How To Solve The Problem Of Scumming (Dirty Plate) In Offset Printing

1. What Is the Definition of Scumming?

Scumming refers to a fault in offset printing in which the non-image areas (blank areas) of the printing plate become abnormally ink-receptive, so that ink spots or dirty marks appear in the blank areas of the printed sheet, or the whole sheet looks gray, foggy and unclean. It is often accompanied by secondary problems such as dot bridging (filling-in), thickened strokes and loss of tonal definition.

Offset printing works on the fundamental principle that "ink and water do not mix": the image areas of the plate are oleophilic (ink-attracting) and water-repellent, while the non-image areas are hydrophilic (water-attracting) and ink-repellent. In printing, water is applied first and ink afterwards; the fountain solution firmly occupies the non-image areas so that ink stays only on the image areas. Once this balance is broken and the non-image areas begin to accept ink, scumming occurs.

One common misunderstanding needs correcting: the subject of scumming is the plate surface, not the ink itself. Offset printing ink is merely the party that is "wrongly accepted." If the fault is understood as "dirty ink," the whole troubleshooting effort will go in the wrong direction.

Scumming usually appears on the printed sheet in three forms:

Irregular ink spots and dirty marks in blank areas, with shifting positions and uneven density;

Overall fogging or graying of the sheet, giving a hazy look with obviously insufficient whiteness;

Dirty spots whose position and density are not fixed and that easily recur after wiping.

These symptoms share one common feature: ink appears where it should not be.

2. How to Distinguish Scumming from Tinting

This is the most important professional dividing line in the whole article. In many pressrooms the two kinds of "dirt" are confused, and the more you adjust, the worse it gets.

Behind this table lies a field-usable test: wipe the blank area with a wet sponge. If it can be wiped off, it is tinting; if not, it is scumming.

Why is this single wipe so critical? Because the causes of the two faults point in almost opposite directions: tinting mostly indicates "excess water causing offset printing ink emulsification," while scumming mostly indicates "insufficient water losing its protection" or "a contaminated plate surface." If you judge wrongly, every subsequent adjustment works in reverse.

3. How to Judge at a Glance on Site

Experienced operators can tell where the problem lies at a single glance at the plate. It is not magic, but a few teachable observation points.

Look at the plate's reflection. In normal printing the plate reflects a grayish-white light, which means the water feed is moderate; if the plate is bright like a mirror with a harsh reflection, that is a sign of too much water. This "grayish white" is the visual signature of a dampening film of the right thickness, and it is the most direct window for judging ink-water balance.

Look at the ink fountain and the gripper edge. Excessive water also leaves a string of related traces: the ink fountain roller shows de-inking, fine water droplets can be seen in the ink fountain, obvious mottled white marks appear at the gripper (feed) edge of the sheet, and water droplets hang from the plate clamps and the blanket's trailing edge after a stop. These are all on-site evidence of "too much water."

Finger test. Run a finger lightly across the non-image area of the plate. If it feels rough or sticky, the plate surface has been contaminated or oxidized — a healthy non-image area should feel clean and hydrophilic. If the dirty marks disappear after wiping the plate with a sponge dipped in concentrated fountain solution, the problem lies at the dampening level; if wiping has no effect, the plate most likely needs cleaning, gumming, or even remaking.

Scumming is rarely caused by a single factor. More often it is the result of ink-water balance, mechanical condition, consumable quality and environmental conditions "giving way" together after long running time. In order of frequency and troubleshooting priority, the causes fall into six layers.

1. Ink-Water Imbalance

Insufficient water → scumming. When the water supply is inadequate, the non-image areas of the plate are not sufficiently dampened, the hydrophilic protective "defence line" is lost, and ink is adsorbed directly onto the plate. This is the most direct path to scumming.

Excessive water → tinting. Conversely, with too much water the offset printing ink becomes over-emulsified, forming hydrophilic ink particles that deposit on the plate's non-image areas and produce a different picture: a uniform, foggy tinting.

Here lies the accuracy bottom line that must be held throughout: too little water causes direct scumming, too much water causes emulsification and tinting. The two causes run in opposite directions while looking similar, so they must be separated by the test of whether a wet sponge can wipe the marks off.

Scumming at both ends. There is another typical scenario: the water on the plate simply cannot be reduced — the moment you cut the water, both ends of the plate scum. This is mostly because the image coverage at both ends of the plate is sparse or completely blank, so offset printing ink accumulates and emulsifies at the ends of the ink rollers while water keeps being supplied, finally forming tinting on both sides. This kind of "two-end scumming" is especially common in newspaper and book printing.

2. Are the Fountain Solution Parameters Normal?

The fountain solution is the "shield" of the non-image areas. When the shield fails, dirt arrives.

pH value. When the pH is too high (above 5.5), the fountain solution is on the alkaline side, which weakens the hydrophilicity of the plate's non-image areas and lets ink slip in. "Alkaline side tends to scum" is basic consensus in the industry. Conversely, a pH that is too low (below 4.5) goes to the opposite extreme: excessive acid corrodes the plate surface and causes plate wear (image weakening), and it also destroys the cobalt and manganese driers in offset printing ink, slowing down drying of the printed film.

Conductivity. Conductivity measures how much dissolved salt is in the fountain solution. During printing, paper dusting and linting, and ink contamination, will gradually raise the conductivity. Once conductivity exceeds the limit, it means the fountain solution has been contaminated, the dampening effect drops, and plate filling-in and scumming follow.

Alcohol and temperature. Alcohol (isopropyl alcohol, IPA) lowers the surface tension of the fountain solution, allowing the plate to be dampened more fully, which greatly reduces water consumption and suppresses emulsification. If its content is too low, protection is insufficient. In addition, loss of control over fountain solution temperature also weakens plate stability.

One thing to note: more alcohol is not always better. Too much intensifies the dissolution of the paper surface, generating paper dust and lint, and promotes over-emulsification of the ink.

3. Is There a Problem with the Plate Itself?

Even the best ink-water balance cannot withstand a plate with a weak foundation.

Platemaking defects. Insufficient exposure or poor developing means the hydrophilic base of the non-image areas was never truly established. The moment such a plate goes on press, its non-image areas are in a "semi-oleophilic" state, and scumming is almost inevitable.

Oxidation during stops. If a press stop is not followed by timely wiping and gumming for protection, the non-image areas of the plate are exposed to air and oxidize, their hydrophilicity declines, and scumming appears on restart. This is why many presses "always scum after a stop."

Residue on the plate. Residual ink, paper dust and dried fountain solution deposits adhere to the plate and form local contamination points, which are a direct source of dirty spots. Selecting a plate with fine, uniform grain and adequate mechanical strength is the first line of prevention.

4. Is the Ink Performance Suitable?

On the offset printing ink side, problems usually come from being "too soft" and "too thin."

Too soft and too thin. Adding excessive amounts of varnish, reducer or other additives lowers the ink's viscosity and body below the normal working range. Thin, soft offset printing ink easily crosses the water-film barrier under impression pressure and spreads beyond the image boundary — the defence line is "washed out."

The correct remedy is not to keep thinning it, but to add a heavy-body gloss varnish (gloss paste) to build up the ink's body.

Poor water resistance. Offset printing ink with poor water resistance, coarse particles and a tendency to emulsify will lose control of emulsification whenever the water feed is slightly high, and then scum. Experienced process technicians remember this: some low-grade inks have coarse particles and strong water absorption; on the surface it looks like there is too much water, but the real root cause is the ink.

5. Are the Rollers and Blanket Clean?

This is the layer most easily misjudged as "the machine is broken," when in fact it is a chronic condition.

Scaling on dampening rollers, glazing on ink rollers. Scaling on the dampening rollers changes the uniformity of water feed, while glazing on the ink rollers changes the amount of ink applied; both push the ink-water balance out of balance. Once roller surfaces harden and glaze, neither water nor ink transfer remains stable.

Blanket glazing. Over time a blanket accumulates paper dust and ink, its surface glazes and loses resilience, and under pressure the accumulated deposits transfer very easily to the blank areas of the paper, forming dirty plates. The blanket position corresponding to the paper edge especially needs focused cleaning.

Roller setting and pressure. Incorrect roller setting and excessive printing pressure both aggravate friction and build-up. It is worth stressing: dirty rollers infect the blanket, and a dirty blanket in turn contaminates the plate — this is a chain of contamination. If you only wipe the plate without breaking the chain, the dirt will certainly come back.

6. Are the Paper and Environment Up to Standard?

The last layer is often the one most easily overlooked.

Paper dusting and linting. If the surface strength of the paper is poor, dusting and linting during printing are severe; the dust and lint accumulate on the plate and blanket and finally return to the sheet in the form of dirty spots. Running a comparison test with another batch of paper is the direct way to identify a paper problem.

Temperature and humidity. When ambient humidity is high (above 60%–65%), the paper absorbs moisture and expands, machine parts condense moisture, and the extra water breaks the ink-water balance, significantly raising the risk of scumming. In the rainy season the pressroom must run dehumidifying equipment, and paper should be brought into the pressroom in advance for conditioning and stabilization.

Speed, temperature and pressure. There is one more empirical rule: higher ambient temperature, higher printing speed and heavier printing pressure all make the plate more prone to scumming. The reason is that higher temperature increases ink fluidity and releases more free fatty acids, while higher speed causes the ink rollers to heat up through friction and thins the ink. Therefore, in these situations the fountain solution concentration should be raised appropriately to strengthen anti-scumming capability.

Once the causes are known, the solution has a path. The core approach is: stop the loss quickly first, then bring the parameters back into position, and finally build a long-term mechanism.

1. How to Troubleshoot on Site

Four-step troubleshooting method. On-site handling should follow the principle of "fast before slow, low cost before high cost," and you must never start by changing the plate or the ink.

Step 1: test the fountain solution first. pH, conductivity, alcohol content and temperature — these four indicators are the fastest and cheapest to check, and they are the most frequent points of failure, so they must be checked first. After the parameters are brought back into range, run twenty or thirty sheets and observe.

Step 2: fine-tune the water feed. After a small increase, run 20–30 sheets and see whether the dirt clears. When adjusting, always follow the minimum-water principle: use as little water as possible on the premise that the non-image areas do not scum. Never blindly add ink the moment dirt appears — once the vicious circle of adding water and adding ink is formed, the ink becomes severely emulsified and the situation becomes far harder to fix.

Step 3: judge the plate next. If the non-image area feels sticky when a finger is drawn across it, it is contaminated; test by wiping the plate with a sponge dipped in concentrated fountain solution — if the dirt clears, the problem is controllable; if it persists after repeated wiping, the plate needs cleaning and gumming, or remaking if necessary.

Step 4: check the rollers and blanket last. Clean the dampening and ink rollers, deglaze glazed roller surfaces, correct roller settings, clean the water tank and replace the filter pads. If the fountain solution is obviously contaminated (conductivity seriously over the limit), drain the whole tank and prepare a fresh solution.

After these four steps, the great majority of scumming cases can be located.

2. What Values Should the Key Parameters Be Kept At?

Parameter reference table. Turning experience into numbers is the key step that moves scumming from "black art" to "under control."

A few supplementary notes:

How to adjust pH: if the pH is too low, reduce the proportion of concentrate and dilute with water; if it is too high, check whether alkaline substances have been mixed in, and add concentrate appropriately.

If conductivity is too high: first drain part of the fountain solution and dilute with pure water; if it seriously exceeds the limit, replace the entire tank.

In preparation: soft water should be used; when adding manually, use standard measuring tools to ensure accurate dosing, and units with automatic dosing devices should be maintained and calibrated regularly.

Important note: all of the above are industry reference values. Actual settings must be based on the equipment manual, the instructions of consumable suppliers and your plant's actual water quality — do not copy them blindly.

3. How to Stay Free of Scumming in the Long Run

First aid only stops the loss; real cost reduction depends on prevention. Four fronts must work at the same time.

(1) Materials. Select offset printing ink with higher body and good anti-emulsification performance; when the ink is too thin, add a heavy-body gloss varnish to build up its body rather than continuing to thin it; hold the line on plate quality by choosing plates with fine, uniform grain and solid image and non-image bases. Compatibility verification should be carried out between different brands of paper, fountain solution and ink, to avoid the situation where "a change of paper batch brings scumming."

(2) Equipment. Implement the industry-summarized "Three Levels" and "Three Minimums":

Three Levels: cylinders level (plate, blanket, impression), ink rollers level, dampening rollers level — ensuring smooth packing and roller surfaces, uniform pressure, and centre distances that meet the standard and remain relatively stable.

Three Minimums: minimum pressure (enough to ensure normal transfer of water, ink and the image), minimum water (reducing the degree of emulsification as far as possible, on the premise that the plate does not scum), and minimum ink (while maintaining ink consistency to strengthen water resistance).

In addition, when renewing or upgrading equipment, priority can be given to continuous-feed alcohol dampening systems, whose ability to monitor ink-water balance is clearly superior to that of traditional intermittent dampening systems.

(3) Operation. Practise the "Five Diligences":

Diligently watch the water on the plate and adjust the water feed in time;

Diligently stir the ink to improve fluidity and maintain good transfer performance;

Diligently clean out the ink fountain so that no dried ink skins or debris remain in it;

Diligently wash the blanket and the plate to remove paper lint, paper dust and residual ink;

Diligently check against the OK sheet, comparing with the approved sheet at any time to find the new ink-water balance.

These "Five Diligences" should run through the whole production process as standardized practice, not be remembered only when a problem appears.

(4) Management. Test and record the fountain solution parameters before start-up on every shift; use a densitometer to continuously monitor the density of each colour on the printed sheet and track the trend of ink-water balance; gradually accumulate data for different inks, papers and environmental conditions, and build a routine data model for each press to achieve data-based management.

4. How to Investigate Recurring Scumming

At one plant printing newspapers, scumming began to appear in the blank areas on both sides of the paper at around the twentieth thousand copy. This "twenty-thousand-copy threshold" is very typical — it is not a single cause, but the combined outbreak of ink-water imbalance, changes in mechanical condition and consumable quality after long running time.

Ink-water imbalance: after long running, ink emulsification intensifies; excessive water forms hydrophilic particles that cause tinting, while insufficient water makes the non-image areas absorb ink directly.

Mechanical condition: wear of the cylinder bearings, excessive backlash in gears and synchronous toothed belts cause vibration and jumping at high speed, destroying the ink-water balance; paper dust and ink accumulate on the blanket at the position corresponding to the paper edge and transfer directly to the blank areas of the paper.

Paper and ink: poor paper surface strength causing dusting and linting; high ink viscosity and poor fluidity, with paper dust and lint accumulating on the plate and blanket.

For this type of scumming that "only appears after a certain print quantity," the following sequence is recommended:

Check the water system first — test the pH, conductivity and alcohol concentration of the fountain solution, calibrate the dampening roller pressure, and ensure the water film is thin and uniform.

Then check the mechanics — use a dial indicator to check cylinder runout, watch for abnormal noise and high temperature; inspect the gears and toothed belt for wear; focus on cleaning or replacing the blanket at the position corresponding to the paper edge; check whether components such as the paper guide rollers have accumulated ink that smears the paper.

Finally check the consumables — run comparison tests with paper of a different batch or brand; talk to the ink supplier and select offset printing ink suitable for high-speed web printing, adding a small amount of varnish when necessary to improve fluidity.