5. COATING

Dip-spinning is a bulk coating process used for coating large quantities of parts. The basket or drum, with a diameter of 400–900 mm, is the primary tool. Parts are loaded into the basket, with the fill level depending on the bulk density of the parts. Depending on the dip-spin parameters and coating settings, an average coating thickness of 3–6 μm per layer can be achieved.

For parts with complex geometries (e.g., internal threads, small components, undercuts), it is advisable to rotate the basket to ensure full surface coverage. The same applies for centrifuging excess coating material after coverage. A tilt angle of ≥ 75° at 5–20 rpm allows better drainage of excess coating. This can be combined with a pre-spin before tilting.

Next, the basket is immersed into the coating bath. Slow rotation of the basket improves coverage by removing bubbles and adjusting contact surfaces.

After dipping, the basket is lifted and rapidly spun or centrifuged so that coated parts move to the outer wall. Centrifugal force causes excess coating to drain back into the bath. Parts are then dried in the oven. For better curing and reduced sticking, parts should be distributed as far apart as possible on the conveyor or tray.

The dip-spin process is influenced by parameters such as immersion time, spin speed, spin duration, basket load, and coating viscosity. Additional factors include tilting or changing rotation direction to improve quality.

Recommended parameters: The basket must be immersed for at least 30 seconds. Low rotation (10–20 rpm) with oscillation improves coverage and removes trapped bubbles.

An important parameter is centrifugal force (F), dependent on rotational speed and basket diameter. To ensure comparable results with different basket sizes, the centrifugal force should be equivalent.

Coating thickness depends on spin speed and basket diameter: higher speed and larger diameter result in thinner layers at the same viscosity.

Spin duration is typically 10–20 seconds per direction, often repeated for complex part geometries.

Basket construction and geometry also influence the process. Closed or double-walled baskets require adjusted spinning parameters.

The part quantity and spin acceleration affect the coating flow (see Figure 12). Ideal flow is seen in examples a) and b). Overloaded or slow acceleration leads to uneven coating and possible sticking, as shown in examples c) and d.

Coating weight is influenced by viscosity. Lower viscosity results in less coating adherence. Adjust viscosity according to part geometry and system design. For solvent-based basecoats, a starting viscosity of 40–42 s is recommended.

A sample coating is applied to verify coating weight and confirm requirements. If requirements are not met, adjust spin speed, spin duration, viscosity, and basket load.

As noted in Section 3.3, viscosity is influenced by temperature, solvent evaporation, and solids content. Maintain constant coating temperature for consistent quality. Temperature can be affected by day/night changes, oven radiation, warm parts, and climate variations.

Establish and maintain a defined processing viscosity window (±2 s). The coating system often consists of multiple layers. Parts are transported between basket and oven; minimize handling to prevent coating damage.

Parts should be gently unloaded onto the pre-curing conveyor or tray to prevent damage. Larger parts or greater drop height increase damage. Rubber/plastic surfaces and reduced drop height help mitigate damage. Vibrations from feeders or post-coating drops can also affect quality.

For re-coated parts, temperature must be ≤30 °C but above dew point. Basecoats and topcoats require separate baskets and immersion tanks. Incorrect thinner can damage the coating.

Regular cleaning of coating baskets is mandatory to remove built-up dried coating. Methods include wet-chemical, thermal, mechanical, or laser cleaning.

In the rack dip spinning process, parts to be coated are placed on a rack at one or more small holding points or, depending upon geometry, the parts may be placed in individual compartments. The entire rack with parts is then immersed, spun, and sent through the curing oven. To ensure consistent quality, the racks must be cleaned on a regular maintenance schedule.

The dip draining process is a relatively simple application method. In this process, a part is immersed in the coating material and subsequently withdrawn at a defined speed. This process is suitable for a wide range of parts (e.g., pipes, metal sheets, or more complex components). During the immersion process, the coating material can be applied to both the inside and outside of the parts provided there are sufficient drainage holes.

Alignment of the parts during dipping and withdrawal is essential to ensure coating quality.

• For complete wetting of the parts inside and outside, air inclusions must be avoided.
• During withdrawal, excess material must drain from the part.
• The layer thickness remains high on horizontal surfaces; tilting is required to control coating thickness.

The most important process parameters are the viscosity of the coating material and the withdrawal speed.

Coating thickness is also affected by the shape and temperature of the part, gravity, surface tension, coating material temperature, and shearing properties.

During immersion, parts must be fully wetted and free of air bubbles.

Withdrawal speed is the critical factor for coating weight or thickness. Low speeds (up to 100 mm/min) allow control of coating thickness; depending on coating, temperature, and viscosity, speeds up to ~300 mm/min may be possible.

If the coating on vertical surfaces exceeds the shear limit, local viscosity decreases and the material begins to run. This effect is similar to spray application with excessive layer thickness: the layer weight is too high and shear forces are excessive.

Substrate roughness and surface tension strongly influence wetting. Excess droplets can be removed using an air knife or by blowing off.

The spray application process is used for parts that require a highly uniform coating thickness or are unsuitable for other application methods. The coating is atomized into fine droplets by compressed air or high-speed rotary processes. This ensures low variation in applied thickness. Spraying can be automatic or manual.

Parts are usually mounted on component-specific racks. To optimize coating distribution, the spray nozzle and part move relative to each other. Coating that does not hit the part is overspray. Electrically charging the coating material attracts droplets to the grounded part, reducing overspray. Overspray is captured with vacuum, filters, or water curtains to prevent environmental contamination.

The coating is delivered via hoses using a pressure or pump system, minimizing air and moisture entry. Continuous stirring or circulation prevents solid settling. Filters must be used. When multiple coatings are applied, separate lines or thorough rinsing is required between changes.

Compressed air must always be dry and oil-free.

Spray process parameters include:

• Viscosity
• Coating temperature (see technical data sheet)
• Air pressure
• Material pressure
• Spray nozzle/disc
• Ambient temperature
• Relative humidity
• Distance to part
• Surface tension
• Leakage current

Equipment also affects the process, including:

• Spray gun / high-speed rotary bell
• Manual or automated application (lift, swivel, or robot arm)
• Diaphragm pump or pressure vessel
• Rack shape and size
• Hose inner diameter
• Spray booth dimensions and airflow
• Type of electrostatic charge

All parameters and equipment must be coordinated for optimal results.