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White spots often occur in solventless lamination process. After a large amount of practical experience accumulation and multiple discussions and summaries by industry professionals, we have largely controlled this problem in practical operation. However, there are differences in product types, materials used, and processes among each enterprise, and the reasons for white spot problems are also different. To comprehensively understand and effectively solve such problems, and to maximize the control of solvent-free composite white spots, let’s summarize and analyze the control and response measures for operational problems.
One of the advantages of solvent-free process compared to dry coating is the low amount of glue applied. However, in practical operation, the low amount of glue applied can indeed cause white spots in the composite.
The light colored areas with special ink layer transitions are prone to white spots due to uneven penetration resistance during the stacking process of the adhesive layer and ink layer. Areas with thick ink layers have higher resistance, while areas with shallow ink layers have lower resistance, resulting in inconsistent extension strength of the adhesive layer and relative unevenness between the adhesive layer and ink layer. The adhesive layer in the light colored areas is relatively thinner after extension, making it easy to form obvious white spots.
Low glue application and poor leveling after glue coating result in insufficient filling of the uneven ink layer, causing white spots.
When the ink thickness of the printed pattern is uneven, it is difficult for the glue to completely fill the ink pits during the coating and curing process.
5. Consider the matching of ink and glue
In the past, this kind of white spot problem was mostly considered to be too loose or insufficient glue. If we consider this problem from the perspective of ink and glue matching, plus the impact of machine speed on both ink and glue, the white spot problem is really related to the matching of ink and glue. If the matching of ink and glue is poor, with no white spots at low speeds but white spots at high speeds, it indicates that this ink and glue combination cannot meet the requirements of solvent-free high-speed production and affects production efficiency.
It is not objective to simply say that there is a problem with the glue or ink in this situation. Sometimes the defects of the ink can be compensated for by certain properties of the glue, and sometimes the advantages of the ink can compensate for the defects of the glue. When the two complement each other strongly, there will be no problems; If there is a problem, it can only be said that the defects and advantages of the two cannot complement each other well, and the combination is not suitable and more in line with reality.
One of the key advantages of solvent-free compounding is that it is faster and more efficient than dry laminating machines. However, solvent-free high-speed production requires certain conditions to be guaranteed, such as sufficient glue amount, suitable glue viscosity, temperature conditions, printing patterns, etc., which need to be suitable for high-speed production. Otherwise, it can only be controlled within a certain range of machine speeds. In fact, whether solvent-free can fully utilize the advantages of high-speed production is also related to the printing film. For example, due to the printing speed being too fast, the residual solvent in the printing film affects the degree of ink curing, and there are also variables such as variations in the thickness of the ink layer. These are the reasons why high-speed lamination cannot be achieved.
Only after eliminating the factors that limit the high-speed composite can the speed be appropriately increased. Blindly producing at high speeds will bury the hidden danger of white spots.
The influence of printing pattern design is often overlooked, but it is actually a very common influencing factor. Improper control on patterns such as excessive ink depth, solid and hollow areas, the presence or absence of white ink as a base, and clear boundaries between dark and light colors can greatly increase the likelihood of white spots appearing. Especially in areas with significant thickness differences in the ink layer, white spots are more likely to appear. Of course, when special printing patterns are combined, white spots can also be prevented through other operational methods.
When composite thick PE film, due to uneven material thickness and uneven winding force, such as deviation in raw material thickness causing loose and tight feeding, the two layers of film are subjected to inconsistent horizontal and vertical forces after composite, which not only has the possibility of wrinkles but also affects the formation of composite white spots. The appearance of white spots in this situation is the result of the combined effect of low initial adhesion without solvent and inconsistent winding tightness. Especially for composite products with two-layer structures, the impact of large deviations in film thickness is more prominent.
For example, in the autumn and winter seasons in the north, when the ambient temperature is low, the white spot phenomenon cannot be cured under conventional curing conditions. Multiple factors can be combined, and environmental conditions can amplify the problem. Therefore, the curing conditions should be adjusted according to the actual environmental conditions and standardized technical guidance documents should be formed.
Ink manufacturers will adjust the ink formula according to seasonal changes. If white spots appear due to solvent-free compounding, it may also be a problem with adjusting the ink formula. For example, adding slow drying or fast drying solvents, the ink itself changes the solubility of the printing film or the dryness of the ink, or the ink composition changes and there are problems with the fusion and matching of the original glue.
After the printing film machine is not stored for a certain period of time, it continues to be compounded. In theory, the ink resin is not fully cured and the adhesion strength to the printing substrate is not optimal. The direct compounding of ink and glue by the printing film machine is greater than the strength after the ink is fully cured. If it is compounded after being stored for several hours, the strength of the fusion reaction between the ink and glue after bonding is relatively small. So there is a hidden danger of white spots appearing immediately when the printing film under machine is compounded, especially when other factors that cause white spots also exist, the risk is very high. Usually, after being stored at room temperature for 24 hours, it is best to composite within 48 hours. However, if conditions do not permit, it should be stored for at least 12 hours.
The processing environmental conditions have a significant impact on both glue and film. The same production process can produce products with different appearances under high temperature and high humidity conditions, as well as dry and low temperature conditions. The environmental conditions in the printing workshop can affect the state of the printing film, which will definitely affect the composite effect. Therefore, in solvent-free composite processes, it is not enough to only focus on the environmental conditions in the composite workshop. It is also necessary to pay attention to the correlation between the printing film and white spots when the environmental conditions in the printing workshop change, and take corresponding preventive measures.
It is very difficult to find a single universally applicable solution to the white spots produced by solvent-free composites. For most solvent-free composite products, in order to achieve foolproof results, it is necessary to comprehensively consider and avoid various adverse factors. In addition to the most commonly used control methods and paying attention to some key details, specific problem analysis is also necessary in actual operation to identify the true cause of the problem and make precise treatment. For occasional white spot problems, it is important to investigate thoroughly and identify the actual variable influencing factors, in order to accumulate valuable experience for thoroughly and effectively controlling white spots.
Longdapac engineers are very professional in these fields. The lamination technology is very mature and with good quality. The lamination for mono-material PE/PE laminating film are also with good quality. It is flat, stable, tidy and clean.