The copper foil is as thin as 6 micrometers. What does that mean?
A human hair is approximately 60 to 80 micrometers in diameter.
The foil on your hand is ten times thinner than a hair.
It emerged from the spool, passed through dozens of guide rollers, went through the coating head, the drying oven, and the take-up spool. At any point along the way, if the tension window was slightly reduced, there would be a dull thud, and it would break.
But the issue of the tape breaking is not something that happens randomly.
It's because the parameter windows for the unwinding, tension control, deviation correction, and winding processes were gradually reduced.
1. Why is thin foil more difficult to handle than thick foil?
The differences in the behavior of copper foil and aluminum foil during coating are rooted in their mechanical properties.
The tensile strength of 6μm electrolytic copper foil typically ranges from 300MPa to 500MPa, and the elongation varies between 4% and 10%, depending on the grade and process conditions.
At the same thickness, the elongation of copper foil from different manufacturers can differ by more than double, because the cutting processes and testing conditions of downstream manufacturers are inconsistent with those of the upstream manufacturers, resulting in significant shrinkage in the test results.
The situation for aluminum foil is even more special.
For 1 series aluminum foil, such as 1060-O state, the tensile strength is only 60 to 110MPa, much lower than that of copper foil, but the elongation can reach 15% to 18%, with better plasticity.
Aluminum foil is softer, meaning it is more tolerant of tension fluctuations, but it is more prone to wrinkling.
Copper foil is harder and more brittle, and it breaks under tension.
These differences between the two materials determine that: with the same tension setting value, it may be just right on copper foil, but it will be too large on aluminum foil.
The unwinding tension of the aluminum foil should be set at the lower limit.
Conversely, the deviation correction response for the aluminum foil needs to be more sensitive because soft materials are more prone to drifting during the tape running process.
II. The operator should remember this one sentence: Copper foil is afraid of being pulled, while aluminum foil is afraid of drifting.
1. Roll out steadily
Rolling out is the first step in unwinding the tape.
When the foil roll is changed from a full roll to an empty one, the roll diameter changes, but the tension cannot change accordingly.
The rolling out tension is usually controlled by a magnetic powder brake or a servo motor, using an open-loop method. A torque value is set, and the tension is maintained constant through conical compensation.
The problem is: every time the roll diameter is reduced by half, under the same braking torque, the actual tension will double.
If no roll diameter compensation is done in the PLC, the tension at the end of the roll will soar out of control.
The starting reference value for copper foil rolling out tension is taken as the lower limit for aluminum foil.
This range is very wide because the specific value depends on the width of the foil material.
The tension settings for narrow 500mm and wide 1300mm are completely different.
Action for putting the tape down:
After changing the material, run empty first. Touch the foil surface at the rolling out section and feel if there is any local tension or sagging.
If there is a tense feeling, reduce the torque; if there is a sagging feeling, increase the torque.
This tactile judgment is faster than looking at the instrument.
2. Tension Flex
The goal of tension control is not to have the maximum tension for the steepest stability, but rather stable tension, reasonable zoning, and matching with the material modulus and thickness.
The tension zoning of the coating machine includes the unwinding zone, coating zone, drying zone, and winding zone.
The tension requirement in the coating zone is the most stable, as tension fluctuations directly cause fluctuations in the coating surface density.
This zone is controlled by closed-loop PID, with the lever position as the feedback signal.
What the operator can do:
If the lever in the coating zone frequently swings up and down significantly, it indicates that the P value is too high.
If the lever position slowly drifts and does not return, it indicates that the I value is too small.
Reporting these phenomena to the process engineer is much more useful than just saying the tension is unstable.
3. Deviation Control
For ultra-thin foil materials, deviation during the belt running is not gradual but sudden.
The deviation control system responds half a beat later, and the edge of the foil has already touched the end face of the guide roller, causing either slight wrinkling or severe tearing.
The current mainstream deviation control systems can control the accuracy within ±0.1 millimeters and the response speed reaches the level of 5 milliseconds.
This speed sounds fast, but if the sensor installation position is too far from the deviation control actuator, the signal transmission plus mechanical response time will eat up this advantage.
There is a principle for sensor installation: try to be as close to the deviation control actuator as possible to shorten the feedback path.
Ultrasonic sensors are suitable for metal foil materials like aluminum and copper, as they are not affected by surface reflection.
Photoelectric sensors may be interfered by the oxide layer on copper foil surfaces.
Key points for implementation:
Check if the sensor probe is contaminated by slurry splashing when the machine is stopped. If the probe is dirty, the detected edge position will drift, and the deviation control system will drive the foil in the wrong direction.
4. Head Coating Flatness
The risk points of breakage vary depending on the coating method.
For slit extrusion coating, a closed material supply system is used, and the slurry flows within the die head cavity and is extruded onto the foil surface through a slit.
This method is the most sensitive to tension fluctuations. Any change in tension will cause a change in the gap between the foil and the die lip, and the coating thickness will also change.
If the gap is small enough for the foil to adhere to the die lip, the local tensile force will suddenly increase.
For micro-凹 coating, the slurry is transferred through the embossing roller, with relatively lower sensitivity to tension, but if the pressure between the embossing roller and the back roller is too high, micro-cracks are likely to form at the gap between the thin foil and the roller.
These cracks will be the starting point of breakage in subsequent tension fluctuations.
5. Drying Chamber Slowdown
The drying chamber is a high-incidence area for foil breakage because it has both temperature and tension variables.
The thermal expansion coefficient of copper foil is approximately 17 ppm/℃. For every 1℃ increase in temperature, the foil elongates by 0.0017%.
The drying chamber temperature is usually between 80 and 140℃, and the thermal elongation of the foil in the chamber cannot be ignored.
If the tension in the drying chamber is not compensated for temperature, the foil will elongate due to heating, and the tension will rise when it cools in the drying chamber, causing a tightening and loosening cycle, making the thin foil prone to fatigue and breakage.
The tension control in the drying chamber usually adopts a feedforward compensation method, adjusting the tension set value according to the drying chamber temperature.
The higher the temperature, the lower the tension set value should be.
The tension in each section of the drying chamber should gradually decrease as the drying process progresses, leaving space for stress release of the coating.
Key points for implementation：
Observe the foil surface at the drying chamber outlet. If there are obvious longitudinal stripes or edge waves on the foil surface, it indicates that the tension in the drying chamber section is too high.
If there are horizontal relaxation stripes, it indicates that the tension is too low.
6. Winding Uniformity
Winding is the final window for breakage and is also the most easily overlooked step.
During winding, the diameter of the roll increases from around 100mm to 400mm or even larger.
If the winding tension does not change with the roll diameter, the pressure exerted by the outer foil on the inner foil will accumulate continuously, causing the inner foil to develop wrinkles, and the tension on the outer foil will increase, eventually breaking at a certain critical point.
Conical tension control is the standard practice.
The conical coefficient is typically set between 0.8 and 0.9, meaning the tension in the winding increases proportionally as the roll diameter increases.
The pressure of the pressing roller also needs to be coordinated.
The function of the pressing roller is to press the foil tightly onto the core and remove the air between layers.
If the pressure is too low, the roll will become loose and twist; if the pressure is too high, there will be indentations or even cracks on the foil surface.
Operator's actions:
After the roll-up is completed, the hand presses on the roll surface.
If there is a significant elastic rebound when pressed, it indicates that the roll is too loose. The next roll-up tension can be appropriately increased or the taper coefficient can be reduced.
If pressing is hard and there is no rebound, it indicates that the roll is too tight and the roll-up tension is too high.
III. How to build the data window: Small batch comparative test
The parameter boundaries are not determined by intuition, but are determined through experimentation.
Step 1: Build the record table.
Record the following parameters each time during coating: unwinding tension setting value, coating zone tension, oven temperature in each section, roll-up tension taper coefficient, coating speed, deviation system alarm times.
When a break occurs, record the break position and the fracture morphology.
Step 2: Make variable comparisons.
Fix other parameters, only adjust the unwinding tension. Run short rolls for comparison, fluctuating the current value by 10% up or down.
Observe the changes in the stability of the foil roll and the frequency of breakage.
Step 3: Find the window boundaries.
Gradually approach the upper and lower limits, record the critical values at which wrinkling or breakage occurs.
The window boundaries are the physical limits of the parameters.
Step 4: Leave a safety margin.
The actual operating parameters cannot run close to the window boundaries. At least a 15% to 20% margin should be left.
Conclusion:
The continuous thin foil production line doesn't rely on quick hands, but on keeping all the processes (roll unwinding, tension control, deviation correction, coating, drying, and roll collection) within the parameter boundaries.
What is saved is not just one roll of foil, but the overall line yield and delivery time.
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