In the practical operation of powder coating production and spraying, many practitioners encounter various functional problems—such as insufficient film hardness, bubbling after boiling, poor powder adhesion at corners, and powdering of pearlescent pigments. These problems not only affect the appearance and performance of the coating but can also lead to a surge in rework rates and increased costs. Today, we will break down the causes of these high-frequency functional problems one by one and share practical solutions to help you avoid pitfalls and improve product qualification rates.

1. Insufficient Powder Coating Hardness

Coating hardness is a key indicator of coating durability. Insufficient hardness leads to easy scratching and poor wear resistance. Solving this problem requires starting with formulation design and focusing on two key aspects:

First, accurately select the resin and curing agent. Try to use film-forming systems with high glass transition temperatures and high functionality. These systems have high crosslinking density and high glass transition temperatures after curing, resulting in higher film hardness. For example, when choosing carboxylated polyesters, prioritize resins with high acid values; when choosing hydroxyl polyesters, select resins with high hydroxyl values; when using blocked polyisocyanate curing agents, prioritize varieties with high NCO content.

Secondly, optimize the selection and ratio of pigments and fillers. On one hand, you can directly select pigments and fillers with high hardness, which can quickly adjust the coating hardness; on the other hand, appropriately increasing the mass percentage (or volume percentage concentration) of pigments and fillers can also significantly improve hardness, with adjusting the filler ratio being easier to implement. Porous quartz powder is a highly recommended filler, containing stable and high-hardness zirconium dioxide (ZrO₂), and its unique porous structure enhances coating hardness while maintaining good impact resistance, making it an ideal choice for improving hardness.

2. Blistering after Boiling in Water

Blistering of the coating after boiling in water is a common problem of insufficient water resistance, and in severe cases, it can be accompanied by loss of gloss, discoloration, poor adhesion, or even peeling. The core causes are mainly: insufficient cross-linking density of the coating film, incomplete curing reaction, and excessive residual electrolytes; low glass transition temperature (Tg) of the coating film, and excessive residual hydrophilic groups and substances; poor temperature resistance and water resistance of the coating film. These problems allow moisture to easily penetrate the coating film and seep into the space between the substrate and the paint film, leading to blistering and other adverse phenomena.

The key to solving this problem is to improve the density and hydrophobicity of the coating film. It is recommended to add a special water-resistant additive. The functional groups of this type of additive can form strong chemical bonds with the resin and substrate through chemical reactions, significantly improving the coating density and water resistance, while also enhancing the hydrophobicity of the coating. In practice, the additive addition amount should be controlled at 0.3%-1.5% to achieve the desired effect.

3. No powder adherence at 90-degree groove corners

The core reason for powder adherence at complex structural areas such as 90-degree groove corners is the Faraday cage shielding effect of electrostatic spray guns—these areas form electrostatic shielding zones, preventing effective powder adsorption, and conventional additives are generally unable to solve this problem.

The most effective solution is to use a friction gun with friction powder for spraying. Frictional charging differs from traditional corona charging; the powder acquires a charge through friction as it passes through the spraying device, effectively avoiding the Faraday cage shielding effect, making it particularly suitable for spraying complex-shaped parts. In practice, using thermal spraying yields even better results; additionally, applying an electrostatic liquid to corners and other hard-to-reach areas before spraying can also help improve powder application.

4. Pearl Pigment Powder Spillage

The core reason for pearl pigment powder spillage is its inherent characteristic—non-conductivity—making it difficult to carry a charge. During spraying, pearl pigments cannot effectively carry a charge like the main powder. This difference in charging properties causes the pearl pigment to separate from the main powder, and some uncharged pearl pigment cannot firmly adhere to the workpiece surface, ultimately resulting in "powder spillage."

5. Wax Powder Selection and Application

Wax powder is mainly used in powder coatings for scratch resistance and anti-settling, but improper selection can lead to problems such as smoke and particle formation. In practical application, please pay attention to the following points:

5.1 Avoid the drawbacks of low-priced wax powders: Cheap wax powders are prone to smoking during baking. For scenarios requiring high quality, it is recommended to choose high-purity wax powders.

5.2 Selection based on scratch resistance requirements: To improve the scratch resistance of the coating, prioritize wax powders with high molecular weights, such as those around 6000. These are used in textured and regular powder coatings, offering good scratch resistance and cost-effectiveness.

5.3 Differences in characteristics among different wax powders: Polyamide waxes have strong thixotropy, effectively balancing the sagging and leveling of thick-film coatings. They are not easily soluble in solvents, have no risk of particle formation, and are easier to disperse than organobentonite. However, they are more expensive, and some varieties may affect interlayer adhesion. Polyethylene waxes are mainly used as anti-settling agents. They have no temperature limitations, no risk of particle formation, good suspension properties for large pigment particles, and are widely used. They also do not significantly thicken while preventing sedimentation.

Essentially, polyamide wax (PA) and polyethylene wax (PE) have completely different chemical structures: polyamide wax is a product synthesized from acids and amines, while polyethylene wax is a hydrocarbon polymerization product. Although both can be used as lubricants to improve coating hardness, polyamide wax commonly used in powder coatings has a lower molecular weight (below 400), while polyethylene wax typically has a molecular weight between 500 and 5000. resulting in superior abrasion resistance. Furthermore, after modification, both can be used for anti-settling and thixotropic applications. In the spraying of aluminum profiles and cast iron workpieces, polyamide wax can also help degas the substrate due to its low molecular weight, low viscosity, and incompatibility with epoxy polyester, but it does not contribute to scratch resistance.

6. Surface Color Floating and Underlying Material Exposure in Wrinkle Powder

Surface color floating (especially in darker powders) and underlying material exposure are frequent problems in wrinkle powder production. Surface color floating is mainly caused by the separation and migration of hydrophobic pigments and surface tension imbalance; underlying material exposure is mostly caused by improper coating viscosity and excessively long coking time. The countermeasures can be approached from three aspects:

6.1 Pigment selection: Prioritize hydrophilic pigments to reduce the risk of pigment separation and migration;

6.2 Filler and mixing process: Select fillers with finer particle sizes (≥1200 mesh). During mixing, premix the pigment and filler evenly first, then add polyester resin. Adjust the color until it is similar before adding wrinkle-reducing agent;

6.3 Auxiliary agents: Add appropriate amounts of pigment dispersants and wetting agents to improve pigment dispersibility, balance surface tension, and alleviate floating color problems.

7. Key Issues and Solutions for Transfer Powder

The core challenges of transfer powder printing lie in wood grain clarity, paper/film release effect, and surface defects. Specific solutions are as follows:

7.1 Key Factors Affecting Wood Grain Clarity

Several factors affect the clarity of the wood grain after transfer, requiring careful control in practice: Higher amounts of matting agent result in a more blurred wood grain; the type and amount of catalyst significantly impact wood grain clarity; different manufacturers' polyester resins and different types of barium sulfate or calcium carbonate produce varying degrees of wood grain clarity; more filler is not necessarily better, as excessive amounts reduce wood grain clarity; different amounts of HAA curing agent have some impact on wood grain clarity; titanium dioxide has a significant impact and should not be used excessively; while the type and amount of wax powder, leveling agent, and degassing agent have relatively minor effects on wood grain clarity.

7.2 Difficulty in Paper/Laminator Removal After Heat Transfer

The main causes are fourfold:

7.2.1 Poor surface quality of the substrate, such as inability to withstand high temperatures, leading to paper/film adhesion;

7.2.2 Incomplete curing of the powder coating, resulting in re-curing during heat transfer and subsequent sticking;

7.2.3 Poor quality heat transfer adhesive, or improper mixing concentration or application amount;

7.2.4 Solution: Adding an appropriate amount of release agent (such as silicone oil) to the heat transfer adhesive can effectively improve paper removal.

7.3 Spots or Foggy Patches Appearing After Heat Transfer Printing

Main Causes and Solutions:

7.3.1 Spots or foggy patches already exist on the substrate surface;

7.3.2 The substrate is not heat-resistant, causing paper or film adhesion;

7.3.3 The powder coating has not fully cured;

7.3.4 Poor quality heat transfer adhesive, improper mixing ratio, or incorrect application amount (high-quality adhesive will automatically evaporate at high temperatures, leaving no residue);

7.3.5 Remedial Measures: If the problem is caused by adhesive sticking to paper, the substrate can be wiped with water or briefly baked in an oven; if the problem is caused by the heat transfer film, it can be wiped with a low-concentration organic solvent such as acetone.

7.4 Uneven Product Surface Color

Core Causes:

7.4.1 The transfer paper (film) is not tightly adhered to the aluminum surface, causing it to lift during baking, or the tubular bag leaks during vacuuming;

7.4.2 Uneven temperature field inside the oven;

7.4.3 Inconsistent substrate loading time in the oven, resulting in excessively long transfer times in some areas;

7.4.4 Uneven adhesive application on the transfer film, resulting in "adhesive overspray" (adhesive accumulation), preventing the transfer paper from fully adhering to the substrate.

8. Summary

Functional problems in powder coating production and spraying are mostly related to formula design, material selection, and process details. By mastering the three core principles of "precise formula, suitable materials, and refined processes," most functional problems can be effectively solved, improving powder coating production efficiency and product quality.