Solvent-free composite production, we must pay attention to the uniformity of coating!
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The economic significance of uniform coating
Regarding the economic significance of the uniformity of coating in solventless composite production, the author compares the cost through the following data analysis.
It is assumed that the amount of glue applied at 1.50 g/m2 is the minimum requirement for the composite appearance and peel strength of the flexible package.
The uniformity of the coating of the solvent-free composite device A is ±10%, and the average coating amount required is 1.67 g/m2, that is, the local minimum point is 1.50 g/m2, and the local highest point is coated. The amount was 1.84 g/m 2 (10% to 15% on the basis of the average coating amount).
The uniformity of the coating of the solventless composite device B is ±5%, and the average coating amount required is 1.58 g/m2, that is, the local minimum point of the coating amount is 1.50 g/m2, and the local highest point is applied. The amount is 1.66 g/m2. That is to say, in the same flexible packaging composite product, the equipment A is required to have a coating amount of 1.67.58±=0.09 g/m2 per square meter of the equipment B, that is, it consumes 5.7% of the solvent-free adhesive.
Calculate the average amount of glue used in a solvent-free composite equipment of 4.0 tons/month. Equipment A should use 4.0×12×5.7%=2.736 tons of adhesive in one year, and the unit price of solvent-free adhesive is 23 yuan/ In kilograms, the direct cost loss is 2.736×1000×23=62928 yuan, and the direct economic loss in 5 years is nearly 320,000 yuan.
There are also some solvent-free composite equipment on the market that can only achieve a coating uniformity of ±15%. Although such equipment has relatively low procurement costs, it often means more solvent-free adhesives, so it is cheap. Not equal to the economy.
Quality loss
Most of China's flexible packaging design often uses more large color blocks, mainly ink-based overprinting white ink, and some parts have hollow design, this pattern design will increase the difficulty of solvent-free compounding.
In the solventless composite production, it is assumed that the minimum amount of glue required for the red ink and white ink overprinting portion of the sample is 1.5 g/m2, and the coating amount of 1.0 g/m2 in the transparent portion can satisfy the appearance and strength requirements. When the amount of glue applied is too large, the transparent portion will be gelatinized (lower transparency) and even bubbles will appear. Therefore, if the above-mentioned solvent-free composite equipment A (partial highest point of coating amount is 1.84g/m2) and solvent-free composite equipment B (partial highest point of coating amount of 1.66g/m2) are used, respectively. The sample, compared to transparency, produced a composite film that was significantly lower than the composite film produced by Equipment B. The lower the viscosity of the adhesive used, the more obvious the phenomenon of the rubber film produced by the composite film. It can be seen that the equipment with poor uniformity of coating has a more obvious influence on the composite quality of flexible packaging. The key is that it can not effectively meet the quality requirements of the composite film overlay and transparent. Faced with this situation, some industry people will adopt a method to increase the average amount of glue, which will not only improve the composite quality, but will increase the cost of glue. In addition, increasing the amount of solvent-free composite coating, there are technical risks such as increased friction coefficient and anti-sealing failure for PE composite film.
Technical safeguards for uniformity of rubber coating
1. Five-roll coating system
The five-roll coating system is the general structure of solvent-free composite equipment in China, as shown in Figure 1. There are mainly metering steel rolls R1 (fixed metering steel rolls), transfer steel rolls R2 (rotary metering steel rolls), transfer rubber rolls R3, coated steel rolls R4, and coating rolls R5. Among them, R1 is always fixed in the solventless composite production process, and it is combined with R2 and the adjustable glue blocking mechanism on the sides of the two rolls to form a glue storage device for storing solvent-free adhesive. The rubber roller can be manually turned while the drum is being scrubbed or the foreign matter is removed. Among them, R5 and R3 are rubber rollers, which are used to reduce the friction between R3 and R2 and R4 during production, and reduce the friction heat, thereby effectively avoiding the phenomenon of "transpiration and fogging" of the adhesive.
By continuously adjusting the speed difference between the rolls, the accuracy of the low amount of glue can be ensured. In order for the adhesive to form a uniform film on the coating roll, it is necessary to set different line speeds for the metering roll and the transfer roll, and the line speed of the coating roll is also different, which is lower than the line of the coating roll. Speed (ie host line speed), such as a reasonable line speed ratio between R3 and R4 should be 0.25. The change in the amount of the solvent-free adhesive applied depends on the linear velocity ratio between the transfer roller and the coating roller.
For the solventless composite equipment using the gear belt connection of the geared motor, the linear velocity ratio of R2 and R3 is constant, that is, the surface rate of R3 varies with the amount of glue applied, and when the amount of glue is different, The line speed ratios of R3 and R4 are also different, but this problem does not exist if independent servo-controlled, solvent-free composite equipment is used. It can be seen that the reasonable linear speed ratio of R3 and R4 is an important factor to ensure the uniformity of the coating of the five-roll coating system.
2. Precision static metering gap
With the continuous improvement of the processing level of equipment in China, the processing precision of equipment parts has been greatly improved. The surface finish of R1, R2 and R4 in the five-roll coating system can reach ±0.0002mm, and the concentricity can reach ±0.005mm. Since R1 is fixed and does not rotate, theoretically speaking, the deviation of the static metering gap is directly dependent on the concentricity deviation of R2 without considering the assembly error. The assembly error has many influencing factors, which depends on the machining level and assembly level of equipment manufacturing enterprises, such as CNC machining precision, bearing precision, assembly level and assembly precision of wallboard.
3. Eliminate factors affecting dynamic metering gaps
(1) The pressure of R2 and the vibration of R3
During the operation of the solvent-free composite equipment, R3 transmits pressure, so R2 will be deformed when subjected to pressure from R3, so the pressure applied to R3 should not be too large. The sufficient rigidity of R2 can reduce the amount of deformation caused by the applied pressure and reduce the gap change with other rollers.
In the solventless composite equipment processing, because R2, R3, R4 have a certain degree of concentricity deviation, R3 will inevitably vibrate when the solventless composite equipment is running (this is also related to the large and small pressure on the R3). Therefore, it directly affects the uniformity of transfer of R3. The good concentricity and dynamic balance of each roller can reduce the problem of uneven rubber coating caused by vibration and improve the durability of each roller.
(2) Metering roller temperature stability
The metering roller has a diameter of 175 mm, and a temperature difference of 1 ° C per phase produces a gauge gap deviation of 2.1 μm. Moreover, the larger the temperature difference is, the larger the deviation of the metering gap is, and the metering is not accurate, and the roller surface is damaged in severe cases. It can be seen that the heating stability of the mold temperature machine used for the metering roller is particularly critical, the display temperature must be consistent with the set temperature, and the temperature fluctuation during the working period should be <±1 °C.
After the solvent-free composite equipment is operated for a period of time, if the circulating water pipe inside the metering roller is semi-blocked, there will be a problem that the temperature of the metering roller is inconsistent, which is manifested by prolonged heating and warming time. The most serious situation is that when the gap is adjusted, the temperature on the left and right sides of the metering roller is consistent (and consistent with the set temperature), and the temperature on the left and right sides is deviated during the startup process, resulting in inconsistencies on both sides of the metering gap. The uniformity of the amount of glue also changes, the most direct consequence is the lack of winding.
(3) Pressurization of the rubber block
The pressure of the block is equivalent to the force that spreads the adjusted metering gap, and the inappropriate pressure obviously changes the adjusted metering gap. At present, the pressure applied to the rubber block is mainly the self-weight of the pressure bar, and the force value should not be too large considering that there is a certain mechanical locking force.
(4) The problem of glue penetration at the edge of the rubber block
As the glue at the edge of the block is prolonged, the viscosity will increase rapidly, the resistance of the R3 will increase, and the metering gap will become larger, which will cause the glue amount to change and deviate from the preset value.
In addition, the glue-free parts of the solvent-free composite equipment are usually designed to be closed operation, which makes the squeezing of the edge of the rubber block more troublesome. If the cleaning is performed during the startup operation, there is a great safety hazard; Cleaning up will lead to an increase in scrap rate and a decrease in efficiency.
Therefore, we must fundamentally understand the main factors causing the bleed, in order to truly solve the problem of bleed, for this, the author summarizes the following three factors.
1R1 and R2 have poor processing accuracy. The deviation is the diameter deviation and the concentricity deviation. When the diameter deviation is large, the gap with the arc surface of the rubber block increases. When the deviation of the concentricity is large, the contact gap between each part and the arc surface of the rubber block is unstable when the rotation is one week. The gap change between the two creates conditions for the penetration of small molecule solvent-free adhesives. 2 The processing accuracy of the rubber block is poor. It directly affects whether the contact gap between the arc surface of the rubber block and the surface of the steel roller is tight. 3 metering gap adjustment accuracy and actual gap change. The rubber block is dimensioned according to the standard metering gap, so the change of the metering gap will directly affect whether the contact between the arc surface of the rubber block and the surface of the steel roller is tight.
(5) Reliability of the metering gap locking mechanism
After the metering gap is adjusted, there are two kinds of locking methods for the metering steel roller, one is mechanical locking, and the other is gas pressure locking. If the locking mechanism of the metering steel roller is not reliable, the change of the metering gap is easily caused by the external force (such as the pressure of the rubber block and the extrusion of the edge glue).
(6) Long-term use stability
R1, R2, and R4 should have sufficient rigidity to avoid deformation after repeated compression for a long time. The surface of R2 and R4 is ceramic-plated, the surface wear resistance will be multiplied, and the theoretical life is 10 times that of chrome plating. Some chrome-plated transfer steel rolls have chrome-plated problems on the surface after 2 to 3 years of use, which makes it difficult to guarantee the original metering gap accuracy.
(7) Glue stability near the block
Solvent-free A/B glue (the temperature after mixing is 35-45 °C) will increase the viscosity near the block, which is visually expressed as whitening and bubbles. At this time, the amount of glue applied to the film will be transferred. Significantly reduced (because the bubbles occupy a certain volume). At the same time, the chemical reaction between the glue and the water vapor in the air, consumes a part of the -NCO group, and in severe cases, the composite film edge is not cured, and the peel strength is low.