Scumming in Offset Printing: Causes and Solutions

Scumming in offset printing occurs when non-image areas of the printing plate accept ink and transfer unwanted marks to the printed sheet. Areas that should remain clean can develop a dirty background, patches or streaks.

Ink and water balance is a useful starting point, but persistent scumming can also involve roller settings, plate preparation, fountain solution, piling or ink compatibility. In this guide, I explain the checks I use to work through those possibilities.

Scumming produces unwanted ink in areas that should remain clean.

What causes scumming in printing?

In conventional dampened offset printing, non-image areas must remain ink-repellent. Too little effective dampening or an excessive ink film can upset that balance. However, adding water does not repair a damaged plate or correct a mechanical setting.

Common areas to investigate include:

• Incorrect ink and water balance.
• Dirty, worn or incorrectly set ink and dampening rollers.
• Plate preparation problems or loss of protection in non-image areas.
• Incorrect or contaminated fountain solution.
• Piling, plate wear or excessive contact pressure.
• An ink and fountain solution combination that does not perform well together.

Start by identifying the problem

Inspect the printed sheet, plate and blanket before making several adjustments. Note whether the problem affects one unit or several, appears immediately or develops during the run, and began after a change of paper, ink, plates or fountain solution.

A magnifier can help reveal small unwanted marks before they are obvious to the naked eye. We have used this inspection tool; another inspection option is also listed on Amazon. Check current specifications and availability before buying.

These are affiliate links. I may earn a commission from qualifying purchases.

How to fix scumming in offset printing

1. Check ink and water balance

Check that the dampening system delivers an even film and that the ink film is appropriate for the job. Make controlled adjustments and inspect the result. Avoid continually increasing water to compensate for another fault: excess dampening can introduce emulsification and other quality problems.

For the underlying principle, see ink and water balance concepts.

2. Inspect the rollers and press settings

In my experience, settings are one of the first things worth checking. Examine roller condition and cleanliness, including the metering and water pan rollers. Check roller contacts using the press manufacturer's specified procedure and settings.

Also check plate-to-blanket pressure and packing against the machine specifications. Use the correct procedure for your press rather than applying a universal stripe width or packing value. See my notes on checking roller settings.

Carry out inspections, cleaning and adjustments using the press's approved safety procedures.

3. Check plate preparation and protection

We once traced sensitive plates to gum that was not being applied properly in the plate room. It was a useful reminder that a problem visible on press may begin in prepress.

For processed plates, check development and finishing against the plate supplier's instructions. During stoppages or storage, use the protection method recommended for that particular plate. Computer-to-plate describes how the image is made; it does not mean every plate requires the same processing or gumming routine. Processless plates have their own handling instructions.

If the problem remains in the same area and the plate appears damaged, involve prepress and consider replacing it rather than repeatedly changing the ink and water settings.

4. Check fountain solution

Confirm the concentrate dosage and check the condition of the solution. Use the supplier's recommended measurements and operating range for your system, including pH and conductivity where applicable.

If contamination is suspected, follow the approved cleaning and replacement procedure. Ask the supplier to confirm compatibility with your ink, plate and water supply. Do not adjust chemistry to an arbitrary target borrowed from a different pressroom.

5. Investigate piling and plate wear

Buildup on the blanket or plate can affect printing and contribute to wear. Damage to a plate's non-image surface can leave it accepting ink where it should repel it.

In our experience, paper is a useful starting point when investigating piling. Look for lint, coating particles or other deposits, and note whether the problem began with a new stock. A different suitable paper may help, but a higher price alone does not establish the cause.

See how to keep blanket piling to a minimum and troubleshooting piling problems.

6. Review the ink with your supplier

Good ink needs a workable ink and water balance window. If you need an unusually heavy ink film to reach the required colour, or the problem started with an ink change, discuss strength and compatibility with your supplier.

Provide samples and details of the paper, plate, fountain solution and operating conditions. Avoid adding reducers or other additives without an approved recommendation.

What if adding water does not stop the scumming?

Return to the diagnosis. Check water delivery, roller contacts, plate condition and fountain solution instead of continuing to increase dampening. If several units are affected, investigate shared materials and systems; if one unit is affected, inspect that unit closely. These observations help narrow the search, but do not prove a cause by themselves.

My approach to persistent scumming

I do not always follow the same order. Settings are usually my first suspect, followed by ink, but the timing and pattern of the defect matter. Change one variable at a time, record what happens and use your pressroom experience to decide the next check.

This guide combines my own observations with general troubleshooting principles. For manufacturer guidance, consult Sun Chemical's coldset scumming guide, heatset scumming guide and sheetfed troubleshooting guide, alongside your press and consumable suppliers' instructions.