What is trapping?
Trapping is a pre-press technique used to compensate for registration errors in lithographic printing and is the process of adding a slight overlap between adjacent areas of colour to avoid gaps caused by misalignment. A litho press requires each colour separation to be laid down one at a time over the next in a particular order. In four colour process printing, this order is KCMY or in other words, black followed by cyan, then magenta and lastly yellow. Each plate needs to be perfectly aligned to avoid what we call misregistration.
The exact registration of each colour or perfect aligning of each plate can be very difficult for many reasons. Colours may become misaligned and overlap incorrectly giving the appearance of unwanted colours or paper may show through in places there should be print. Pictures and type may appear out of register and the final printed product can look unsightly. Therefore, trapping is necessary to allow a certain overlapping tolerance between the colours to eliminate these problems.
What causes registration errors in print?
Misregistration is caused by a number of factors.
- The largest area of problems used to be in the distortion or inaccurate planning of film during the platemaking process. These problems have largely been bypassed with the advent of CTP (computer to plate) platemaking.
- Changes in the temperature or humidity of the environment, which can affect how paper behaves under pressure.
- Stretching of plates during exposure or fitting to the press.
- Inaccuracy of the printing press.
- Human error on press.
- Bad batches of paper.
- Paper bounce and stretch.
How do I know what to trap?
Trapping is required when two colours meet. It is usually the lighter colour that is spread underneath the darker colour, which should hold the overall shape of the element. Sometimes, gaps are left to prevent two colours mixing.
Most up-to-date reprographic software traps by default at a setting of somewhere in the region of 0.04mm. However, it is not safe to assume that every problem will be solved by this. It is always best to be on the look-out for issues such as small type in one spot colour against a different spot colour background, for instance, where traps might need to be increased manually to ensure the best results. The main ideas to bear in mind when thinking about trapping are;
Overprinting is the simplest of trapping methods. If something is set to overprint, the colour beneath remains untouched and the colour that sits on top will print over this – when this happens with a light “top” colour, this can cause unexpected or unwanted results. However it is usually a safe setting for black type, as 100% black type will only appear slightly darker printed onto most coloured backgrounds.
If an object of one colour sits on another colour, the bottom colour will knockout, leaving a hole or cutout of itself in the colours/plates beneath. This happens with Pantones too.
The cyan star has been set to knockout, so it leaves a hole or cut out of itself on the colours/plates below
If the background colour is lighter than the colour in the foreground, the lighter colour is extended under the darker colour. This is called choke.
If the background colour is darker than the colour in the foreground, the lighter colour on top is extended over the darker colour. This is called spread.