With the improvement of people's living standards, the requirements for the outer packaging of goods are getting higher and higher, and the post-press processing technology has increasingly attracted people's attention, such as laminating, hot stamping, and polishing. At present, there is a popular method of finishing - hot melt printing, that is, the use of embossed powder on the printed matter to form a raised effect. This article will briefly introduce the processing process and the problems that should be noticed on the offset printing products.

Powder spraying process

1. Equipment

The hot melt printing machine is mainly composed of two parts: a printing part and a powder spraying part. The printing part completes the printing process of the product; the powder spraying device (the SUNRAISE S-5000 series duster currently used more often) is connected behind the offset printing press, ie, the delivery table of the offset printing press serves as the paper feeding table of the powder jetting machine. This continuous operation ensures that the prints are not completely dry before dusting.

2. Braille powder

Now more used is the United States imported transparent TM14 braille powder. This is a kind of white special baby powder, suitable for a variety of graphic, size size. At temperatures of up to about 1200F (650°C), the Braille powder reacts with the resin-based binder in the offset ink to form a raised, three-dimensional, high-gloss, satin-like film.

3. Powder spraying process

(1) Dusting: The printed matter conveyed from the printing station of the offset printing press is conveyed to the dusting machine, and the vacuum blowing device is turned on. Braille powder flows out from the feeding port. The size of the feeding port can be based on the size and the powder spraying area. Size adjustment. Braille powder passes through a filter before it flows to the print. The filter removes impurities that may be mixed into the braille powder.

(2) Curing of Braille powder: The curing process is generally divided into two times. The first time the temperature is adjusted to 700-900F. At this time, Braille powder has completed the first step of curing, and it is adsorbed to the graphic printing ink. The area has not been completely cured. The resulting printed material has a phenomenon of imaginary flowers; then a second cure is performed. At this time, the dusting port is turned off, and the curing temperature is raised to 1100 to 1200F, so that the bran powder is adsorbed. The printed matter passes through the curing channel again, and the convex powder is fully heated and melted at a high temperature and reacts with the binder in the ink to form a convex effect with good texture and brightness.

(3) Recycling Braille Powder: In the process of powder spraying, because the Braille powder is uniformly discharged, Braille powder is sprayed on the graphic and non-graphic areas of the printed matter, and the blank portion of the Braille powder is only Covered on the surface, no change occurs, so a vacuum suction device is installed on the dusting device to suck all the convex powder distributed on the blank area of ​​the printed matter back into the convex powder container and be reused.

4. Drying

Generally, the drying passage is opened 5 to 10 minutes in advance to preheat, and the temperature reaches 700 to 900 F. Then, the conveyer belt carries the printed product printed with braille powder in the heated drying passage.

5. Cooling

After the print passes through the drying channel, it is cooled, and the cooling part is placed on the air blower so that the high-temperature-cured print can be returned to room temperature as much as possible. At this point you can see a three-dimensional fine print.

The problem of easy spraying and the solution

Paper fire

Cause: The temperature is too high.

Solution: Braille powder curing process is performed twice, and the first temperature can be set slightly lower to ensure that the braille powder can be adsorbed. Because the paper is flammable, the ignition point is very low.

2. Dusting uneven

Cause: The speed of the powder is uneven; the convex powder agglomerates.

Solution: In the horizontal direction of the duster, adjust the feeding port to the same amount of powder. After the convex word powder is used up, it should be sealed and stored to prevent it from agglomerating.

3. Graphic false flowers

Cause: Dust is uneven and the curing temperature is not high enough.

Solution: try to make the next powder evenly; on the premise of not causing paper fire, if the electric heating equipment can properly increase the curing temperature.

4. Graphical strokes are uneven

Reasons: uneven spraying; paper smoothness is not high enough; convex word fineness is not high enough; convex words mixed with impurities; print dried too quickly.

Solution: try to make the powder evenly; use high-smooth paper, general hot-melt printing printing convex products using more than the amount of 240g/m2 paper jam, smoothness of more than 250 seconds; according to the size of the font size can choose fine Degree of braille powder; Braille powder should be kept sealed; no driers in printing, because braille powder is only combined with semi-dry ink layer.

5. Paper warpage

Cause: After the paper is cured at high temperatures, the water content changes.

Solution: For a relatively high-quantity paper jam, do paper treatment before printing to adjust the water content of the paper to the best condition, which is generally lower than the moisture content of the paper that is not subjected to hot-melt processing. The water content is generally low. It is 4% to 8%.

Hot melt printing development and prospects

1. The scope of application

With the development of personalized and high-grade packaging products, hot-melt printing and other printing methods have a wide range of markets, such as business cards, letterheads, stationery, office supplies, and certain special areas of the packaging that need to be emphasized.

2. Development of equipment

Now more powder spraying equipment is used in the market is to add a hot melt printing device after the printing press, take the electric heating, mostly in sheet-fed form. In 2001, the World Hot Melt Printing Committee (WPT) launched a roll-type hot melt printing press. The hot melt printing machine can be connected to any type of web printing press such as offset printing, flexo printing, gravure printing, etc. to achieve on-line operation. The heating method is changed from electric heating in the past to ultraviolet heating, which not only has high efficiency, but also improves product quality, making the product that has been sprayed with Braille powder have a better texture.

3. Development of Braille Powder

There are many kinds of Braille powders produced in the United States. From the perspective of fineness, there are now Braille powders that satisfy different font sizes and different fineness; from the viewpoint of glossiness, there are high-gloss braille powders that form a satin feel, and also matte convex powders; from the perspective of color Consider transparent braille powder and bran powder of various colors. In addition, there are UV Braille powders for use with new hot melt printers.

Industry insiders predict that hot melt printing will develop rapidly with the development of the packaging and printing industry, and will meet and surpass consumer expectations from the printing effect.

Author: Guoling Hua, etc.

Reprinted from: Printing Technology