The influence of pressure on the viscosity
when considering the influence of the pressure of the melt viscosity, also should pay attention to the sensitivity of the viscosity on the pressure with different polymer will be different. Is commonly believed that the greater the compression ratio of polymer melt, the viscosity of pressure sensitivity. Differ as a result of the polymer melt viscosity on the pressure sensitivity, therefore, when the injection molding, rely on increasing the injection pressure to improve the method of the melt flow or mold filling capacity is not very appropriate.
so, thermosetting polymer viscosity change in the molding process and the condition of the thermoplastic has essential difference, the rheological properties of his main refers to viscosity change with the crosslinking reaction. The characteristics of temperature is thermosetting plastic injection molding in the injection molding machine cylinder and mould respectively in different temperature, the injection molding is the best temperature to produce minimum viscosity without cause rapid crosslinking reaction temperature, while the runner and the mold temperature should be conducive to rapid hardening temperature.
the need to increase viscosity and unfavorable use cooling situations, adopt the method of improve the pressure solution should be considered. Consider the effect of pressure on viscosity in molding production, the efficiency of production, must be taken into account comprehensively the reliability of the equipment and mould and the quality of plastic parts, to ensure the molding process has the best injection pressure and injection temperature. Thermoplastic polymer molding process is basically a physical change, namely by heating, make the material to viscous flow with linear macromolecule structure after forming, and then make products by cooling curing.
in the process of forming polymer may be due to the partial degradation or crosslinking have some chemical change, but these changes on the basic properties of the polymers, especially will not result in irreversible changes of viscosity. Therefore, thermoplastic polymer can be repeatedly heating cooling forming, the waste can be recycled. Thermosetting polymer molding process is different from thermoplastic polymer, it besides physical changes, also accompanied by chemical reactions.
macromolecular polymer long chain structure is complex, pile under free state density is low, have larger free space between each other, so in the three-dimensional space under the action of static pressure, equivalent to the value of free space will be compressed to reduce between macromolecules, macromolecular chain will be close to each other, each other will also strengthen, so the macro will show the volume shrinkage or specific volume decreases, and at the same time the deformation flow resistance will also increase. Injection molding processing of polymer melt in the molding pressure, melt by static pressure will increase, and accompanied by the melt volume contraction, it also improved its viscosity.
heating thermosetting polymer, is not only in order to make the original presents linear macromolecule structure of prepolymer can produce deformation under pressure after melt flow in the mold in the shape of the plastic parts, but also must make filling the cavity of the prepolymer can melt under a certain temperature cross-linking reaction occurs, so that they could cure finalize the design for the plastic parts. Thermosetting polymer as crosslinking reaction, and its linear structure into a shape structure, even heating, macromolecular also won't tangle and sliding, so the viscosity becomes infinite, which always lost deformation flow ability.
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