What are the reasons for the “empty glue” and incomplete glue layer on the back of the book in adhesive packaging? how to handle

After the wireless adhesive packaging of books and periodicals is taken offline, sometimes the book back feels “empty” and not full. When viewed in front of the light, there are local missing glue and dents in the adhesive layer, and in severe cases, the inner pages will scatter with just one flip. This phenomenon of “glue not filling the back of the book” is commonly referred to in the industryEmpty glue/insufficient glue layerIt is not the same as “uneven book back” (2847) and “glue overflowing and dirtying book pages” (2188) – empty glue isToo little glue, not fully filledThe latter two have too much shape or glue. Below, explain the reasons and solutions based on the four aspects of “glue quantity temperature book back pressure”.

1、 First, let’s answer directly: What is empty glue and why do we need to manage it

Empty glue=The adhesive layer on the back of the book is partially not filled, forming gaps/depressions. Lightly, the back of the book feels empty and not full, while in severe cases, there is no solid adhesion between the book stickers, and the inner pages are loose and falling off. The reason is usually not “poor glue”, but ratherGlue amount, glue temperature, book back treatment, palletizingThese four steps were not coordinated well.

2、 5 common triggers (by frequency of occurrence)

1. Insufficient glue application (most common)

The gap between the glue pot is too large, the pressure of the coating roller is insufficient, or the glue level is too low, resulting in too little glue being eaten by the back of the book, which naturally cannot fill the gaps in the book. There are recommended adhesive thickness ranges in the book and magazine usage instructions (such as 10-12mm book core about 0.4mm, ≥ 20mm about 0.8-1.0mm, subject to the process card), below which there will be empty spaces.

2. The glue temperature is too low/viscosity is too high, and the glue cannot enter the book seam

Normal operating temperature of book adhesive 160–180℃(According to the accompanying process card). When the temperature is low, the glue becomes thick and has poor fluidity. When applied, it only hangs on the surface and cannot penetrate into the gaps of the book, leaving the center of the book back empty. In the training, ‘90% of after-sales service is caused by equipment temperature control failure’ – the actual temperature displayed at 160 ℃ is only 100 ℃, which is the root cause of this type of ‘glue cannot enter’.

3. The back milling and hair pulling are not in place, and the surface of the book sticker is not opened

The book core needs to be milled back and roughened first, and the paper edges should be “roughened” to give the glue a grip. If the milling back is too shallow and the bristles are not clear, the surface of the book sticker will still be smooth and compact, and the glue cannot hang or seep in, so the book back cannot be filled tightly.

4. There is dust/oil on the back of the book, and the glue does not wet it

The paper powder produced by milling or the book back is stained with oil, and the adhesive liquid shrinks on the surface and does not wet, resulting in a lack of adhesive in some areas. This is similar to the phenomenon of “paper falling off powder causing poor adhesion” (2759) – both surface conditions affect the adhesion and spreading of the adhesive.

5. Imposing pressure/timing is not right

After gluing, use a pallet to press the glue tightly into the back of the book. Insufficient pressure, or too early (glue is still thin) or too late (conjunctiva has formed), will not press firmly, leaving gaps on the back of the book.

3、 Key distinction: Do not confuse these two issues

Simple note:Look at the article with missing glue, the one with uneven shape, and the one with overflowing glue.

4、 How to handle and prevent

1. Adjust the amount of glueCheck the gap between the glue pot, the pressure of the coating roller, and the glue level, and fill the glue layer to the recommended range according to the process card.
2. Calibrate adhesive temperatureUse a thermometer to check the “displayed temperature vs actual temperature” to avoid temperature control failure that may cause the glue to fail to enter; raise the temperature to the process card range and ensure normal fluidity before starting the machine.
3. Check and mill the back for roughnessConfirm that the milling depth and roughening effect are in place, and open the surface of the book.
4. Clean up dust/oil stainsEnsure that the book back is clean and the glue can wet and spread properly.
5. Adjustment and hittingMatch the pressure and timing with the rhythm of the machine, and press the glue tightly into the back of the book.
6. Light illness products that have been taken offlineAssessment of insensitivity to appearance, re pressing and adhesive repair; The loose inner pages need to be reworked and re glued.

5、 Relationship with Sunlife’s real business

Sunlife hot melt adhesive covers copperplate paper/adhesive paper/double adhesive paper/thin paper and other scenarios, corresponding to different models (292/296/295/298, 396/397/398, etc.). When customers provide feedback on-site that the book back is empty or not full, priority should be given to judging based on the four aspects of “glue quantity temperature book back pressure”. Most of these issues are related to adjustable process matching and not due to the failure of the glue itself; If it repeatedly occurs, it should be checked together with incoming material acceptance and model debugging. Sunlife provides “Instructions for Use/Process Card” along with the goods, and regards “Temperature Gun Calibration+Process Training” as a support item to reduce rework in temperature control.

Explanation: The temperature, adhesive layer thickness, and cleaning cycle described in this article are general process points. Specific machine parameters are subject to Sunlife’s accompanying “Instructions/Process Card”; Different models have different opening times and wax quantities. Please refer to the actual model technical data for selection.