A comprehensive analysis of partial matte printing technology for packaging bags

Currently, based on market demand, many food, cosmetics, and even pharmaceutical companies prefer to achieve a partial matte effect on their outer packaging to enhance product grade and improve anti-counterfeiting functionality. Generally, partial matte is achieved through reverse printing matte varnish. It can be said that reverse printing matte varnish technology achieves a similar effect to aluminum washing through reverse printing matte ink.

Matte ink also has another function: it can replace the original matte film, more flexibly meeting customers' personalized needs.

This technology is already a very mature process technology in some flexible packaging production companies. Below, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will analyze what partial matte printing technology is.

I. Precautions for Reverse Printing Production

1、 Raw Materials

The printing surface of the plastic film needs to be corona treated, and the corona value must be at least 38 dynes or more to allow the ink to have sufficient adhesion to the film. Therefore, the film selected for matte varnish reverse printing must have a sufficient corona value. Therefore, in the selection of film raw materials, PET and BOPA are generally preferred.

Normally, BOPP material is not suitable for reacting with matte varnish. Conversely, the corona value of the non-corona treated side of PET and BOPA can reach 38 dynes or more, so it can meet the requirements. However, during use, double corona treatment is still recommended for better assurance, ensuring that the adhesion of the matte varnish printing meets the quality requirements of scratch and wear resistance.

2、 Plate Making

When designing printing plates, the pattern design of the matte varnish plate is reversed because it is printed on the film substrate, so its layout design is reversed, that is, the intaglio plate is in reverse order. Normally, surface-printed matte varnish is a solid printing method, so the depth of engraving of the copper plate will have certain requirements.

The depth of engraving of the plate is determined according to the parameters provided by the matte varnish supplier used by each company. Conventionally, this type of reverse printing matte varnish does not use screen printing, light screen printing, or transition screen printing, because these processes themselves will weaken the main prominent part of the local gloss.

3、 Ink

The matte varnish itself needs to achieve a matte effect. First, the degree of matte will have different requirements depending on different needs. On the other hand, since it is printed on the film substrate, the reverse-printed matte varnish is directly exposed to the packaging surface. During packaging, transportation, and sales, it will directly contact the outside world, so its scratch resistance must exceed that of ordinary intaglio inks. The risk of being scratched must be evaluated to determine whether it meets the high hygiene risk performance requirements of food or non-food products.

This aspect tests the technical strength of the matte varnish supplier and the equipment and process of the flexible packaging production enterprise.

4、 Printing Equipment

Generally, matte varnish is not used alone on the outer packaging, but is used with the inner layer to highlight the unique effect of reverse printing matte varnish.

Therefore, the gravure printing machine needs a reverse printing reversing device to cooperate with the overprinting requirements. In the past, some companies have tried to achieve the surface printing effect through two intaglio printing processes, but it has been proven that it is difficult to achieve the original intention in terms of quality requirements and registration accuracy.

II. Problems That Easily Occur During Printing

1、 Reverse-printed matte varnish is scratched.

This type of abnormality mainly occurs on the equipment, especially on the reversing frame and guide rollers. This is caused by uncleanness and foreign objects on the reversing frame and guide rollers.

2、 Printing materials are easily wrinkled.

Normally, because the material needs to pass through the reversing frame, it is a process from intaglio printing to surface printing. The smoothness and material passing smoothness of the reversing frame are very critical. If you are not careful, it is easy to cause wrinkles in the semi-finished substrate.

3、 Reverse-printed matte varnish clogs the plate.

Because matte varnish requires scratch resistance, its viscosity is relatively high. On the other hand, the particles of matte varnish are much larger than those of ordinary intaglio inks, so it is very easy to clog the plate and fail to apply matte varnish. Therefore, during the printing machine adjustment process, when starting the machine, pay close attention to whether the plate is clean and ensure that it is not clogged.

4、 Solvent residue.

Generally, the matte varnish plate is solid, and the area is generally relatively large, and the amount of ink is relatively large. On the other hand, due to the combination of intaglio and surface printing, according to the order, matte varnish is the last color sequence, so the time spent in the oven is very short, and after winding, the intaglio and surface printing will become a bonding surface of ink between films, which is easy to allow solvents to penetrate each other, hidden in the matte varnish or intaglio ink layer, difficult to volatilize, thus becoming more difficult to eliminate residual solvent, resulting in the phenomenon of exceeding the standard of solvent residue. According to this characteristic, the machine speed should be reduced and the ventilation effect should be improved during the lamination process to reduce the total amount of solvent residue.

5、 Matte varnish stickiness

Similar to the comprehensive factors in point 4, when the matte varnish drying capacity is insufficient, it is easy to cause the matte varnish itself to stick, and at the same time, it will cause the matte varnish and intaglio ink to stick to each other. Therefore, the drying capacity of the oven on the printing equipment is very critical.

6、 Poor matte effect of matte varnish

During printing, if the viscosity of the matte varnish is not well controlled, the matte effect of the matte varnish will be poor; at the same time, if the matte effect of the matte varnish itself is not ideal, this problem will also occur.

7、 Scratch resistance of matte varnish

Currently, matte varnishes are divided into ordinary matte varnishes, high-concentration matte varnishes, and high-temperature resistant matte varnishes. Enterprises should choose different grades according to their needs. Currently, some domestic suppliers are doing well. The adhesion of the matte varnish determines the scratch resistance effect.

III. Lamination Precautions

1、 Stickiness appears in matte varnish during the composite process

The main reasons should be investigated from the aspects of matte varnish wear resistance, matte varnish drying degree, composite oven temperature, and hot press roller temperature.

2、 Solvent residue exceeds the standard after lamination

This problem is similar to point 4 of the printing problems, but there are also differences. In the lamination process, this kind of problem will be more prominent, therefore, the oven temperature, lamination speed, and exhaust of the machine must be carefully controlled.

3、 Composite peel strength

The peel strength of the substrate with reverse-printed matte varnish is lower than that of other structures without reverse-printed matte varnish. Based on current experience analysis, this is mainly due to a smooth exudate in the matte varnish, which is more obvious especially when the substrate is PET material. This aspect needs to be changed through the function and amount of adhesive, but fundamentally, it needs to be improved from the matte varnish itself to cure the root cause.

IV. Common problems in bag making

1、 Matte varnish on the edges turns white

This is mainly due to two major reasons: firstly, whether the matte varnish resistance meets the requirements, and secondly, whether the heat sealing knife on the bag making machine is at too high temperature and too much pressure.

2、 Delamination

When reverse-printed matte varnish is applied to zipper bags, delamination is prone to occur at the zipper head. This is related to the composite strength, which in itself is related to the reverse-printed matte varnish.

We once conducted a group of experiments using the same structure and materials. When comparing, except for the matte varnish process changes, other adhesives, printing, bag making machine temperature, and pressure were consistent. It was found that bags without matte varnish did not show delamination at the zipper position, while those with reverse-printed matte varnish did. Based on this experiment, matte varnish has a certain impact on composite strength and the temperature resistance of bag making.

Therefore, for such zipper bag products with reverse-printed matte varnish, the production process control issues should be fully reviewed before production to avoid unnecessary abnormalities.

3、 Matte varnish is scratched

Reverse-printed matte varnish is directly printed on the film substrate. If the various parts of the machine are not cleaned effectively and promptly, the matte varnish will be easily scratched. Therefore, the cleaning work of the machine must be thoroughly done in the finished product process.

Conclusion:

Partial matte finish can greatly enhance the product grade and is being increasingly used in flexible packaging. To master the rotogravure reverse printing matte varnish technology, pre-press design (layout design), raw material selection, and various stages of production all need to be fully reviewed to reduce the risk level to the lowest controllable range.