Glazing and Calendering Technology in Printing
A. What is glazing?
Glazing is to smear (or spread and print) a layer of transparent paint (glazing oil) on the surface of printed maters. After being leveled, dried and calendered, the surface of printed maters would form a thin, well-distributed and transparent glossy layer. Glazing includes full-glazing, part-glazing, luster glazing, matte (delustering) glazing and special paint glazing etc.
B. Why glazing?
Glazing can beautify and protect printed maters, and can enhance the effect and functional value of printed maters. After being glazed, printed maters would form a smoother surface which enables even refraction of incident light and brighter ink layer. It is a coating finishing with glazing oil, and a protective treatment for the surface of paper or paper printed maters before final procession. The surface of printed maters would be covered with a layer of glossy film after being glazed. Glazing can be applied in:
① book decoration, such as book jacket, cover, interleaf, calendar, monthly calendar, poster, propaganda sample etc. Printed maters would have better luster and brighter color effect after being glazed.
② package-décor paper maters: such as paper bags, envelopes, trademarks etc, able to beautify and protect products.
③ daily necessities and food: such as trademarks of cigarettes, food and lotion, able to achieve damp-and-mildew proof effects.
④ copper foil on hard cover, able to beautify the product with gold-like high brightness. Glazing can also produce better tack force between copper and base.
C. Technical requirement for glazing oil
Perfect glazing oil shall be colorless, tasteless, glossy, fast-dried, chemical-proof and the following:
① film shall be highly transparent and color-fast. Whether there is a layer of transparent film is crucial to good glazing effect of decorative printed maters, meanwhile, figures and text shall not change colors after being dried, nor change color and yellow after being exposed to sunshine or used for a long time.
② The film shall be wearing-resistant to some extent. Some glazed printed maters are required to be resistant to wearing and scraping, for surface of printed maters is bound to suffer from friction through the production line including high-speed box machine, carton box packing machine and book jacketing machine etc.
③ Certain flexibility and elasticity shall be ensured. Glossy film formed on the surface of printed maters by glossy oil shall have good elasticity so as to suit the flexibility of paper or board, and prevent damage, cracking and shedding.
④ Environmental-resistance of film shall be good. Some glazed printed maters are used to produce various kinds of packages. In order to ensure good protective function to the packed products, the glazing film shall have sound environment resistance. E.g. packages of food, cigarette, cosmetics, clothes etc shall be damp-and-mildew proof. Besides, dried film shall have stable chemical performances, and can not change its property by contacting weak acid and base substances in the environment.
⑤ Ensure certain adhesiveness to the surface of printed maters. Influenced by ink layer density of figures and text, surface adherability would be greatly spolied. Therefore, in order to prevent the dried film from cracking and falling off, film shall have strong adhesiveness.
⑥ Ensure good leveling and smooth film surface. Printed stock materials are various, plus influence of printing figures and text, the surface absorbency, smoothness and moisture differ a lot. In order to enable glazing paint to form a sooth film on different product surface, glazing leveling shall be conforming and the shaped film surface shall be smooth.
⑦ Ensure wider application scope of post-press finishing. Printed maters usually need to undergo post-press procession after being glazed, such as embossment processing, hot foil stamping etc. Therefore, post-press application scope of glazing film shall be wide. For example: heat-resistance shall be good. Tacking shall be avoided after hot foil stamping; solvent resistance shall be high. Blistering, crinkling and sticking phenomena shall be avoided when the film is dried.
D. How to choose glazing oil?
Glazing oil should be selected in the principle of being scientific, economic and functional. Scientific means to study whether glazing oil can suit each physicochemical performance during glazing and using of printed maters. For example: cover and jacket of books, magazines and albums of painting require glazing oil with high-transparency, sound scraping & folding resistance; food packages requires nontoxic glazing oil with damp & mildew proof function; packing boxes require glazing oil with highly stable chemical functions without color-change and yellowing under sunshine or lamp light, nor tacking phenomenon due to stacking. Economic refers to matching glazing oil with printed maters to avoid using high-class glazing oil to process medium or low-class printed maters. For example: up-grade printed maters could apply high-cost and quality calendering, UV curing glazing oil or solvent-based glazing oil with acrylic resin as the main solvent. In contrast, mass-produced textbooks can select medium cost ordinary solvent-based glazing oil with proper quality that can satisfy procession requirement. Functional means matching glazing oil with glazing equipment. For example, solvent-based glazing oil only suits normal glazing machine, while UV-curing glazing oil must be applied in machines with UV drying devices. For ethanol soluble or water soluble glazing oil, glazing machine shall be able to meet their drying requirement (the drying passage is generally required to be longer than 6m and temperature is required to be above 65℃). Moreover, labor and material saving and mechanical mass-producing demand shall also be considered in order to cut production cost and maximize output. Glazing oil shall be odorless, nontoxic and tasteless in order to meet the requirement for ensuring safety, sanitation and environment harmless. Selection scope could be wider in production site with good conditions and sound equipment ventilation. In contrast, glazing oil with balmy substances as the solvent must not be used, otherwise, operators’ health and environment would be harmed.
E. Requirement on the quality of glazing oil coated layer
In order to get satisfactory glazing effect, coated layer of glazing oil shall meet the following requirements in coating procession: ① even distributed coated layer, no sand hole, no blister and no un-glazed part. ② proper amount of coating. Coated layer can shape into film after being dried under certain temperature and coating speed. ③ Coated layer is not affected by ink performance, text & figures area, and integrated density. Good leveling and certain adhesiveness with printed mater surface shall be ensured. ④ Coated layer can be pasted onto the calendered surface in calendering belt, and is able to be peeled off without difficulty.
F. Elements influencing glazing quality
Elements that influences glazing quality are mainly: paper performance, temperature, ink and printed matter crystallization etc.
① Influence of paper performance. Glazing quality is affected by paper smoothness. Very sooth paper would have obvious glazing effect, while low smoothness paper or board would not obtain satisfactory glazing result, because glazing oil is almost absorbed by tough surface of paper or board. In order to solve this problem, you could apply casein foundation before glazing, or do glazing twice.
② Influence of temperature: Temperature between 18℃～20℃ would render perfect effect. Glazing oil is easy to solidify in winter, thus lead to uneven glossy film of the surface of glazed product. In order to solve this problem, glazing oil shall be preserved in heat-insulation places. If temperature of glazed oil is too low, i.e. lower than 10℃, you must add some solvent to dilute the original solution of glazing oil if it is un-usable after being kept for a day under 20℃.
③ Influence of printing oil: Printing oil used for product to be glazed shall be solvent-resistant and heat-proof, otherwise, the text and figures of printed maters would turn grey or crinkle. The solution is to pay attention to the following points in selection of ink: A . Select ink being resistant to ethanol, ethanol solvent, acid and alkali. B. select color-fast ink with excellent luster. C. select ink with good adhesiveness to paper.
④ Influence of crystallization. Print crystallization is mainly caused by overtime store of printed maters, too big area of base ink, too much dryer etc. Ink film crystallization on the surface of paper often cause printing failure of glazed oil or faults like “mottled face” and “pock-marked face”.
In order to solve the problem, we usually add 5% milk acid into glazing oil and mix them. The mixed glazing oil could damage the crystallized film on surface of printed matters, and enable the glazing oil to be evenly coated onto the printed matters and form glossy film.
G. What is calendering?
Calendering is operated in coated glazing unit and hot press unit. Printed matters are coated with glazing oil in ordinary glazing machine, and hot pressed by stainless steel belt of the calendering machine after being dried. Then the surface of printed matters would form mirror reflection effect and gain high luster after being cooled and stripped.
H. Elements influencing calendering effect
Elements influencing calendering effect include: Calendering temperature, pressure and machine speed. ① Temperature: Appropriate calendering temperature can increase the thermal motion of paint film molecule and accelerate diffusion speed, which is favorable for the main solvent molecule in paint to re-moistening, gluing and permeating to surface of printed matters. Moreover, proper temperature could raise the plasticity of paint film, and greatly enhance the smoothness of surface under the performance of pressure. Appropriate calendering temperature can enhance the quality of calendering film layer, while too high or low temperature is unfavorable to calendering. When temperature is too high, viscosity of paint layer would be decreased, and moisture content of paper would plunge, which are not good for glazing and stripping. When temperature is too low, paint layer can not be fully plasticized, nor be well pasted onto calendering plate and surface of printed matters. Calendering effect would be poor, and it’s not easy to form film layer with high smoothness. ② Pressure: Pressure functions to condense and cut the thickness of paint layer and make it to form smooth surface layer. Over-pressure would damage the ductibility, plasticity and flexibility of printed matters. Difficulty of stripping would not happen easily. ③ Speed: Pressure speed refers to solidification period of glazing oil in calendering. If the solidification period is short (high speed), glazing oil molecule can not perform fully with ink layer on the surface of printed matters. Smoothness of the dried film layer is poor, and so does the viscosity between glazing oil layer and ink layer.
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