Archive for the ‘Printing’ 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 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

Calculating ink coverage based on weight

January 31, 2018

Ed asks,

“We are creating a program to quote customer requirements as quickly and accurately as possible but we are stuck with ink / printing issues.

Is there a conversion that may help us translate grams of ink to square centimeters / inches of printing?”

Your ink supplier should be able to tell you how many sq. in. or sq. cm. a pound or gram of ink will cover. This will change depending on the viscosity, substrate, anilox roll volume, ink manufacturer and ink type. So in your calculator (estimator) you may want to provide the option to select from a pre-programmed list of inks, or allow the estimator to enter a pounds per inch or grams per centimeter variable.

Then of course you’ll need to know the sq inches of coverage for each color. You could simply calculate the area of the sheet and then assign a light, medium, or heavy coverage variable to simplify the calculation of inches of coverage.

UPDATE: Roger Poteet, Poteet Printing Systems, has reached out to share their experience and knowledge with us through the Poteet Ink Estimator complied and provided by Poteet Printing. I also want to thank Andrew at Compass Packaging for sharing his knowledge and experience with the Poteet Estimator in the comment below.

– Ralph

Printing on Hot Corrugated

March 3, 2015

Jason asks,

Could you explain why it’s not recommended to print on hot corrugate fresh from the corrugator?
I know this isn’t common practice, but there are times when it is necessary when a rush order comes in.

Once corrugated sheets come off the corrugator they should be allowed a bit of time to cure. This will allow the sheet time to stabilize and adjust to the relative humidity of the environment, etc. Slight size changes can take place during this time. If you go straight from the corrugator to the converting process, it is possible that you could end up with a finished product that doesn’t meet the customer’s specification.

Basically ink dries by absorption and evaporation. The chemical characteristics of the ink, mainly pH levels, and temperature, greatly influence drying as well. When you induce significant temperature change to the equation, such as hot board, you change the entire drying process. This will most likely result in ink drying before it has time to absorb into the fibres of the sheet, or it may cause the ink to dry on the plate. Both situations can result in poor coverage/print quality and excessive build up on the plates.

Hot sheets can also cause “baking” of the printing plates which will significantly reduce the plates’ ability to properly transfer ink as well as the overall life of the printing plates.

A hot sheet could also possibly experience more crush as it goes through the converting process than a cured sheet would, and therefore result in a degradation of product quality. A sheet that isn’t properly cured may also be more likely to create rolled scores.

Converting equipment can also be affected by hot sheets. The functional characteristics of urethane components such as feed belts or wheels, transfer belts, pull collars, and in some cases even cutting die rubber can be decreased by hot sheets. The coefficient of friction of feed belts and wheels can be reduced causing registration and skew issues. Die cutting rubber can be overheated causing reduced ejection rates and rubber life.

There are probably other issues that we’ve overlooked here, but I hope this gives you some idea of why running hot corrugated is not a good practice.

— Ralph