Why Ink Smears or Flakes on PP Bottles: Surface Treatment Explained

A run of PP bottles leaves the machine looking perfect, then a fingernail takes the ink off in one clean strip. Or the print looks wet-edged and smeared the moment it clears the screen. Both complaints arrive described as "the machine is printing badly" — usually it is doing exactly what it was told, and the variable that changed is the bottle surface.

Polypropylene is the most common bottle material in personal care and household packaging, and the least friendly to screen printing: no natural chemical grip for ink, and the treatment that gives it one is invisible, temporary and easy to get wrong. This guide covers what happens at the ink-to-PP interface, how to test it with a number instead of an opinion, and how to set a flame station so adhesion stops being a daily argument.

Smearing and flaking are two different faults

Buyers and operators use one word for both, which is why the first repair attempt often makes things worse. Smearing is a wetting problem at the moment of transfer. Flaking is an adhesion problem that appears hours or days later. Different causes, different tests, different fixes.

Treat the table as triage, not diagnosis. Keep one untreated bottle from the same lot as a control and test it whenever a complaint arrives: if control and production part behave identically, the treatment station is not your problem.

Why PP is the difficult one

Screen ink bonds by wetting a surface, then locking onto it mechanically and chemically as it cures. For that, the liquid ink has to spread rather than bead up, which depends on the surface energy of the bottle relative to the surface tension of the ink.

Polypropylene is a non-polar polymer with very little at the surface for an ink to hold onto. Its surface energy is commonly quoted in the low 30 mN/m range (numerically the same as dyn/cm), while most solvent-based screen inks for containers need roughly 38 mN/m or above to wet out reliably. That gap is the whole problem in one sentence: the ink is trying to spread on a surface that repels it.

Three things make it worse:

• Additives migrate. Slip agents, antistatic agents and mold-release residues sit at the surface by design, lowering surface energy while staying invisible. A bottle that printed well from one molder can fail from another using the same resin with a different additive package.
• Treatment decays. A bottle treated at the blow molding stage does not stay treated; the effect fades in storage, faster in warm or dusty conditions.
• Geometry hides the problem. Shoulders, necks and oval bodies meet the flame at different angles, so a setting that treats the body can leave the shoulder under-treated — which is why many failures start near the top.

PET, glass and rigid PVC are more forgiving, which is why teams moving from one material to another are surprised by PP. Glass bonds mechanically to almost anything; PET usually arrives with enough surface energy to print; PP needs help.

What flame treatment actually does

A controlled oxidizing flame oxidizes the outermost molecular layers of the plastic. Polar groups form, surface energy rises, and ink that beaded now spreads. The affected depth is nanometres — the bottle looks and feels identical afterwards, which is exactly why the process needs measurement rather than a visual check.

Two failure modes sit on either side of the correct setting:

• Under-treatment leaves the surface unchanged; prints look fine on the machine and fail days later, so ink or dryer gets blamed.
• Over-treatment heats the part: loss of gloss or a hazy patch, distortion on thin walls, a burnt smell, sometimes a powdery surface that prints worse.

Setting a flame station on a rotary bottle line

The points below are illustrative starting values, not specifications, for a premix burner on cylindrical PP bottles: every burner, gas supply and wall thickness moves the window. Validate each change with a wetting tension test before production.

• Flame character. Aim for a short, stiff, slightly oxidizing flame with a defined inner cone. A long yellow reducing flame deposits carbon on the nozzle and the part, and carbon on the part is an adhesion failure waiting to happen.
• Gap. Hold the tip of the inner cone a few millimetres off the surface; an 8 to 15 mm working gap is a common starting point. Too far and the part sees hot air instead of reactive flame; too close and thin walls distort.
• Dwell and rotation. The bottle must spin so the whole circumference sees the flame evenly, with exposure typically a fraction of a second per pass. Even rotation matters more than speed: a slipping mandrel creates a treated stripe and an untreated stripe on the same bottle.
• Gas and air. The premix ratio sets flame chemistry, and pressure fluctuations change it through the shift. If your gas supply wanders, treatment level wanders with it — and the print room sees a fault that comes and goes.
• Safety and extraction. An open flame on a line needs a gas train with flame detection and interlocks, plus extraction sized for the burner. Settle it before the machine ships, not after installation.
• Maintenance. Nozzles block slowly; a burner that has drifted for a month looks fine and treats nothing. Put cleaning and a wetting tension spot check on the weekly schedule.

One process rule prevents most confusion: flame before printing, never after, and never twice on a printed part.

Let the dyne test settle the argument

Wetting tension is measured with test inks or pens of known surface tension, following the wetting tension method described in ASTM D2578. It is simple enough to run at the machine: draw a swab across the surface and see whether the film holds as a continuous wet line for a couple of seconds or breaks into droplets.

1. Test the actual print zone — body, shoulder and neck separately. A pass on the body can hide an under-treated shoulder.
2. Test a treated production part and an untreated control from the same lot; the difference between the two readings is the real treatment level.
3. Test immediately after treatment, then again at 24 hours. A surface that reads well and decays overnight fails in a warehouse, not on your line.
4. Record lot, reading and burner settings together. Without the settings beside the number, the record cannot be reproduced.

Most solvent screen inks for containers want 38 mN/m or better at the moment of printing, with a margin above the ink's own surface tension. If the treated part reads below that, no amount of squeegee pressure fixes adhesion — you are printing on a surface that will not hold the film.

Wetting tension is a proxy, not proof. Direct adhesion is measured by a tape test on the fully cured print, using a cross-hatch and tape method such as ASTM D3359. Run both: wetting tension tells you whether the surface is ready, the tape test tells you whether the finished print is staying.

When the machine looks guilty but is not

Because adhesion failures surface days later, the printer and the ink get blamed first. Rule these out before touching the machine — each produces symptoms that mimic poor treatment:

• Ink viscosity drift. Thinner evaporates through the shift and the deposit changes; a heavy deposit looks smeared and traps solvent, weakening the film underneath.
• Squeegee condition. A worn or nicked squeegee leaves an uneven film, and the thin patches fail first.
• Off-contact. Too little gap on a cylindrical part makes the screen drag and smear ink before the flood stroke finishes.
• Static and dust. Plastic bottles attract dust, a bond breaker, and static throws fine hairs of ink outside the print area.
• Cure, not adhesion. Undercured ink feels dry and flakes later; check the heat or UV dose reaching the print, not the dryer setpoint.
• Fixture repeatability. If the mandrel does not hold the bottle in the same position every cycle, flame gap and print gap both vary; servo-driven indexing with a repeatable fixture removes a variable operators otherwise chase with pressure adjustments.

A pre-production sequence you can run in one afternoon

1. Ask the molder for the resin grade, whether the compound carries slip, antistatic or release additives, and whether bottles were treated at molding and when.
2. Test incoming bottles with wetting tension pens; write the reading on the carton.
3. Treat a sample set and re-test the same three zones. If the reading will not move, the burner is not reaching the part.
4. Print a short run at production settings, with the real ink and mesh count.
5. Cure, then wait 24 hours before judging — adhesion failures need time to appear.
6. Tape test the cured print, and add a rub or short soak test if the bottle will meet alcohol, oil or a wet environment in service.
7. Record burner settings, ink batch, mesh, cure settings and results against the bottle lot; that record makes the next run repeatable.

What to specify when you buy the line

Surface treatment is often specified last, which is why it becomes the bottleneck. Three decisions belong in the machine specification:

• Integrated or standalone. An integrated flame station keeps treatment and printing in one pass, with no window for decay between them; a standalone treater suits plants that treat at goods-in and print later.
• Speed matching and dwell. The station must hold dwell across the full speed range, at every diameter you run. Ask for dwell at minimum and maximum speed, not just rated output.
• Consumables and local support. Nozzles, electrodes, gas train spares, and the procedure for re-setting the flame after a nozzle change. Treatment faults are usually solved by a small part and a documented setting; if neither is available locally, a small fault stops the line.

For cylindrical and oval PP bottles, fixture design and treatment station decide whether a print stays on. The CNC Bottle Automatic Screen Printing Machine is built around programmable bottle motion with a matched fixture set — the part that holds flame gap and print gap constant from the first bottle to the ten-thousandth. For tight registration on thin-wall or decorated parts, the High-Accuracy Servo Control Automatic Screen Printing Machine is the servo-controlled option, and the wider plastic bottle printing machine selection covers semi-automatic and rotary builds for lower volumes.

Frequently asked questions

Can an adhesion promoter replace flame treatment?

Sometimes, and it is legitimate for heat-sensitive parts or very low volumes. A primer or an ink with an adhesion-promoting additive adds a process step, a consumable cost and a flash-off window, and it can change gloss and resistance. It does not remove the need to test: primer over a contaminated surface fails the way untreated ink does.

How long does flame treatment last?

The effect decays rather than expiring on a fixed date, affected by storage temperature, dust, handling and the resin itself. Print as soon as the process allows, re-test treated stock that has been sitting, and never assume last month's reading applies to this month's carton.

Why does only one side of the bottle flake?

Almost always a treatment shadow. The mandrel or a gripper masks part of the surface, the burner is offset from the centre line, the part slips instead of rotating evenly, or an oval body presents a different distance to the flame as it turns. Map the failing face against the burner position and the pattern usually names the cause.

Do PET bottles need the same treatment?

Usually not. PET generally arrives with enough surface energy for standard container inks, which is why teams switching from PET to PP are caught out. Exceptions are recycled content, heavy mold-release use and long storage, any of which can push a PET bottle below the threshold — test before assuming.

Next step

If ink is smearing or flaking on your PP bottles, send three things: bottle material and shape, the wetting tension readings you get today, and your target output per shift. We can then tell you whether the gap is treatment, fixture or ink, and which configuration of the CNC Bottle Automatic Screen Printing Machine fits the bottle you actually run. For faults that are not adhesion-related, the screen printing machine troubleshooting and repair guide covers the mechanical and registration side, and the UV ink vs solvent ink comparison explains how the ink system changes the surface requirement.