User talk:Hongchao

Hongchao (talk) 10:04, 18 January 2011 (UTC)Hongchao (talk) 09:51, 18 January 2011 (UTC)Condition for [Injection and molding]

The large molecular structure of polymer and the features of molecular thermodynamic movement have determined that polymer has such states of aggregation as the glassy state, highly-elastic state as well as viscous flow state etc. The transformation of aggregative state is influenced by chemical composition, molecular structure, the stress applied whereon and the environment temperature,and is mainly related with temperature when the composition of polymer is certain. When the temperature is below the virtrification temperature(Tg), the polymer is under glassy state, which is hard solid,wherein energy from the molecular thermodynamic movement is below, the elastic modulus is large, and the deformation is reversible. Under such aggregative state, it is not suitable for molding with largescale deformation. When the temperature is above the vitrification temperature(Tg), the plastic will become softer, and therefore, the vitrification (TG)is the maximum temperature that can be used for plastic. The lower limit temperature applied to plastics is called the brittle temperature, below which the plastic will easily get cracked and damaged under stress. When the temperature is between vitrification temperature and viscous flow temperature(TG), the polymer shall be highly–elastic state,wherein energy of the molecular thermodynamic movement increases and the elstic modulus greatly reduces. For amorphous polymer, certain materials can be molded through pressurization, bending, hollow molding or vaccum molding. For crystalline polymer, strech and drawing of the membrane and fiber can be made between vitrification temperature and melting point(Tt~Tm) When the temperature is above viscous flow temperature(or melting point), viscous flow will happen to the polymer under external force; calendering molding, certain extrusion and blow molding are usually adopted whe the temperature is within a range slightly higher than the viscous flow temperature. When at a temperature much higher than the viscous flow temperature. When at a temperature much higher than the viscous flow temperature, flow deformation of the fusant will occur under small external force. The deformation herein is mainly nonreversible viscous deformation which can be permanently preserved upon cooling of the fusant. Hence, this range of temperature is usually used for injection molding. Excessively high temperature, however, will greatly reduce the viscosity of polymer, thereby bringing difficulty to molding and worsening the quality of products. Moreover, when the temperature is higher than the decomposition temperature(Td), decomposition and metamorphism of the polymer will occur. Moldability refres to the plastics' ability of deformation under the action of temperature and pressure as well as their molding ability in the mold cavity. Platic with moldability can be made into plastic products of various shapes through such molding methods as injection, mold pressing and extrusion etc. The moldability mainly depends on such factors as the rheology, thermal property, physical mechanical property of materials, the chemical property of thermoset plastics, technical factors(such as temperature, pressure and molding period etc )as well as the structural dimension of molds. Exceedingly high temperature will produce sound fluidity yet will also result in high shrinkage and even decomposition; excessively low temperature will lead to low fluidity and formability, and the performance of products may be reduced; excessively high pressure will cause overflow and increase the internal stress of products; excessively low pressure will instead lead to insufficient filling in the mold and sudsequent material shortage. The area formed by the four curves in the figure is the optimal molding area.