UV varnishing is a common surface finishing process in the packaging printing industry. Color box printing enterprises (especially those with export business in countries and regions such as Japan, Europe, and the United States) often encounter such problems: After applying UV varnish to the printed products, the wear resistance and adhesion of the UV varnish film layer are not good. This problem is particularly prominent when the printed products are printed with metallic inks (gold ink or silver ink) before UV varnishing. 

Generally speaking, printing companies for color boxes usually solve this problem by changing the UV varnish. In fact, we should not limit our thinking to merely changing the UV varnish; instead, we should broaden our thinking and shift our focus to modifying the production process of the color boxes or adjusting the relevant process parameters. 

Solution Approach 

The conventional production process for corrugated colored boxes is as follows: plate making and paper cutting → ordinary printing → applying UV base oil → applying UV top oil → gluing corrugated material → die-cutting → assembling the box. After numerous experiments, the author has summarized the methods for improving the production process of corrugated colored boxes as follows. 

(1) Plate making and paper cutting → UV ink printing + online UV varnishing. Tests have shown that this production process can significantly enhance the wear resistance and adhesion of UV varnish on metal UV ink. However, metal UV ink is relatively rare in the market and requires special customization, which is expensive. Moreover, during printing, UV printing machines must be used. 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 is not very conducive to widespread promotion. 

(2) Plate making and paper cutting → Ordinary printing → Screen printing with UV varnish. This production process can also solve the problems of poor wear resistance and adhesion of UV varnish on metal ink. However, the cost of screen printing with UV varnish is higher, and its environmental performance is not as good as water-based UV varnish. Moreover, many color box printing enterprises do not have fully automatic screen printers; they only have semi-automatic screen printers with lower production efficiency. Therefore, this production process can only be applied to some color box printing enterprises and is not worth being widely promoted. 

After repeated verification, we clearly understood the solution to this problem: use high wear-resistance UV surface oil to enhance wear resistance, and ensure the adhesion of the UV base oil by adjusting process parameters or replacing materials. Due to the two layers of varnish applied, the varnish film layer is relatively thick, which makes the wear resistance of the high wear-resistance UV surface oil more guaranteed. The next step is to focus on solving the key factors that affect the adhesion of UV varnish - printing ink and UV base oil. 

Solutions/Measures 

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

(1) To test the wear resistance of the UV varnish on the packaging box, the following method can be used: ① If the packaging box needs to be transported by sea, a box containing the actual product or a box containing a product model with the same weight specification as the actual product can be packed in the actual customs declaration box, and then placed on the simulated transportation test machine (vibration test bench) for vibration for more than 2 hours. The packaging box is considered qualified if there is no obvious fall-off of the UV varnish or ink layer on the surface; ② Place the packaging box sample on the abrasion resistance tester, and use a 22N weight to wipe back and forth 80 times (the weight and number of times can be determined according to the customer's requirements). The packaging box is considered qualified if there is no obvious fall-off of the UV varnish or ink. 

(2) To test the adhesion of the UV varnish on the color box, the following method is used: ① Press a 3M transparent tape firmly onto the surface of the color box, then quickly and forcefully tear the tape at a 45-degree angle to ensure that the UV varnish film layer or ink layer does not come off; ② Use an HB pencil core to forcefully draw back and forth on the surface of the UV varnish film layer of the color box, and ensure that the UV varnish film layer or ink layer does not come off. 

The structure of the silver powder contained in the silver ink currently sold on the market mainly consists of two types: one is a granular structure that is nearly circular; the other 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 display the metallic luster and texture of the ink. However, the structure of the gold powder contained in the gold ink currently sold on the market is only one type: a sheet-like structure similar to "fish scales". Therefore, the adhesion of UV base oil to the gold ink is much worse than that to the silver ink, and the problem is more difficult to solve. I also learned that: the adhesion of oil-based UV base oil to metal inks is better than that of water-based UV base oil. However, in the improved process, the same oil-based UV base oil is used, but the adhesion problem of this base oil to metal inks still has not been solved. Therefore, we have to shift our focus to the improvement of metal inks. 

When testing the adhesion of the UV varnish on the problem packaging box, we found that not only the UV varnish layer but also a part of the ink layer was peeled off by the 3M transparent tape. Moreover, we discovered that after completing the regular printing process and when the metallic ink was completely dry, when the finger gently rubbed over the printed surface, a thin layer of metallic powder would remain on the finger. Based on this, we concluded that it might be due to insufficient binder in the metallic ink. Therefore, we added some transparent ink to the metallic ink and conducted another test. It was found that the adhesion of the UV base varnish on the metallic ink had significantly improved, but it still did not meet the customer's requirements. In response to this, we replaced the added transparent ink with quick-drying varnish. As a result, the adhesion problem of the UV base varnish on the metallic ink was basically solved. However, the addition of the quick-drying varnish diluted the color of the metallic 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 metallic ink for re-blending. In addition, in the three processes after UV polishing, the moisture content of the packaging box would have an impact on the wear resistance of the UV varnish. Therefore, certain control is also required for it. 

Summary of the Plan 

Finally, the author summarizes the technical points regarding the wear resistance and adhesion of UV varnish on metal ink as follows. 

(1) Production process route: Plate making and paper cutting → Ordinary printing → Applying UV base coating → Applying UV surface coating. 
(2) Use high-maintenance UV surface coating and oil-based UV base coating. 
(3) Add 10% to 20% of quick-drying varnish to the metallic ink (the exact proportion should be determined by observing that after the metallic ink has completely dried, a gentle swipe of your finger across the printed surface leaves almost no metallic powder on your finger. If the color of the metallic ink becomes diluted, you can adjust it by adding four-color ink). It is important to note that UV primer should only be applied after the metallic ink has completely dried. 
(4) All the quick-drying varnishes contain ink hardening agents. If the metal ink still takes a long time to dry after being coated with quick-drying varnish, an appropriate amount of hardening agent with the same drying properties as the white varnish (with an addition rate of ≤ 3%) can be added. It is strictly forbidden to add red varnish, as red varnish can only accelerate the drying speed on the surface of the ink layer, but cannot accelerate the drying speed within the ink layer. 
(5) In the subsequent process after coating, the moisture content of the color box must be kept below 16%. 
(6) Fix the brands of various materials in the production process route, and keep the process parameters of the common printing, UV primer application, and UV topcoat application processes stable. Also, make proper records.