Oily glazing and surface finishing of prints

The light and dark effects (ghosting) often occur in formal printing, which only occurs when the substrate is printed on both sides with an oxidatively dried ink and polish. The front printed image leaves a print on the reverse side of the printed surface.

The following conditions have a lot to do with the generation of bright and dark effects:

1. The back-printed ink is in contact with the part of the frontal printing surface that has been printed and unprinted in the stack (for example, full-face printing on the reverse side and text on the front side).
2. The deeper the full version of ink, the more obvious this effect is.
3. The coating composition and absorption ability of the printing materials play a role.
4. The time interval between the front printing and the back printing is too short, so that the front printing has not yet dried out, which may cause a dark effect on the back printing surface (the more fresh the ink film or the glazing film, the greater its absorption capacity).
5, if the time interval is larger, will have a bright effect, dry ink film as the absorption capacity of the light film is almost always less than the printing material coating.
6. This kind of effect often occurs when a dry ink on the back is printed or glazing again the second time through the machine. This danger also exists in the back monochrome printing. Dark colors are particularly sensitive in this regard.
7. After the phase separation described in the previous paragraph occurs, the ink and polish are oxidized and dried.
8. The surface of the printing material before the backside printing has been partially affected by the liquid and gaseous lysate adsorbed by the front printing ink. After the back printing, these parts may be reflected through the change of the printing image infiltration.

If the ink and polish are printed on the back-printed dried ink during the second printing, the conventional phase separation and oxidative dry mixing are performed. Since the absorbency of the substrate has been inhibited by the printing ink, neither the liquid nor the volatile components can penetrate. As a result, the surface will build up, and in the stack will be in direct contact with the frontal print. If the situation shown in mode 2 occurs, these components cannot be volatilized at all in the direct contact with the front printing. Absorption of liquid components and oxidatively-dried lysates can occur at unprinted sites on the front side. In this way, the back-printed surface is affected differently in the drying process, resulting in a difference in brightness. As to whether the difference in the surface structure of the backside printing causes a bright/dark effect, it depends on the time interval between the front printing and the back printing. In most cases, this interval can be controlled when the front has dried and the light effect has appeared.
How to prevent dark effects:

From the listed preconditions for the formation of light and dark effects, some methods of prevention can be summarized. Before a batch of live printing, you should check the following points:

• Can the ink and polish be wet on the reverse side - wet printing?
· Underneath can be used in the case of a large stack of ink on the back to improve penetration.
• To avoid rapid phase separation, the ink used should not penetrate quickly.
• No desiccant should be added to the printing ink and polish. If the desiccant is too soluble, a large amount of oxidatively drying cracking products will form too quickly.
· If possible, the entire surface of the back of the light printing, before the front printing, so that the front of the ink in the paper stack is in contact with a full-surface glossy, to avoid the formation of light and dark effects.
• Is the paper stack ventilable between each print?
If the bright and dark effect has already formed, it can only be refinished with a water-based polish or a laminating film. In some cases, this effect can be remedied with matt glazing.

Difficulties when applying light on embossed substrates

Embossed substrates are particularly troublesome when they are applied to oily polishes. The reason is basically the kind of mechanism that leads to a bright and dark effect. Embossed substrates are often processed into book covers. The pressure of embossing on the printing material will cause the printing material of the pressed part to be compressed, reducing its absorbing ability.

In the two printing processes of the two inks, the liquid produced in the phase separation of the second color ink is the concave portion where the base color has been printed, and the absorption capacity is weakened, so that it cannot be infiltrated, and therefore it is formed in the concave line. Liquid accumulation.

If the coating agent is applied during the third printing process in order to improve the abrasion resistance of the printed matter, the liquid phase separated in the second printing process and the resulting cracked product will be sealed by the oily glazing film in the recessed part. in.

After the curing agent is cured, it is not in direct contact with the printing ink in the recessed area (see the figure above, indicating light diffusion), resulting in a white/gray effect on the embossed area, making the printed product unusable.
If such a print is heated, the thermoplastic polish will "float" on the ink, eliminating this interference.

Hot stamping fastness

The hot-stamping property of the oil-based polish is limited by the structure of the binder. However, even a special oil-based polish does not achieve the fastness of the water-based polish. Especially for cellophane MS polish, the hot-stamping fastness is insufficient.

For prints with high hot-stamping fastness, it is important to understand the hot stamping conditions and the hot stamping foil before glazing with an oil-based polish.

Hot stamping conditions refer to:

· Stamping temperature (°C);
· Stamping pressure (K Pa) (1K.Pa = 0.01 kg.cm-2).
· Stamping time (seconds).

Glazing surface bonding

The application of hot-melt adhesives to the surface of the oil-based polish is problematic, and only special adhesives may be used. The lack of adhesion and long bonding time are the root causes of the problem. Therefore, it is advisable to leave the paste empty. Although the use of a dispersion adhesive can greatly reduce the adhesion problem, it generally requires that the bonding site be left unglazed. Folding carton processing cycle is generally very short, if the dispersion of the coating agent is coated on the surface of the printer and has been coated, due to poor absorption capacity will make the bonding time is too long, the folding cartons will be re-adhesion Collapsed.

Polishing dose control

The amount of coating agent is a universal problem. In practice, there are only two control methods:

1, visual judgment directly after the delivery.
2, brightness measurement.
Both of these methods have drawbacks because the gloss of the coating film drops rapidly after coating, whether it is wet-wet printing or wet-dry printing. This phenomenon is called the "backlash" effect.
At 0.25, 1, 2, 3, 5, and 20 hours after the coating was applied, the brightness was measured at the same site and one would find that the brightness decreased by 10% (relative) within 2-3 hours. This time is also the time it takes for the polish to transition from liquid to solid. The decrease in brightness indicates that the coating surface has changed during the drying process.

Source: Changshu Changyi Instrument Co., Ltd.