Streaks in offset printing are color density fluctuations or contrast variations, usually perpendicular to the print direction. These are visually disturbing, especially when printing even surfaces. In offset printing, a slight streaking cannot be eliminated 100% due to the nature of the system. The causes of streaks can be very versatile. All process parameters, from substrates to inks, machine settings and condition, can be considered as causes. Often several components in combination lead to streaking. The elimination of the malfunction requires a structured procedure in the exclusion process.
Below you will find a list of the most common causes and remedies. This list is not complete because the possible number of causes for streaking in printing is so extensive.
Cause 1 – Underpacking:
The blanket packing must be set exactly to the manufacturer’s specifications. In addition, for some press types the actual total packing height must be measured manually and entered in the press console for each printing unit. This is the only way to correctly control the printing pressure.
- Check the blanket packing and renew if necessary
- Measuring the packing height
Cause 2 – Blanket
Blankets may be responsible for streaks in the printout. This is typically indicated by uneven stripes with smooth edges across the sheet width.
- Installing new blankets
- Use different blankets
- Mount the blankets correctly
Cause 3 – Rollers
Printing rollers and their correct adjustment are important for even printing and constant ink transport. Roller streaks are typically contrast-rich and appear at short intervals. Depending on which rollers or roller pairs are affected, different strip patterns appear in the print.
For standard inking units:
- Adjustment of all printing rollers according to manufacturer’s specifications. Usually all rollers are first adjusted to the distributors. Then to the printing plate.
For anilox inking units:
- Replace doctor blades
- Adjust doctor blades according to manufacturers recommendation
Cause 4 – Damage to the bearings of the printing rollers
- Remove affected rollers and replace bearings. Check bearing shells/receivables, replace if necessary.
Cause 5 – Rollers calcinated
Over time, printing rollers accept particles from the dampening solution and become hydrophilic. This disrupts ink transport and results in uneven ink transfer. Typical indication: Print streaks that occur at irregular intervals across the print run.
- Cleaning of the rollers. Decalcifying the rollers with a deglacer. Ensure that all rollers are in contact during the application. For some machine types, the roller bridge must be deactivated when idling.
- Change osmosis water hardener
In most cases, the osmosis water is hardened with calcium prior to dampening solution conditioning. Over time, calcium settles on the rollers and makes them hydrophilic.
Alternatively, a magnesium-based hardener can be used to delay the water accepting effect.
Cause 6 – Substrate
Some substrates tend to cause visible streaks in the print due to their absorbance.
In particular, uncoated papers or laminated cardboard are more prone to this than other substrates.
Typical indication: Aprubated ink density changes perpendicular to the printing direction with the length of half or full circumference of the ink form rollers.
- Changing the substrate
- Reduction of ink application (reduction of ink density)
- Priming of the substrate
- Pre-moistening of the substrate in the upstream pressure unit
Cause 7 – Dampening units
The constant and uniform dampening of the printing plate is essential for correct ink transfer and for preventing overemulsion of the printing ink. Normally, only one dampening form roller is used for film dampening units. The dampening unit must therefore be set very accurately to the manufacturer’s specifications.
For film dampening units:
- Adjusting the dampening rollers according to the manufacturer’s specifications
- Checking, if necessary renewing the metering rollers and dampening form rollers
For spray dampening systems:
- Reducing the dampening solution quantity
- Optimizing the pulse frequency
Cause 8 – too fine screens
If the selected screen ruling is too fine for the substrate, this increases the system-related tendency to form streaks.
Typical indication: Only certain area coverages are affected by stripe formation.
- Choosing a coarser screen
Cause 8 – Excessive printing pressure
If the printing pressure between the blanket cylinder and the substrate is set too high, printing streaks are produced by wringing and driving out the blanket surface or substrate. Typical indication: streaking from the sheets half length and especially towards the format edges.
- Reduce printing pressure (Kissprint to determine optimum printing pressure)
- Checking, if necessary correcting the blanket cylinder underpacking
Cause 9 – bearer ring pressure too high
If the pressure between the bearers is set too high, this can lead to vibrations and bearing damage.
Typical indication: Regular, sawtooth-shaped stripes across the entire print format.
- Adjust the bearer ring compression according to the manufacturer’s recommendations (perform a Kissprint between the printing plate and the blanket to determine the optimum pressure level)
Cause 10 – Drive
The drive of the printing press must be constant and with a consistent torque. Variations in the drive power produce uncontrollable streaks in the printed image.
Typical indication: Strong uneven streaking, especially in the printing unit closest to the drive. The stripe effect decreases in the following printing units.
- Check and adjust the belt tension between the main motor and the drive wheel of the printing press.
- Contact Manufacturer Service
Cause 11 – Broken Rollers or bearers
Broken rollers, bearers or damaged shafts may cause serious vibration and/or serious ink-transfer disruption.
- Check ink-train and dampening system drive components
- Replace worn-out parts
- Adust settings accordingly to manufacturers recommendation
This listing is incomplete and will be extended in the future
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