Chapter 5. Pre-press

5.4 Trapping

Wayne Collins

Trapping can be a very complex procedure in pre-imaging software for certain imaging technologies. It is an electronic file treatment that must be performed to help solve registration issues on certain kinds of press technologies. Generally, if a substrate has to move from one colour unit to another in the imaging process, the registration of one colour to another will not be perfect. That mis-registration must be compensated for by overlapping abutting colours. As soon as two colours touch in any two graphic elements we must create a third graphic element that contains both colours and overlaps the colours along the abutment line. That third element is called a trap line and can be generated many different ways that we will review.


First let’s look at the differences between the four most common imaging technologies and determine where and why we need to generate these trap lines. Electrophotography, or toner-based digital printers, generally use only process colours. Each time an electrostatic drum turns, it receives an electrical charge to attract the toner colour it is receiving. The drum keeps turning until all colours of all toners are on the drum, and then all colours are transferred to the substrate at one time. There is no chance for mis-registration between the cyan, magenta, yellow, and black toners as they are imaged at the resolution of the raster generated by the RIP, and the placement of the electronic charge for each colour can be adjusted until it is perfect, which makes it stable from image to image.


Let’s compare electrophotography to the lithographic print process. In lithography, a printing plate is generated for each colour and mounted on a plate cylinder. The plates are registered by manually turning wrenches to hold plate clamps, so the plate-mounting procedure can generate registration errors. Each colour printing unit is loaded with a separate ink, a plate that has been imaged to receive that ink, and a blanket that offsets the image from the plate before it transfers it from the blanket to the substrate. This is another mechanical transfer point that can cause registration errors. Most high-end lithographic presses have servo motors and cameras that work together to adjust for mechanical registration errors as the press runs. The substrate must travel from one printing unit to the next, and it is here that most registration errors occur. There are slight differences in the substrate thickness, stability, lead (or gripper) edge, and a different rate of absorbing ink and water that cause slight mis-registration. Also, consider that most sheet-fed litho presses are imaging around 10,000 sheets per hour, and we are only talking about movements of one-thousandth of an inch. On most graphic pages, however, the naked eye can see a mis-registration of one-thousandth of an inch, so the process must be compensated for. The solution is generating trap lines to a standard for lithography of three one-thousandths of an inch. This trap line allowance in abutting colours allows for mis-registrations of two-thousandths of an inch that will not show on the final page.


Inkjet is the next imaging technology we must assess and compare. The print heads on all inkjet machines are mounted on the same unit travelling on the same track. Each ink is transferred one after the other and the substrate does not move after receiving each colour. It is like electrophotography in that mis-registration between print heads can be adjusted electronically, and once in register remain stable for multiple imaging runs on the same substrate. If the substrate is changed between imaging, the operator must recalibrate to bring all colours into registration, and ensure the placement of abutting colours is perfect and no compensation is needed. As a result, no trapping will be needed for most inkjet imaging processes.


Flexography is the fourth imaging technology we need to assess. This technology has the most points where mis-registration can occur. The printed image must be raised on the plate to receive ink from an anilox roller that can deliver a metered amount of ink. The computer graphic must be reduced (or flexed) in only one direction around the plate cylinder. A separate printing plate is developed for each colour and mounted on a colour unit that includes an ink bath, anilox roller, doctor blade, and a plate cylinder. The substrate travels from one print unit to the next on a continuous web that is under variable amounts of tension. If a graphic has a lot of white space around it, the substrate can be pushed into the blank space and cause distortion and instability in the shape and pressure of the raised inked image on the substrate. Flexography is used to image the widest range of substrates, from plastic films to heavy corrugated cardboard. This process absolutely needs trap lines generated between abutting colours. Standard traps for some kinds of presses can be up to one point (1/72 of an inch, almost five times our standard litho trap). Graphic technicians need to pay particular attention to the colour, size, and shape of the trap lines as much as to the graphic elements. In most packaging manufacturing plants, there are pre-imaging operators that specialize in creating just the right trapping.

Let’s examine some of the ways these traps can be generated. The simplest way is for a graphic designer to recognize that he or she is designing a logo for a package that will be imaged on a flexographic press that needs one-point trap lines generated for all abutting colours. The designer isolates the graphic shapes that touch and creates overprinting strokes on those graphic elements that contain all colours from both elements. That doesn’t even sound simple! (And it’s not.) It becomes even more complicated when the graphic is scaled to many different sizes on the same package or used on many different packages. So most designers do not pay attention to creating trap lines on the graphics they create and leave it to the manufacturer to create trapping for the specific documents on the specific presses they will be reproduced on.

There is specialized software that analyzes a document, determines where abutting colours are, and generates the tiny graphic lines as a final layer on top of the original graphic. This is done before the document goes to the RIP so it is raster-image processed at the same time as the rest of the document. Most RIPs process PDF files these days, and there are specialized plug-ins for Adobe Acrobat that will analyze a document, generate trap lines, and let an operator examine and edit the thicknesses, shape, and colour of those lines. It takes a skilled operator to examine the extra trap lines and determine if they are appropriate for the press they are going to be printed on. Press operators also need to determine the trap values of their inks. This refers to the ability of one printing ink to stick to another. Inks vary in viscosity depending on the density and types of pigments they are carrying. The trap characteristics and transparency of a printing ink are part of what determines the printing order in which they are applied to the substrate. For example, a process primary yellow ink is very transparent and will not stick (trap) well if printed on top of a heavy silver metallic ink. The metallic silver is thick and very opaque, so it will hide everything that it overprints. A graphics technician must generate trap lines for a graphic that has metallic silver abutting to a process yellow shape. The technician will increase (spread) the shape of the yellow graphic to go under the abutment to the silver. The silver shape will not be altered, and when it overprints, the yellow ink will stick to and hide the yellow trap line shape. The best analogy, we have heard is from a press person — the peanut butter sandwich analogy. We know the jelly sticks to the peanut butter and the peanut butter will not stick to the bread if the jelly is spread first. If a press person does not know the trap values of the inks, he or she can make as big a mess of the press sheet as an upside-down peanut butter and jelly sandwich makes on the front of your shirt! For this reason, trapping should be left to the specialists and is usually applied to a final PDF file before it is sent to a RIP. Ninety percent of trap lines for lithographic and flexographic imaging reproduction are generated automatically by specialized trapping software. Operators are trained to recognize shapes and colour combinations that will cause problems on the press. They will custom trap those documents with the Acrobat plug-ins we talked about earlier.

Special Consideration for Black

There is one trapping combination that should be considered and applied to all four imaging technologies. It is the way that black ink is handled in the document and applied on the imaging device. Most type is set in black ink, and much of it overprints coloured backgrounds. In all four imaging technologies, black is the strongest ink and hides most of what it overprints. It is still a transparent ink and most process black ink is more dark brown than the rich dark black we love to see in our documents. If the size of the black type or graphic is large enough, we will be able to see the black colour shift as it overprints stronger or weaker colours under it. Graphic designers should pay attention to setting whether black type or graphics overprint the background, or knock out the background to print a consistent black colour. A useful rule of thumb is that type above 18 points should be knocked out and boosted. Raise this threshold for very fine faces such as a script where larger point sizes can overprint, and reduce it for excessively heavy fonts like a slab serif. If the graphic is large enough, it should also be ‘boosted’ with other process colours.

The way we handle black ink or toner deserves special consideration in all four imaging technologies. Black is a supplemental colour to the three primary process colours. It is intended to print only when the other three colours are all present in some kind of balance. In all imaging technologies, we must watch that our total ink coverage does not approach 400%, or 100% of each ink overprinting the other inks in the same place. This is usually too much ink or toner for the substrate to absorb. As a result, it will not dry properly and will offset on the back of the next sheet, or bubble and flake off the media in the fuser. Therefore, we must pay attention to how our photographs are colour separated, and how we build black in our vector graphics.

Colour Separations

When colour separating photographs, we can build in an appropriate amount of GCR or UCR to give us the right total ink coverage for the imaging technology we are using for reproduction. UCR stands for under colour removal. It is applied to very dark shadow areas to remove equal amounts of cyan, magenta, and yellow (CMY) where they exceed the total ink limit. For example, in sheet-fed lithography, a typical total ink limit is 360. In areas that print 100% of all four colours, UCR will typically leave the full range black separation, and remove more and more CMY the deeper the shadow colour is. A typical grey balance in shadows may be 95% cyan, 85% magenta, and 85% yellow. Including a 100% black, that area would have a total ink coverage of 365. Other imaging technologies have different total ink limits, and these can vary greatly from one substrate to another within an imaging technology. An uncoated sheet will absorb more ink than a glossy coated sheet of paper and so will have a different total ink limit.

GCR stands for grey component replacement, and it is intended to help improve grey balance stability in a print run and save on ink costs. GCR can affect far more colours than UCR as it can be set to replace equal amounts of CMY with black all the way into the highlight greys. This is particularly useful in technologies like web offset newspaper production. Grey balance is quickly achieved in the make-ready process and easily maintained through the print run. Black ink for offset printing is significantly cheaper than the other process colours, so there are cost savings for long runs as well. GCR is used in photos and vector graphics produced for other imaging technologies as well. Any process where grey balance of equal values of the three primary colours could be an issue is a smart place to employ GCR.

You may be wondering how we can check the shadow areas of every photo we use. These GCR and UCR values can be set in ICC profiles by linking the shadow and neutral Lab values to the appropriate CMYK recipes. When the ICC profile is applied for a given output device, the shadows get the proper ink limits, and the grey tones get the prescribed amount of black, replacing CMY values.


Black keylines, or outline frames for photos, are common in many documents. This is another place where a document should have trapping software applied for every imaging technology. Outline strokes on graphics can also have a ‘hairline’ setting, which asks the output device to make the thinnest line possible for the resolution of the device. This was intended for in-house studio printers where the resolution is 300 dpi — so the lines are 1/300th of an inch. But the same command sent to a 3,000 lspi plate-setter will generate a line 1/3000th of an inch, which is not visible to the naked eye. These commands must be distinguished in PostScript and replaced with lines at an appropriate resolution — trapping and preflight software will do this.

Knock outs

The use of solid black backgrounds is becoming more popular in documents, which can cause problems in reproduction with all imaging technologies. The first problem is with filling in details in the graphic shapes that are knocked out of the solid black background. Fine type serifs, small registered or trademark symbols, or the fine hairlines mentioned above will all fill in and be obliterated when imaged. The problem is multiplied when we boost the black colour by adding screened values of cyan, magenta, or yellow to the colour block. When white type or graphics knock out of these background panels, any slight mis-registration of any colour will leave a halo of that colour in the white type or graphic. This problem can also be solved with trapping software. Essentially, the trapping engine outlines all the white type with a small ‘black only’ stroke that knocks out the process colour that boosts the black, making the white type fatter in that colour separation. This ‘reverse trapping’ works well when applied to the four imaging technologies we have been examining: lithography, flexography, electrophotography, and inkjet.