Overprint Varnishes

Overprint Varnishes

Whatever printing method is chosen, an overprint varnish enables:

● The print work to be enhanced.

● The rub resistance and the mechanical resistance to be improved.

● The chemical resistance of the ink film to be improved.

This chapter outlines the 4 major families of overprint varnishes and summarises their main properties.

1. Conventional offset / letterpress varnishes
2. Acrylic varnishes
3. UV varnishes
4. Flexo / Gravure varnishes
5. Summary tables

Conventional offset /letterpress varnishes

Conventional “paste” type varnishes have numerous advantages, including excellent protection against humidity, ease of use and excellent rub and chemical resistance.
Formulation:
The oleoresinous formulation of thesevarnishes is of the same chemical nature as conventional offset / letterpress inks. Their siccativity is adjusted to optimise the drying time. The principal characteristics of the varnish are obtained by the incorporation of various additives.

Use:
They are extremely easy to use and do not require any specific equipment. They are applied from the ink duct, with or without fount solution, either wet-on-wet or wet-on-dry over conventional offset / letterpress inks.
Characteristics:
Drying takes place through penetration and air polymerisation and therefore involves some delay before handling and cutting / folding operations can be carried out on the printed material. Application with fount solution allows gluing reserves to be made.
Their compatibility when applied over conventional inks is optimal.

Incidents

1. Hold out / not smooth

2. Set off

3. Gluing problems on varnished prints

Poor drying

Probable causes and remedies

1. Hold out / not smooth (back to incidents)

2. Set off (back to incidents)

3. Gluing problems on varnished prints (back to incidents)

4. Poor drying (back to incidents)

Acrylic varnishes

More and more widely used, acrylic varnishes have met with great success thanks to their low cost and their ease of use.
Formulation:
There are two types of formulation based on acrylic resins: water based dispersions and solvent based dispersions. Their viscosity is adjusted as a function of the application method.
Use:
Water based acrylic varnishes can be applied using the fount solution tank, a separate varnishing machine or a varnishing unit (the most widely used method is a scraping chamber with an anilox cylinder with a screen ruling of 80-100 l/cm and a capacity of 6, 9 or 13 cm3/m2).
Solvent based acrylic varnishes are dry offset applied using the ink duct.
Acrylic varnishes can be applied in-line over conventional offset / letterpress inks.
In all cases (water or solvent based), it is recommended to use light fast inks because certain pigments may be chemically altered.
Characteristics:
Drying is rapid or even instantaneous (hot air or IR drying).
They are colourless and non-yellowing and they have high gloss.
These varnishes can be glued.

Incidents

1. Hold out / Not smooth

2. Discoloration of the ink film

3. Set off

Probable causes and remedies

1. Hold out / not smooth (back to incidents)

2. Discoloration of the ink film (back to incidents)

3. Set off (back to incidents)

UV varnishes

UV varnishes are top of the range varnishes. They have the best performance in terms of gloss and chemical resistance.
Formulation:
UV varnishes are formulated using the same chemical principles as UV inks, in other words they contain reactive resins that polymerise and form a hard film under the effects of UV radiation. The compatibility of UV inks with UV varnishes is therefore optimal.
Use:
UV varnishes can be applied by using of the ink duct, a varnishing machine or a varnishing unit ( > See “Fastnesses of inks") and require specific UV drying equipment.
They may be used in-line on UV or Flexo inks and off-line on conventional offset / letterpress inks after drying ( > See Pulsar Technical Data Sheets)
As with acrylic varnishing, it is recommended to use light fast inks because certain pigments may be chemically altered.

Characteristics:
UV varnishes have the best performance in terms of resistance, gloss and instantaneous drying under the effects of UV radiation.

Incidents

1. Hold out / not smooth

2. Discoloration of the ink film

3. Gluing problems on varnished prints

4. Poor drying

Probable causes and remeies

1. Hold out / not smooth (back to incidents)

2. Discoloration of the ink film (back to incidents)

3. Gluing problems on varnished prints (back to incidents)

4. Poor drying (back to incidents)

Flexo / Gravure varnishes

Their principal advantage is their low cost.
Formulation:
Two types of flexo formulation are available: water based acrylic resin solutions or dispersions (water based flexo varnishes) and solvent based cellulosic resins (solvent based flexo varnishes).
Gravure varnishes are solvent based cellulosic formulations.
Use:
These varnishes are applied using conventional flexo and gravure printing units.
They are used for in line wet-on-wet varnishing of flexo and gravure inks respectively.
Characteristics:
Drying by coalescence (water based varnishes) or by evaporation (solvent based varnishes) is rapid or even instantaneous (hot air dryers are used).

Summary tables

** Drying times required to obtain complete drying of the varnish (hard drying). These times are only given as an indication since they depend on the printing and varnishing conditions.

** Drying times required to obtain complete drying of the varnish (hard drying). These times are only given as an indication since they depend on the printing and varnishing conditions.

* UV varnishing of conventional offset or flexo inks after drying: precautions need to be taken (please refer to the Technical Data Sheets).
** Drying times required to obtain complete drying of the varnish (hard drying). These times are only given as an indication since they depend on the printing and varnishing conditions.

** Drying times required to obtain complete drying of the varnish (hard drying). These times are only given as an indication since they depend on the printing and varnishing conditions.