Problems in offset printing
Offset printing offers high quality and repeatability. However, various issues may appear on press and reduce the visual effect. Therefore it is worth knowing the most common offset printing problems and how to limit them in daily production. Below we focus on set-off, ink scuffing and ink repulsion, and we show practical ways to keep the print stable.
1. Set-off – when fresh ink transfers to the next sheets
Set-off appears when fresh ink transfers from lower sheets to upper ones in the pile. As a result, unwanted marks or shadows show up on solids and images. Usually several reasons work together, so it is helpful to analyse ink laydown, paper and delivery settings step by step.
Main causes of set-off and how to reduce them
- Too much ink on low-absorbent paper – First, try to return to standard ink feed and avoid over-inking. Additionally, you can add an ink additive, use powder spray on the pile and adjust the way sheets are delivered.
- Delivery pile too high – When the pile becomes heavy, air escapes and pressure increases. Therefore it is better to work with lower piles, use more frequent pile changes and combine this with careful powder application.
- Wrong delivery settings – On heavy boards, poor adjustment of fall height and gripper opening often leads to set-off. In this case reduce the fall height, adjust the timing of the grippers and ensure that sheets fall tightly into the delivery without excessive bouncing.
- Poor flatness of the substrate – Wavy edges or uneven sheets make set-off more likely. Consequently, choosing a better quality paper with good flatness helps stabilise the result.
- Electrostatic charge in the pile – Static builds up especially on dry or coated stocks. Therefore you should work with smaller, looser piles and, if needed, use antistatic devices to keep sheet separation and print quality under control.
Altogether, set-off has many possible sources, yet careful control of ink, paper and delivery usually keeps the problem at a safe level.
2. Ink scuffing – when the print rubs off
Ink scuffing appears when pigment rubs off during handling or finishing. As a result, prints lose density and details, especially in dark areas. Therefore it is important to check ink formulation, paper sizing and the amount of binder in the ink.
- Ink thinned too much – When the ink becomes overly fluid, the vehicle penetrates the paper too quickly and leaves pigment poorly anchored. In this case you can print an additional layer of vehicle or use an oil-based overprint varnish to bind pigment to the surface.
- Poorly sized, highly absorbent paper – If the paper absorbs the vehicle too fast, pigment cannot bond properly. Then an oil-based overprint varnish often improves rub resistance. Additionally, you may use a printing paste or a high-gloss oil-based varnish when the job is already printed.
- Too little binder in the ink – Sometimes the ink itself contains too little vehicle. Here the solution is simple: add binder to the ink or run an extra pass with vehicle only to strengthen the image.
3. Ink repulsion – when one ink rejects the next one
Ink repulsion appears when the first ink layer dries too hard and no longer accepts the next colour. Consequently, the second ink beads on the surface and the image loses coverage and sharpness. Usually the reason is excessive drier or too long a pause between printing the first and second colour.
- For short runs – In short runs you can gently rub the surface with cotton wool. Thus you break the hard skin and help the next ink adhere.
- For long runs – In longer runs it is better to apply an oil-based overprint varnish and then print the next colour at the moment when the varnish is setting. In addition, you may reduce or remove drier to keep the first layer open for the second one.
When you diagnose the cause correctly, you can choose a matching method and avoid ink repulsion both in small and large runs. In the end this leads to clean, readable and visually attractive offset prints.
Do you face problems in offset printing? Get in touch with us – we will analyse the job, suggest adjustments and help you choose a stable production setup.