raw material:raw materials in the form of particles are pumped into the barrel of the injection machine.(2)The transportation of the material in the molten state:as the result of the rotating reciprocating-screw,the plastic particles are transported into the heating area of the barrel,then plasticized and getting into the molten stage uniformly. (3)The injection of supercritical fluid and the blending process between SCF and the polymer melt:The SCF is injected into the barrel in the measurement area of the screw and the melt-gas solution can be formed at a high plastic pressure.(4)The melt injection into the mold:Open the needle valve at the nozzle,the melt-gas solution is injected into the mold and the nucleation is occurred because the pressure drop in the mold cavities,ultimately the cells is formed,with small size and uniform size distribution in the mold cavities.Many physical transform process take place in the microcellular injection molding,like blending,nucleation, thermal transfer,mass transfer,flow,deformation,et al,so the theoretical system is rather difficult.In order to describe the processing mechanism accurately and throughly,one should have great insights into thermaldynamics,fluid dynamics,and the Polymer rheology.Unluckly,these disciplines are tiresome to treat with in both scientific and engineering area.When the basic laws for the elucidation of a given phenomena can be summed up to a series of differential equations and the analytical solution to them are hard to derive,there is no choice but to adopting numerical methods and this brings large quanities of computation task for the research.In this dissertation,with the help of Amon-Denson unit cell model widely used in the bubbe growth problem research work and appropriate assumptions,coupled with the phase equilibrium equation,force equilibrium equation and the duffusion equation, the cell growth model during the filling stage in the microcellular injection molding process is well established.The phase equilibrium equation is proposed accounting for the driven force to the cell growth,while the force equilibrium equation is proposed in case of the cell deformation and collapse.In the flowing direction of polymer melt,the way of mass transfer regarding the gas solute is mainly convection,while in the direction perpendicular to the flowing directon, the way of mass transfer is mainly diffusion. The establishment of diffusion equation is based on the different ways of mass transfer prevalent in different direction.For a rectangular area and a axis-symmetrical area,the diffusion equation have different forms.After figuring out the gas concentration,in combination with the phase equilibrium equation and the force equilibrium equation,the formula regarding the gas concentration and the cell radius can be established effectively.Before probing the analytical solution to the diffusion equation for the two types of mold cavities,the related parameters in the diffusion equation are translated into dimensionless form.Different strategies are applied for the solving of the diffusion equation.In the rectangular area,the approach for probing the analytical solution is Laplace transformation,wihle in the axis-symmetric area,the variable separation method is applied.The solutions are in the form of dimensionless parameters. The visualization of the concentration distribution function in the rectangular area is accomplished on the Mathematica software. Treating with the analytical solution,the gas concentration distribution curves can be derived. Although the definition of dimensionless concentration for the rectangular area and the axis-symmetric area are different,the dimensionless concentration were fading as the filling time increasing.According to the concentration curve by Mathematica,the sharp peak representing a large gradient is lowered as the melt filling the mold cavity owning to the gas solute diffusion along the width direction. Compared with the rectangular area,the concentration curve in the axis-symmetric area is more uniform,without fierce vibration along the radius direction.Combining the concentration formula and the polymer melt constitutive equation,the formula regarding the cell radius can be derived with ease,and the study of cell growth in rectangular area is launched.From this useful formula,key factors in the cell growth process are the following parameters:the supersaturation degree of gas,the melt temperature,the static hydraulic pressure in the polymer melt,and the rheological properties of the melt.Certainly,it can be concluded from the formula that the cell size does not vary in most part of the rectangular area and cells with different size along with flow direction are restricted in a certain area.In comparison with the SEM images for different sections from a standard ASTM sample manufactured by microcellular injection molding,the results given by the model established in this paper can provide a good explanation for the cell size distribution along the flow direction.The microcellular injection molding in rectangular area is also investigated by numerical simulation in Moldflow at different gas concentration,and the results have accorded with the outcome given by the theoretical model very well. Based on the key factors in the cell growth given by the model and the optimum processing parameters design is achieved.The inner door for cars is manufactured by microcellular injection molding,and the cells mophorlogy is characterized by SEM.From the SEM images,the radius of the cells is about 1 micron,and the plastic product satisfies the application area.