All-round comparison of plastic hose offset printing and screen printing process

In the past 10 years, in the cosmetics, medicine, food and other industries, plastic hose packaging has been favored by consumers because of its light weight, convenient use, and high quality and low price.

As the name suggests, the raw material of plastic hose is plastic, more specifically, mainly polyethylene material. According to the structure, the packaging hoses can be divided into single-layered tubes, multi-layered tubes and composite tubes of various raw materials. As far as packaging is concerned, the part that attracts customers the most is a printed pattern. Therefore, the printing process of plastic hoses is mainly introduced here.

Hose printing process

The most common current hose printing processes are: dust removal - surface pretreatment - printing (UV ink) - coating - varnish UV curing (or thermal curing). A special hose press can basically realize the above process, such as hose presses produced by companies such as POLYTYPE in Switzerland and HINTERKOF in Germany.

The first step is dust

One of the common problems with plastic materials is that they are prone to static electricity. During the production of hoses, the polyethylene raw materials are constantly in contact with the metal molds, causing static electricity on the surface of the hoses. Static electricity makes the surface of the hoses easily absorb dust. Therefore, before printing It needs to be dust-removed first.

Step 2 Surface Pretreatment Because polyethylene is a non-polar material, surface pretreatment is required before printing. There are two main methods of treatment, one is corona treatment, and the other is flame treatment. Both of these methods can achieve the purpose of attaching ink and varnish firmly.

The main factors influencing the corona treatment effect are: discharge power, the distance between the electrode and the treated surface, and the speed of the hose passing through the corona processor.

The main variables affecting the flame treatment effect are: the air/fuel gas ratio, the distance of the treated surface from the visible blue top of the flame, and the speed of the hose through the flame.

Usually the surface tension of the hose is required to reach 3.6×10-2N/m or more, and the specific value is also related to the characteristics of the ink and varnish used.

A simple way to verify the surface treatment effect is to put the treated hose into water and lift it from the water to see if there is a uniform film of water on the surface of the hose. If the water film is even, it means that the hose surface is well treated. Otherwise, the factors that affect the effect of the surface treatment should be adjusted appropriately.

The third step of printing At present, there are two common printing methods for plastic hoses. One is offset printing. The ink is transferred to the printing plate by a set of ink rollers, and the printing plate transfers the ink on the printing pattern to the blanket. Then, the complete printed pattern is once transferred onto the substrate by a blanket.

Hose offset presses are different from what we commonly call offset presses. The so-called offset printing plate is a lithographic printing plate. The printing ink is supplied to the printing plate by the principle of immiscible ink, and then transferred from the printing plate to the blanket. Although waterless offset printing has been developed, the printing plate is still lithographic or gravure . The offset printing plate used for the printing of the hose is a relief printing plate, so the more accurate name is the letterpress printing.

Hose offset printing plates are usually made of light-curing resin plates, which are produced through processes such as main exposure, development, rinsing, drying, and post exposure. The production time of a printing plate is about 30-40 minutes, and the resistance force is more than 100,000 pieces. Since this type of printing plate is a resin plate, and the raised portion of the graphic and text portion is relatively high (approximately 0.5 to 0.6 mm), the high-light portion can only retain more than 5% of the dots when making a half-tone version, so the printing on the hose is required. The pattern is not as fine as a flat offset, and it is for this reason that there has been a phenomenon of labeling the hose package instead of directly printing a logo or the like on the hose.

At present, the speed of hose offset printing has reached 12,000 pieces/hour, and the operation process has become more and more simple. With the increase of production speed and the decrease of offset printing costs, offset printing has been widely used for printing hoses.

Another way of hose printing is traditional screen printing. Domestic multicolor screen printers are usually made up of several monochrome screen printers connected in series, with a large area, a long hose conveying path, poor positioning accuracy between color and color, and poor printing speed. Also lower, making its cost high. At present, in Europe, screen printing machines for hose printing have been developed and produced, such as ISIMAT in Germany, HINTERKOF, and MOSS in Italy. The printing accuracy and printing speed have been greatly improved. Production speed Up to 5400 pieces/hour, overprint accuracy of ±0.2mm, printing color up to 6 colors. Due to the thicker layer of screen printing ink, it is currently mainly used for the printing of color hoses or partially uncovered varnish products.

In the hose printing mode, it is also possible to combine the above two printing modes. A fully automatic hose production line, which realizes the combined production of offset printing and screen printing, makes the number of printing colors of the hose reach 12 colors.

With the popularization and improvement of UV curing technology, UV curing inks are commonly used for both hose offset printing and screen printing, making ink control easier.

The fourth step of glazing is equipped with a set of glazing devices on the hose offset press and screen printer, consisting of two steel rollers and one rubber roller. The fastness of varnish is also directly related to the size of the tension on the plastic surface after pretreatment. Varnish is usually a UV curable varnish, with various types such as bright and matt oils. In addition, there are also heat-cured varnish and three-in-one varnish for coating after hose offset printing.

Step 5 Varnish Curing For UV curable varnish, the degree of varnish curing is related to the wavelength and UV intensity emitted by the UV lamp. For products with subsequent processing such as stamping, the degree of curing of the varnish will have a great influence on the subsequent processing quality.

common problem

In the printing process of the hose, several problems that easily arise and solutions are as follows.

1. Insufficient adhesion of varnish and ink This problem is mainly related to the surface preparation of the printing material. The surface tension can be increased to 3.6 x 10-2 N/m by increasing the corona-treated power and the flame-treated air/combustion gas ratio. However, the surface tension is not as large as possible. Some inks also have poor adhesion when the surface tension reaches 4.0×10 -2 N/m. Therefore, the best surface tension value should be found for different inks.

2. The phenomenon of ink and odor in the graphic text This phenomenon is usually caused by excessive printing pressure. There are two pressures on the hose press that affect the final product's print quality. One is the pressure of the ink roller on the plate and the other is the pressure of the plate on the blanket. If the pressure is too high, the ink in the middle of the picture and text will be squeezed to the edge of the picture and the ink will appear on the edges of the printed pattern and the dots will become larger. Therefore, it must be ensured that the printing pressure is moderate.