Do books and magazines have a grainy, small bumps, and uneven surface on the back/cover after being glued together? Reasons and Handling

After the adhesive packaging of books, magazines, or picture books is taken offline, when flipping to the back or cover of the book where the adhesive is pressed, occasionally you may feel a layer of small “graininess” or see a few small bumps the size of rice grains, and the surface is no longer smooth and even. In most cases, thisIt’s not that the book is’ useless’But instead, foreign objects have been mixed into the adhesive layer or localized crusts have appeared. It is not the same as “uneven book back and raised spine” (shape defects), nor is it the same as “the appearance of the purchased hot melt adhesive particles themselves”. Below, explain the reasons and solutions clearly based on the four aspects of “paper glue equipment environment”.

1、 First, let’s answer directly: What are graininess and small bumps

The granular texture/small bumps on the surface of the finished adhesive product are essentiallyUnnecessary solids have been mixed into the adhesive layerDuring coating, the adhesive solution may form local crust or protrusions after cooling. Common sources include coking particles, dust, incompatible precipitates, and local crusts. It generally does not affect the overall page count of the book, but it can have an impactAppearance and feelEspecially sensitive to hardcover, picture books, and light colored covers.

2、 5 common triggers (by frequency of occurrence)

1. Carbonized coke particles in the adhesive groove fall off and mix into the adhesive layer

If the glue pot is not cleaned for a long time and old materials are repeatedly returned to the pot, a layer of coking material (carbide particles) will accumulate at the bottom. These burnt particles are carried to the back of the book by the glue, appearing as hard small black spots/bumps. The instructions for using pearl cotton thread suggestClean the adhesive groove about once every two weeksAnti carbonization, the same goes for book and magazine lines. This is the most common and easiest to investigate.

2. The temperature of the glue pot is uneven or locally overheated, causing the glue surface to crust

The normal operating temperature of book adhesive is 160–180℃(Please refer to the accompanying process card for details). If the glue pot is locally overheated or the edges of the glue pot are exposed for a long time with uneven heat dissipation, a layer of “skin” will form on the surface of the glue liquid, which will become lumps when scraped off during gluing. In the training, ‘90% of after-sales service is caused by equipment temperature control failure’ – the actual deviation displayed at 160 ℃ is large, which is the root of such problems.

3. The milling back dust was not fully absorbed and mixed into the adhesive layer

Milling and pulling the back of the book core will produce paper powder. If the vacuuming is not smooth, the paper powder will fall onto the glue pot or book back, and mix with the glue to make it “astringent” and have a grainy feeling. The instructions for the use of books and periodicals emphasize that the milling back and pulling hair are in place, but dust control is also crucial.

4. Thick adhesive layer and uneven cooling

The thicker the adhesive layer, the easier it is for internal bubbles and impurities to remain on the surface; Thick adhesive layers cool slowly and are more prone to surface unevenness. There is a recommended range of adhesive layer thickness for normal book cores (based on the process card), and excessive thickness is not only adhesive intensive but also prone to lumps.

5. Mixing different brands/models, incompatible precipitation

Mixing hot melt adhesives of different formulas in the same glue pot may result in local incompatibility and particle precipitation. The pearl cotton instructions clearly state that “different brands and models should not be mixed”, and the book and magazine lines should also be implemented.

3、 Key distinction: Do not confuse these two issues

Simple note:If there are “dots” on the surface of the finished product, please refer to this article. If the overall “shape” is not correct, please refer to the article on shape defects. If there are problems with the rubber particles themselves, please refer to the incoming materials.

4、 How to handle and prevent

1. Periodic cleaning glue potThoroughly removing the bottom coking material is the fastest step to eliminate black spots and lumps; It is recommended to clean it every two weeks (depending on the actual amount of glue used).
2. Calibrate the temperature of the glue potCheck the “displayed temperature vs actual temperature” with a thermometer to avoid local overheating and skin formation caused by temperature control failure.
3. Check for vacuum and dustConfirm that the dust collection at the milling back station is normal and the paper powder does not fall onto the glue pot and book back.
4. Control the thickness of the adhesive layerSet the adhesive amount according to the process card, without blindly thickening.
5. One pot of glue, not mixedSeparate pots for different brands and models, or clean the pots when changing the glue.
6. Minor finished products that have been taken offlineFor those with low appearance requirements, re pressing can be reduced; Fine decoration/picture books and other appearance sensitive categories need to be evaluated for adhesive replacement or process adjustment.

5、 Relationship with Sunlife’s real business

Sunlife hot melt adhesive is mainly based on EVA system, and corresponding models are available for book binding (copperplate paper/adhesive paper/double adhesive paper/thin paper), graphic and text fast printing, pearl cotton (EPE) and other scenarios. When customers provide feedback on-site that there is a granular feeling on the adhesive surface, priority should be given to judging based on the four aspects of “paper adhesive equipment environment”. Most of them belong to controllable foreign matter/crust problems and do not need to be directly judged as waste; If it repeatedly occurs, check it together with the incoming material inspection (particle appearance).

Explanation: The temperature and cleaning cycle described in this article are general process points, and the specific machine parameters are subject to Sunlife’s accompanying “Instructions/Process Card”; The wax quantity and opening time vary for different models. Please refer to the actual model technical data for selection.