Industrial Painting Systems 2
Paint is a coating material that adheres to the surface it is applied to, protecting the surface from mechanical and chemical effects while also providing it with an aesthetic appearance.
The process of painting is not merely the application of paint to a surface alone but rather a series of steps with different procedures, collectively forming a comprehensive process known as “painting systems.”
Paints consist of a mixture of components (adhesives, color pigments, and additives) dissolved in a solvent (water or solvent). The solvent is removed from the application surface during drying to obtain the painted surface. The best paint formulations contain higher amounts of adhesives and color pigments.
The type of adhesive used in the paint often determines its name. For example, if cellulose resin is used as the adhesive, the paint is called cellulose paint; if acrylic resin is used, it is called acrylic paint; and if epoxy resin is used, it is called epoxy paint. Adhesives impart crack resistance, adhesion, and durability properties to the paint and are generally transparent.
Pigments are powders with particle sizes ranging from 0.01-1 µm. They are classified as organic, inorganic, and special pigments.
Paint systems can be grouped into two main categories: powder coating applications and liquid paint applications. Liquid paint applications are further divided into solvent-based and water-based coatings.
Application Stages:
Paint systems comprise three fundamental application stages: surface preparation, painting, and drying.
Surface Preparation: The application varies depending on the material. For instance, sandblasting is used for steels, brushing for aluminum, and flame treatment for plastics to prepare the surface. This ensures that the surface to be painted is free from dust and unwanted debris, and devoid of oil, grease, and moisture.
Painting Process: Painting techniques can be categorized into four main methods:
a. Brush Painting Method: This is the oldest and simplest painting method, applied manually and entirely dependent on the operator’s skill.
b. Dip Method: The part to be painted is continuously immersed in a tank of paint, allowing excess paint to drain back into the tank after removal.
c. Curtain Method: Primarily used for painting flat surfaces, the paint flows in a curtain-like manner, coating the part passing beneath it.
d. Spray Method: Spray painting applications are performed using three techniques: air, airless, and electrostatic. Application methods include:
- Manual
- Hard automation: Implemented using fixed or rotating guns attached to fixed or three-axis positioners.
- Robotic application: Achieves high production volumes for parts with complex geometries using robotic manufacturing systems.
In solvent-based paint spray applications, equipment must be explosion-proof. This includes conveyor drive elements, fixture drive elements, all paint, pneumatic, electrical equipment used inside the booth, and robots.
In enamel spray applications, it is essential to select equipment resistant to metal dust and glass particles, as enamel containing these materials can be highly abrasive at high speeds.
Air Method:
This method has the widest range of applications, suitable for both manual and robotic applications. Paint, pressurized by a pump, is sprayed onto the part surface through a gun, achieving desired surface characteristics through atomization and fan air.
In manual applications, the amount of paint applied to the product is visually controlled to achieve the desired surface quality. Operators adjust paint or atomization pressure and control application by moving the gun closer or farther from the part, relying on their experience to obtain good surface quality.
In robotic applications, visual control occurs after the spray process, once the painting process is complete. System parameters can be adjusted to minimize defects in the finished product. These parameters include:
- Paint Pressure: The appropriate line pressure and flow rate should be maintained with a suitable pump, constantly monitored. Line pressure and flow should be determined to provide the required pressure and flow rates for the guns.
- Paint Characteristics: Viscosity determines the amount of paint adhering to the surface. Changes in viscosity make paint adherence difficult. Similarly, weakening of chemical bonds in the paint makes it difficult to adhere to the surface.
Density: Paint density should be kept constant at a set value. Density control should be performed for each newly prepared paint, and the density of paint left in the tank for a certain period should be checked before use.
Maintaining paint viscosity and density within specified values is the most critical parameter to consider in setting up a paint line. It should be remembered that all adjustments can be disrupted only by changes in paint viscosity. Therefore, paint temperature must be controlled from the tank or pump to the point where the paint reaches the part surface.
- Atomization air pressure: It atomizes the paint coming out of the gun nozzle, carrying it onto the product surface. High atomization pressures should be used to paint distant surfaces. Spray painting with high atomization pressure from close range can lead to unwanted defects on the part surface. Paint defects that may occur on painted surfaces will be discussed in the “Painted Product Defects” section.
- Fan air pressure: Determines the spray width. Increasing fan pressure allows for a wider surface scan.
- Cabin conditions: The cabin temperature should be kept at a constant value, referring to normal ambient temperature. It should be remembered that paint quality will deteriorate above 80% relative humidity.
Proper air circulation should be provided in the painting environment, and overspray returning from the part surface should be removed from the environment. Thus, paint defects occurring over time by adhering to moving surfaces inside the cabin and drying will be prevented.
- Robot speed: When creating a painting program, the part to be painted must be introduced to the robot point by point. The robot, passing through these points at designated speeds and with designated atomization, fan, and paint pressure settings, paints by opening or closing the gun at designated points on the part. After the painting process, paint thicknesses on the product surface can be measured to determine which points need to increase or decrease paint volume.
Airless Method:
Based on the principle of spraying paint onto the part surface using small-diameter nozzles under high pressure. Since the paint is not carried with air, the overspray rate (paint escaping from the surface) is lower. Therefore, there is no need for a very powerful cabin ventilation system. However, because the nozzle diameter is very small, it is essential that the paint used is clean.
Electrostatic Method:
After atomization, the paint is charged negatively, and the surface to be painted is charged positively immediately after gun exit. This way, the paint is pulled directly onto the model surface, reducing the amount of overspray. Electrostatic guns are specially designed for electrostatic applications. Electrostatic guns make it much easier to paint difficult-to-reach areas.
In electrostatic applications, paint consumption is low, and overspray is minimal. However, achieving high film thicknesses is difficult. Therefore, it is mainly used for the first coat applications.
Painted Product Defects:
Paint manufacturers and users enumerate more than twenty paint defects. Retroactive elimination of defects arising from pre-paint preparation of the product’s surface quality is possible. The main defects and their causes are explained below.
- Sagging or excessive paint accumulation on the surface:
Viscosity is low (more fluid paint), the gun is too close to the surface (air atomization function cannot be effectively used), or the film thickness is high (excessive material amount). If sagging continues despite reducing the