Liquid crystal display has three major materials: liquid crystal, ITO glass and polarizer. Each production of a liquid crystal display requires two polarizers, attached to the upper and lower sides of the liquid crystal glass box respectively. Polarizer can be divided into transmissive polarizer and reflective polarizer, figure 1 shows the basic structure of transmission polarizer. The main function of polarizer is to make out the polarization of natural light generating polarization, into polarized light with liquid crystal molecular torsion characteristics, to control the passage of light or not, so as to improve the transmittance and the visual angle range, forming antiglare and other functions, it is a kind of products in the field of upstream raw materials is the panel important.
Liquid crystal display must rely on polarized light imaging, all the LCD display has two pieces of polarizer close to the liquid crystal glass, composed of a total of about 1mm of liquid crystal.
1. Influence of bubbles on liquid crystal display
Polarizer bubbles are the gases that remain between the polarizer and the surface of the liquid crystal glass box when the polarizer is attached to the surface of the liquid crystal glass box due to the attached environment, adhesion process and adhesion mode. When using the attached device polarizer attached to the liquid crystal glass box surface, because there are a lot of gas between the two, only by sticking to the extrusion of all gases out from between the two, and the attached device and some artificial factors will ring attached when the effect in a certain extent.
When the polarizer is attached to the liquid crystal glass box, a certain number of tiny bubbles will inevitably remain, and some bubbles are hardly visible to the naked eye, as shown in figure 2. The light passing through the polarizer becomes a direction of light passing through these tiny bubbles, which will make the part of the light deviate from the original direction due to refraction, leading to the inconsistent color of the part and other parts of the color, which will affect the display effect of the display. Defects such as LCDs, especially large TFT LCDs, are not allowed.
2. Polarizing plate defoaming
2.1 process flow
In the actual production, the attached display polarizer will usually add Polaroid special defoaming device, keep a period of time in a certain temperature and pressure will remain after the bubble eliminated, so as to eliminate the bubbles enhance the LCD display effect to. The process of removing bubbles by polarizing plate is shown in figure 3.
2.2 process parameters
In addition to global process, the main effect of temperature is polarizing film and adhesive attached polarizer by softening, so that bubbles more easily in the internal flow, heat resistance, but also need to set the temperature parameters of the polarizer itself the heat resistance and the polarizer protection film is considered, when the temperature parameter is not appropriate will seriously affect the defoaming effect. Therefore, in addition to global process, the temperature will affect the polarizer bubble removing effect in two aspects, one is the temperature that is in addition to the studio within reach of the global temperature process, the general value according to the production of liquid crystal display the size and material of different formulation; two is the internal temperature uniformity the equipment, namely the temperature difference at different positions of the apparatus, which requires in addition to control equipment to achieve global system.
The main problem of uneven temperature is brought in defoaming equipment inside the studio box area of the LCD glass, and the inferior position due to the big difference in temperature between the LCD glass boxes placed in different areas of the quality problems, such as part of the region within the global LCD glass boxes except as protective film or LCD glass even inside the device box softening phenomenon in different regions at the same time the above two kinds of circumstances phenomenon. This defect will cause some serious problems such as two times of defoaming and partial glass box damage in some liquid crystal glass boxes, which will greatly increase the production cost. Generally, the requirement of temperature uniformity is 5 degrees centigrade.
To solve this problem, we developed the equipment within the studio is divided into two parts of heating area and work area, the mixing mechanism of special hot air heating area to the work area, the work area of air pumped into the heating zone, to achieve the temperature uniformity, the built-in temperature recorder to record the internal temperature of each test point the test point distribution curve, as shown in Figure 4, a B and C three, the parameter is set to a temperature of 50 DEG C, the pressure 0.5MPa and holding pressure holding time of 10 min, the temperature curves as shown in Figure 5, temperature uniformity is + 2 DEG C, fully meet the technological requirements of defoaming.
In the process of removing bubbles of polarizer, the main function of pressure is to be the motive power of removing bubbles, that is, to squeeze the bubbles, to smooth the polarizer and press the adhesive pressure sensitive adhesive between the polarizer and the liquid crystal glass box, so that the two can be firmly bonded. There are two aspects of stress control:
One is the lower limit of pressure, that is, when the pressure in the chamber is lower than the lower limit required by the pressure setting, the chamber must be filled with air to ensure the stability of the pressure in the chamber;
Two is the upper limit of pressure, that is, when the pressure in the chamber exceeds the upper limit required by the pressure setting, the chamber must be released.
If the lower limit of pressure is achieved without the operation of air supplement, the bubble may not occur, leading to the increase of product cost; if the upper limit of pressure is not controlled, it may lead to product damage and even affect the safety of the equipment except the foam.
To solve this problem, we adopt double output digital pressure switch and electric contact pressure gauge double control method. The double output pressure switch can provide an upper limit value and a lower limit setting for the pressure setting value, respectively.
When the pressure interface of the pressure switch is connected with the chamber and the pressure in the chamber is detected, the output signal is connected with the PLC to control the opening and closing of the valves on the defoaming device. When the detection value is less than the lower limit, the intake valve opens for the chamber air, and closes when the set value is reached; when the detection value is greater than the upper limit value, the exhaust valve is opened, and the exhaust of the workroom is discharged to the setting value. Since the upper limit of the pressure is related to the safety of the equipment, an electric contact pressure gauge is added to control the upper limit of the pressure in the chamber two times. The pointer on the dial to set the upper limit of electric contact pressure gauge, the detection and also communicated with a working port, an electrical output signal is connected with the PLC, when the pressure exceeds its detection limit pressure set value, the actual pressure exceeds the upper limit pointer pointer position, open the exhaust valve exhaust, below the upper limit stops to prevent. Safety accidents caused pressure switch failure. The system has been widely used in the defoaming machine, and the effect is good.
In the process of foam removal, there are three time periods: pressure rise time, holding time and exhaust time. The three time periods make up the whole process flow time. In order to produce more products in a shorter time, reduce costs and gain profits, process time must be shortened as much as possible.
The holding time must be strictly in accordance with the time specified by each product, otherwise there will be the phenomenon of defoaming. Since the exhaust time is simply to release the pressure inside the chamber to the atmospheric pressure, the proportion of the process is very small, so shortening the time of the process is in fact need to shorten the heating and boosting time.
The time of equipment boost is generally realized by increasing the air inlet pipe diameter of equipment. For example, when the inlet pipe diameter is 1.27cm and the air supply pressure is 0.5MPa, the internal pressure of the chamber needs to be about 10min when the inlet pipe diameter is and the air supply pressure is 0.7MPa. When the intake pipe diameter is 2.54cm, the intake time is only 2min.
At present, most of the heating methods used by domestic LCD manufacturers are external heating. When heating this way, the outer wall of the equipment should be heated first, and then the heating inside the studio can be realized. This greatly prolongs the heating time of the chamber except the foam equipment.
In order to solve the problem that the temperature of soaking machine is slow, especially the heating time is too long when the equipment is heated for the first time, we can solve the problem by changing the heating way of the equipment, that is, using the internal heating type, as shown in figure 4.
The inner heating method is that the heating pipe is installed inside the equipment to directly heat the air in the workroom, and the time of the outer wall of the heating chamber is omitted, so the processing time is greatly shortened. The interior of the work room is divided into heating zone and working area. The hot air in the heating zone is sent to the working area by special stirring mechanism, and the air in the working area is sucked into the heating zone. The heating method has been put into use and the effect is good.
Figures 6 and 7 respectively for the temperature curve of external heating and heating methods, process parameters were set at a temperature of 60 DEG C, the pressure of 0.5MPa, the initial temperature is 30 DEG C, from Figure 6 and Figure 7 can be seen outside the heating mode from 30 rises to 60 DEG C to about 22min, about the internal heat 5min. The 20min of each process was calculated by 5min, and the temperature of each heating process was 47min. The internal heating time was 30min, and the time was reduced by about 36%.
Thus, the improved heating method can greatly reduce the process time, greatly improve the production efficiency.