Industry News

Varnish, as a post-press process of flexo printing machines, is widely used in packaging production lines. Varnish/Coating is a transparent liquid layer applied to the surface of printed materials or substrates, which forms a protective film after drying or curing to enhance both appearance and physical performance. It is commonly used for products such as water bottle labels, snack packaging bags, and paper cups. This article will provide an in-depth analysis from four aspects: varnish properties, types of varnish, varnishing machines, and common problems and solutions.

Flexo Press for Nutrition Labels

Properties of Varnish

An ideal varnish should not only be colorless and odorless, with high gloss, fast drying, and good chemical resistance, but also offer high transparency, excellent abrasion resistance, good flexibility, strong environmental resistance, and good leveling performance.

High Transparency

For flexo printing products such as beverage labels, coffee cups, and coffee packaging bags, achieving an excellent varnishing effect depends on forming a colorless and transparent film on the surface. After drying, the printed graphics must remain unchanged in color and should not yellow or discolor due to sunlight exposure or long-term use.

Flexo Printer for Food Packaging Labels

Good Abrasion Resistance

After varnishing, flexo printing products must have a certain level of abrasion and scratch resistance. During post-press processes such as high-speed folder gluers, paper packaging machines, and paper cup forming machines, unavoidable friction will occur on the surface of printed products like tea packaging bags, gift boxes, and beverage paper cups.

Good Flexibility

The varnish film formed on the surface of products such as wine bottle labels, milk cartons, and ice cream cups must maintain good flexibility. It should match the flexibility of paper or paperboard to prevent cracking, brittleness, or peeling.

Flexographic Printer for Fast Food Wrappers

Good Environmental Resistance

Varnished printed products such as beer labels, juice cartons, and candy packaging wrappers can provide effective protection for the packaged goods. After drying, the coating film should have stable chemical properties and must not be affected by contact with weak acids, alkalis, or other environmental substances.

Good Leveling Performance

Due to the wide variety of substrates and the influence of printed graphics, differences in surface absorption, smoothness, and wettability are common. Therefore, the varnish must have good leveling performance to ensure a smooth and even film after coating.

Label Printing Machine for Biscuit Packaging

Types of Varnish

According to the curing method, varnish can be classified into water-based varnish, UV varnish, and solvent-based varnish.

Water-Based Varnish

Water-based varnish uses water as the solvent, making it environmentally friendly and non-toxic. It can be dried naturally or by hot air drying, featuring low odor, high safety, and relatively low cost. It is widely used for liquid food packaging, disposable paper bowls, kraft paper bags, as well as other food packaging paper, paper cups, and paper bags.

Flexographic Press for Bakery Paper Bags

UV Varnish

UV varnish is cured instantly by ultraviolet light, offering high gloss, strong abrasion resistance, and extremely fast drying speed. It can be further classified into high-gloss UV varnish, matte UV varnish, and spot UV varnish. It is widely used for cosmetic labels, shampoo labels, luxury packaging boxes, and cosmetic packaging boxes, especially in high-end packaging, labels, and folding cartons.

Solvent-Based Varnish

Solvent-based varnish uses organic solvents as the carrier, providing good film formation and strong adhesion. However, it has a strong odor and is less environmentally friendly. It is mainly used for plastic films and specialized packaging.

Flexographic Printing Machine for Gift Boxes

Varnishing Machines

Different materials require different varnishing machines. The most common substrates in the printing industry include paper, film, and aluminum foil.

Paper

Paper and paperboard have good absorbency. For products such as shopping paper bags and gift paper bags, water-based varnishing machines or UV varnishing machines are commonly used.

Film

Plastic films such as BOPP, PET, and PVC have low surface energy and require treatment. Products like perfume labels and medicine bottle labels are well-suited for UV varnishing machines or coating machines, and corona treatment is required.

Aluminum Foil

Aluminum foil has a dense surface and requires high adhesion. For products such as chocolate foil wrapping, butter and cheese foil, candy foil wrap, ice cream lid foil, and coffee capsule lids, specialized metal varnishing machines and UV curing systems are required.

Flexo Printing Machinery for Juice Cartons

Common Varnishing Problems

Poor varnishing quality, low gloss, and incomplete drying or sticking (blocking) are the most common issues in the varnishing process.

Poor Varnishing

Problems with the varnishing plate, low-quality varnish, improper machine settings, or incorrect pressure (too high or too low) can all lead to poor varnishing results. In such cases, it is recommended to use higher-quality varnishing plates and varnish, and properly adjust the varnishing equipment.

Flexographic Machine for Folding Cartons

Low Gloss

Low gloss is usually caused by insufficient varnish brightness, excessive coating thickness, or poor paper surface smoothness. Selecting the appropriate varnish gloss level, coating thickness, and paper quality can effectively improve gloss performance.

Incomplete Drying and Blocking

Excessive machine speed or overly thick varnish can result in insufficient drying. In UV varnishing, inadequate UV lamp power or lamps exceeding their service life can also cause blocking. Solutions include reducing machine speed, adjusting the coating weight, replacing UV lamps, or increasing lamp power.

Flexography Machine for Book Covers

Varnish is indispensable in the production lines of food packaging and labels. By fully understanding the types of varnish, its properties, varnishing machines, as well as common problems and solutions, its performance can be maximized to support the production of skincare labels, pharmaceutical labels, and perfume boxes.