Quality Control Checklist for Offset Printing

Offset printing is a printing method that involves transferring, or offsetting, an inked image from a plate to a rubber blanket before it is printed to the desired printing surface. It commonly uses a planographic plate where the image to be printed gets the right dose of ink from constantly moving ink rollers. Quality control is an important consideration in offset printing to ensure good print quality.

Check your ink settings for each job. The standard CMYK colour scheme is almost always used in most offset printing jobs, and each of these colour schemes should be at the required Delta E so you only produce the desired colour. CMYK refers to Cyan, Magenta, Yellow and Key (Black). It works as a subtractive colour model scheme by partially or entirely "masking" or layering colours on a lighter background -- preferably white. When an incident light beam hits the print, it absorbs the wavelengths resulting in a visible colour that has been "subtracted" or reduced. Delta E, on the other hand, refers to how accurately a colour mix matches a standard colour wheel.

Dotgain is a basic concept when using the CMYK colour printing scheme. It refers to a phenomenon whereby printed materials look darker than intended. The cause for this is halftone dots that settle in the area between the original printing layer and the final printed piece. Also known as Tonal Value Increase because of its effect to printing "tone," dotgain has to be calculated based on the properties of the used ink material. Printed dots will naturally grow as the printing sets, and this growing action can be estimated and compensated for beforehand. In offset printing methods, the ideal dotgain settings for a file print with 40 per cent of CMY colours should be 53 per cent when measured on paper. For files with 40 per cent of K, this should be set to 56 per cent.

Consistency is a very important consideration in printing. You want to have your printouts come out exactly the same way every time. For this reason, you should not only consider dotgain targets at 53 per cent and 56 per cent, respectively, but also the variation for each. You need to print a sample batch of up to at least five sheets, measure the dotgain variation from sheet to sheet and check whether these values fall within 3 per cent for CMY and 6 per cent for CMYK combined. If not, equipment maintenance is necessary on the printer so it can run smoothly.

With monochrome colour schemes such as grey, Delta E becomes less important and Delta H takes centre stage. Delta H is the same as Delta E, except that it is affected only by "hue" or intensity or contrast and not colour mixes. The calculation for Delta H formulae is most often done using automated programs. For grey balance parameter calculations, make sure your Delta H value is less than 1.5 to ensure the best print quality.