9. DOCUMENTATION

As a function of the quality system, each applicator will document and maintain records for the coating processes and coating related functions performed at their facility. A few examples of the aspects of the coating process which are documented include the following: the quantities and properties of the parts received for coating, incoming and outgoing part inspections, the type of pre-treatment and coating system that are applied, and the coating line and production equipment utilized to apply the coating systems. As the process documentation will be customized by the coating applicator to suit their specific needs and operations, this section is provided for general guidance only and does not include information on all of the process related functions that are documented. For example, records for measuring instruments and other inspection tools are not discussed in this section.

During the coating application process, it is necessary to add new coating material or solvent to replace the materials which have been lost due to consumption or evaporation. As a result, the coating level must be checked on regular frequency. The quantity of the material that has been added, the type of material that has been added, and the lot number of the material that has been added should be recorded. In addition, the date and time of the material addition should also be recorded. Following the material addition, the measurements of coating viscosity and solids content should also be documented on the same form.

Identifying characteristics of the parts to be coated are documented on an application card (e.g. job card, work order, traveler tag) which remains with the parts throughout the entire process (i.e. from the time of part receipt to return shipment to the customer). The part specific characteristics usually listed on the application card include the following:

  • Type of part/part geometry
  • Weight/volume/bulk density of the parts
  • Delivery condition (oily, heat-treated, etc.)
  • Pretreatment
  • Description and thickness of the coating system to be applied

In addition, the parameters that are to be used to apply the coating materials can also be listed on the application card or a separate coating process instruction. Examples of some of the process parameters which may be listed on the card are as follows:

  • Coating viscosity
  • Program number
  • Load weight (or loading requirements)
  • Basket or rack descriptions
  • Curing requirements

Upon receipt at the coating applicator, the parts to be coated must be inspected to verify the description and quantities received are in agreement with (i.e. match) the terms of the work contract. In addition, the condition of the received parts must also be inspected to ensure they are in a suitable condition to be processed. The inspection should include an examination of part surface properties (e.g. excess oil, rust-free, scale) and presence of foreign materials (e.g. mixed parts, debris). If necessary, incoming inspection for specific part attributes such as ability to gauge may also be performed.

Upon return delivery to the customer, the coated parts must be inspected to ensure the requirements of the work contract have been fulfilled. Prior to shipping the coated parts to the customer, a series of tests will be performed to assess the quality of the coating system. Depending upon the specific requirements applicable to the parts, these tests may include assessments of corrosion resistance, ability to gauge, and coefficient of friction as well as several other coating system attributes which may be tested.

Detection of solvent-based zinc flake coatings
Titanates present in the solvent-based zinc flake coatings can be detected by means of a simple drop test.

To determine if a solvent-based zinc flake coating is applied to the coated article, use a pipette to place a drop of sulfuric acid (20–25%) onto the surface of the applied coating system. The acid will attack the zinc and the binder in the basecoat and gas bubbles will be visible within a matter of a few seconds. If a topcoat has been applied over the zinc flake basecoat, carefully remove the topcoat layer with a knife.

A drop of hydrogen peroxide solution (H2O2, 20-25%) is then added to the area where the first drop had been applied. If titanates are present in the coating material, the drops will turn yellowish to orange in color.

Detection of chromium (VI)-free systems
To detect the presence of chromium (VI) within the applied coating system, scratch the coated article with a sharp scribing tool to activate the surface. Apply a drop of test solution onto the surface and thoroughly rub it into the scribed location using a cotton swab. After 2 minutes, a red color will appear if the coating contains chromium (VI).

Test solution: 6 ml of deionized water, 5 ml of acetone, 5 ml of ethanol, 4 ml of H3PO4 (86%), and 0.1 g of diphenylcarbazide.

Checking the oven temperature v. time profile
Commercially available test equipment is available for measuring the oven temperature versus time profile (see Appendix C "Recommended Measuring Instruments and Test Equipment" on page 52).

To measure the oven temperature versus time profile, place the data logger (i.e. recording device) and probes into the oven together with a load of coated articles. Upon removal from the oven, the measurements recorded by the data logger are downloaded to a computer and the temperature v. time diagrams are created by a software program. To ensure the data logger is not damaged during the recording process, it is placed in a well-insulated thermal barrier which protects the test equipment from exposure to heat.

This type of measurement is suitable for batch ovens or ovens with conveyor belts that do not have drops greater than 0.5 m. If the conveyor belt has a large drop which could damage the recording device during the fall, temperature probes connected to the recording device by long cables may be used as an alternative. In all instances, the temperature measuring probe should be inserted into a representative part. For this purpose, Dörken utilizes a M10 screw which has a small hole drilled into the head of the bolt where the temperature probe is inserted. This ensures that the actual temperature of the metal is measured and not the temperature of the air within the oven. In addition, it is recommended to measure the temperature at multiple locations within the operating zone of the oven when it is fully loaded with coated parts.

Check for proper curing
After the curing process, there are several test methods which may be used to determine if the coating system has been properly cured. These test methods are described below:

One method to inspect the curing properties of the coated parts is to subject them to salt spray testing to determine if the requirements of the customer have been fulfilled. This is the most effective method for assessing the curing properties of the coating system but also the most time-consuming.

Another method which may be used to determine if the coating systems have been properly cured is the MEK test. The MEK test is fast and simple to perform. To conduct the test, moisten a piece of cloth with MEK (methyl ethyl ketone) and press the moistened cloth onto a flat surface of the coated part with your thumb. Rub the cloth back and forth over a short distance with a force of 1 kg and count the number of double rubs needed to remove the coating system. If the coating is adherent to the substrate, it has been properly cured. The one disadvantage of this method is that it cannot be used to assess the curing properties of zinc flake basecoats as these materials do not react with MEK.

Another test which is very easy to perform and is suitable for determining the curing properties of all coating materials is the adhesive tape test. To conduct this test, firmly press a piece of adhesive tape onto the sample to be tested using your fingers or an eraser and then remove it rapidly at an angle perpendicular to the substrate. The percentage of coating material removed by the tape is then assessed. If there is only a slight amount of coating that has been removed by the tape, the coating system has been properly cured. For further details please refer to section "7.2.7. Adhesion".

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