A few missing dots or broken fine lines may not look serious during a short printing trial.

Once the line returns to normal production speed, the same film may start showing bare spots, uneven solid areas or incomplete small text.

When this happens, the ink is often adjusted first. But the defect may also come from the film condition or from the way the ink and substrate work together.

The Defect Usually Gives the First Clue

Poor wetting does not always affect the whole printed area.

It often appears first in areas that leave little room for variation:

• Fine lines become broken
• Small characters lose definition
• Solid areas develop isolated bare spots
• Ink pulls back shortly after transfer
• Fisheyes appear in local areas
• Defects increase as line speed rises

If the print looks acceptable at low speed but changes during production, it is worth checking more than the ink viscosity alone.

The film being used on the machine, the pretreatment condition and the actual defect pattern can all help narrow down the cause.

PP, PE and PET Need Different Treatment

Plastic films may look similar, but they do not behave the same way during printing.

PP and PE normally need more attention before printing. A clean-looking film can still give uneven ink coverage if its printable surface is not in the right condition.

PET behaves differently and uses a different primer approach.

If the film has already been corona treated, the material currently running on the press should still be checked rather than assuming that an earlier treatment record represents its present printing condition.

For production troubleshooting, the actual print result is more useful than treating PP, PE and PET as one group.

Primer Selection Starts With the Film

Primer selection should follow the substrate and the printing system, not just solids, viscosity or appearance.

For corona-treated PE, PP, OPP and BOPP, Anhui Sinograce Chemical offers SA-248G, a water-based modified acrylic printing top coating.

Typical data:

Solid content: 40±1%

pH: 6.0–8.0

Viscosity at 25℃: 100–200 mPa·s

Tg: 30℃

SA-248G is designed for good leveling and adaptability to UV ink printing. For applications with additional surface-performance requirements, it also provides high gloss, water resistance and scratch resistance.

For corona-treated PET/BOPET, SA-235 uses a different modified acrylic system.

Typical data:

Solid content: 35±1%

pH: 7.0–8.0

Viscosity at 25℃: 100–200 mPa·s

Tg: 28℃

SA-235 is designed for PET/BOPET UV printing, with good ink acceptance and leveling.

The two products have similar viscosity ranges, but they are intended for different film surfaces. Similar numbers on a technical data sheet do not make the products interchangeable.

Checks to Make on the Printing Line

Before making a major change to the ink formulation, compare the actual production conditions:

• Film type
• Pretreatment condition
• Location and shape of the printing defect
• Trial speed and normal production speed
• Ink viscosity and surface tension
• Primer coating uniformity
• Primer drying condition

If the defect changes with line speed or film type, that information can be more useful than changing several ink parameters at the same time.

For PP, PE, OPP, BOPP and PET/BOPET printing applications, Anhui Sinograce Chemical recommends evaluating the primer with the actual film, ink system and production conditions.

A technical data sheet can narrow down the product choice. The final printing test shows whether the system is suitable.