Coating common problems and solutions
1. Uneven coating amount
Reasons: poor uniformity of base paper and uneven banner; too low liquid level in the trough on the coating head (Kun type), resulting in excessive paint pressure in the local wedge area of the scraper; improper adjustment of the gap between the feeding material and the back or scraper wear, etc..
Measures: improve the uniformity of the base paper; increase the liquid level of the trough on the coating head, appropriately reduce the viscosity of the paint, clean the paint nozzle; observe the wear of the scraper, and replace the new scraper in time.
2. Scraper streaks
Reason: The pigment has too large particle size particles or dry paint particles (recommended instrument: scraper Fineness Meter), fibers fall off the surface of the base paper, etc., so that debris remains on the blade of the scraper; the solid content of the paint is too high, and it does not move under the scraper. The paint particles form streaks; the water retention of the paint is poor, the coating is overly dehydrated; the quality of the scraper is not good, etc.
Measures: check the particle size distribution of the pigment; improve the surface strength of the base paper to reduce the fiber falling into the paint; clean the scraper in time; reduce the solid content of the paint and improve the water retention of the paint. In addition, when streaks appear, the scraper at the streak can be pressed locally to remove the particles under the scraper with the running of the paper.
3. Coated orange peel spots
Reasons: The uniformity of the base paper is poor, resulting in uneven absorption of the coating; the scraper angle is too low or inappropriate; the solid content of the paint is too high or the fluidity and water retention of the paint are poor; the drying speed is too high.
Measures: improve the uniformity of the base paper; adjust the angle of the scraper, usually greater than 20 degrees; replace the appropriate hard scraper; reduce the solid content of the paint, reduce the viscosity of the paint (related instruments: Viscosity Cup), or increase the coating amount.
4. Pit points or strip pits on the coating surface
Reason: The design of the paint process is unreasonable. There are situations where the paint receives high turbulence, impact or high-speed shearing, resulting in more bubbles. After the scraper, lines or pits lacking paint are formed on the coating.
Measures: By transforming the paint process, slow down the impact of the paint during transportation and reflow; avoid the use of high shear stirring and pumps; select the appropriate type and dosage of defoamer.
5. Coating spots
Reasons: The back is damaged or there are hard sundries such as dry paint blocks, tape or shredded paper; the base paper has holes, and the paint is bonded to the back after coating; the scraper has foreign bodies or damage, etc. Due to the above reasons, the local pressure of the scraper is too large or too small, and spots of uneven feeding appear.
Measures: replace the scraper in time; clean the scraper; reduce the frequency of holes in the base paper.
6. No peel strength after coating and laminating
Reasons and exclusions:
Check whether the film corona treatment of the layered surface meets the requirements to determine whether the ink reaches the peel strength. Generally speaking, printing inks use composite inks, but some composite inks cannot reach the peel strength. At present, many ink manufacturers have specially produced special inks for printing BOPP coated woven fabrics to solve this problem, which will increase the peel strength and firmness.
Delamination is separated from the coated fabric and the woven fabric. The corona treatment of the woven fabric and the selection ratio of the coated fabric should be checked to find out the reasons and eliminate them in time.
7. Wrinkling phenomenon occurs when coating and laminating
Reasons and exclusions:
When the position of the substrate is skewed, it will lead to wrinkles during coating and compounding, and with the accumulation of deflection, the degree of wrinkles generated will increase relatively. It is necessary to adjust the position of each substrate on the line, and it should not be skewed to avoid wrinkling.
Severe moisture on the printing substrate can also lead to wrinkling and poor peel strength. The printing substrate should be properly kept to prevent moisture.
Improper tension setting and control will make the tension of each part uncoordinated and unmatched, resulting in wrinkling. The tension setting and control of each part should be consistent and synchronized. Make unwinding and winding adapt and match each other.
