Uneven thickness of book glue thread leads to a significant decrease in the pass rate of finished products? Deep analysis of reasons+solutions

In the wireless adhesive packaging process of books, the uneven thickness of the adhesive wire is a quality problem frequently encountered by printing factories and graphic and text fast printing factories. One side of the adhesive layer is too thick and overflowing, while the other side is too thin and debonding. This not only makes the appearance of the book look unsightly and causes page flipping to lag, but also leads to problems such as loose pages, glue peeling, and uneven page edges. This directly lowers the finished product qualification rate, skyrockets the rework rate, and increases the cost of consumables and labor simultaneously. Based on the operation scenarios of wireless glue assembly machines, the characteristics of hot melt adhesives, equipment operation, and process flow, the core causes of uneven thickness of glue lines are comprehensively dismantled below, and corresponding rectification plans are provided. At the same time, we recommend specialized hot melt adhesives suitable for book glue assembly.

1、 The uneven thickness of the adhesive thread is mainly manifested in two forms

Localized thickening: glue overflow on both sides of the spine and the beginning and end pages, with adhesive layer accumulation and protrusions, making it uneven and prone to sticking to the pages and dirtying the cover.
Localized thinning: The adhesive layer in the middle and sides of the spine is missing, and the bonding area is insufficient, making it easy for the pages to peel off or scatter after flipping through.

2、 Analyze the six core reasons one by one

（1） Book itself and page layout issues (fundamental issues at the source)

1. The book core pages are uneven and the page breaks are not in place
After cutting and organizing the pages, the edges are staggered in height, and the overall thickness of the book core is inconsistent. When applying glue to the rubber wheel, the high position is repeatedly glued, resulting in a thicker adhesive layer, and the concave position has insufficient contact with the adhesive, directly forming a thickness difference. When multiple books are glued together continuously, if the elasticity and flatness of each book are not the same, the problem will continue to escalate.

2. Abnormal clamping force of the book core
The pressure of the book clamping mechanism of the glue binding machine is too low: the book core is loose, the pages are displaced, the overall force after gluing is uneven, and the distribution of the glue layer is disorderly;
Excessive clamping pressure: The spine of the book is deformed and partially depressed due to compression, and the adhesive layer cannot be evenly covered during coating, resulting in thin adhesive areas.

（2） Mechanical failure and improper debugging of the glue assembly machine (the main cause)

1. Wear, eccentricity, and unstable rotation of the adhesive wheel
Long term use of rubber wheels may result in wear, grooves, bumps, or eccentric rotation due to loose shaft cores. During the rotation process, the contact pressure between the rubber wheel and the spine of the book varies greatly, and the amount of adhesive carried fluctuates, resulting in uneven thickness of the adhesive line. The surface of the rubber wheel has accumulated glue and carbonized hard blocks, which can also hinder the uniform adhesion of the glue.

2. Rubber wheel height and levelness deviation
The rubber wheels are uneven in height on both sides, with an overall horizontal deviation, and the spine and rubber wheels cannot make uniform contact throughout the entire width. One side is tightly adhered with a thick adhesive layer, while the other side is in contact with a thin suspended adhesive layer, which is a typical cause of abnormal adhesive lines on one side.

3. Offset of book tray and guide rail position
The height of the book tray is uneven, the gap between the guide rails is inconsistent, and the books tilt and deviate during transportation. The spine angle of the book is skewed, and the adhesive force is uneven, resulting in an imbalance in the thickness of the adhesive line.

4. The actions of the book delivery and page flipping mechanisms are not synchronized
The connection of the assembly line glue machine station is stuck, and the tightness of the transmission chain is uneven. The book conveying speed fluctuates between fast and slow, and the length of contact with the glue wheel is different. The glue accumulates thicker at the positions where it contacts for a long time, and the glue layer is thinner at the positions where it passes quickly.

（3） Hot melt adhesive selection and adhesive state issues

1. Hot melt adhesive viscosity mismatch
Choosing book specific adhesive with high viscosity: poor fluidity of the adhesive, uneven spreading of the adhesive wheel with adhesive, and easy accumulation into thick adhesive blocks;
Adhesive with low viscosity: Strong fluidity, rapid flow after coating, thinning of the adhesive layer in the middle of the spine, and overflow of adhesive at the edges.

2. Melt glue temperature out of control
Low temperature: incomplete melting of colloids, uneven adhesion of semi-solid colloids, and local agglomeration forming thick gel areas;
Excessive temperature: The glue becomes thinner and the flowability increases, causing the adhesive layer to flow away. The adhesive layer in the middle is thinner, and overflow accumulates on both sides.
At the same time, insufficient stock of glue in the glue box and mixed use of new and old glue can also cause inconsistent overall melting state, affecting the uniformity of glue application.

（4） The milling and roughening processes do not meet the standards

Before wireless adhesive installation, milling and roughening are key pre processes.

1. The depth of milling back varies: the milling surface of the spine is uneven, and the adhesive layer follows the undulations of the base surface, with a significant difference in thickness;
2. Uneven roughening force: Some pages have completely opened fibers and a large amount of adhesive, while some areas have insufficient roughening and less adhesive, resulting in an imbalanced distribution of adhesive layers;
3. Wear and angle deviation of the milling back cutter head can directly cause uneven milling surfaces, laying hidden dangers at the root.

（5） Cover fitting and pressing process issues

1. Cover positioning offset
The cover conveying and positioning baffle are skewed, and when closing the book, the cover is misaligned left and right, front and back, squeezing the spine glue and pushing the glue layer to one side, causing local glue thickness and local glue thickness.
2. Uneven pressure of the compression roller
The pressure on the upper and lower pressing rollers is not consistent, and the roller surface is worn and uneven. When pressing the adhesive, the squeezing force at different positions is different, and the adhesive is squeezed and flows. Eventually, the thickness of the adhesive line is disordered, accompanied by bubbles and adhesive problems.

（6） Influence of environment and operating habits

Large temperature difference and high humidity in the workshop: Low temperature accelerates colloid cooling, resulting in poor fluidity and easy agglomeration; High humidity can cause the surface of the book pages to become damp, resulting in poor adhesive adhesion and insufficient adhesive hanging in certain areas.
When operators work in batches, books are placed askew and stacked unevenly, which can also exacerbate the problem of uneven glue lines.

3、 Corresponding rectification solution (quick implementation)

1、Regular book coreStrictly carry out page breaking and page organization to ensure that the four sides of the page are neat; Unified book clamping pressure should be maintained with the book core not loose and the spine not deformed.
2、Maintenance and debugging equipmentRegularly polish and replace worn rubber wheels, and tighten the transmission shaft; Calibrate the levelness and height of rubber wheels, book trays, and guide rails; Check the transmission chain to ensure uniform and stable conveying speed.
3、Optimize milling back and rougheningRegularly replace milling cutters and deburring teeth, unify milling depth and deburring force, ensure smooth spine base surface and uniform fiber fuzzing.
4、Control the status of hot melt adhesiveStrictly set the melt adhesive temperature according to the glue parameters, maintain stable glue volume in the glue box, and avoid mixing a large amount of new and old glue; According to the speed of the glue machine and the material of the book, replace the book glue with a viscosity that is suitable.
5、Adjust compression and positioningCalibrate the cover positioning mechanism to ensure accurate alignment of the book; Adjust the left and right pressure of the compression roller to achieve even force distribution.
6、Control the workshop environmentMaintain a constant temperature and dryness in the workshop to reduce the impact of temperature and humidity on colloids and paper.

4、 Choose the right specialized book hot melt adhesive to reduce the defect rate from the source

After repeated debugging of the equipment and standardized operation, if the problems of uneven glue lines, glue overflow, and delamination still occur frequently, the core is that the hot melt adhesive does not match the process. For wireless adhesive binding scenarios such as books, brochures, bidding documents, and test papers, we recommend Guangzhou Sunlife Book Special EVA Hot Melt Adhesive. With the support of Foshan Jucai New Material Production Base, we have been focusing on graphic binding and book binding for many years, perfectly adapting to various high-speed adhesive binding machines.
Guangzhou Sunlife Book Hot Melt Adhesive Core Advantages

1. Moderate viscosity and even spreading
The formula is customized for the book binding process, with excellent melt flowability and stable adhesive wheels. After coating, the adhesive can be evenly attached to the surface of the spine without clumping or flowing recklessly. It improves the problem of uneven thickness of the adhesive line and one-sided overflow from the root, greatly improving the appearance qualification rate of the finished product.

2. Strong adhesive properties, suitable for milling back and roughening process
Colloid has good permeability and can fully penetrate the paper fibers. Even if there are slight undulations on the milling surface of the spine, it can be evenly wrapped. The thin adhesive area is not prone to peeling or loose pages, and the bonding is firm and long-lasting.

3. Wide temperature range and strong weather resistance
Adapt to the commonly used melt adhesive range of 55 ℃ -175 ℃ for conventional glue assembly machines, and there will be no sudden change in fluidity even with slight temperature fluctuations; Low temperature does not crack, high temperature does not flow, stable production status throughout the year, and is not afraid of changes in the workshop environment.

4. Reasonable curing speed, suitable for assembly line operation
The opening time is moderate, and after gluing, it will not quickly solidify, causing incomplete spreading, nor will it solidify too slowly, resulting in glue flow. It is matched with various types of glue installation machines at high, medium, and low speeds, balancing production efficiency and product quality.

5. Environmentally friendly and odorless, with compliant quality
Using food grade standard raw materials, smoke-free and odorless, passing multiple environmental tests, long-term workshop operations do not stimulate the human body, suitable for various scenarios such as printing factories, graphic stores, school printing rooms, etc.
Adaptation scenario
Various books, brochures, magazines, bidding documents, contracts, test papers, notebooks, manuals, etc. are wirelessly glued together, compatible with disc gluing machines, linear gluing machines, and fully automatic assembly line gluing equipment.

The uneven thickness of the book glue line is a quality problem caused by the combination of multiple factors such as the book core, equipment, process, and glue. Prioritize troubleshooting hardware and operational issues such as page sorting, milling back, rubber wheels, and pressing in daily production; If there are no abnormalities in the equipment, it is necessary to replace the suitable hot melt adhesive for books and periodicals in a timely manner.
As the source manufacturer of Guangzhou Sunlife Hot Melt Adhesive, we can provide free samples and one-on-one technical selection guidance based on your adhesive machine model, production speed, and paper material. With professional after-sales technical support, we can help you completely solve common adhesive problems such as uneven adhesive lines, overflow, and peeling, reduce rework rates, and improve overall production efficiency!