Archive for the ‘Finishing/Converting’ Category

Skip Feed Repeat and Max Print Area

November 15, 2018

Chuck asks,

My question, is there a formula that determines how long of a print area (thru the machine) can be printed without being printed a second time when running skip feed. We have a 66” rotary diecutter with skip feed. We did a test on a sheet size of 79-1/4, mounted a 28″ plate, centered and it did not reprint.

I reached out to a few of my industry contacts. My colleague Dwayne Shrader put together some info for this post.

Max Sheet Length Before Print Repeat

On a 66 inch machine the print is going to repeat every 66 inches from the lead edge of the plate. That’s the key to determining the length of the sheet before repeat… ‘from the lead edge of the plate’.

To determine the maximum sheet before print repeat where …

Cylinder Circumference with plate installed = CC
Lead Edge Offset = LEO
Max sheet Before print Repeat = MBR

Then…CC + LEO = MBR

You say you mounted it in the center, I assume you mean you centered it around the cylinder. So the lead edge of the plate would have been at 19 inches behind zero register… (66 – 28)/2=19. This being true, then CC + LEO = MBR, or 66 + 19 = 85. Therefore, you could run a sheet just under 85 inches before the print would repeat.

Maximum Print Length

Now, determining the maximum print length, is just a little different because the cylinder circumference is not the maximum machine print length. Keep in mind that the lead and trail edge lockup takes up some of the circumference (or print area). On the typical 66 inch cylinder your maximum machine print length is going to be between 61 and 64 inches depending on machine design, lockup type, etc. Your machine manufacturer should be able to provide the maximum through machine print length.

To determine the maximum print length where …

Maximum Machine Print Length = MMPL
Lead Edge Offset of printing plate = LEO
Maximum Printable Length through machine = MPL

Then… MMPL – LEO = MPL.

For example, let’s say the maximum machine print length is 62 inches and the lead edge is offset by 5 inches… 62 – 5 = 57 inches of available print length. Consequently the maximum sheet before repeat would be 71 inches in this case.

Sometimes a converter may have a plate that is made for a normal feed job, but they want to use it to print a larger skip feed job. In this case the plate is mounted in the normal position and then the register is retarded to offset the distance from the lead edge to the start of the print. The same formula for maximum print length is used here as well just substituting the register offset from zero register.

To determine the maximum print length where …

Maximum Machine Print Length = MMPL
Register Offset from Zero = ROZ
Maximum Printable Length through machine = MPL

Then… MMPL – ROZ = MPL.

Hope this is helpful.

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