"Back sticking" problems are likely to cause greater harm, and color printing plants will often encounter such problems. This article mainly starts from the characteristics of ink performance, analyzes the factors that cause back sticking problems and how to deal with them. I hope that I can communicate with you to promote the improvement of technical water products in our color printing industry.
Back sticking refers to the phenomenon that the printing layer adheres to the back side of the film or the printing layer adheres to each other during the process of compounding or bagging after the plastic film is printed and wound. Due to the transfer or detachment of the graphic and text, it often results in a large number of scrapped prints. Back sticking is prone to occur during periods of high temperature, especially high humidity. The reason is often the result of a combination of environmental, ink, solvent, film, and printing process factors. To solve the back-adhesion problem well, it is necessary to analyze the specific situation and rationalize the factors from a comprehensive perspective. Interaction relationship.
The inks commonly used for printing plastic films include polyamide surface printing inks, chlorinated polypropylene composite inks, and polyurethane inks. Among these three types, chlorinated polypropylene composite inks have a lower probability of back sticking because chlorinated polypropylene compound inks are generally used to print BOPP films that are non-polar materials and the corona treatment on both sides is quite different. And the corona surface is only about 32 dyne, which has poor affinity with the ink; in addition, the chlorinated polypropylene composite ink has good solubility in common solvents, and it is easy to adjust the balance of solubility and volatility rate during printing. .
In the case of back adhesion caused by the use of polyamide printing inks, the printing film is often a corona-based LDPE tube, or it can be packaged and placed together after bag making. In terms of ink properties, the softening point of the polyamide linking material is generally about 110 C Torr, and it tends to be sticky in a relatively high temperature and high humidity environment. During printing, because the printer considers the effect of electric heating on the overlay accuracy of the LDPE film, it does not dare to heat, or open the cold wind, resulting in excessive residual or surface drying of the solvent in the printing layer. At the time of winding, the elongation rate of the LDPE film is relatively large. , And the regularity of the printing pattern, with the increase in the diameter of the film roll, "burst" phenomenon, triggering the back of the ink layer. To solve this kind of problem, it is best to use high-grade ink with relatively low viscosity and high color concentration so that the printing ink layer is thinner. Use idle printing units to increase film drying time and cooling distance. The printed film is made as early as possible and dispersed in the box for a period of time.
Due to its high gloss, good temperature resistance, oil resistance, and weak acidity and alkalinity, polyurethane ink has developed rapidly in recent years, and its usage has been rising rapidly. In the summer, when polyurethane ink is used to print NY and PET films, back adhesion is more likely to occur. In the actual printing process, the phenomenon of ink back sticking caused by unqualified films is relatively easy to eliminate. However, due to the inability to master the use of ink, the improper setting of process parameters leads to the phenomenon of back sticking of the printed film, which is why our printing technicians need to spend time. Great mind to solve. Polyurethane inks generally use keto solvents such as methyl ethyl ketone as the true solvent, and are used as the binder resin for dissolving inks. For example, the commonly used solvents include butanone, isopropyl alcohol and toluene. Among these three solvents, butanone has the fastest rate of volatilization. During the printing process, the amount of butanone is not replenished in a timely manner, causing the ink in the ink tank to swell due to the poor solubility of the resin, and the viscosity increases. The adhesion of the ink to the film. Therefore, when using a polyurethane ink, butanone must be maintained in a relatively sufficient amount in the dilution solvent. When the polyurethane ink is printed in the summer, the viscosity of the ink rises quickly. The operator must continuously replenish the diluent solvent and keep the ink in the ink tank always at a relatively low viscosity.
A great deal of practical experience shows that the phenomenon of ink back sticking caused by the ink layer “dry†is common in polyurethane inks. To solve this problem, the ink is required to be printed at a low viscosity. In the low-viscosity state, the printing ink consumption is small and the ink layer is thin. Under the effect of a large amount of dilution solvent, the polyurethane ink connecting material molecules are relatively loose, and the shielding ability against solvent volatilization is weak. When the ink is transferred to the film, the solvent can quickly and completely separate from the ink layer in the drying tunnel, and it is not easy to produce a "dry surface" phenomenon. After a reasonable adjustment of the solvent ratio and viscosity of the ink, the following points need our operators attention. First, bake the temperature and air volume. The temperature and air volume should be balanced. When the temperature is high, the air volume is relatively large, and the temperature of the light or the light energy of the light easily leads to “dryâ€, and the proper temperature and sufficient air volume are maintained before the ink layer can be completely dried. Second, the printed film is cooled. After the final color of the film is printed, it needs to be properly cooled and rewinded. Excess heat is present in the printed film, which may cause softening of the ink layer and increase back adhesion. Third, winding tension and film roll diameter. The linking material of polyurethane ink is similar to the molecular structure of PET film. The affinity of the ink for the PET film is particularly good. The PET film is a rigid film with good gas barrier properties. Even a very small amount of solvent residue in the printing ink layer may cause an adhesion effect between the ink layer and the back film. Therefore, in practical applications, the PET film is more likely to exhibit back adhesion than the NY film. The winding tension should be as small as possible when the film roll is not loose, and the small roll diameter also ensures that the film fastening force at the inner diameter part is not large.
Source: China Ink Technology Network