Powder coating

A Brief Analysis of the Hammered Powder Coating Process

Abstract:

Hammered powder coatings, also known as artistic powder coatings, are characterized by a texture resembling the pattern formed by hammering a metal surface. This type of powder coating not only offers the common advantages of thermosetting powder coatings but also provides a beautiful texture, smooth film, and strong decorative qualities. It can also cover surface defects such as roughness and unevenness. Hammered powder coatings have a soft color, flexible, hard, and durable film, which makes them widely applied in fields such as instrumentation, distribution cabinets, security doors, home appliances, furniture, lighting fixtures, and medical devices.

1. Introduction

Hammered powder coatings are a type of decorative coating that creates a texture similar to the pattern formed by hammering metal surfaces. These coatings are not only advantageous for their usual thermosetting powder coating properties but also for their ability to enhance the surface finish, making it smooth, flexible, durable, and aesthetically appealing. The coatings can cover defects like roughness or uneven surfaces, improving both the appearance and performance of the coated items. These coatings are commonly applied to various metal surfaces in industries such as instrumentation, distribution cabinets, security doors, home appliances, furniture, lighting fixtures, and medical devices. For more on the performance of textured powder coatings, check out A Study on the Gloss Stability of Textured Powder Coatings.

2. Formulation of Hammered Powder Coatings

Hammered powder coatings are formulated using the principles of changes in melting viscosity, surface tension, and curing speed during the curing process. These changes cause the surface of the coating to shrink, forming the characteristic hammered texture. There are several methods for producing hammered powder coatings, each with its own benefits and considerations.

2.1 Adding Fillers

One method of producing hammered powder coatings involves adding fillers to the formulation. By increasing the filler content or using high oil absorption fillers, the normal flow of the resin during melting is hindered. This results in a coating structure where the filler particles act as a framework. However, the amount of filler must be carefully controlled, as excessive amounts can affect the mechanical properties and chemical resistance of the final coating. To explore a deeper issue regarding powder coating caking, see A Brief Analysis of the Caking Problem in Epoxy-Polyester Powder Coatings.

2.2 Adding Incompatible Substances

Another method involves adding incompatible polymers with a higher melting point than the resin base. These polymers do not mix well with other components during extrusion, and when the coating cures, the resin forms around the polymer particles, creating a textured surface. This method offers good chemical and corrosion resistance but requires strict control over the type and amount of incompatible substances used.

2.3 Adding Rheology Modifiers

Rheology modifiers can also be added to the formulation to achieve the desired hammered texture. These modifiers increase the viscosity of the coating during the curing stage, preventing it from flowing and allowing the formation of the hammered effect. This method provides a cost-effective solution while maintaining good decorative effects and improving the protective properties of the coating. For further details on powder coating coverage, refer to A Brief Analysis of Factors Affecting the Powder Coating Coverage Area for Aluminum Profiles.

2.4 Adding Hammered Agents

The most commonly used method in the production of hammered powder coatings involves adding hammered agents. These agents reduce the surface tension of the powder coating, allowing the resin to wrap around the agent particles during curing, resulting in an even distribution of the hammered texture. This method offers superior mechanical properties, stable texture reproduction, and excellent aesthetic and protective effects, although it is more expensive.

3. Powder Coating Process for Hammered Coatings

Hammered powder coatings require special handling during the application process due to their unique formulation. Adjustments to equipment and application techniques are necessary to ensure the formation of the desired texture and a uniform coating.

3.1 Process Flow

The typical process flow for applying hammered powder coatings includes surface pretreatment of the workpiece, drying, filling and scraping putty, sanding, drying again, and finally spraying the coating.

3.2 Surface Pretreatment

The surface of the workpiece must be thoroughly cleaned and treated to remove oil, rust, and other contaminants before spraying. Common pretreatment methods include phosphating, anodizing, or chemical oxidation, depending on the type of material. A clean and dry surface ensures good adhesion and corrosion resistance for the coating.

3.3 Putty Filling and Scraping

Before applying the powder coating, it is important to fill any gaps, seams, welds, air holes, or other imperfections on the workpiece surface. The putty used should be easy to apply, have good adhesion, high hardness, fast drying, and should not shrink or crack during the curing process.

3.4 Application Parameters

Several factors affect the quality and hammered effect of the coating, including spraying environment, curing conditions, and coating thickness. For further reading on powder coating quality, see How to Identify the Quality of Powder Coating?.

4. Application Considerations and Common Issues

4.1 Considerations

Ensure even coating thickness to avoid unclear texture boundaries or exposed substrate.
Spray in the correct order, starting with the secondary surfaces and finishing with the primary ones.
Use a screen with slightly larger mesh size and adjust the powder flow, air pressure, and spraying distance to achieve optimal results.

4.2 Common Issues

Flow Marks: Caused by excessive coating thickness or slow heating, resulting in poor texture formation.
Uneven Texture: Caused by inconsistent spraying speed or varying coating thickness.
Unclear Texture Boundaries: Typically a result of excessive coating thickness.
Exposed Substrate: Occurs when the coating is too thin.
Pinholes: Caused by improper pretreatment or contamination in the air.

5. Conclusion

Hammered powder coatings offer a simple production process with stable quality and excellent decorative properties. They are widely used for coating metal surfaces across various industries. The development of hammered powder coatings continues to evolve, providing new and cost-effective solutions for surface finishing applications.

Factor	Optimal Condition/Range	Impact on Hammered Powder Coating Quality
Powder Coating Formulation	Varies based on fillers, rheology modifiers, or incompatible substances	Affects the texture formation, film smoothness, and durability of the coating. Proper formulation ensures a high-quality hammered effect.
Filler Type and Amount	Varies depending on filler oil absorption and particle size	Controls the structural framework of the coating. Incorrect filler amounts can affect the mechanical properties and cost-effectiveness.
Surface Pretreatment	Oil removal, rust removal, phosphating, anodizing	Ensures good adhesion, corrosion resistance, and a clean, dry surface. Proper pretreatment prevents defects such as poor bonding or peeling.
Curing Temperature and Time	Generally between 180°C and 200°C, depending on resin	Ensures proper curing and crosslinking, affecting the final texture quality and mechanical properties. Insufficient curing results in defects.
Coating Thickness	Typically between 70µm and 100µm	Thickness influences the size and clarity of the hammered texture. Too thick or too thin can distort the texture or leave the substrate exposed.
Spraying Parameters	Static voltage between 60-80kV, air pressure 4.9-15×10⁴ Pa	Affects powder adhesion, texture uniformity, and coating thickness. Proper parameters ensure even texture formation and prevent defects.

1. What is the difference between Hammered Powder Coating and regular Powder Coating?

Hammered powder coating creates a decorative, textured "orange peel" or dimpled finish that hides surface defects and adds, while regular (smooth) powder coating provides a sleek, uniform finish. Hammered finishes are generally thicker, more durable against scratches, and better for hiding imperfections on metal.

2. How does the choice of filler affect Hammered Powder Coating?

Different Types of Powder Coating TexturesThe choice of filler significantly affects hammered powder coating by influencing texture definition, durability, and surface smoothness. Inert fillers like barium sulfate maintain high-gloss, consistent, and smooth, yet textured finishes. Improper, non-heat-resistant fillers can cause outgassing, leading to pinholes or cracking during the curing process.

3. What are the benefits of using Hammered Powder Coating in industrial applications?

Hammered powder coating offers industrial applications superior durability, exceptional corrosion and abrasion resistance, and the ability to conceal substrate imperfections due to its textured, "hammered metal" finish. It provides a long-lasting, low-maintenance, and eco-friendly protective layer that resists chemicals, UV radiation, and heavy wear.

4. How does curing temperature impact the quality of Powder Coating?

Curing temperature is the most critical factor in powder coating, directly determining durability, adhesion, and finish quality. Optimal temperatures (typically \(160^{\circ }\text{C}\) to \(200^{\circ }\text{C}\) or \(320^{\circ }\text{F}\)–\(392^{\circ }\text{F}\)) ensure proper, smooth cross-linking. Incorrect temperatures result in:

5. What is the ideal application technique for Powder Coating to achieve a uniform hammered texture?

The ideal application technique for a uniform hammer texture in powder coating involves using an electrostatic spray gun with lower gun current (kV) and reduced powder flow (lower air pressure) to ensure a consistent, moderate film thickness, typically applied at 2.0-3.0 mils. Key techniques include:

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