The Glossing and Calendering Process in Printing

What is Glossing?

Glossing involves applying a layer of colorless transparent coating (varnish) onto the surface of printed materials, which, after leveling, drying, and calendering, forms a thin and uniform transparent glossy layer on the surface. Glossing techniques include overall glossing, partial glossing, glossy glossing, matte (dull) glossing, and special coating glossing.

Why Gloss Printing Materials?

Glossing serves to beautify, protect, enhance promotional effects, and increase the practical value of printed materials. The surface of glossed printed materials appears smoother, ensuring uniform light reflection and a brighter ink layer. Glossing involves the application of varnish onto the surface of printed materials. It is a protective treatment applied to paper or paperboard printed materials after printing but before final processing. Glossed printed materials are covered with a glossy film on the surface, and their applications include:

  1. Bookbinding, such as book covers, dust jackets, inserts, as well as calendars, advertisements, promotional samples, etc., which can enhance the gloss and color brilliance of printed materials.
  2. Packaging decoration paper products, such as paper bags, envelopes, labels, etc., where glossing serves to beautify and protect the merchandise.
  3. Cultural items, such as playing cards, postcards, and printed gold patterns, where glossing provides resistance to mechanical friction and chemical corrosion.
  4. Everyday items and food, such as cigarette packs, food packaging, detergents, etc., where glossing can prevent moisture and mold.
  5. Copper foil stamping on hardcovers, which enhances appearance and brightness, resembling gold. Proper glossing can improve adhesion performance, especially when copper and substrate bonding is inadequate.

Technical Requirements for Varnish:

Ideal varnish should possess characteristics such as being colorless, odorless, with strong gloss, rapid drying, and chemical resistance, along with the following properties:

  1. High transparency and color stability of the film layer: Excellent glossing results depend on forming a colorless transparent film on the surface of printed sheets, which does not change color after drying, even with prolonged exposure to sunlight or usage.
  2. Wear resistance: Some glossed printed materials require a certain level of wear and scratch resistance, especially those produced through high-speed box-making machines, paperboard box packaging machines, book covers, etc.
  3. Elasticity: Glossing varnish must maintain good elasticity on the surface of printed materials to adapt to the flexibility of paper or paperboard, preventing damage, cracking, or peeling.
  4. Environmental resistance: Glossed printed materials, particularly those used for various packaging boxes, must exhibit good environmental resistance to ensure protection, such as moisture and mold resistance for food, cigarettes, cosmetics, and clothing packaging.
  5. Adequate adhesion to the surface of printed materials: Glossing varnish should have strong adhesion to compensate for the reduced surface adhesion of printed materials due to ink density variations, preventing cracking or peeling of the film layer during use.
  6. Good leveling properties and smooth film surface: To ensure uniform film formation on different product surfaces, glossing varnish requires excellent leveling properties, resulting in a smooth film surface after drying.
  7. Broad suitability for post-printing processes: Glossed printed materials usually undergo subsequent processing, such as embossing or hot stamping with electroplated aluminum. Hence, glossing varnish should have wide compatibility with post-printing processes, exhibiting good heat resistance and solvent resistance.

How to Choose Varnish?

The selection of varnish should adhere to basic principles of science, economy, and practicality. Scientifically, varnish should meet the physical and chemical properties required for glossing and usage on printed materials. Economically, varnish selection should match the grade of printed materials to avoid using expensive varnish for low-grade printing. Practically, varnish selection should be compatible with glossing equipment. Additionally, safety, hygiene, and environmental protection requirements must be considered. Varnish should be stored safely, non-flammable, odorless, and non-toxic, with good storage stability.

Requirements for Varnish Coating Quality:

To achieve ideal glossing effects, the varnish coating should meet the following requirements during application:

  1. Uniform coating without sand eyes, air bubbles, or missed spots.
  2. Appropriate coating quantity allowing drying and film formation at a certain temperature and coating speed.
  3. Resistance to the effects of printing ink, printed image area, and ink density, exhibiting good leveling and adhesion to the 4. surface of printed materials.
  4. Adherence to calendering belts during the calendering process, easily peelable after cooling.

What is Calendering?

Calendering is the process where glossing is performed using both varnish application and heat pressing units. Printed materials are first coated with varnish using a standard glossing machine. After drying, they undergo hot pressing with a stainless steel belt in a calendering machine. Upon cooling and peeling, the surface of printed materials achieves a mirror-like reflective effect, thus obtaining high gloss.

Factors Affecting Calendering Quality:

Key factors influencing calendering quality include temperature, pressure, and machine speed:

  1. Temperature: Appropriate calendering temperature increases the molecular thermal motion of the varnish layer, facilitating diffusion and penetration of main agent molecules into the surface of printed materials. Additionally, proper temperature enhances the plasticity of the varnish layer, resulting in improved surface smoothness under pressure. However, excessively high or low temperatures are detrimental to calendering quality. High temperatures reduce adhesion strength and lead to decreased paper moisture content, affecting glossing and peeling. Conversely, low temperatures prevent proper varnish plasticization, resulting in poor adhesion to calendering plates and printed material surfaces, leading to inferior glossing effects.
  2. Pressure: The role of pressure is to compress the varnish layer to form a smooth surface layer. Excessive pressure may reduce the extensibility, plasticity, and toughness of printed materials, making peeling difficult.
  3. Speed: Calendering speed refers to the curing time of the varnish during calendering. A shorter curing time (faster speed) results in inadequate interaction between varnish molecules and ink layers, leading to poor surface smoothness and weak adhesion between varnish and ink layers.