Wuhan Handern ?Orientation of plastics in the molding process

日期:2020/12/7 9:08:25 / 閱讀: / 來源:本站

Wuhan Handern Orientation of plastics in the molding process 
1. Flow orientation

(1) In the isothermal flow area, due to the small cross-section of the pipe, the shear rate at the pipe wall is the largest, and the degree of orientation is the highest in the melt close to the pipe wall; in the non-isothermal flow area, the melt enters the cavity with a larger section After the pressure gradually decreases, the shear rate in the melt also gradually decreases from the maximum value at the gate to the minimum value of the flow front. Therefore, the degree of molecular orientation in the melt front region is low. When this part of the melt first contacts the mold wall with a much lower temperature, it is rapidly cooled to form a frozen layer with little or no orientation structure. However, the melt near the frozen layer is still moving, and the viscosity is high, and the shear rate is large when flowing, so this part of the melt in the subsurface layer (0.2~0.8mm from the surface) has a high degree of orientation. The melt in the center of the mold cavity has a low shear rate during flow and a low degree of orientation. At the same time, due to the high temperature and slow cooling rate, the molecular de-orientation has time to develop, so the final degree of orientation is extremely low. www.dressup9x.com Hollow Profile Sheet Machinery
(2) In the mold cavity, since the shear rate in the melt decreases along the flow direction, the degree of molecular orientation in the flow direction decreases gradually. But the most oriented area is not at the gate, but at a position not far from the gate. Because the melt fills up here first after entering the mold, there is a long cooling time, the frozen layer is thick, and the molecules are subjected to the greatest shear here, so the degree of orientation is also the highest. Therefore, during injection molding and extrusion molding, the effective orientation in the product mainly exists in the subsurface layer that is cooled earlier.

2. Stretch orientation

During the molding process, if plastic molecules (such as a film into monofilaments) are stretched in one direction to several times their original length at a certain temperature, the molecular chains in them will be aligned along the stretching direction, which is the stretching orientation . www.dressup9x.com Hollow Profile Sheet Machinery
Stretching orientation should be carried out between the glass transition temperature and melting point of the plastic. This is because the molecules must be higher than the glass transition temperature to have sufficient activity. The deformation that occurs during the stretching orientation is essentially composed of three parts: ①The instantaneous elastic deformation-reversible deformation is the twist of the molecular bond angle It is caused by the deformation and the straightening of the molecular chain; ②The high elastic deformation is the result of the unwinding of random coils, which is a required part of the stretching orientation process; ③The viscous deformation is the result of mutual slippage between molecules , Is irreversible deformation. When the film into a single filament is quickly drawn at slightly higher than the glass transition temperature, the first part of the elastic deformation occurs quickly, and when the second part of the highly elastic deformation proceeds, the elastic deformation begins to shrink. Part ③ The viscous deformation must lag behind the high elastic deformation in time. If the high elastic deformation has been fully developed, and the viscous deformation is still small, the film or monofilament is suddenly cooled, so that a larger molecular orientation can be obtained. If the stretching temperature is increased, the amount of viscous deformation will increase, and the relaxation of high elastic deformation will be greater than that at low temperatures, so the opposite side of the degree of orientation will be smaller. From the actual process, the following rules can be always:
(1) At a given stretching ratio (ratio of the length after stretching to the original length) and stretching speed, the lower the stretching temperature (not lower than the vitrification schedule), the better. Its purpose is to increase high elastic deformation and reduce viscous deformation. www.dressup9x.com Hollow Profile Sheet Machinery
(2) Under a given stretching ratio and a given temperature, the higher the stretching speed, the higher the degree of molecular orientation obtained.
(3) At a given drawing speed and constant temperature, the drawing ratio is larger and the degree of orientation is higher.
(4) Regardless of the stretching conditions, the greater the quenching rate, the higher the degree of maintaining orientation.

作者:admin


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