Powder coating is applied electrostatically as dry powder. After coating part is cured under heat to allow powder to flow and form a skin. Powder coating improves part corrosion resistance. Chromate conversion coating, zinc plating, and anodizing are used as base coat to powder coating. In this article we will discuss powder coating process, it’s types and advantages.
Powder Coating Process
Powder coating process involves three basic steps to achieve quality Finish.
- Powder Coating
Step 1: Pretreatment For Powder Coating
This Process involves metal parts cleaning and application of conversion coating. For powder coating 7 tank pretreatment process is highly recommended.
Mechanical and Chemical Cleaning
Mechanical cleaning involves sand blasting and other abrasive actions to remove corrosion and other irregularities in metal parts.
Whereas chemical cleaning involves removal of oil, dirt, grease present on part surface using chemicals.
For different materials different conversion coatings are recommended. For example for CRCA steel, chromate conversion coating is recommended. Conversion coating offers following benefits:
- Improves Powder Coating Life.
- Improves Powder coating adhesion.
- Provides in process corrosion protection.
Step 2: Applying Powder Coating
Powder coating to metal parts is applied using electrostatic gun. Powder Gun charges powder with positive charge. Parts to be powder coated are grounded. Powder is sprayed towards parts using compressed air.
Step 3: Curing
Powder coating requires curing at elevated temperatures for cross linking of polymers. Curing temperature and time vary according to powder coat manufacturer specifications.
Convection and infrared cure ovens are used to cure powder coating parts.
Types of Powder Coating
According to dry powder chemical composition and cross linking structure. They are classified in two categories.
1) Thermoplastic Powder Coating
Thermoplastics are synthetic materials that can be formed multiple times. High temperature can melt thermoplastics. Therefore they are not recommended to use in high temperature or heat environment. But they have good wear and chemical resistance.
Following types of thermoplastic coatings are available
Nylon powder coating are durable and impact resistant. They offer good protection against a wide range of chemicals, solvents and oils. FDA approved food contact grades are available. Due to low friction coefficient, nylon coating is used on gears and conveyor equipment.
Polyesters coating has very good adhesion, abrasion and UV resistant properties. Therefore recommended for outdoor applications. Such as furniture and automobile parts.
c) Polyvinyl Chloride (PVC)
PVC coatings are pliable, sleek and durable coatings. They have very good electrical resistance. FDA approved food grade PVC coatings are also available.
Polyolefin powder coat are resistant to abrasives and chemicals. These coatings results in a smooth surface and excellent adhesion properties. Polyolefin coatings are used on components that require regular cleaning.
2) Thermoset Powder Coating
Thermoset creates a non-reversible chemical cross-linking reaction. That result in the change in physical properties.
Epoxy Coatings are very durable, offers excellent hardness, very good chemical and corrosion resistance. These coatings provides smooth and uniform appearance.
But when epoxy coating comes in contact with sun they fade and chalk. Therefore epoxy coating are recommended for indoor applications.
Acrylic coatings performs better than epoxy coating in sunlight. They give a high gloss, well looking finish. Clear / Transparent acrylic coating are also available.
c) Epoxy Polyester Hybrid
Epoxies and polyesters are mixed to foam hybrid coatings. They offer very good mechanical properties, corrosion and chemical resistance. Due to presence of polyester, Hybrid coatings chalks less and offer better weathering in outdoor conditions.
Powder Coating vs Liquid Paint
Powder Coating process is similar to painting process. But it offers following advantages compared to liquid paint.
- Does not contain volatile organic compounds (VOCs)
- Thicker coating can be achieved compared to liquid paints.
- Gives a uniform visual appearance across coated area regardless of part orientation.
- Good scratch resistance, toughness and hardness.
To sum up, Dry powders are available in various materials, colors and special effects such as stain, glossy etc. They have applications in automobile, telecom and construction industry. Low temperature cured dry powders are used for coating plastic parts.
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