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The root cause of UV ink printing surface not sticking to hot melt adhesive isThe surface energy of the cured UV ink layer is extremely low (usually below 34 dynes), and the adhesive cannot wet and spreadIt can only stop on the surface to form a “false adhesion”, and when subjected to a slight force, the entire piece peels off, and the peeling surface is clean and free of fibers. There are only two directions to solve the problem: either remove ink or increase energy on the paper surface, or use adhesive particles that can climb onto low surface energy substrates.
Tear open the failed adhesive and observe the cross-section:
| adjustment item | Direction | Principle |
|---|---|---|
| Melt Temperature | Raise by 5-10 ℃ | Reduce viscosity and improve wetting |
| Compression pressure | Appropriately increase | Forced spreading and contact of adhesive solution |
| Compression time | Extend by 0.3-0.5 seconds | Leave time for wetting |
| Adhesive application amount | Slightly increased but not excessive | Excess will form a slip layer, which is actually worse |
It should be noted that parameter adjustments can only improve mild failures. If the surface energy drops below 30 dynes, no parameters can be salvaged and surface treatment or redesign must be carried out.
Use a dyne pen to draw a line in the adhesive area: if the liquid film does not shrink for more than 2 seconds, it indicates that the surface can reach the dyne value; If it immediately shrinks into a droplet shape, it will be lower than this value. It is recommended to keep two Dain pens, 34 and 38, as they are low-cost but can save a lot of troubleshooting time.
Q: Why are some boxes from the same batch sticky and some not?
Answer: It is mostly due to uneven UV curing or uneven light exposure that causes surface energy differences. It is a stability issue at the printing end that can be located by using a Dain pen to measure at different positions.
Q: Can increasing the amount of glue solve this problem?
Answer: No, it’s even worse. When the wetting is poor, the larger the amount of adhesive, the easier it is to form a whole film at the interface. When peeled off, the whole film falls off, and the fastness does not increase but decreases.
Q: Is there any adhesive specifically designed to deal with UV surfaces?
Answer: There are relatively more suitable formulas, but there is no “universal glue”. When Guangzhou Sunlife Trading encounters such demands, it is usually recommended that customers send UV printed finished product samples, test multiple models of Sunlife hot melt adhesive, and then recommend them. At the same time, suggestions for leaving blank or roughening processes should be provided, which is a more reliable approach.
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