What is the viscosity of hot melt adhesive? How does viscosity affect machine operation and bonding

When selecting glue and using the machine, the parameter “viscosity” is often seen. Some people ask, “What is viscosity? Is high or low better?” Simply put:Viscosity refers to whether hot melt adhesive is thick or thin after melting. It directly determines whether the glue can penetrate into the paper seam, whether the glue is applied smoothly, and whether it can be pulled or not. Explain it clearly below.

1、 What is the viscosity of hot melt adhesive

• Flow resistance in molten stateHot melt adhesive becomes a fluid after heating and melting, and its viscosity measures its internal friction/flow difficulty, commonly measured in mPa · s (millipascals · s). The higher the temperature, the lower the viscosity – this is a characteristic of thermoplastic materials (see 2779 “What is EVA”).
• It is a “molten state” parameter, not a solid hardnessViscosity refers to “how thin and thick it becomes after melting”, which is not the same as whether the adhesive layer is hard or not after curing. Different models have different viscosities, corresponding to different production line speeds and paper.
• Don’t mix it with “melting point/softening point”The melting point determines the degree at which it begins to melt, and the viscosity determines how well it flows after melting. The combination of the two determines the performance on the machine.

2、 The influence of viscosity on adhesion

• Low viscosity (dilute)Good penetration, easy to feed into seams and wet fibers – friendly to thin paper and high-speed production lines; But if it is too rare, it will seep into the pages, overflow glue, and ruin the words.
• High viscosity (thick)Thick stacking and strong seam filling – suitable for thick book cores, porous/coated paper; But it is too thick and has poor penetration, easy to draw, difficult to spread, and cannot be spread out during the opening time (see 2689 for drawing problems).
• Temperature is a key variableTraining points – The viscosity and wettability are optimal in the range of 160-180 ℃; Insufficient temperature and high viscosity result in poor drawing/spreading, while too high temperature can lead to thermal decomposition and degradation of performance, as well as carbonization and brittleness (see 2757/2689).

3、 How to choose viscosity based on operating conditions (corresponding to Sunlife model)

• High speed production line (>4000 books/hour)To achieve low viscosity and fast penetration, the corresponding high-speed opening time is 2-7s; if the viscosity is too high, it will not have enough time to spread and form a film.
• Thick book core/porous/copperplate paperChoose a moderately high viscosity sealant and ensure the adhesive layer (special adhesive for copperplate paper such as 396/397, the model is subject to the process card).
• Thin paper/educational bookletLow viscosity should be avoided to prevent glue penetration and smudging.
• The specific mapping of “model x paper x speed” can be found in 2715 “How to choose hot melt adhesive models for different papers”. This article only discusses the parameter “viscosity” itself and does not repeat the model list.

4、 Practical attention to viscosity during machine operation

• The temperature needs to be in placeInsufficient temperature → high viscosity → poor drawing, spreading, and penetration; Overheating → carbonization, viscosity reduction, and brittleness. First, ensure the true temperature of the glue pot (calibrated with a thermometer, see the glue pot temperature diagnosis document), and then discuss viscosity.
• Don’t mix brandsThe viscosity systems of different brands/models are different, and mixing them can easily cause clogging and unstable performance (see 2714 “Storage and Maintenance”).
• According to the accompanying process cardThe recommended temperature and viscosity range for each batch of adhesive are written on the process card, and are compared and set before use, which is more stable than adjusting by hand feel.

Sunlife’s positioning: Clearly explain the parameters and mix the adhesive according to the working conditions

Sunlife specializes in EVA hot melt adhesive, with corresponding models for books, graphic printing, packaging, and pearl cotton. There is no absolute “high good low good” viscosity, only “whether it matches your production line speed and paper”. Tell us the device speed, paper type, and thickness, and Sunlife will match the model and provide the process card according to the actual working conditions, without taking viscosity as the sole criterion.

Frequently Asked Questions

Q: Is it better to use glue with lower viscosity?

A: No, it’s not. Low viscosity penetration is good, but it is prone to overflow and smudging; High viscosity joint filling is strong but easy to draw. Matching your paper and speed is the best.

Q: Does the same adhesive feel thicker and more brushed in winter?

A: In winter, the ambient temperature is low, and the glue cools down quickly to the spine of the book. The opening time is compressed, which subjectively makes it feel thicker and easier to draw; Ensuring the true temperature of the glue pot and the workshop environment temperature is crucial (see the glue pot temperature diagnosis article).

Q: Where can I find viscosity data?

A: In the accompanying process card/testing data, the actual batch markings shall prevail; When selecting, ask the supplier to recommend the corresponding viscosity range based on your working conditions.

The “viscosity” of hot melt adhesive is a practical parameter, not the higher the better. Look at it together with temperature, opening time, and paper, and then give the working conditions to Sunlife to make the glue that is truly easy to machine and stick firmly.