How to solve the problem of air bubbles in adhesive products? A comprehensive summary of the experience of a glue binding master!
With the continuous development of printing technology, wireless adhesive binding occupies an important position in textbooks, teaching aids, and book binding due to its high binding efficiency and low cost. However, the problem of air bubbles on the back of adhesive bound products has always plagued various printing companies. This issue not only reduces the firmness and service life of books, but also seriously affects the appearance quality of books, leading to customer complaints and returns, causing economic losses to the enterprise.
The author has many years of experience in the operation of adhesive bonding linkage lines. Through continuous learning and in-depth analysis of the entire adhesive bonding process, the author will explore the causes of air bubbles from multiple perspectives such as equipment, process, materials, and operation, combined with actual production cases, and propose suggestions for optimizing the adhesive bonding process. The aim is to provide practical and feasible solutions for the printing industry and improve the overall quality of adhesive bonding products.
The manifestation of air bubbles in adhesive products
The air bubbles in adhesive products mainly appear as obvious honeycomb shaped small holes or bubbles at the top and bottom of the book, and there is a feeling of unevenness when pressing the back of the book with force. The specific manifestations are as follows:
01/Uneven distribution of hot melt adhesive
As shown in Figure 1, in the cured adhesive layer, there are obvious voids formed by bubbles, or blank spots in some areas that are completely uncovered by hot melt adhesive. Bubbles cause insufficient bonding area in the area, and the hollow part completely loses its bonding force. By cutting open the book from the middle of the back, it can be seen that there is no hot melt adhesive covering or the hot melt adhesive is not solid in some areas of the back, and there are voids.
02/Inconsistent adhesive layer thickness
This issue causes the surface of the book back to be uneven (wavy), manifested as irregular undulations, bulges, or depressions on the cured surface of the book back, no longer in a smooth and uniform state (as shown in Figure 2), and there may also be significant differences in the thickness of the adhesive layer on the top and bottom of the book (as shown in Figure 3).
Figure 2: The surface of the book back is uneven and uneven
The impact of air bubbles on the quality of adhesive bonded products
01/Affects product appearance
One is to cause the overall unevenness or partial depression of the book back, which affects the flatness and overall beauty of the book back. The second is the air bubbles in the sky and feet, which affect the reading senses of customers or readers.
02/Severe cavitation affects firmness
Severe air bubbles form empty glue, making the pages easy to separate, lacking firmness, and failing to meet tensile testing standards. In severe cases, it can cause serious quality accidents such as book back breakage and loose pages.
03/Causing returns and customer complaints
Bubbles larger than 1mm in size at the top and bottom of the product may result in customer returns, complaints, or even penalties, which can affect the company's reputation. If the air bubbles are particularly severe, the product will be judged as unqualified in quality inspection, which will have a serious impact on the reputation of the enterprise.
Analysis of the causes of air bubbles in adhesive products
01/Improper adjustment of the gluing mechanism
(1) Improper control of adhesive temperature: Excessive or insufficient adhesive temperature can directly affect the fluidity and viscosity of hot melt adhesive, resulting in uneven coating and affecting the quality of book core coating. The difference in the number of printed sheets can also affect the curing and cooling time of hot melt adhesive, so it is necessary to adjust the adhesive temperature according to seasonal and environmental temperature changes.
(2) Inappropriate height position of the glue wheel: If the glue wheel is too high or too low, it will cause uneven distribution of hot melt adhesive, resulting in empty glue and affecting the bonding strength of the book.
(3) Inappropriate thickness and viscosity of hot melt adhesive: The coating thickness should be adjusted according to the thickness of the book, usually controlled within the range of 1.0 ± 0.2mm (adjusted according to the thickness of the book core, as shown in Figure 4 where the adhesive layer thickness is too thin). The inappropriate viscosity of hot melt adhesive can also affect the uniformity and adhesion of the coating.
Figure 4: The thickness of the adhesive layer is too thin
(4) If the glue wheel breaks at the wrong position, it will cause a lack of glue on the back of the book (as shown in Figure 5).
02/Improper adjustment of milling back cutter
(1) The milling back blade is severely worn, and if the surface of the book back after milling is not milled clean, uneven, or has milled fragments, it will affect the quality of the adhesive coating (as shown in Figure 6).
(2) The gap between the milling back blade and the pressure plate is too large, which may cause the end of the book block to curl and result in uneven milling back.
(3) Improper adjustment of the vertical distance from the horizontal plane of the milling back blade to the book clamp: If the distance is too large, the exposed part of the binding will lack a certain amount of tension pressure, which will cause looseness between the paper sheets, increase the depth of penetration of the hot melt adhesive, and increase the thickness of the book back. This will cause the thickness of the binding to be higher than that of the incision, resulting in defects such as air bubbles, creases, and bow wrinkles on the book back, and also making it difficult to cut the finished product; If the distance is too small, it will affect the penetration depth of hot melt adhesive and affect the bonding strength.
03/Insufficient adjustment of the palletizing forming mechanism
(1) Inappropriate height or balance of the palletizing mechanism: If all bubbles appear on the back of the book, the height of the palletizing mechanism needs to be adjusted; If there are air bubbles at the top or bottom of the ground, a level should be used to adjust the balance of the support.
(2) When the clamping action time of the palletizing forming mechanism does not match: "top first, then clamp", "clamp first, then top", or "simultaneously clamp", it can also cause air bubbles. At this time, it is necessary to adjust the clamping action time of the palletizing.
04/Improper paper selection and folding
(1) Lightweight paper: Lightweight paper is softer than ordinary laminated paper, and the pages are not fully flattened, forming air pockets that can easily cause air bubbles on the back of the book.
(2) The difference in paper weight between the cover and the main text: using paper with a weight of 200g/m2 or more for the cover while using paper with a weight of 65g/m2 or less for the main text can cause the cover to drag the content after being set, resulting in air bubbles on the back of the book.
(3) Improper folding and stacking: Improper folding or stacking can cause creases or uneven surfaces on the book cover (especially curled book backs), which can result in uneven adhesion of hot melt adhesive and void formation on the book back.
05/Improper slot adjustment
(1) The spacing and depth of the grooves do not meet the standard requirements, which can lead to insufficient filling of the adhesive on the back of the book, resulting in air bubbles on the back of the book.
(2) The slotting knife is severely worn, and the depth of the slotting on the back surface of the book varies. Hot melt adhesive is not easy to fill, which will affect the quality of the adhesive coating (as shown in Figure 7).
Figure 7: The slotting knife is severely worn, and the depth of the slotting on the back of the book varies
(3) The angle between the slotting knife and the book back is incorrect, which can cause paper scraps and paper hairs to block the slot after slotting, making it impossible for hot melt adhesive to penetrate into the slot, resulting in insufficient and uneven glue on the book back, leading to air bubbles on the book back.
06/Inconsistent adjustment of book clip clamping force
When the clamping force of each book clip in the glue binding machine is inconsistent, the book core may be easily dragged by the pressure plate through the milling back station in the book clip with too little pressure, causing the milling back to be uneven (tilted). After passing through the groove, the depth of the groove in the front and back of the book core will be inconsistent, and the supporting force will also be unbalanced, resulting in air bubbles on the back of the book.
07/Improper use of hot melt adhesive
(1) Not selecting the correct type of hot melt adhesive based on paper type: Different types of paper should use the corresponding type of hot melt adhesive, such as copperplate paper, which should use copperplate adhesive. Mixing different types of hot melt adhesive is not allowed (the glue pot must be cleaned thoroughly when changing the adhesive), otherwise it will cause uneven adhesive on the back of the book, resulting in bubbles.
(2) The machine speed does not match the hot melt adhesive: the curing time of high-speed adhesive is 3-5 seconds and the opening time is 5-7 seconds; the curing time of low-speed adhesive is 8-13 seconds and the opening time is 9-15 seconds. Improper selection of hot melt adhesive, too fast or too slow curing time and opening time, can also affect the quality of the adhesive and cause air bubbles.
(3) Unstable quality of hot melt adhesive: Some hot melt adhesives have poor quality and high impurities, which can cause impurities to adhere to the heating rod of the glue pot or block the rubber hose and wheel, resulting in uneven gluing and the generation of air bubbles.
Solution for air bubbles in adhesive products
01/Optimization and adjustment of adhesive mechanism standards
(1) Adjust the glue temperature: Adjust the glue temperature according to the ambient temperature to ensure that the temperature difference is controlled within ± 5 ℃. For example, the glue temperature can be set to 160 ℃ in summer and increased to 170 ℃ in winter.
(2) Adjust the height position of the rubber wheel: Check and adjust the height of the upper rubber wheel and the uniform rubber wheel to ensure that the pressure of the rubber wheel is uniform and appropriate, avoiding being too high or too low.
Suggestion for 1 # bottom glue pot: The distance between the 1st glue wheel and the book core is 0.50-0.80mm; the distance between the 2nd glue wheel and the book core is 0.80-1.00mm; and the distance between the uniform glue wheel and the book core is 0.85-0.90mm.
Suggestion for 2 # bottom glue pot: The distance between the 1st glue wheel and the book core is 0.80-1.00mm; the distance between the 2nd glue wheel and the book core is 1.80-2.00mm; and the distance between the uniform glue wheel and the book core is about 2.00mm
(3) Optimize the thickness and viscosity of hot melt adhesive: Adjust the coating thickness according to the thickness of the book, choose a stable quality hot melt adhesive, and adjust the position of the glue wheel breaking. The thickness of the book, the thickness of the adhesive layer, and the reference for instructions are as follows: if the thickness of the book is less than 10mm, the recommended adhesive layer thickness is 0.6-1.0mm. Thin books should avoid hardening the spine due to excessive adhesive layer thickness; The thickness of the book is 10-30mm, and the recommended adhesive layer thickness is 1.0-1.5mm. For medium thickness books, it is necessary to consider both the permeability and strength of the hot melt adhesive; If the thickness of the book is greater than 30mm, it is recommended to use a glue layer thickness of 1.5-2.0mm. For thick books, it is necessary to increase the amount of glue to fill the book groove, and also to cooperate with the groove pulling process (groove depth ≥ 1.5mm).
02/Improvement of milling back process
(1) Regularly inspect the milling back blade: The inspection standard is that the appearance is free of defects, the blade edge is not passivated, and the milling effect (book back flatness, roughness) and process parameters (depth consistency) are comprehensively verified. If necessary, the blade can be ground or replaced (as shown in Figure 8).
Figure 8 Regular inspection of milling back blade edge
(2) Adjust the gap between the milling back cutter and the pressure plate: Adjust the gap between the front and rear cutters and the milling back blade to 0.1-0.3mm (based on the actual measurement with a feeler gauge), with a maximum of 0.3mm, to ensure that the milled book back is flat (as shown in Figure 9).
Figure 9 Adjusting the gap between the milling back cutter and the pressure plate
(3) Correct the vertical distance from the horizontal plane of the back milling cutter to the lower edge of the book clip: Correctly correct the exposed part of the book clip at the book core binding, usually recommended to be 10mm. The exposed part of the binding=milling amount+10mm. If the exposed part of the binding is 13mm, then the milling amount is 3mm.
(4) Milling depth: Generally, the thickness of paper is 1.5-3.0mm (80g/m2 paper is about 2.0-2.5mm thick). Take waste book cores for trial milling and check whether the milling depth is uniform (depth difference ≤ 0.1mm is qualified).
03/Adjustment of palletizing forming mechanism
(1) Adjust the height of the palletizing forming mechanism: Adjust the height of the palletizing forming mechanism according to the amount of book slide extension. Taking the Precision Cambridge 12000 model as an example, the milled back book core is stopped at zero position above the packaging table. At this time, the packaging table is at the highest point, and the distance from the working surface of the guide rail insert to the low-voltage plate of the packaging table is 138.5mm, with an error of 0.1mm. At this time, the height of the support molding mechanism is just as solid as the book back after gluing.
(2) Adjust the balance of the palletizing forming mechanism: Adjust the balance of the palletizing according to the position of the air bubbles on the top and bottom of the book, ensuring uniform forming of the book back with a front to back error of 0.1mm (as shown in Figure 10).
Figure 10 Adjusting the balance of the palletizing forming mechanism
04/Paper optimization process measures
(1) Optimization measures for lightweight paper: It is necessary to compact and bundle lightweight paper before binding, and leave it for more than 8 hours; Choose the appropriate hot melt adhesive; Adjust the speed, pressure, and other parameters of the adhesive binding machine appropriately to ensure that the hot melt adhesive is evenly distributed and fully penetrated.
(2) Flatness control of book labels: Adjust the level of the book label platform to ensure the flatness of the milling back and the depth of the groove, ensuring that the book labels are neat, without wrinkles or curls.
(3) In the early stage of communication with the publishing house, it is recommended to minimize the use of paper with a significant difference in weight between the cover and the book core when selecting materials for the design. If it is indeed necessary during the design process, it is recommended to increase the use of inserts or liners to transition the tension between the two.
05/Slot mechanism adjustment
(1) Increase the groove depth to around 1.2mm and ensure that the hot melt adhesive fully penetrates into the groove.
(2) Control the slot spacing between 5.0-7.0mm.
(3) Regularly inspect and replace the milling back groove cutter and groove milling back shear block on time.
(4) The standard requirement for slot spacing is 5.0-7.0mm; the standard requirement for slot depth is 1.5 ± 0.5mm.
06/Book clip clamping force adjustment
(1) Determine the relationship between book thickness and clamping force, with reference to the standards shown in Table 1.
Table 1 Relationship between book thickness and clamping force
(2) Different devices can detect book data through thickness sensors, and the controller automatically matches the displacement of the clamping plate to accurately control the clamping gap. For example, when a book thickness of 15mm is detected, the clamping plate moves to a gap of 15mm+adhesive compensation amount (approximately 0.5mm). Or by adjusting the torsion spring pressure of each book clip on the host, the clamping force of each book clip can be made consistent.
(3) If the shrinkage of the book core after clamping is ≤ 2.5mm, it is qualified. The back of the book is randomly inspected for defects such as wrinkles, bubbles, exposed glue roots, and overflow glue.
The solution to the problem of air bubbles in adhesive products needs to start from multiple aspects such as equipment, process, materials, and operation. Through systematic analysis and optimization, the stability of adhesive product quality can be effectively improved. The solution described in this article has been validated in actual production, aiming to provide ideas for peers to solve wireless binding process problems, and has reference value for binding machine leaders and quality technicians.