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The paint film undergoes creep under pressure and temperature, and the surface is replicated by the opposite back paint/rough surface, resulting in a sinking of gloss and the formation of marks; It can restore gloss indentation, permanent indentation, and is most easily produced in high gloss and dark color systems.

Differentiation: Roller printing is directly pressed out when passing through the roller; The indentation on the roll is gradually visible during storage and transportation after it is rolled up.

1、 Formula system

1. The Tg of the paint film is low and the cross-linking density is insufficient. After curing, the Tg is close to/lower than the storage temperature. It is easy to soften in high temperature storage in summer, and the molecular chains slip under pressure, resulting in a dull luster; Insufficient cross-linking, low MEK abrasion, poor creep resistance, and easy collapse.

Phenomenon: There are fog spots and uneven gloss after uncoiling, and the gloss recovers after hot air heating=reversible pressure spots; Even if heated, it cannot be eliminated=permanent deformation

1. The residual cosolvents/film-forming agents are too high and the baking is not sufficient. The solvents are locked inside the paint film, which is equivalent to internal plasticization, greatly reducing the actual Tg of the paint film, and causing a sharp drop in adhesion and pressure resistance marks (directly related to the cosolvents you asked earlier)

2. The gloss/hardness of the topcoat and back coat do not match the high gloss of the topcoat, and the back coat is rough/matte; The rough texture of the back paint is directly copied onto the smooth surface paint, which is a very classic spot pressing mechanism in color coating

3. Issues with the combination of additives

Incorrect selection of wax powder: low melting point wax, precipitation type wax, easy to stick and imprint under high temperature pressure;

Poor dispersion and settling of fillers: uneven modulus of paint film, making it easier to press out marks locally;

Excessive lubricant can cause the surface to become sticky and the interlayer adhesion to be too tight.

1. There are too many flexible polyesters and high hydroxyl low cross-linking systems for resin selection, which have good T-bending but poor pressure resistance; High Tg rigid polyester has good pressure resistance, but it is prone to brittleness and poor T-bending, which is a difficult point to balance in the formulation.

2、 Baking and curing process

1. Insufficient PMT peak temperature, insufficient baking time, surface dryness, incomplete internal cross-linking, high solvent residue, and false drying;

2. Insufficient cooling, the winding temperature is too high, and the paint film is still in a soft state above Tg, so it is directly wound, with continuous interlayer compression, almost inevitably resulting in indentation; Industry standard: recommended winding temperature<40 ℃, stricter requirements for high gloss system

3. The temperature difference in the oven is large, the plate temperature is uneven horizontally, and the degree of curing on the left and right sides of the same roll is not consistent, resulting in high residual solvent in some areas and local indentation.

3、 Roll up, tension, equipment factors

1. The winding tension is too high, the taper setting is unreasonable, the inner layer pressure is extremely high, and the indentation at the core position is the heaviest; The tension does not decrease in taper, with internal tightness and external looseness;

2. The pressure of the pressure roller is too high, the pressure roller is uneven, and the eccentric contact roller pressure is too high, which directly presses the roller print on the soft paint film after leaving the oven; Wear and scaling of the roller surface, periodic imprinting;

3. Fluctuations in plate thickness, unevenness of the substrate, localized stress concentration due to bending of the plate shape, and point indentation;

4. There are impurities, dust, and zinc particles in the interlayer, which are easily accompanied by scratches.

4、 Storage and transportation environment

1. When the stacking temperature is high and the ambient temperature exceeds the Tg of the paint film, even under normal tension, long-term compression will gradually produce pressure marks;

2. Flat stacking of rolls is easier to crush than vertical storage;

3. In high humidity environments, the system absorbs water and plasticizes, causing the paint film to soften and intensify the pressure.

Quick and easy judgment method

1. Hot air gun heating indentation position: Gloss recovery → reversible pressure spot; Heating remains unchanged → permanent plastic indentation

2. Look at the location: concentrated near the core → tension issue; Uniform fog spots on the entire roll → Tg/residual solvent; Consistent with the circumference of the roller → roller surface issue

3. Comparison: Same formula, back paint replaced with high gloss and same hardness back paint, reduced indentation → Copy type indentation

Brief improvement direction

1. Formula: Improve Tg and crosslink density after curing; Select high melting point polyethylene wax; Control the slow volatile components of co solvents; The hardness and gloss of the topcoat and back coat should be matched as much as possible

2. Process: Ensure PMT meets standards and fully crosslinks; Strengthen cooling and strictly control the winding temperature; Reduce winding tension and enable reasonable taper

3. Storage: Try to store vertically and avoid high-temperature stacking