Paper Drying Cylinder
In a paper dryer section, multiple drying cylinders heat the moving paper sheet to evaporate water. Steam heats the drying cylinder shell from the inside. The cylinder surfaces are kept clean using oscillating doctors.
The surface of the drying cylinder can deteriorate due to wear and corrosion, leaving marks and other defects on the paper sheet. The paper can also stick to the cylinder surface, causing the paper to break and wrap around the cylinder.
A traditional approach to providing the necessary resistance to wear and corrosion, as well as release (anti-sticking) properties for the drying cylinder surface, was an application of a hard chrome coating with up to 20% PTFE (Teflon-type). However, doctoring results in the rather quick loss of the Teflon-type treatment and wear of the chrome coating.
HVOF spraying of the WC-10Co4Cr inner coating layer and air spraying of the Teflon-type layer is a popular alternative. During operation of the dryer section, the coating is polished by the paper sheet to the necessary roughness due to the removal of the top Teflon-type layer down to the peaks of the WC-coating.
The coating performs even after complete removal of the Teflon layer as the inner WC-layer becomes smoothly polished.
A major problem is non-uniform wear of the coating due to doctoring and the known HVOF technology restrictions for processing high-quality WC-coatings. The application of HVOF WC-coatings is the most time consuming, resulting in a high cost of the coating.
Teflon-type materials have quite low adhesion to WC-based coatings, even to porous ones, resulting in coating failures.
A Kermetico HVAF WC-CoCr – FEP Coating
The Kermetico HVAF process to apply a WC-layer and a Fluorinated Ethylene-Propylene polymer layer for release properties may substantially prolong the life of a drying cylinder surface.
While keeping a similar technological approach for on-site coating applications, the WC-10Co-4Cr coating layer being sprayed with the Kermetico HVAF technology results in the highest quality coating:
- For processing a dense inner layer, the spray particle size is reduced to 5-30 microns while the particle velocity is increased 1.5 times versus regular HVOF spraying, thus depositing a practically non-porous coating with high hardness (1,450-1,600+ HV300) and high ductility.
- The outer layer is sprayed with a particle size of 15-53 microns, using a long chamber in the HVAF gun.
- The Kermetico HVAF process produces WC-based coatings with the highest fracture toughness, exceeding known HVOF coatings by 10-fold. Since the primary mechanism of wear for WC-based coatings is attributed to the material’s brittleness, the Kermetico HVAF coating exhibits the highest wear resistance. This gives our HVAF coating the longest service life.
Kermetico HVAF technology provides a much higher spray rate:
- With Kermetico HVAF equipment spray rates are increased 4-5 times (up to 33 kg (73 lbs.)/hour)
- Grit blasting for surface preparation is performed with the same Kermetico HVAF gun, increasing blast rate by a factor of 10 while decreasing grit consumption to 1/100 of that used by vacuum grit blasters.
Kermetico HVAF equipment is portable, providing an on-site coating option.
Fluorinated Ethylene-Propylene (FEP) possesses about the same anti-sticking properties and corrosion resistance as Teflon at a temperature up to 200°C (392°F). However, contrary to Teflon-type materials, the FEP coating has good adhesion to the WC-CoCr coating. Its baking procedure is shorter and requires a lower heating temperature (120°C – 248°F), ensuring better control of the properties and a further reduction of application time.
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