In the entire process of powder coating production and application, the free flowability of the finished powder is one of the key indicators determining the coating effect. Good flowability not only ensures the smooth operation of processes such as electrostatic spraying and fluidized bed dip coating, reducing problems such as spray gun clogging and uneven powder application, but also improves the smoothness and consistency of the coating film and reduces production losses. For manufacturers, how to accurately control the free flowability of the finished powder is a core issue for ensuring product quality and improving customer satisfaction. Based on practical production experience, the following four core solutions can effectively solve this problem, taking into account both practicality and operability.
1. Temperature Control and Cooling
Currently, the powder coating industry generally uses ACM mills (air classifier impact mills) for grinding and pulverizing. This equipment can achieve simultaneous grinding and powder classification, resulting in high efficiency. However, in actual production, we have found a common pain point: when the ACM mill operates continuously for a long time, or in hot summer environments with high temperatures, the temperature of the finished powder after sieving will rise significantly, usually reaching above 40℃. If packaging is performed directly at this temperature, the powder particles will adhere to each other due to increased thermal motion, easily forming clumps after settling, severely affecting the performance of subsequent coating applications. To solve this problem, after numerous production trials, we developed a simple and effective improvement solution – installing a refrigeration device at the air inlet of the ACM mill. This device pre-cools the air entering the mill, removing the heat generated during grinding and keeping the temperature of the finished powder below 25°C. This prevents the powder from clumping due to residual heat after packaging, significantly improving its looseness and flowability, resulting in more even dispersion during subsequent spraying and increased powder application rate. This method is simple to operate, cost-effective, and a fundamental means of controlling powder temperature and ensuring flowability in production.
2. Adding Post-mixing Flow Additives
To further improve the free flowability of powder coatings, adding post-mixing flow additives is a widely adopted and effective method in the industry, with fumed silica being the most commonly used high-quality additive. We typically incorporate fumed silica evenly during the extrusion and pulverization of sheet materials. Powder coatings treated in this way, when observed under an electron microscope, exhibit clearly defined, non-sticky particle boundaries, a strong granular texture, and significantly improved flowability. The principle behind this phenomenon is quite intuitive: fumed silica consists of finer, less dense colloidal particles. These particles are evenly dispersed among the powder particles, acting like a "lubricating film" on the particle surface, while also functioning as "ball bearings," reducing friction and adhesion between powder particles, thus effectively preventing agglomeration and allowing for smoother powder flow. From a chemical structure perspective, fumed silica suitable for powder coatings is divided into hydrophilic and hydrophobic types. Comparative experiments have shown that hydrophobic silica not only excels in preventing agglomeration but also significantly enhances the electrostatic properties of powder coatings. This is crucial for electrostatic spraying processes—improved electrostatic properties allow the powder to adhere more firmly to the workpiece surface, reducing waste. Regarding the dosage, excessive amounts are sufficient to achieve the desired effect; typically, adding 1‰ to 2‰ of the total powder volume is adequate. Excessive addition may negatively impact coating performance. In actual production, we have summarized two practical addition methods:
A: Add a dedicated feed inlet to the pulverizer's feeding section, equipped with a specialized metering feeder. Add fumed silica precisely and evenly according to a set ratio, allowing it to enter the mill together with the extruded semi-finished product for co-pulverization. This method ensures thorough mixing of the additive and powder particles, resulting in more uniform dispersion.
B: Add a feed inlet at the rotating sieve opening of the ACM mill. Use a mechanical feeding device to stably feed fumed silica from the feeder, allowing it to pass through the sieve with the ground finished powder, completing the mixing process. This method offers flexibility and is suitable for adjusting the additive dosage in real-time according to the flowability requirements of different batches of powder.
3. Precise Adjustment of Powder Particle Size
The size and distribution of powder particles directly affect the free flowability of the powder. Different types of powder flakes and different environmental conditions (such as temperature, season, and humidity) have different requirements for particle size. Fortunately, ACM mills themselves have the function of adjusting powder particle size. We can achieve precise control of particle size by optimizing operating parameters, thereby ensuring flowability. Specifically, the adjustment methods mainly include three aspects: First, adjusting the input amount of flakes. If the input amount is too large, the grinding will be insufficient, resulting in larger and unevenly distributed powder particles, which will affect flowability; if the input amount is too small, the efficiency will be too low and the cost will increase. The optimal input amount range needs to be found according to the mill model. Second, adjusting the rotation speed of the main and auxiliary mills. The higher the rotation speed, the stronger the grinding force and the finer the powder particle size; the lower the rotation speed, the relatively larger the particle size. Adjustments need to be made in combination with the characteristics of the powder type. Third, controlling the airflow of the induced draft fan. The airflow affects the airflow velocity inside the mill, which in turn affects the powder classification effect. Reasonable adjustment of the airflow can make the particle size distribution more concentrated and improve flowability. To ensure accurate particle size control, we use a laser particle size analyzer for real-time monitoring during production. This allows us to monitor the distribution of powder particle size and adjust operating parameters accordingly, ensuring the finished powder meets flowability requirements and is suitable for different coating scenarios and environmental conditions.
4. Strict Moisture Control
Powder coatings are extremely sensitive to moisture absorption. Moisture absorption directly leads to a sharp decrease in free flowability, a critical issue that must be addressed during production and storage. The hygroscopicity (moisture content) of powder coatings directly affects their dielectric constant, a key factor determining the powder's electrical properties. When powder absorbs even slight moisture, its electrical properties decrease significantly, resulting in reduced powder application rate and poor flowability. This can lead to clumping and uneven dispersion during spraying, resulting in an uneven coating surface and even difficulty in adhering to the workpiece. If moisture absorption is severe, the powder will clump together, forming hard lumps of varying sizes, making electrostatic spraying impossible and potentially clogging the spray gun, impacting production progress. Furthermore, when moisture-absorbing powder is baked into a film, the moisture evaporates due to heat, leading to defects such as bubbles and pinholes in the coating, severely affecting product quality. Therefore, humidity must be strictly controlled throughout the entire process of powder coating production, application, and storage. The relative humidity in the production workshop should be controlled below 60%, and the spraying environment must also be kept dry. During storage, powder coatings should be sealed in packaging and stored in a dry, cool, and well-ventilated warehouse, avoiding contact with humid air and keeping them away from water sources and damp areas. Unused powder after opening should be sealed and stored promptly; if necessary, a desiccant can be added to prevent moisture absorption.
5. Conclusion
The free flowability of the finished powder is a crucial indicator of powder coating quality. Controlling this is not a single-stage operation but requires coordinated efforts from multiple dimensions, including production, additives, particle size, and storage. Temperature control and cooling are fundamental, preventing powder from agglomerating due to high temperatures from the outset. Adding flow aids such as fumed silica is a core method, directly improving powder bulk and flowability. Precise particle size adjustment is crucial, allowing the powder to adapt to different environments and process requirements. Strict moisture control is a vital line of defense for stable flowability, preventing performance degradation due to later moisture absorption. In actual production, manufacturers should flexibly combine these methods based on their product characteristics, production equipment, and environmental conditions, continuously optimizing parameters through experimentation to find the most suitable control scheme. Simultaneously, a comprehensive quality inspection mechanism must be established to regularly test the flowability of the finished powder, promptly identifying and adjusting any problems. Only in this way can powder coatings with excellent flowability be consistently and stably produced, ensuring a smooth transition to subsequent coating processes, ultimately providing customers with high-quality products and gaining a competitive edge in the market.
