How to solve the poor wear resistance and adhesion of UV insulating ink?

During the printing process, sometimes we come into contact with surface finishing techniques such as UV insulating ink, and the development of this technique is inseparable from printing materials such as UV varnish. Color box printing companies often encounter such disturbances: after UV coating the printed products, the wear resistance and adhesion of the UV coating layer are poor, especially when the printed products are printed with metal ink (gold ink or silver ink) and then UV coated, this phenomenon is particularly prominent.

In general, color box printing companies often solve this problem by replacing UV varnish. In fact, we should not only limit our thinking to replacing UV varnish, but also broaden our thinking and shift our focus to changing the production process route of color boxes or adjusting relevant process parameters.

Solution ideas

The conventional production process route for corrugated color boxes is as follows: plate making and paper cutting → regular printing → UV primer coating → UV surface coating → corrugated mounting → die-cutting → box pasting. After multiple experiments, the author summarizes the methods for improving the production process of corrugated color boxes as follows.

(1) Plate making and paper cutting → UV ink printing+online UV polishing. Experimental results have shown that this production process can greatly improve the wear resistance and adhesion of UV varnish on Jinjie UV ink. However, metal UV ink is relatively rare in the market, requires special customization, is expensive, and requires the use of UV printing machines during printing. Generally, most small color box printing enterprises do not have UV printing machines, so this production process is only suitable for some color box printing enterprises and has little significance for widespread promotion.

(2) Plate making and paper cutting → regular printing → screen printing with UV coating. This production process can also solve the problem of poor wear resistance and adhesion of UV varnish on Jintu ink, but screen printing UV varnish has a higher cost and lower environmental performance than water-based UV varnish. In addition, many color box printing companies do not have fully automatic screen printing machines, only semi-automatic screen printing machines with lower production efficiency. Therefore, this production process can only be applied to some color box printing companies and is not worth promoting widely.

After repeated verification, Jiexun finally identified the solution to the problem: using high wear resistant UV surface oil to improve wear resistance, and ensuring the adhesion of UV base oil by adjusting process parameters or replacing materials. Due to two rounds of polishing, the varnish film layer is thicker, which ensures the wear resistance of the high wear resistant UV varnish. The next step is to focus on solving the key factors that affect the adhesion of UV varnish - printing ink and UV base oil.
 

Before solving the problem, it is necessary to first understand the customer's requirements and testing methods for the color box.

(1) To test the wear resistance of UV varnish on color boxes, the method is as follows: ① If the color box needs to be shipped by sea, a box of actual products can be packed in the cardboard box used for actual customs declaration, or a box of product models with the same weight and specifications as the actual products can be packed, and placed on a simulated transportation testing machine (vibration test bench) for more than 2 hours of vibration, with no obvious UV varnish film or ink layer falling off the surface of the color box as the grid; ② Place the color box sample on the friction tester and rub it back and forth with a 22N weight for 80 times (weight and frequency can be determined according to customer requirements). It is considered qualified if there is no obvious UV light oil or ink drop.

(2) To test the adhesion of UV varnish on the color box, the method is as follows: ① Apply 3M transparent tape firmly to the surface of the color box, and then quickly and forcefully tear off the tape at a 45 degree angle until the UV varnish film or ink layer does not stick off, which is considered qualified; ② Use an HB pencil lead to rub back and forth forcefully on the surface of the UV gloss oil film layer in the color box, until the UV gloss oil film layer or ink layer is not scratched off, which is considered qualified.

At present, there are two main structures of silver powder contained in silver ink sold in the market: one is a nearly circular granular structure; Another type is a sheet-like structure similar to fish scales. The former is more conducive to the adsorption of UV base oil, while the latter can better demonstrate the metallic luster and texture of ink. At present, the gold powder in the gold ink sold on the market only has one structure: a sheet-like structure similar to "fish scales". Therefore, the adhesion of UV primer on gold ink is much worse than on silver ink, and it is also more difficult to solve. The author also learned that oil-based UV base oil has better adhesion on metal ink than water-based UV base oil. However, oil-based UV base oil was also used in the improved process, but its adhesion problem on Jintu ink has not been solved. Therefore, we have to shift our focus to the improvement of metal ink.

When testing the adhesion of UV varnish on the problem color box, we found that not only the UV varnish film layer but also a part of the ink layer was stuck off by 3M transparent tape. In addition, we also found that after completing the conventional printing process and the metal ink is completely dry, when gently tracing the surface of the printed product with fingers, a thin layer of metal powder will be left on the fingers. From this, we conclude that it may be due to insufficient bonding material in the metal ink. So we added some transparent ink to the metal ink, and after testing again, we found that the adhesion of the UV base oil on the metal ink had significantly improved, but still did not meet the customer's requirements. We will replace the added transparent ink with fast drying varnish, which basically solves the adhesion problem of UV primer on metal ink. However, the addition of fast drying varnish diluted the color of Jintu ink, causing a slight change in the color of the printed product. Therefore, a small amount of four-color ink needs to be added to the metal ink for re blending. In addition, in the three processes after UV polishing, the moisture content of the color box mainly affects the wear resistance of the UV varnish, so it needs to be controlled to some extent.