How to improve the wear resistance and adhesion of UV varnish?

UV glazing is a common surface finishing process in the packaging and printing industry. Color box printing enterprises (especially color box printing enterprises with export business in Japan, Europe, America and other countries and regions) often encounter such problems: after UV varnishing the printed products, the wear resistance of the UV varnish layer and Poor adhesion, especially when the printed product is printed with metal ink (gold ink or silver ink) before UV glazing, 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 should diverge our thinking and change the thinking to change the production process route of color box or adjust relevant process parameters.

Solutions
The conventional production process route of corrugated color box is: plate making and paper cutting → ordinary printing → UV base oil → UV face oil → corrugated mounting → die cutting → paste box. After many tests, the author summarizes the methods to improve the production process of corrugated color boxes as follows.

(1) Platemaking and paper cutting → UV ink printing + online UV glazing. Tests have proved that this production process can greatly improve the wear resistance and adhesion of UV varnish on metal UV inks, but metal UV inks are relatively rare in the market, require special customization, are expensive, and must also use UV when printing Printing machine. In general, most small color box printing enterprises do not have UV printers, so this production process is only applicable to some color box printing enterprises, and the general promotion is of little significance.

(2) Plate-making and paper cutting → ordinary printing → screen printing UV glazing. This production process can also solve the problems of poor wear resistance and adhesion of UV varnish on metal inks, but screen printing UV varnish has a higher cost and environmental protection performance is not as good as water-based UV varnish. In addition, many colorful box printing enterprises do not have fully automatic screen printing machines, only semi-automatic screen printing machines with low production efficiency, so this production process can only be applied to some color box printing enterprises, and is not worthy of general promotion.

Finally, after repeated verification, we clearly clarified the idea of ​​solving this problem: the use of high wear resistance UV surface oil to improve wear resistance, and by adjusting the process parameters or replacing materials to ensure the adhesion of UV primer. After two glazings, the varnish film layer is thick, which makes the wear resistance of the high-wear-resistant UV surface oil more secure. The next step only needs to focus on solving the key factors that affect the adhesion of UV varnish-printing ink and UV primer.

solution
Before solving the problem, first need to understand the customer's requirements for color boxes and detection methods.

(1) To test the abrasion resistance of color box UV varnish, the method is as follows: â‘  If the color box needs to be shipped by sea, you can use the carton used in the actual customs declaration to pack a box of actual products, or install a box of weight specifications consistent with the actual product The product model is placed on a simulated transportation test machine (vibration test bench) and vibrated for more than 2 hours. The product is qualified if there is no obvious UV varnish film or ink layer on the surface of the color box; â‘¡Place the color box sample in the friction tester On the top, use a 22N weight to wipe back and forth 80 times (weight and times can be determined according to customer requirements), no obvious UV varnish or ink drop is qualified.

(2) Check the adhesion of the UV varnish of the color box, as follows: â‘  Apply 3M transparent tape to the surface of the color box, and then tear the tape quickly and forcefully at a 45-degree angle so as not to stick the UV varnish film The layer or ink layer is qualified; â‘¡Use HB pencil lead to scratch back and forth on the surface of the color box UV varnish film layer, so as not to scratch the UV varnish film layer or ink layer.

At present, there are two main structures of silver powder contained in silver inks sold on the market: one is a nearly circular granular structure; the other is a flaky structure similar to "fish scale". The former is more conducive to the absorption of UV primer, the latter is more able to show the metallic luster and texture of the ink. However, the structure of the gold powder contained in the gold ink sold on the market is only one type: a flaky structure similar to "fish scale". Therefore, the adhesion of UV primer to gold ink is much worse than silver ink, and it is more troublesome to solve. The author also learned that the adhesion of oily UV primers to metal inks is better than water-based UV primers. However, the oily UV primer is also used in the improved process, but the adhesion problem on the metal ink has not been solved, so we have to shift our focus to the improvement of the metal ink.

When detecting the adhesion of the problem color box UV varnish, we found that not only the UV varnish film layer but also a part of the ink layer were stuck by the 3M transparent tape. In addition, we also found that after the normal printing process is completed and the metal ink is completely dried, when a finger gently scratches the surface of the printed product, a thin layer of metal powder will be left on the finger. From this we judge: it may be that the connecting material in the metal ink is not enough. So we added some transparent inks to the metal ink, and once again found that the adhesion of the UV primer to the metal ink has been significantly improved, but it still fails to meet customer requirements. In this regard, we replaced the added transparent ink with quick-drying varnish. In this way, the adhesion problem of the UV primer on the metal ink is basically solved. However, the addition of quick-drying varnish dilutes the color of the metal ink, resulting in a slight change in the color of the printed product. Therefore, a little four-color ink must be added to the metal ink for redeployment. In addition, in the three processes after UV coating, the water content of the color box will affect the wear resistance of the UV varnish, so it needs to be controlled.

Solution summary
Finally, the author summarizes the technical points of the wear resistance and adhesion of UV varnish on metal ink.

(1) Production process route: plate-making and cutting paper → ordinary printing → UV base oil → UV surface oil.

(2) Use high-wear-resistant UV top oil and oil-based UV primer.

(3) Add 10% to 20% of quick-drying varnish to the metal ink (the specific ratio is after the metal ink is completely dried, the finger gently scratches the surface of the printed product, and the metal powder is basically not left on the finger, if the metal ink The color is diluted, it can be adjusted by adding four-color ink). It should be emphasized that you must wait for the metal ink to dry completely before passing the UV primer.

(4) Quick-drying varnish contains ink drier, if metal ink is still difficult to dry after adding quick-drying varnish, you can add a drier with the same drying properties as white dry oil (addition amount≤3% ), Do not add red dry oil, because red dry oil can only speed up the drying speed of the surface of the ink layer, but can not speed up the drying speed inside the ink layer.

(5) In the process after glazing, the moisture content of the color box must be controlled below 16%.

(6) The brand of various materials in the production process route is fixed, and the various process parameters of the general printing, UV primer, and UV surface oil procedures must be stable and relevant records should be made.

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