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Direct answer: The root cause of the curling of these graduation yearbooks is not machines, but thick copperplate paper and high-temperature workshop conditions, where the original adhesive particles had a short opening time and insufficient initial adhesion. After switching to hot melt adhesive granules specifically designed for book binding with longer opening time and higher initial adhesion, the curling rate decreased from 12% to 0.6%.
Last June, a printing center at a university in southern China received 4200 graduation commemorative books, with 157g copperplate paper on the inner pages and 250g laminated film on the cover. The trial production was normal for the first two days, and on the third day of mass production, the temperature in the workshop rose to 34 ℃. The finished product had a large area of curled edges at both ends of the spine – when the book was placed flat, there were 3-5mm openings in the front and back, and a slight cracking sound could be heard when flipping pages.
After excluding the equipment, focus on the glue itself. After taking out the glue from the glue pot and conducting a wire drawing test, it was found that the glue lost its stickiness about 3 seconds after leaving the glue roller – and the actual interval from glue application to the top cover of this machine was 4.5 seconds. That is to say, when the cover is pressed on, the adhesive has already started to solidify.
| Condition | Pilot production phase | Batch stage |
|---|---|---|
| workshop temperature | 27℃ | 34℃ |
| Paper status | Just unpacked, moisture content is normal | Open for two days, slightly dry |
| takt time | Slow, with an interval of about 3 seconds | Accelerate the work, with an interval of 4.5 seconds |
The superposition of three variables completely consumed the originally sufficient opening time. This is also a typical cause of many problems where there are no issues during normal times but accidents occur during peak seasons.
After Sunlife’s hot melt adhesive technicians arrived at the site, it was recommended to replace the adhesive with a book binding type with an opening time of 6-8 seconds and higher initial adhesion. At the same time, the adhesive temperature should be lowered to 168 ℃ to reduce viscosity decay at high temperatures, and the compression time should be extended to 3 seconds. On the afternoon of the same day, a trial production of 300 books was conducted, and after cooling for 24 hours, a random inspection was conducted
The remaining 3900 books were run according to the new parameters without any delay in delivery.
Seasonal concentrated rush work activities such as graduation yearbooks and annual meeting yearbooks should not be evaluated based on normal workshop conditions when selecting glue, but should be reserved for extreme conditions such as the hottest, most rushed, and driest paper. The opening time would rather be longer, and the compression time would rather be more than half a second.
Question: Is the curled mouth necessarily a problem with the glue?
Answer: Not necessarily. Shallow milling back, incorrect paper grain direction, and insufficient compression pressure can all cause warping. Suggest conducting a replacement test first to eliminate equipment factors.
Q: What are the special requirements for the inner pages of thick copperplate paper?
Answer: The surface coating of copperplate paper has poor absorption and requires a higher initial adhesion and better permeability adhesive. It is best to add milling grooves behind the milling to increase mechanical bite.
Q: Can it be solved solely by adjusting the temperature?
Answer: It can alleviate but not cure. The opening time is determined by the formula, and the temperature can only be adjusted slightly.
Guangzhou Sunlife Trading provides peak season adhesive selection evaluation and machine debugging services for printing, graphic and text fast printing, and book binding enterprises in universities.
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