Written By Mark Batson Baril
"If the glued part of the product ever fails then we are not selling boxes, we are selling flat pieces of nicely printed paper." Quote from a very determined folding carton Glue Department Manager.
WHY do glue assists work better when they run across the grain as opposed to with the grain? Most people say they just work better - most don't know why - they just do. Is there anything written down explaining why?
Let's start this answer with another question -
WHAT is a glue assist?
A glue assist is well known in the folding carton and printing industries and rather unknown in most of the rest of the diecutting and converting industry. Glue assists were developed as a method to break through the clay coating, varnish coatings, UV coatings etc… of paperboard so that the water soluble glue could penetrate the soluble fibers of the inner core of the board. A series of knives are placed into the die (usually in the glue flap area). These knives are set at a height typically 30% of the overall stock thickness lower than the through cut knives. This partial cut gives the glue more adhesion between the two glued surfaces. If the board is pulled apart, the top layers of the carton board (covered by the slick coating) must tear apart before the carton surfaces will separate.
The penetration of the knife also adds to the actual surface area that the glue has to adhere to. This exposed area must be formed in a way that exposes fibers and stays open through the gluing process. A cross grain cut will tend to stay open were a with grain cut will tend to close. By running the specialty rule that is forming the glue assist pattern across the grain, we force more fibers to be exposed. For example if you were to take a piece of pine, lets say 1/2" thick, and break it with the grain then the woods' cellulose fibers break in long strings. Should you take the same piece of wood and break it across the grain the cellulose fibers will splinter in longer slivers and expose more of the interior of the wood. The same happens with the paper board. By penetrating the material across the grain the pressure on the inner fibers forces the same cellulose fibers to break and splinter exposing the inner fibers to the soluble glue allowing for greater adhesion.
One concern is that a number of companies add glue assists to the flaps of their seal end cartons and then seal the carton using hot melt non-porous glue in their cartoning process. Although not as big a help as with a penetrating glue, glue assists still help in that the hot melt glue will form around the broken and exposed fibers. The general rule is that unless the product is being spot glued in only a couple of small areas, the use of glue assists will help the strength of the box, not hurt it.
Most die shops and diecutting shops have a very specific pattern they use that they know is better than the competitions'. This is to say that there are many patterns that are common and each has it's own reason for effectiveness. Some shops use a simple straight perforating rule that cuts in just one direction. Others use wave perfs or half a zipper rule that cuts in both directions to the grain. Whatever the case may be in your shop, keep in mind the cross grain factor to help make your decision.
"If the glued part of the product ever fails then we are not selling boxes, we are selling flat pieces of nicely printed paper." Quote from a very determined folding carton Glue Department Manager.
WHY do glue assists work better when they run across the grain as opposed to with the grain? Most people say they just work better - most don't know why - they just do. Is there anything written down explaining why?
Let's start this answer with another question -
WHAT is a glue assist?
A glue assist is well known in the folding carton and printing industries and rather unknown in most of the rest of the diecutting and converting industry. Glue assists were developed as a method to break through the clay coating, varnish coatings, UV coatings etc… of paperboard so that the water soluble glue could penetrate the soluble fibers of the inner core of the board. A series of knives are placed into the die (usually in the glue flap area). These knives are set at a height typically 30% of the overall stock thickness lower than the through cut knives. This partial cut gives the glue more adhesion between the two glued surfaces. If the board is pulled apart, the top layers of the carton board (covered by the slick coating) must tear apart before the carton surfaces will separate.
The penetration of the knife also adds to the actual surface area that the glue has to adhere to. This exposed area must be formed in a way that exposes fibers and stays open through the gluing process. A cross grain cut will tend to stay open were a with grain cut will tend to close. By running the specialty rule that is forming the glue assist pattern across the grain, we force more fibers to be exposed. For example if you were to take a piece of pine, lets say 1/2" thick, and break it with the grain then the woods' cellulose fibers break in long strings. Should you take the same piece of wood and break it across the grain the cellulose fibers will splinter in longer slivers and expose more of the interior of the wood. The same happens with the paper board. By penetrating the material across the grain the pressure on the inner fibers forces the same cellulose fibers to break and splinter exposing the inner fibers to the soluble glue allowing for greater adhesion.
One concern is that a number of companies add glue assists to the flaps of their seal end cartons and then seal the carton using hot melt non-porous glue in their cartoning process. Although not as big a help as with a penetrating glue, glue assists still help in that the hot melt glue will form around the broken and exposed fibers. The general rule is that unless the product is being spot glued in only a couple of small areas, the use of glue assists will help the strength of the box, not hurt it.
Most die shops and diecutting shops have a very specific pattern they use that they know is better than the competitions'. This is to say that there are many patterns that are common and each has it's own reason for effectiveness. Some shops use a simple straight perforating rule that cuts in just one direction. Others use wave perfs or half a zipper rule that cuts in both directions to the grain. Whatever the case may be in your shop, keep in mind the cross grain factor to help make your decision.
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