What is the appropriate amount of hot melt adhesive to apply? The relationship between adhesive layer thickness and bonding strength

People often ask on bookbinding machines, “How much hot melt adhesive is needed to make it firm? How thick is the book backing adhesive layer enough?” The answer is not “the more, the stronger” – there are fewer pages falling off, more glue overflow, sticking knives, and waste. Let’s clarify the parameters of “glue amount” and “glue layer thickness” below.

1、 What is the relationship between the amount of glue applied and the thickness of the glue layer

• The amount of glue applied determines the thickness of the adhesive layerThe amount of glue produced by the glue pot, the gap between the glue scraper, and the amount of glue carried by the book back when passing through the glue roller collectively determine how thick the book back adhesive layer is after final curing. Thickness is measured in millimeters.
• The thickness of the adhesive layer has a reasonable range, not the thicker the betterThe reference provided in the training materials is that for book cores with a thickness of 10-12mm, the adhesive layer is about 0.4mm; for thick book cores with a thickness of ≥ 20mm, the adhesive layer is about 0.8-1.0mm. Thin book cores have the strongest adhesive layer, and blindly thickening them can actually lead to more problems.
• ‘Thin book cores are the strongest’ is a true ruleThin book pages have fewer fibers and shorter penetration paths, and can be easily grasped with an appropriate thin adhesive layer; Thick book core fibers and high stress on the spine require thicker adhesive layers for reinforcement.

2、 What will happen if the amount of glue is insufficient

• Dropped or scattered pagesThe glue amount is not enough, and the glue layer does not completely wrap around the back fibers of the book. When flipping or falling, the glue opens directly.
• Edge glue/back glue connection is brokenThe back glue is so thin that it cannot cover the spine of the book, and the edge glue has not been patched, causing the “bones” of the entire book to break.
• More deadly for thick booksThick book cores already require a 0.8-1.0mm adhesive layer, and when the amount of adhesive is insufficient, the thick books will scatter first.

3、 What happens if too much glue is applied

• Excess glue, eating wordsGlue is squeezed out from the back of the book and seeps into the pages, causing the characters to be pasted off and resulting in an unsightly finished product.
• Adhesive knife, adhesive tapeToo much glue is not enough to cure before entering the press/cutting process, with glue on the blade and wire drawing on the back of the book.
• The opening hours have been consumedThick adhesive layer cools slowly, which is equivalent to indirectly compressing the opening time. The faster the production line, the more likely it is to have problems.

4、 How to adjust the amount of glue to the appropriate level

• First, determine the target adhesive layer based on the thickness of the book core10-12mm book core should be placed at 0.4mm, and books with a thickness of ≥ 20mm should be placed at 0.8-1.0mm; Don’t add randomly based on the feel.
• Adjust the gap between the scraper and the adhesive amount of the rubber rollerThese are two physical knobs for the amount of glue applied; Re calibrate after changing paper/model.
• The temperature needs to be in place, otherwise ‘even if the amount is enough, it won’t be secure’The temperature is not enough, the glue is too thick, and it cannot penetrate the paper seam. It seems that the glue is applied sufficiently, but in reality, it does not bite the fibers; Calibrate the true glue temperature with a thermometer, and then talk about the glue amount.
• Don’t mix brandsDifferent brands have different viscosity systems, and mixing them can change the flow and adhesive content, which can lead to clogging and loss of control over the amount.
• According to the accompanying process cardThe recommended glue amount/layer thickness range for each batch of glue should be written on the process card, and it is more stable to compare the settings before machine use than to rely on experience.

Sunlife’s approach: Mix the glue according to the book core and explain the “quantity” clearly

Sunlife produces books and magazines, quick printing of graphics and text, packaging, and pearl cotton hot melt adhesive. Different book core thicknesses, paper sizes, and production line speeds correspond to different adhesive types and application ranges. Don’t rush to add glue if you can’t stick it firmly – tell us the thickness of the book core, paper, and equipment speed. Sunlife will match the model and provide the process card according to the actual working conditions, and set the amount of glue and the thickness of the adhesive layer in one go, without shedding pages or overflowing glue.

Frequently Asked Questions

Q: Is it true that the thicker the glue, the less likely it is for the pages to fall off?
A: No, it’s not. Excessive thickness of the adhesive layer can lead to overflow, sticking, and slow cooling; Thin book cores are already around 0.4mm the strongest, thickening is not beneficial. The correct solution is to determine the adhesive layer according to the thickness of the book core.

Q: Why are the pages still missing even though the glue amount has been increased?
A: Most likely, it’s due to insufficient glue temperature – the glue is too thick and seeps into the paper seam, but it doesn’t bite the fibers when measured properly. First calibrate the true temperature of the glue pot, and then adjust the amount of glue.

Q: Where can I find the data on the amount of glue applied?
A: In the accompanying process card/testing data, the actual batch markings shall prevail; When selecting, ask the supplier to recommend a range based on your book core thickness and production line speed.

There is no “universal maximum value” for the amount of glue applied to hot melt adhesive, only a “suitable value corresponding to the thickness of your book block”. Only by providing Sunlife with thickness, paper, and speed can the adhesive produced be truly accurate and firmly adhered.