Archive for the ‘Finishing/Converting’ Category

How Much Washboarding is Acceptable

April 24, 2018

Paul asks,

I would like to know about washboarding in corrugated – how much variation in surface flatness (in microns) is typically seen for the types of corrugated board used in applications like shelf ready packaging?

I don’t know that there is a published standard on how smooth the surface of a corrugated sheet should be for the type of printing used for display or shelf ready packaging, but I think we are safe to say pretty darned smooth.

One of the key elements of printing the type of quality high-graphics usually found in display work and shelf ready packaging is minimizing the impression (pressure) between the plate and the substrate. The greater the impression pressure the more the printing plates distort and that distortion results in growth and distortion of the dots that create your image. Typically in high-graphics printing we are looking for a kiss impression, just enough that the printing plate just touches or “kisses” the surface of the board and transfers the ink. For the best print results we want .0015” to .003” (~38 to 76 microns) impression when printing. Oh course the lightest impression we can get away with.

Now, if the high and low points of the board surface exceed this kiss impression depth then additional pressure will be necessary in order to obtain coverage in the low points caused by fluting. Then, as stated above, as we add more impression our printing starts to suffer. With large block print you might think you can get away with over impression and perhaps you can a tad more than you can with process or fine lines and text, but excess impression on solid coverage will result in color variation through striping of the print.

We should also note that typically washboarding is less prevalent on small flute (such as a E, F or N) because there are more support points and they are closer together than a C or B flute.

Digital printing may be a little more forgiving than offset, but we still have to remember that the smoothness of the surface contributes to the overall aesthetics of the packaging and not just the print quality.

—Ralph

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

Assembled Corrugated Blank Tolerances

February 23, 2018

Kevin asks,

Is there an industry-standard tolerance that would be applied to the assembled dimensions (Length, Width, Depth) of a 2pc carton?  I would question whether multiple processes should lead to alternate tolerances when compared to a single process 1-piece carton.  I would also wonder how carton size would play into the tolerance and if there are any steadfast rules and/or guidelines throughout the industry.

The particular carton I am trying to apply tolerance to is a 2pc Partial Overlap Top, HSC bottom where the width panels are die cut and the other scoring off of a press and then offset stitched within the length panel for reference.  I am trying to hold +/- 1/8” on all dimensions now.  These tend to be around 76 x 32 x 50” with 2” POL.

According to the Joint PMMI/FBA publication PMMI B155-TR2.2-2011,  the tolerance of a RSC panels are +/- 1/16 inch per panel with an overall blank tolerance of +/- 1/8 inch for both length and width. Also, I believe there is a TAPPI publication of dimensional accuracy which may make reference to tolerance.

Slot depth should be within 1/8 inch from the center line of the corrugator score. Slots should also be centered within 1/16 inch of the aligning scores.

No allowance is given for carton size, caliper, substrate or complexity of design in this specification.

I haven’t found any reference to multiple piece cartons which would lead me to believe that there would be an additional allowance for a multi-piece carton.

So, based in this information, it would make sense that the overall assembled dimensions (interior and exterior) should be within, and controlled by, the +/- 1/8 inch overall tolerance of the RSC or die-cut blank.

One other note to toss in here. While the PMMI/FBA publication above may note that +/- 1/8 inch over all is acceptable, today’s customers are typically demanding something closer. With today’s drive and anvil technology +/- 1/16 inch (1/8 total) may generally be considered the norm, but more and more customers are demanding closer tolerances and expecting little to no variation.

— Ralph