Introduction

The width of the particle size distribution also depends on the design and the efficiency of the grinding system. Important factors include the dimensions of the mill, to avoid overmilling; the type of grinding elements, to achieve a high grinding intensity; optimized airflow conditions; and a highly efficient classification. Following is a description of the powder coatings production process and the most important influences on that process.

Powder grinding system

The feeding of chips out of a vessel is done by a dosing rotary airlock. The chips are transported by airflow into the ICM, where the grinding and classification process takes place. The ICM grinds the powder with hammers that have a tip speed of up to 130 m/s. The solids are comminuted in the milling chamber at the exterior circumference of the grinding disc and the profiled grinding surface of the internal wall of the housing.

An internal circuit transports the ground particles to an impeller wheel classifier, which is integrated in the housing of the ICM. The classifier allows particles with the desired size to pass. Coarse particles are rejected and transported again into the grinding zone. The main task of this classifier is to determine the top size of the product. A high cut sharpness of the integrated air classifier guarantees a well-defined top size free of spattered particles. Furthermore, the internal classification achieves a short residence time inside the mill and avoids overmilling of the product. Final product with the desired particle size will not be transported into the grinding zone again, thus producing a narrow particle size distribution.

Depending on the plant's concept, the product that leaves the classifier mill is either separated from the transportation airflow by means of a cyclone or, if a reduction of fine particles is required, an additional classification process step is installed. This is realized by a cyclone classifier, which produces good flow properties in the product.

A residual dust collector (RDC) separates the very fine particles from the airflow after the cyclone, where particle size is in the range of a few microns. A fan produces the airflow for the transportation of the solids through the system.

Figure 1: Particle Size Distributions of Powder Coatings Produced by an Impact Classifier Mill

Particle size distributions

Figure 2: Standard Values of Particle Size Distributions After Reduction of Fine Particles

The material properties depend on the powder coating components that are used and the extrusion process, which fixes the brittleness or ductility of the material. Brittleness or ductility is described by the physical parameter, called elastic modulus. A brittle material is characterized by a low elastic modulus. It increases with increasing ductility.

New applications in the automotive industry require a low top size and a very small amount of fine particles. In some cases, the amount of particles below 10µm is even less than 3%, or about 1.5% below 8 µm.

To avoid a decrease in product quality, the high amount of fine material has to be reduced. This requires a classification process behind the impact classifier mill. It is done by an additional classification using a cyclone classifier. In the case of grinding system retrofits, a change from a separation process to a classification process can be done by the exchange of the vortex finder of the cyclone and the installation of a classifier rotor and a secondary air inlet at the bottom of the cyclone housing. Due to the design of the classifier housing, an easy cleaning is guaranteed. Some examples of classified particle size distributions produced by a cyclone classifier are shown in Figure 2.

Figure 3: Maximum Particle Size of Particle Size Distribution With the Same x₅₀-Value

A highly efficient classification process avoids deflected particles or particles that are too coarse, which will have a negative influence on the quality of the powder coated surface. Figure 3 shows the results of classifications with different geometrical situations inside an impact classifier mill. In this case, differences in the top size of the particle size distribution is about 35 µm, although the x₅₀-value has the same particle size.

ICM compact classifier mill system ICM 3.8

Grinding systems for small batches

These grinding systems allow for fast production of small product quantities, which are necessary for the easy development of new powder coatings or the production of specimen samples for customers. Table 1 lists standard values for charge quantities and throughput for these ICM Systems.

Both compact systems are a link between the laboratory mill in the research laboratory and the production plant. It is a complete process unit that consists of an aggregate for the air-cooling, a pneumatic dosing device by means of rotary airlock, the classifier mill ICM 4.8, the downstream cyclone CAR 6 and a filter for the fine dust separation. The installation of an air classifier instead of a cyclone leads to a lower amount of fine particles to achieve good flow properties. Such a design with an air classifier is shown in the photo.

Table 1

• Small space requirement: 1.95 meters by 2.44 meters and a height of 2.52 meters.
  
• Roller seating assures good mobility of the plant.
  
• Fast cleaning is obtained by easy-opening mechanism and easy disassembly or changing of built-in parts.
  
• Throughput range includes larger laboratory and smaller production quantities (Table 1), so that material samples for the customer's check can be produced more easily.
  
• Steep particle distributions are generated that can additionally be optimized by the use of a cyclone classifier.
  

Summary

This paper is reprinted with permission from Vincentz Verlag. It was first published in Powder Coating Europe 2002.