What is the reason for the formation of skin (film) on the surface of hot melt adhesive pot? how to handle

The hot melt adhesive in the glue pot will form a thin layer of skin or film on the surface after a long insulation time, which is called “skinning” in the industry.It is a thin film formed by oxidation on the surface of the melt after contact with air, not carbonization – carbonization is the process of burning, blackening, and becoming brittle due to excessive temperature. The skin itself may not necessarily damage the adhesive strength, but the layer of skin mixed into the back of the book will affect its appearance and cleanliness, and it will also waste a lot of glue. Please explain how it is generated, handled, and prevented below.

1、 First distinguish: crust ≠ carbonization

Many people mistake surface crust for carbonization, and the two treatments are completely different:

• Crust formationAt working temperature, the surface film is thin and has a color close to the original adhesive. Scratching off this layer can still be used;
• carbonizationIf the temperature exceeds the upper limit and causes thermal decomposition, the whole pot will become deep, have a burnt smell, and become brittle. It must be cleaned and restarted (see cleaning process for details)How to clean the glue pot when changing glue or wire）。

Simple judgment: only on the surface, no significant color change, no burnt smell → crust formation; The whole pot is deep, has a burnt taste, and becomes crispy → carbonized. Don’t over clean the crust as carbonization, and don’t mix carbonization with crust.

2、 Why does it crust

Sort by frequency of occurrence:

1. The insulation temperature is too high.Long term insulation with an upper limit of 180 ℃ results in the fastest oxidation; Taking the middle and lower limits is clearly better.
2. The insulation time is too long.Leaving it on continuously after work or in standby mode without cooling down, and the surface is constantly in contact with the air.
3. The glue pot has a large opening.The pot has an open mouth, no insulation cover, and a large area in contact with air.
4. The glue pot does not stir.The surface is stationary and does not flow for a long time, making it easier to form conjunctiva.
5. The glue itself is prone to conjunctiva.Some formulas contain easily oxidizable components, which make them more prone to peeling than other adhesives during operation.

3、 How to deal with skinning

• Remove the surface film before productionUse a filter or scraper to remove that layer of film,Don’t just stir it back into the pot——Stirring it back will mix into the book back and block the glue system (see troubleshooting for glue system blockage)Root causes and investigation of adhesive blockage and breakage）。
• Cooling and insulationWhen not in production, lower the glue pot to the insulation level (significantly lower than the working temperature) to reduce oxidation; Short term standby can maintain working temperature, while long-term shutdown will definitely decrease.
• Reduce exposureClose the lid of the pot and add a thermal cover to reduce air contact.
• regular cleanup: PressHow to clean the glue pot when changing glue or wireDo periodic cleaning of the pot, don’t wait for the skin to become thicker and thicker.
• For those that have already thickened and mixed with impurities, follow the cleaning process and do not repeatedly stir the skin back into the melt.

4、 How to prevent (key in temperature management)

The root cause of skin formation is often temperature, not the adhesive itself – this also echoes the industry’s experience that “90% of after-sales service is due to equipment temperature control failure”: it shows that 160 ℃ may actually only be 100 ℃, and conversely, high temperature will accelerate skin formation and carbonization.

• Equipped with temperature gun calibrationBefore class starts, use a temperature gun to check the actual temperature of the glue pot, don’t just look at the panel (see method)What should I do if the temperature of the glue pot is not accurate? Calibrate with a thermometer）；
• Long term insulation with a middle and lower limitThe recommended working temperature for book adhesive is 160-180 ℃, and for long-term insulation, it is recommended to set the middle and lower limits to maintain fluidity and slow down the formation of conjunctiva;
• Standby graded managementShort term standby maintenance, long-term shutdown to reduce insulation temperature.

5、 What can Sunlife do to help

More than 90% of the crust formation is due to equipment and process temperature issues, not the glue itself. Sunlife will provide temperature and opening time recommendations based on your paper, machine speed, and book thickness when using hot melt adhesive for books and magazines. It also supports sending samples, making samples on the machine, and determining the process according to the actual effect of the film and book back; Easy conjunctiva batches can be evaluated and replaced with lower conjunctiva formulas (such as models with low polyolefin odor and no wire drawing). Specific working temperature and opening hoursConclusion of accompanying process card and sampleBased on experience, do not rely on hard estimation.