Some of the problems that happen when binding books and may lead to customer complaints can be reduced if the bookbinder knows how the adhesive works.
Bindery operators cannot modify the Hotmelt adhesive formulation to adapt it to different kinds of paper or book formats but they can change a very important parameter: the temperature of the adhesive.
Each adhesive is designed with different characteristics. Some of the most important are:
- Softening point
- Setting time
- Open time
One may think that when an adhesive (let’s say our Goma Soli L-94, with a very short open time) is working fine in summer with 30-40 ºC in a binding machine with a speed of 5000 books/hour, we already have the right conditions and adhesive and we do not need to take care of anything else anymore. However, this is not completely true since there are other variables during the bookbinding process that may produce books with binding flaws.
Some of these variables are:
- Bookbinding speed
- Variable room temperature
- Type of paper, paper weight, varnish
- Milling cutters in milled books
- Amount of adhesive applied – Spine width
- Unexpected machine stops
Here, we are going to explain why and how all these variables can cause binding problems if appropriate measures are not taken into account. Besides, we will recommend how to modify the parameters of the binding machine to avoid them.
When using a specific Hotmelt adhesive, for example Goma Soli L-94, at different binding speeds we should consider that the open time is being modified as the working conditions are different.
Usually, the binding of a thick book requires slowing down the machine (from typical 5000 books/hour to 1500-2000 books/hour, for example), which implies that the time between the adhesive is applied on the book until the moment the cover is on and the pressure is applied against the spine (key point for a good penetration) increases. This means that an adhesive that is working perfectly for bookbinding speeds around 5000 books/hour may not work under these new conditions because it gets too cold and there is no good penetration. The open time of the adhesive has been exceeded. What can we do then? We only need to increase the application temperature of the adhesive. In this way, the open time is increased, taking longer to cool, and this compensates for the extra time it takes to put the cover on due to the slowing down. An increase of 10-15 ºC is usually enough. Another solution would be the use of a different adhesive with a longer open time (like Goma Soli L-90-B) but this would not be really necessary.
Variable room temperature
The temperature changes throughout the year and the extreme temperatures during summer and winter affect the Hotmelt adhesives for bookbinding.
When the facilities do not have a constant temperature during the year, which is quite usual, the open time of the adhesive is affected.
Considering the same application temperature of the adhesive, in summer it will take longer for the adhesive to cool down whereas in winter the adhesive will cool down much faster. Therefore, the room temperature modifies the open time.
Having this in mind, it is advisable to modify the application temperature during summer and winter as long as there are notable temperature differences inside the facilities. For example, if in winter the adhesive is applied at 175 ºC, in summer it should be lowered by 10 ºC, i.e. at 165 ºC.
Type of paper, paper weight, varnish
The type of paper, the paper weight (g/m2) and the presence of varnish or coating make the paper absorption different. The adhesive penetrates deeper into standard paper and with good absorption than in varnished or coated papers, which show less absorption.
For difficult papers like gloss-coated or varnished papers, it is advisable to increase the application temperature of the adhesive up to 180-185 ºC. This way, the adhesive is more fluid and can penetrate better despite the lower absorption of this type of papers due to their treatment.
In oil-based varnished papers, it is recommended to wait 48 hours after the varnish application until the adhesive is used, or at least 24 hours. If the adhesive is applied before, the varnish is still wet and has not completely penetrated into the paper. The varnish then acts like a nonstick layer repelling the adhesive. Besides, the oily components of the varnish soften the adhesive.
Sometimes, in order to save money, low quality paper with a lot of varnish is used, so the bright appearance makes the paper look of better quality. This is why a strong smell of varnish is noticeable when opening some books. This is counterproductive for inks and also for adhesives.
The high quality adhesives with hydrogenated resins usually show better performance for difficult papers than cheaper adhesives.
Books should be milled in order to increase the contact surface of the adhesive with the spine so the adhesive can penetrate better giving a greater resistance and preventing pages from easily tearing. It is advisable a milling of 6-7 mm.
The milling makes the adhesive and the paper form a uniform block very difficult to break when opening, even at 360º (cover and back cover together). If the milling cutters are not used, the penetration of the adhesive is less and the book can break due to the lack of bonding of the adhesive with the edge of the pages.
For this reason, we recommend the use of milled cutters for non-sewn books, whatever the spine size is, because this generates a better binding between the spine and the cover, offering better resistance.
If the milled cutters are not sharp enough, a clean cut is not obtained and the penetration of the adhesive is remarkably reduced. It is advisable to replace them when the cut is not clean (it seems that it breaks instead of cutting). In case of sharpening, it should be done with an effective system such as pressure sandblasting systems.
The temperature and viscosity of the adhesive are very important since the more fluid the adhesive is, the better it penetrates into the holes made by the milling cutters, thus achieving a stronger binding between the adhesive and the paper.
Amount of adhesive applied. Spine width
We should bear in mind that the higher the quantity of adhesive, the longer it takes to cool down, so the open time will be longer. In this case, the temperature of the adhesive should be lowered.
The recommended amount of adhesive in most books is between 1.0-2.0 mm, although they usually have between 0.9-1.5 mm. If a lower amount is used, the pages could be peeled off more easily.
If the spine is very big (great width and /or length), the adhesive consumption is higher than usual so the temperature of the pre-melter tank should be similar to the one of the melter tank. If not, the adhesive cannot reach the right temperature. It will have lower viscosity than desired and the penetration will be less.
It is very important to have in mind that the fact of setting a certain temperature in the machine does not mean that the actual temperature of the adhesive is that one, especially if the consumption is very high.
Due to the high adhesive consumption that a book with wide spine requires, it is preferable to constantly add small quantities of adhesive into the melter tank to keep the right temperature. If a huge amount is added, the temperature could drop and the adhesive penetration would be worse. For this reason, the temperature in the pre-melter tank should be almost the same as in the melter tank in order to avoid a temperature decrease. Our recommendation is 180-185 ºC in winter and 175-180 ºC in summer.
Unexpected machine stops
When the bookbinding machine stops (or slows down), the time between the adhesive is applied and the moment the cover is placed increases and, as previously said, the adhesive gets colder than needed and the penetration into the paper is worse, meaning that the open time has been exceeded.
In this case, the books should be removed and placed again in the bookbinding machine.
In general, the higher the softening point of an adhesive, the shortest its open time.
Besides, the farther the softening point of the application temperature is, the longer it takes to solidify, therefore the open time is longer.
If there is no constant temperature inside the bookbinding facilities, it is better to choose a Hotmelt adhesive with a medium-short open time so you can play with the temperature of the adhesive and adapt to the machine parameters and room temperature. Otherwise, if the adhesive has a too short open time, it could work fine during summer but cause problems during winter or at low binding speeds. On the contrary, if the adhesive has a very long open time, it will work well in winter but in summer the adhesive will be still soft when arriving to the cutting area (blades) and it may cause the book to get dirty or it can even be deformed.
If you have any question or doubt about these topics, do not hesitate to contact us at firstname.lastname@example.org. It will be a pleasure to help you.