8. IN-PROCESS MAINTENANCE INTERVALS

In-process maintenance intervals depend on the rate and volume of production, the respective environmental influences, and the type of parts to be coated. Due to these variables, this section should only be regarded as a general guideline that each coating applicator should adapt according to their specific conditions of use.

Table 7: Maintenance intervals
Maintenance measures Interval
Viscosity + temperature Every 2 hours
Solids content Once per working shift
Filters Once per week
Cleaning the coating baskets Define minimum requirements, then define cleaning intervals
Cleaning the coating racks Define minimum requirements, then define cleaning intervals
Cleaning of all application components Define minimum requirements, then define cleaning intervals
Furnace temperature, furnace profile Once per month
Coating level Define minimum requirements, monitor every 2 hours

The following process equipment components are to be inspected at regular intervals, compared with reference samples (where applicable), and cleaned accordingly:

  • Coating immersion tank (interior and sidewalls)
  • Vibration tables and shaker systems
  • Oven conveyor belts
  • Sheet metal trays
  • Spray application components (e.g. hoses, nozzles)
  • Filters, racks, coating baskets, and all components in direct contact with the coating materials

The coating line should be cleaned once per working shift (e.g. every 8 hours) to remove any residual contaminants in the general vicinity of the production equipment (e.g. centrifuge). Removing debris and other potential contaminants near the coating line on a frequent basis will prevent small residues from sticking to parts or entering the fresh coating material.

Moisture

With water-sensitive coating materials (e.g. solvent-based zinc flake basecoats), moisture can cause the coating to gel or thicken. When using a liquid pretreatment such as phosphating or electroplating, water must not be transferred to the coating material from the treated parts. In the case of zinc flake basecoats this must be avoided and the treated parts must be dry before the basecoat is applied.

  1. The coating tank must be clean and dry before filling. Remove dust, grease and water from the coating tank if it is not clean.
  2. The coating tank must be kept closed with a lid whenever possible.
  3. When adding new coating material to the coating tank, pour the entire contents of the delivery container into the coating tank. Do not store partially emptied delivery containers for later use! The coating material in the partially emptied delivery container could absorb moisture from the ambient air.
  4. To prevent condensation from forming in cold delivery containers, bring the delivery container to ambient temperature before filling the coating tank. Do not use a heater for this purpose! When cold delivery containers are opened in a warm atmosphere, water condenses into the container and the cold coating material gels.
  5. A similar effect occurs if the parts are cold during the coating process. When very cold screws, nuts or stamped parts are brought into the coating facility at ambient temperature, condensation can form on the surface of the parts. In order to prevent moisture from getting into the coating tank, the parts should be warmed up to ambient temperature and completely dried prior to applying the coating.
  6. The room in which the coating material is mixed and prepared should have the lowest possible humidity.

Dew Point

If the coating container or the parts to be coated have a temperature that is below the dew point, water will condense and damage the moisture-sensitive coating materials. To maintain coating material integrity, this condition must be avoided. It is therefore recommended that the minimum temperature of the coating material and the parts be Tmin = dew point + 3 K at all critical control points. For this reason, it is absolutely necessary to check the dew point on a regular frequency.

Dust

After mechanical blasting or during transfer of coated parts (e.g. by parts rubbing against each other), dust can be generated which adheres to the surface of the parts. Before the parts are placed into the coating material, this dust must be removed or poor coating adhesion may result. Additionally, if steel or zinc dust particles become embedded in the dry-film coating layer, rust spots may appear before the substrate metal begins to corrode. Steel and zinc dust particles can also dissolve in water-based coating materials and change their physical properties (see also "2. Surface pretreatment" and "3. Preparation of the coating material").

Hydrogen embrittlement

There is an increased risk of hydrogen embrittlement associated with certain pretreatment methods such as pickling and electrolytic degreasing. Hydrogen atoms can diffuse into the steel and, under certain conditions, lead to a decrease in material strength. When applying the Dörken coating systems, the coating application process may be ruled out as a cause of internal hydrogen embrittlement of steel.

Temporary corrosion protection (rust Inhibitors)

When using temporary corrosion protection (i.e. rust inhibitors), chemicals containing amines, borates and silicates must not be used as these compounds when applied in high concentrations will damage the zinc flake basecoats.

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