How Can the Cobb Rating Affect Flexo Printing?

March 26, 2018

Kim asks,

What effect does the Cobb rating of paper have on printing with flexographic inks?

Cobb determines the paper’s ability to absorb water. The higher the Cobb reading, the more water is absorbed into the paper.

Since flexographic inks are predominately water, the Cobb reading has a great affect on how the ink interacts with the paper. Color, drying and coverage are all affected by the Cobb rating of the paper.

A lower Cobb rating will absorb less liquid into the paper and therefore more ink will stay on the surface of the sheet. If the ink is formulated to match the paper the results can be superior coverage, deep bold colors and remarkable surface effects. If the ink is formulated for a higher Cobb rating it may dry slower, offset from one print station to the next down and color and coverage may suffer.

A higher Cobb rating will draw more liquid into the paper, usually faster, leaving less ink on the surface of the sheet. This has its advantages and disadvantages too. The ink may dry faster minimizing offsetting, but coverage and richness of color may be sacrificed as the inks and solids are drawn into the paper and less stays on the surface.

So, it is very important that the paper, inks and printing plates are closely matched for each job. If you were to run a job on a high Cobb rated kraft and then simply switched to a low Cobb rated high hold-out paper without switching inks and plates, the results would most likely be far from favorable.

Now there is a little leeway and ink viscosity and pH can be adjusted to a certain extent to control drying, transfer and coverage. But it’s always best if the ink is formulated to match the desired results to the characteristics of the paper.

This is a very simple explanation of how Cobb can affect flexographic printing, but it could be a complete seminar on its own. If you have a question about a specific job your ink supplier is a great resource.

— Ralph

Tolerance for Scrap in Load

March 13, 2018

Andrew asks,

I had a customer ask me how much scrap they should tolerate in their product (slots/cutouts/etc). We aim for 100% scrap removal, but this is not always possible. So my question to you is if there an industry standard for this?

The answer is it’s what the customer demands or will tolerate. There is really no industry standard that I’m aware of for the amount of scrap allowed in a load. In the past customers were much more tolerant of “some scrap” in the load. However, as packing lines have become highly automated the amount of scrap a customer will tolerate has continued to decrease. Today many customers and brand owners have zero tolerance for stray scrap. Even a single piece of slot or glue tab scrap can result in costly downtime on an automated packing line. The customers don’t want the downtime, nor do they want to pay their employees to remove the scrap. In some cases with “hands-off” lines there may not be anyone to monitor the incoming boxes and remove stray scrap before it stops the line.

So we have to do our best to eliminate the scrap before it gets to the strapper or unitizer. We need to make sure our tooling is properly designed and the equipment is setup to optimize cutting and scrap removal. On the folder-gluer we need to make sure the slot knives and heads are sharp, properly adjusted and not damaged. The same goes for the tab knives and hand-hole devices as well.

On the diecutter we need to make sure the cutting die is designed and rubbered properly to provide a clean cut and proper scrap ejection. Make sure all rubber is in good condition and that the cutting die rule is not broken or damaged. If there is any impacted scrap in the cutting die, remove it and investigate the rubbering in that area. Also make sure the anvil covers are in good condition and even across the cylinder.

There are rotary diecutter stackers on the market that are specifically engineered to provide superior scrap removal even when running behind the industries fastest diecutters. These stackers are designed to remove scrap before it makes it to the stacking hopper and eliminate the labor involved in manual stripping or scrap picking. The saving in labor and returned product can have a very positive impact on your bottom line.

You’re correct, even though our goal is 100% scrap removal it can be difficult to make sure that one tiny piece of slot doesn’t make its way into the finished product. Sometimes it can become a battle between the customer and the boxmaker as to how much is too much.

Let’s toss these two questions out to our readers. We would like to know your thoughts and experience.

  1. If the customer and boxmaker agree on an acceptable amount of scrap when the order or contract is signed, will/does it help elevate issue down the road?
  2. I know there is an ongoing battle to control costs and offer the customer the best possible competitive price, but can a premium be charged to guarantee zero scrap loads?

— Ralph

California’s Prop 65 Law Update

March 7, 2018

Steve asks,

Would you have any information in regards to changes in California’s Proposition 65 law?  These are changes in labeling and marketing requirements for products that contain ingredients known to the State of California to cause cancer and/or reproductive harm, and I’m specifically interested in any known requirements from corrugated convertors and/or harmful ingredients in corrugated.

Here is the list of Chemicals Known To The State To Cause Cancer Or Reproductive Toxicity sent to us from our customer. They also provided a list of all the products we make for them in which we must disclose any products that have chemicals that appear on the other list.

Below is a related article for your readers’ reference:

https://www.exponent.com/knowledge/alerts/2017/08/prop-65-changes/?pageSize=NaN&pageNum=0&loadAllByPageSize=true

Our understanding is that as long as none of the materials used to manufacture the box (paper, starch, ink, glues, tapes or staples, etc.) have any of the chemicals that appear on the Proposition 65 list then you are okay and should not need to add any additional markings to the box.

Reach out to your suppliers and put the responsibility on them to certify the materials they provide comply with Prop 65. They would be the best source to know what is in the raw materials. You probably already request and keep on hand Safety Data Sheets from each of your suppliers. It’s always good to keep these on file and up to date just in case a question ever arises. Also in today’s legal environment, it’s important to have a paper trail to protect yourself and your company.

As far as the products contained in the box, it is up to your client to disclose their product warnings based on what they put into the box.

If any of our readers would like to share their knowledge and experience with California’s Proposition 65 Law, send us a comment. Our community is best when we all participate!

— Ralph

 

Global Percent of Medium

March 6, 2018

Mike asks,

I have a question on what the % medium should be globally. I’m trying to get a handle on it algebraically so to speak.

Classic corrugated sheet, in my mind, is one 26# medium sheet and two 42# liner sheets.  The medium has a wiggle factor of ~1.3 so,

Medium % = 26*1.3/(26*1.3+42*2) = ~ 29%.

Does this sound reasonable?  As BWs tend downward, my gut tells me there is more % medium overall but I am thinking still less than a third.

Twenty five years ago, here, it was indeed 42/26/42 with C flute having a take up factor of 1.43.  Then the sheet feeders began to request 23# medium and corrugating rolls started to have lower profiles which reduced the take-up factor.  Then we moved to 35# and today that probably is 33/23/33.  Globally the World Containerboard Organization http://www.wco-containerboard.org/ tracks total containerboard production.  Additional information may be available through them.

— Ralph