In the production process of high-temperature cooking bags , many failures often occur. Due to the relatively high cost, the printing company has brought significant economic losses. Common failures include delamination of high-temperature sterilization white ink, printing substrates and composite substrates. Delamination, surface wrinkling after high temperature sterilization, etc. This article combines the actual situation to analyze the phenomenon of wrinkling on the surface after boiling at high temperature.
1. Wrinkled form after high temperature cooking bag sterilization treatment
Whatever composite packaging bags require good flatness, true color, is excellent in the opening property, retort pouch is no exception. There are three kinds of wrinkling phenomena: a. The printed substrate has longitudinal, horizontal or irregular wrinkles; b. The substrate and the composite substrate are wrinkled on the surface together, and the flatness is extremely poor; c. The printed substrate shrinks and shrinks Separated from the composite substrate, with air in the middle, in the form of strips, often with poor peel strength.
2. Classification of common substrates and structures of high-temperature cooking bags
When most companies on the market print high-temperature cooking bags, most of the printing substrates use PET and NY as printing materials, most of the middle layer uses aluminum foil, and the inner heat-sealing layer uses cast CPP or boiling-resistant PE film. Wait.
Among them, PET has good stereotypes and excellent freeze resistance and heat resistance; NY has poor moisture resistance and easily absorbs water. Aluminum foil is an important substrate in the flexible plastic packaging industry. It is calendered at high temperature and has a silver-white luster. Aluminum foil used on flexible plastic packaging is 9 - 15μm, unwinding, rewinding not to use too much tension, tension is easy to tear, not by the excess foil guide rollers, folding susceptible to cracking, good impact barrier The aluminum foil has good thermal conductivity and electrical conductivity, making it widely used in flexible packaging. In high-temperature cooking bags, using aluminum foil as an intermediate layer material can increase the heat transfer rate and make the temperature of the food in the cooking bag uniform.
Aluminum foil has excellent barrier properties, and the surface should not have oil stains. It is best to pre-treat it before printing or compounding, so that the surface tension can reach the required requirements, which can obviously improve its printing fastness and aluminum compound fastness. The extruded cast CPP reinforced film is often used as an inner layer. The cast CPP has good barrier properties, high temperature resistance and boiling resistance, and it has good effect on alkali acid resistance, small heat shrinkage, and excellent heat seal strength, which can reach 121 ℃ 30min sterilization treatment, when 30min at 100 ℃ , heat sealing materials such as LDPE , LLDPE , EVA , mPE can be used .
3. Factors of surface wrinkling after high-temperature cooking bag sterilization
The phenomenon of wrinkle on the surface should be carefully observed to make an accurate analysis, if the printed substrate wrinkles in the longitudinal, horizontal or irregular directions. The analysis is mainly considered from three points: a. When the temperature of the drying tunnel is too high during printing, the constant tension of winding and unwinding is inconsistent, and the tension of unwinding is too large to deform the PET film. b. The core of the rewinding roller is not smooth. Because the volume of the rewinding product continues to increase, vertical or horizontal wrinkles will occur at the bottom of the reel, making the original flat printing substrate uneven. This phenomenon of wrinkling It is not easy to eliminate during the process. c. The printing traction roller has foreign objects or is too dirty. When the printing substrate passes through the foreign objects, lateral sliding occurs. When the sliding substrate passes through the embossing rubber roller, wrinkles are generated. These wrinkles are regular, one paragraph at a time. No.
Solution: a. Adjust the drying temperature according to the temperature and humidity of the workshop, machine speed, and the depth of the actual printing effect pattern. Most retort pouch for printing nylon 15μm, PET of 12μm, it is recommended that the drying temperature preferably at 40-60 ℃. If the drying temperature is too high, the film will not withstand the high temperature and will deform, elongate, and soften. To adjust the constant tension of winding and unwinding, there are now high-speed gravure printing machines that have produced an automatic tension controller. When the limit or set value is exceeded, an automatic alarm and automatic adjustment are convenient. b. Replace the paper tube with a smooth center, and pay attention to the flatness when winding. At the beginning, the winding tension should be small. When it reaches a certain thickness, slowly increase the tension value, so that the winding material will not wrinkle. c. Wipe the traction roller with foreign objects clean.
When the substrate and the composite substrate are wrinkled on the surface together, the flatness is extremely poor, the factors are: a. The wrinkle of the printed substrate itself or the wrinkle cannot be eliminated by printing; b. When the pressure on the two ends of the hot steel roller is inconsistent or compounded The deviation of the thickness of the composite substrate PE and CPP is large. This deviation phenomenon is easy to occur when the deviation is 10μm ; c. The bottom tension is too small when the composite product is wound, and the upper tension is too large. Kind of phenomenon.
Solutions: a. Strictly control the printing quality; b. Adjust the pressure of the steel roller to a balanced state, and the composite substrate should not be biased as much as possible. When the composite is aluminum foil, try not to wear the traction roller as much as possible to prevent breakage; c. Adjust the winding Tension, try to be consistent.
When the substrate has wrinkles and there are bubbles in the middle, the tension is mostly not controlled during dry compounding. The tension is too large and the tension is too tight. It is most likely to occur in NY / PE or NY / aluminum foil / CPP , especially nylon substrate. If the unwinding tension is too large, heat shrinkage and deformation will occur in the drying tunnel, and this phenomenon will occur after the finished product is cooled; the effect of the glue solution, the dry solid amount when coating the high-temperature cooking bag must reach 4.5g / m2 , the wire roller Choose 80μm deep. If uneven coating occurs, or the quality of the glue itself is poor, the glue can easily be separated from the substrate after boiling, which greatly reduces its fastness.
Solution: a. Adjust the unwinding tension, do not dry at high temperature as much as possible when the solvent is completely volatilized, so as not to deform and cool and shrink; b. Increase the operating concentration, choose a glue with a large dry solid content, usually when the solid content is 75 % , The working concentration of 30% is good; c. Choose professional adhesives with high temperature resistance, and it is strictly forbidden to apply general adhesives.
Fourth, the impact of composite technology
1. When printing and compounding, the ambient air must be kept dry, especially the lower the relative humidity, the better, the highest should not exceed 85% , otherwise it will affect the printing compound fastness.
2. Ink and adhesive: When cooking film printing, ink thinner should be added with or without alcohol solvent as much as possible. The quality should be strictly controlled during printing. The ratio of adhesive to curing agent should be correct. , excellent performance, the amount of dry 4 - 5g / m2.
3. Drying temperature, compound temperature and pressure: If the drying temperature is too low, the solvent will not evaporate sufficiently, which will easily produce odor, and it will directly affect the compound fastness. However, if the drying temperature is too high, it will cause the conjunctiva of the adhesive coating to affect the volatilization of the solvent in the coating and also have a certain effect on the film.
Drying at a temperature of generally 60 ℃ - 70 ℃ - the gradient of 75 ℃, the dry composite, composite rolls must be heated, the heated object: soften the adhesive, the viscosity increase, the binder evenly distributed on the membrane, while retaining the reaction of the hot-melt adhesive state, to facilitate bonding, the composite is generally a temperature of 60 ℃ - 70 ℃.
Composite pressure has influence on the two products: one impact fastness, and second, affect the transparency of the composite film, the best composite pressure 2.5 - 4kg / cm2.
4. Aging. The purpose is to make curing reaction sufficiently, the best composite strength, generally the curing temperature controlled cooking film bags at 40 ℃ - range 50 ℃, aging time of about 48 hours.
In summary, in the entire production process of cooking bags, the choice of film, ink, adhesive, and dilution solvent plays a decisive role in the quality of the cooking bag, and it cannot be ignored in production.
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