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This article comprehensively analyzes its impact on composite packaging bags from two aspects of tension and pressure .
1. The effect of tension on composite packaging bags
1. The concept of tension
The tension of the substrate is the resistance of the substrate to the tensile force per unit area of ​​its cross-section, and the unit is kg / m2 or N / m2. The tension displayed on the device is the tensile force on the substrate in kg. The meanings of substrate tension and device display tension are different, so for different types of substrates with different thicknesses, different device display tensions should be set.
2. The principle of tension setting
The feel can clearly feel the difference in tension. Under the premise that the substrate runs smoothly, the smaller the tension, the better. It is better to relax slightly and at the same time, the composite packaging bag does not produce curl.
The effect of tension on the composite packaging bag is very large, especially the oven tension is very sensitive. If the oven tension is too high, it is easy to cause tunneling, aluminum plating layer transfer and other phenomena. The tension of the mask and the bottom film should be matched. The closer the linear expansion coefficient (shrinkage rate or stretch rate) of the composite material, the better. Otherwise, the composite packaging bag will easily curl.
Make a cross on the composite product with a blade, and the four divided film angles should be flat or slightly curved. The composite product will roll to the side of the film with too much tension, and the tension of the corresponding substrate can be adjusted accordingly.
3. Tension and coefficient of linear expansion of the substrate
The dimensional change rate of the substrate per unit length under the action of external force is called the linear expansion coefficient, that is, the shrinkage rate. For elastic materials, such as plastic films, when the external force is stretched, the size will increase, and when the external force is released, the size will shrink. If there is temperature effect at the same time, the degree of retraction will increase.
In the elastic range, the degree of stretching or retraction is proportional to the stretching force (tension). When the yield strength range is exceeded, the substrate size no longer rebounds, and it is really stretched. Different materials have different coefficients of linear expansion.
It is the principle of matching tension to control the minimum amount of stretching of the base material and make the size of the mask and the bottom film uniform after retraction.
Stretching and shrinking of single-layer film:
Stretching and shrinking of double-layer composite film:
The two-layer substrates are each compounded after being stretched to a certain degree under tension and temperature, and tend to return to their original size after compounding. For composite products of unit length, the original size of the two-layer material is not necessarily the same, so the amount of retraction after compounding is not necessarily the same, and the initial adhesion strength of the adhesive also affects the respective retraction ability, which produces different Appearance status.
Therefore, the key to tension matching is to make the original size of the two-layer base material consistent before stretching and to minimize the amount of stretching.
4. Principle of tension control
1). Principle of tension control
Tension is related to the passage of the substrate. Only when the substrate receives a force in the opposite direction at the two ends of the paragraph can tension be generated. The tension of the traction roller and brake roller on the substrate determines the amount of tension. The change in the speed difference between the traction roller and the brake roller determines the change in tension.
The principle of tension control is to control the speed difference between the traction roller and the brake roller and adjust the displacement of the floating roller between the two rollers, thereby controlling the tension to maintain a constant tension.
If a certain tension has been generated between A and B, when the linear speed of the traction roller and the brake roller are consistent, the tension remains unchanged; when the linear speed of the traction roller is greater than the brake roller, the tension increases; when the linear speed of the traction roller is less than the brake When the roller is pressed, the tension decreases. Ideally, as long as the traction roller and the brake roller maintain the same linear speed at all times, they can maintain a constant tension at all times.
In the actual production process, due to the thickness deviation of the substrate, the friction between the guide roller and the substrate passed by the substrate is inconsistent (coexistence of rolling and sliding), and the size of the substrate changes due to stretching or shrinkage due to drying and cooling Due to inconsistent, partial deformation of coil eccentricity, machine speed change, equipment accuracy and other adverse factors, the tension changes from moment to moment, and the range of change may be large, so the tension must be well controlled.
2). Tension detection method
(1), sensor detection method
The sensor is installed at the two ends of the detection roller. When the detection roller is pressed, the load is transmitted to the sensor. The sensor transmits the obtained tension signal to the tension control component for adjustment, thereby realizing tension closed-loop control. The disadvantage of this method is that tension oscillation is easy to occur during tension adjustment, which causes local fluctuation of tension.
(2) Detection method of floating roller potentiometer
Set the appropriate cylinder pressure according to the required tension range of the substrate and control the cylinder diameter, and balance it with the tensile force of the substrate to stabilize the tension.
When tension fluctuation occurs, the gear that rotates coaxially with the end of the swinging shaft of the floating roller drives the potentiometer gear to rotate a certain angle, and the potentiometer transmits the obtained electrical signal to the tension control execution part to reverse the tension, thereby achieving closed loop Tension control. The disadvantage of this method is that it takes up a lot of space, but he overcomes the shortcomings of the sensor sensor detection method, and can effectively absorb the fluctuation of tension.
3). Tension control method
There are three tension control methods for unwinding and unwinding:
Magnetic powder brake clutch method, pneumatic brake clutch method and motor drive method. The magnetic powder brake clutch mode and the pneumatic brake clutch mode both directly control the rotating torque and indirectly control the tension, and the torque control is achieved by setting the tension controller current.
The motor drive method generally controls the speed of the motor by controlling the magnetic field strength of the asynchronous motor, so-called vector frequency conversion control. There are also DC motor control methods or feeding motor control methods. The principle is to use one of the motors as the main motor in the system, and set the speed of the main motor to make other motors follow the main motor.
If the tension changes during the follow-up process, the feedback signal of the potentiometer or sensor will be transmitted to the tension control center, and then the control center will issue a command to make appropriate adjustments to the motor speed to achieve the purpose of tension adjustment. The advantage of using the motor control method is that the diameter of each driving roller does not need to be proportional, and the control accuracy is high, which represents the direction of tension control.
Second, the impact of pressure on composite packaging bags
During the transfer of the adhesive, the pressure that the substrate receives per unit length in the axial direction of the rubber pressure roller is called the composite pressure. The unit is kgf / mm. The compound pressure is related to the length of the pressure roller, not to the width of the mesh roller, and to the width of the substrate.
The role of the pressure roller is to maintain uniform and sufficient contact between the coating and the substrate. It is determined by the size of the pressure, the compressive strength of the rubber, the amount of deformation and the uniformity of the deformation.
Relationship between coating pressure and adhesive transfer
Due to the flexible deformation of the rubber roller, the nip contact area is area contact. In a rolling cycle of the pressure roller (the process from a certain point on the pressure roller from entering the nip area to leaving the nip area), the amount of adhesive transfer follows the following rules:
Section AB: Insufficient pressure section-The pressure is increased from P0 to P1, which is not enough to complete sufficient contact, and the amount of transfer is irregular.
Section BC: Proportional transfer section-The transfer amount increases in proportion to the pressure increase, and the transfer is insufficient.
CD segment: transfer saturation segment-the transfer volume is sufficient and uniform to obtain the best transfer volume.
DE section: The pressure is too large section-as the pressure increases, the coating amount decreases instead, and the uniformity of coating decreases.
EF section: Overpressure section-as the pressure increases, the coating amount rises and the uniformity of coating rises
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