Solutions to common faults in the use of UV varnish

Qi Cheng

Fault phenomenon: unstable glazing effect

Main reasons and solutions: This is a big headache for many flexo printing enterprises. In the process of flexographic printing, in order to keep the viscosity of water-based ink stable, it is necessary to add a certain amount of amines to the ink. Although it can effectively adjust and maintain the viscosity of the ink, some amines will have a negative impact on the curing effect of UV varnish. Sometimes, glazing instability is caused by the amine added to the ink. Solution: try other new methods to control the viscosity of the ink. If the varnish does not match the ink used, you must change the varnish or change the formula of the varnish.

Fault phenomenon: yellow coating

Main reasons and solutions: the main reasons for the yellowing of UV varnish after curing are generally two aspects: first, the UV varnish has turned yellow after being placed for too long, especially the yellowing phenomenon of low-quality varnish is more serious; Second, excessive UV irradiation and UV varnish curing. Solution: if it is the quality problem of UV varnish itself, you must replace UV varnish; If the yellowing is caused by overexposure, reduce the power of the UV light source or increase the glazing speed.

Fault phenomenon: UV varnish is incompatible with ink, with stripes and orange peel

Main reasons and solutions: the specific performance is that UV oil beads on a certain ink base, which looks like orange peel after curing, or although it can be evenly coated without blistering, if you scrape it with a sharp tool, the UV oil layer will be separated from the ink layer. The main reason is that UV varnish is incompatible with ink, and the viscosity of UV varnish is too high; The wire of the anilox roller is too thick (the coating amount is too large) and the surface is not smooth; Uneven coating pressure; The leveling property of UV varnish is poor. The solution is to apply a layer of primer first, then apply UV oil, or apply UV oil after spark treatment, and mix an appropriate amount of alcohol based infrared light oil.

Fault phenomenon: the adhesion of UV varnish is poor, and the oil cannot be coated or scratched

Main reasons and solutions: the main reason is that the viscosity of UV varnish is too small and the coating is too thin; Ink middle note ink oil or dry oil content is too high; Printing ink surface crystallization; Improper additives in printing ink; There are too many anti sticking materials (silicone oil, powder spraying) on the ink surface; The wire of the anilox roller is too thin; The light curing conditions are inappropriate. The solution is to reduce the viscosity of UV varnish and reduce the coating amount; Adjust the pressure evenly; The coating roller should be ground and polished; Add bright leveling agent. For this fault, corresponding measures should be taken when printing, and glazing conditions should be considered in advance; UV varnish can be coated thicker appropriately; Apply primer or special varnish when necessary; Apply primer to the printed product to enhance adhesion.

Malfunction: poor adhesion fastness of the coating on the surface of non absorbent substrate

Main reasons and solutions: if the printing material is a non absorbent or non porous substrate (such as PVC or PET film), there may be poor adhesion fastness of the varnish coating on the surface of the substrate. It is generally believed that the harder the coating, the better the effect, but for non absorbent substrates, the toughness of the coating is the most important, otherwise there will be poor adhesion. In order to obtain a 'soft' coating, some special raw materials need to be used in the varnish formula, and light solid resins and active monomers with good toughness and adhesion should be selected. Solution: the cross cut test method is generally used to test the adhesion fastness of the coating. First, prepare transparent tape and gridding tools, draw cross gridlines on the coating, be careful not to hurt the substrate, and then adhere with tape. The area of the coating torn off by the tape represents the curing degree and adhesion of the coating. If the coating is not torn off at all, it indicates that the adhesion of the coating is quite good. If the coating is torn off in a large area, it means that this kind of varnish is not suitable for use on the non porous substrate, and other types of varnish should be considered.

Symptom: the varnish is not cured well on the black ink, white ink and light emitting blue ink layers

Main reasons and solutions: various unexpected problems often occur when printing varnish on black ink, white ink and light emitting blue ink layers, which is also one of the most headache problems for the majority of printing enterprises. We can understand and treat this problem in this way: the pigment in the ink and the photoinitiator in the varnish absorb the ultraviolet light of the same wavelength. They compete for limited ultraviolet energy together, and the curing of the varnish requires energy, so this has a greater impact on the varnish, which may cause incomplete curing of the varnish.

Solution: increase the UV lamp power, clean the reflector, replace the UV lamp tube, and reduce the printing speed of the varnish, but this can only be used as an expedient measure. From a long-term perspective, we should develop and develop a versatile varnish that can completely cure on any color ink layer.