Glue Assists- Tricks of The Trade

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.

Score Bend Testers

Written By Mark Batson Baril

THE QUESTION:
I have recently been given a SCORE BEND TESTER by my superior, and have been instructed to start using it. I have been in the industry for 20 years and have had no need for this device. Can someone please tell me how I go about implementing this into my daily routine, and what are the parameters for it's use. I do about 25 - 30 make-readies on Bobst Diecutters a week. Thanks in advance for any help...

A Score Bend Tester
Made by Thwing-Albert Instrument Company

The Score Bend Tester is a device used to test cartons, after they have been die cut, for their strength at the scores. The main result the tester is there to calculate is how much force it takes to open the carton up, from its flat, ready to fill, condition. There are other testers out there that measure the board strength before it is converted into a carton or before a score is formed, but for this question we are focusing on the testing of the scored board only. There are several machines on the market The ones we have researched cost between $6,500.00 and $10,600.00 USD.

The main purpose of having and using the Score Bend Tester is to control the quality of machined filled boxes. As companies that use automatic machinery in their packaging lines become more sophisticated, they are demanding equal sophistication from their carton producing vendors. Most of these machines find themselves within the Quality Control and/or testing labs of medium to large sized box shops. Typically, parameters are set-up for how much strength it should take to fold open the carton during the machine filling operation. It is then the job of the carton manufacturer to stay within those parameters. The only way to properly test and document what is actually being produced is to run tests on some type of bend tester. As in any statistical process control situation, every production runs' quality control will vary slightly from one to the other. Many companies will take test measurements at the beginning, middle, and end of the run. Each test sampling will usually have at least ten cartons and again will vary depending on the size of the run, the number up the tool is running, the parameters set-up by the final customer, etc...

We have learned that the testers are used all the way from the sampling process for new cartons, up through the first article inspections done on press, and on to the final production runs. By using the tester as a guide from start to finish, the manufacturer can get controlled information in order to make educated decisions on everything from paper parameters to tooling specifications. To try to insure maximum speeds in their finishing operations, some companies also use the machines to test the flat diecut cartons throughout the run to insure consistency and conformance with their own gluing departments requirements.

So, those are the basics of what the machine is typically used for. As far as putting it to use as a regular part of your day to day operation, it would seem that this will be a combined effort between you, your quality control department and your customer. The same combined effort holds true if you are using the machine for extra information for your own production improvements. Instead of including your customer in the mix, just include anyone effected by the bend strength of that scored paperboard. Sounds like you have your work "cut-out" for you.

Many thanks to the Thwing-Albert Instrument Company, who sells 15 - 20 of these machines worldwide per year, for their pictures and candid information.