What should I do if the hot melt adhesive particles on the UV ink printing surface cannot stick? Cause analysis and solutions

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.

First, make a 30 second judgment

Tear open the failed adhesive and observe the cross-section:

• Smooth interface and complete ink layerTypical wetting failure, the problem lies in surface energy, look below for solutions.
• The adhesive layer is broken and has wire drawingInsufficient cohesion is a selection issue and should be replaced with a high cohesion model.
• The ink layer is lifted up as a wholeThe adhesion between ink and paper is insufficient, and the problem lies in the printing end. We need to find a printing process to solve it.

Five feasible solutions

• 1. Leave blank (preferred).In the design phase, the edge of the glue box, the back glue area of the spine, and the overlapping area of the sealing box are made into unprinted blank strips, with a width 2mm wider than the actual glue width. This is the lowest cost and most thorough method.
• 2. Avoid partial polishing.If the entire surface is coated with UV varnish, allowing the polishing process to partially avoid the bonding area (commonly known as “opening the skylight”) will have the same effect as leaving white space.
• 3. Hair treatment.Install a hair roller in front of the box gluing machine to slightly damage the ink layer in the bonding area and expose the paper fibers. Suitable for urgent orders that have already been printed but cannot be revised in time.
• 4. Coronation or flame treatment.Improve surface energy to over 38 dynes. High equipment investment, suitable for factories that engage in long-term UV surface bonding.
• 5. Replace with high moisture type rubber particles.Choosing a formula with lower surface tension and stronger initial adhesion (such as EVA type or APAO type with specific tackifying resin) can to some extent “climb” onto low surface energy surfaces.

What else can be done in terms of parameters

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.

How to measure surface energy on site

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.

Frequently Asked Questions

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.