The retort packaging bag with BOPA//LDPE structure is widely used in the packaging of pickles and bamboo shoots. BOPA//LDPE boiled bags actually have high technical index requirements. Although a certain scale of soft bag enterprises can make boiled bags, the quality is also uneven, and some will have more batch quality. question. Here, this paper analyzes the key points in the production process of BOPA//LDPE boiled bags.
A. Selection of materials
1. Selection of BOPA film
①Bow phenomenon of nylon film
BOPA film can be produced by tubular film stretching method or by plane biaxial stretching method. Biaxially oriented nylon films produced by different methods have different bow effects, which have a significant impact on the overprinting accuracy of the film and the flatness of the packaging bag (including the appearance of the bag surface before and after boiling).
The specific method to detect the bowing effect of nylon film is to measure the thermal shrinkage of the diagonal. We can also test the wet heat shrinkage rate of nylon film according to the actual use conditions of the boiled bag (such as 100 ℃, 30min). The smaller the difference between the diagonal heat shrinkage rate, the better the balance of the product; 1.5%, there will be no warping angle during bag making.
② Market supply varieties
BOPA film is divided into two types: printing grade and composite grade. Printing grade can be used for printing and composite processes. Composite grade is only recommended for composite processes that do not require printing. The thickness is generally 12μm, 15μm, 25μm two specifications. 15μm for flexible packaging composite film, 25μm for cold-formed aluminum pharmaceutical packaging. Double-sided corona film must be used when it is used for interlayer lamination and for boiling and cooking purposes.
③Key quality requirements of BOPA film
a. If the flatness requirement is high, the synchronously stretched film with small bow effect should be selected.
b. The surface tension of the film is ≥50mN/m to ensure the adhesion fastness of the ink. The processing value is not the bigger the better.
c. Choose a film with good adaptability to relative humidity to ensure overprinting accuracy.
d. Select the film variety with smaller thermal shrinkage rate (wet heat shrinkage rate).
2. Selection of heat sealing layer PE
The difference between the boiled bag PE and ordinary PE: ① better heat sealing strength; ② good heat sealability of inclusions; → Stable heat sealing quality; ⑤ Good transparency, no obvious water streaks; ⑥ No fish eyes, impurities, crystal points that affect the use → Appearance bubbles, or even piercing the PA film → Barrier performance decreased, or oil appeared leakage phenomenon. The first three quality characteristics are mainly determined by the pellet formulation of each layer of the PE film during blow molding.
3. Selection of printing ink
Polyurethane special inks are generally used for nylon film printing: ① Benzene-free and ketone-free series; ② Benzene-free and ketone-free series.
When choosing printing inks, pay attention to:
①The resistance selection of color models, such as F1200 red, 1500 red, F1150 red, F2610 gold red, F3700 orange, F4700 medium yellow and other color inks of a polyurethane ink, it is indicated in the manual that it cannot be used for BOPA/PE Structural boiled film printing, some colors are not resistant to boiled, and the color material is easy to seep out when the water is boiled.
②Gold and silver ink should be used with caution. For gold and silver ink, the ink factory does not recommend using it for boiling purposes in the instructions, but some boiled packaging bags on the market use gold and silver colors. The general practice is to consult the ink factory for formula design before application, and also Be careful not to print in large color blocks.
③ The nylon film must have good ink adhesion fastness, so as to ensure the final peel strength of the ink part.
4. Selection of adhesive
Choose an adhesive that can withstand boiling, and ensure the degree of cross-linking and curing after compounding. In addition, the aged glue should be used with caution (generally should be avoided), because the effective ratio of the main agent and the curing agent group in the glue solution has been unbalanced during the placement process of the aged glue, and the glue layer is prone to dry phenomenon.
5. Quality requirements for ethyl acetate
The water and alcohols in ethyl acetate (not only ethanol, but also the content of methanol and isopropanol should be controlled) will react with the curing agent in the glue, and the curing agent will be consumed, resulting in the phenomenon that the glue layer does not dry out. One of the main reasons for the wrinkling of the rubber layer of the bag.
B. Gravure printing process
1. Selection of specific ink models
It is produced according to the ink type specified by the process. For example, some inks of other colors are not suitable for BOPA//PE printing.
2. When the old ink is reused, it is necessary to add more than 50% new ink, and the deteriorated ink shall not be used.
3. When necessary, a certain proportion of curing agent can be added to the white ink
There are two purposes of adding a certain proportion of curing agent to white ink: one is to improve the heat resistance of the ink; the other is to offset the consumption of the curing agent by the resin groups in the ink and avoid the incomplete curing of the adhesive layer in summer.
Adding method: Dilute with solvent first, then add it to the ink while stirring slowly until it is evenly mixed.
Wrong method: directly add the curing agent into the ink, or add it into the ink tray, which will not mix uniformly, but will not achieve the effect of adding the curing agent.
In addition, pay attention to the timeliness: the timeliness is generally 12 hours, and the overnight ink curing agent has expired, or a certain amount of curing agent should be added again.
4. Moisture-proof management of nylon membrane
Nylon absorbs moisture, and it is prone to ruffles, sloping edges, stripes, difficult coloring, and inaccurate color registration during printing.
When printing, it is best to control the temperature and humidity of the production workshop. When the humidity of the production workshop exceeds 80%, the nylon film is easy to absorb moisture and rise, causing a series of printing product quality problems.
Specifically, pay attention to the following aspects: ① Do not open the package too early before use. ② Try to use it up at one time, and wrap the remaining film with a material with good barrier properties. ③ When printing, the first color group is not on the plate roller, and it is pre-dried. ④ Ensure a reasonable temperature (25℃±2℃) and humidity (≤80%RH) in the production workshop. ⑤ The printed nylon film should be packed with moisture-proof film.
C. Dry composite process
1. Selection of glue amount
Standard gluing amount range: 2.8 ~ 3.2gsm, excessive gluing amount has no meaning on peeling strength, but increases drying load. For composite equipment with insufficient drying capacity, it will increase the probability of delamination and bag breakage after cooking.
When detecting the glue amount, pay special attention to the change of water content before and after the nylon film passes through the drying tunnel, which affects the accuracy of the glue amount detection!
When we produce boiled bags, we should not only look at the amount of glue, but also pay attention to the microscopic uniformity of adhesive coating. The parameters of the mesh roller will directly affect the microscopic uniformity of adhesive coating.
2. Moisture requirements of ethyl acetate
The unqualified quality of ethyl acetate (such as excessive moisture content and alcohol) often leads to wrinkling quality accidents of composite membranes.
The alcohol content in ethyl acetate often does not attract the attention of most flexible packaging enterprises. The ethyl ester test data (barrel solvent) of a flexible packaging enterprise found that only one high-quality product and two first-class products out of 14 batches. The quality is poor, and the soft package factory should pay attention.
3. Ensure that the adhesive is fully dry
We usually only pay attention to the amount of compound glue. In fact, insufficient drying is often the most direct cause of incomplete curing of the adhesive (insufficient heat resistance of the adhesive layer), delamination and wrinkling of the packaging bag when it is boiled. There are a small amount of water and alcohol impurities in the prepared glue. Good dryness can make the moisture and other impurities in the glue layer volatilize as much as possible, and reduce the consumption of the curing agent in the glue layer. In order to ensure that the adhesive is fully dry during dry compounding, pay attention to the following points:
(1) The drying performance of the equipment itself, such as the air intake and exhaust volume of the equipment, and the length of the oven.
(2) Setting of drying temperature.
①The setting of drying temperature in the first zone. The ethyl ester concentration of the drying medium in the first zone is the highest, so the drying temperature of the first zone cannot be set too high (generally not higher than 65°C). Because the temperature is too high, the solvent on the surface of the adhesive layer volatilizes quickly and the skinning inhibits the escape of the inner layer solvent in the drying section of the latter areas.
②Drying temperature gradient setting. The oven temperature should be set according to the law of gradual increase in gradient, the purpose is to accelerate the diffusion and volatilization of the adhesive layer solvent in the hardening area and the odor exclusion area, and reduce the solvent residue in the film.
(3) Adjustment of intake and exhaust air volume.
①In the evaporation area of the drying process, the inlet and exhaust air volume valves should be opened to the maximum, and the return air valve should be closed.
②In the dry hardening area and the odor elimination area, the return air volume can be appropriately increased, which can save some energy consumption.
4. The influence of ambient temperature and humidity
The high-temperature and high-humidity season in summer is a period of frequent occurrence of quality accidents of the two-component polyurethane adhesive dry-process composite adhesive layer. According to an adhesive factory, 95% of the quality feedback received in summer is not related to the adhesive layer. related. Under the environment of high temperature and high humidity, the water content in the air is very high, and it is easy to enter the glue tray through the volatilization of acetic acid to consume the curing agent, so that the ratio of the main agent of the adhesive and the curing agent is unbalanced, resulting in the adhesive after compound curing. The layer crosslinking and curing is incomplete, and delamination and wrinkling appear when boiled in water.
Flexible packaging companies that do not have the conditions to control the temperature and humidity of the workshop should pay attention to the following points in the summer high temperature and high humidity season:
①By detecting the ambient temperature and humidity and the temperature above the plastic tray and solvent barrel, the phenomenon of “dew point” can be avoided. Once the “dew point” occurs, it means that a large amount of moisture in the air enters the plastic tray, and it is very easy for the rubber layer to dry out.
②Boiled bags should avoid high humidity periods for compound processing during production arrangements.
③The ethyl acetate bucket and glue circulation bucket used for compounding should be covered and sealed. If a semi-closed plastic tray is used, the influence of environmental humidity can be further reduced.
5. Maturation process requirements
General aging conditions: temperature 50 ~ 55 ℃, 48 hours.
In addition, pay attention to the uniformity of the curing of the entire film roll: ① whether the displayed temperature is consistent with the actual temperature near the film roll (the actual temperature of the upper, lower, left, and right sides of the film roll); ② whether the air near the film roll can achieve effective convection; ③ winding surface The effect of temperature on curing: There is a certain process of heat transfer, whether the curing conditions of the core composite film meet the requirements. (This may be a major factor in inconsistent quality.)
D. the bag making process
The heat sealing strength of the boiled bag is better, and more importantly, the quality of the whole batch is required to be stable, such as: ① no local bad sealing phenomenon; ② no individual bad sealing phenomenon in the whole batch.
When making boiled packaging bags, pay attention to the following points:
① Under the condition of ensuring the sealing appearance, set a slightly higher heat sealing temperature to avoid the phenomenon of unstable heat sealing quality caused by the deviation of the thickness of the composite film.
② During normal production, the edge sealing should ensure three effective heat sealing times. When the machine is turned off and then turned on again, the surface of the part that has been hot pressed once or twice may be cooled when it is turned on again (the first hot pressing can only play the role of preheating), and the actual number of effective heat sealing is only twice Therefore, it is also necessary to set a higher heat-sealing temperature (the heat-sealing can be good after two times of hot-pressing), so as to avoid a small number of poor sealing phenomenon when the machine is turned off and then turned on.
③Most of the boiled bags are liquid packaging, which requires high drop resistance of the packaging bags. Avoid undercutting of the heat-sealed edge during bag making, especially the edge of the heat-sealing knife should not be too sharp, and should be chamfered or polished appropriately. .
E. Testing requirements
1. Representativeness of Sampling
①When the first sample is confirmed, the quantity of one continuous sampling should cover the length of all sealing knives, so as to effectively avoid the phenomenon of partial poor sealing and missed inspection.
②Sampling is to take samples after the debugging is normal, adjust the heat sealing temperature, pressure, and machine speed, and reconfirm after replacing the film roll.
2. Effectiveness of the heat seal strength detection and judgment method
①The correct method is to cut the heat-sealed edge of the bag into a narrow strip of 20-30mm and tear it in the direction perpendicular to the sealing line.
②There should be no phenomenon that the width of more than 2mm can be torn on the inside of the sealing edge. Otherwise, the strength is qualified during the test on the machine, but the heat sealing layer is not completely integrated, resulting in a large decrease in the sealing strength during boiling and the phenomenon of bag breaking due to boiling. When the bag is detached from the interface between the two inner layers of PE on the sealing edge after being boiled in water, it belongs to the problem that the heat sealing edge is not strong.
3. Main points of boiling test
(1) Sampling method
① After the test machine of the boiled packaging bag is normal, the inspector will randomly and continuously select several sample bags from each row in the test machine bag (the number of samples required to cover the length of the sealing knife), and then carry out the boiling test after being sealed with water.
②When sampling more than one bag, use a marker to clearly mark the bag and the left and right directions so as to accurately determine the position of the sealing knife where the heat sealing is not firm.
③ When the normal production process conditions change significantly, such as machine speed, temperature adjustment, etc., it is necessary to re-sample for boiling test.
④ After each shift, re-sampling should be taken for boiling performance test.
⑤Separate and deal with unqualified products found in the process in time.
(2) Test conditions
①Put 1/3 to 1/2 capacity of water in the packaging bag, and try to vent the air when sealing. If too much air is enclosed, it is easy to cause misjudgment. A lid was added during the boiling test to slightly increase the pressure inside the bag.
②The boiling time is subject to the customer’s use conditions, or according to the relevant testing standards.
(3) Test qualification standard
① There is no overall or partial wrinkling and delamination on the surface of the bag; the peel strength is detected by hand feeling after boiling.
② The printing ink has no discoloration or bleeding;
③ There is no leakage and bag breakage; there is no obvious running edge phenomenon on the sealing edge (the running edge width is controlled within 2mm).
Post time: May-05-2022