The common method for making powder colors is the method of mixing the melt of the raw materials used in the extruder, which are then converted into fine particles.
There are eight distinct steps for the conventional manufacturing method (melt mixing):
1) Weighing raw materials
2) Premix
3) Extrusion
4) cooling and crushing
5) Grinding and particle size control
6) filtering (sieving)
7) Homogenization or dry blending (Dry Blending)
8) Packaging
All the above stages are important, but the three key processes of pre-mixing, extrusion and grinding are the main stages of powder coating production.
1) Pre-mixing
The purpose of this stage is to create a homogenous mixture of raw materials before the extrusion stage, which ultimately creates uniform color and special properties in the powder. Therefore, at this stage, uniformity should be controlled with the help of laboratory extrusion.
2) Extrusion
There are three types of extrusion to make powder paint:
– single screw
– Two turns at the same time
– Tooth Planetary Roller
These are designed to have maximum distribution, minimum shelf life and precise melt temperature control.
Several processes take place during extrusion, which are generally referred to as particle distribution or dispersion. These processes do not work separately and take place simultaneously to some extent.
These processes include:
– Melting, mixing and uniformity of resin components
– Wetting of pigments by resin components and distribution of additives in the melt
– Breakdown of ideally bonded pigments into primary particles.
The initial heat of the extruder is needed to ensure the melting of the resin, but during the continuous production of mechanical energy resulting from the friction of the materials, it is sufficient to provide heat, and the inner wall of the extruder must be kept cold to keep the temperature constant.
The materials coming out of the extruder should be immediately cooled in thin sheets and crushed into chips for use in the mill.
3) Grinding and particle size control
Electrostatic powder paints for conventional applications usually have a particle size distribution in the range of 0-100 microns, but the range that provides good practical properties is 10-50 microns. Large amounts of fine particles below 10 microns provide a powder with low electrostatic charge and poor flowability, and the presence of coarse particles provides coatings with an unfavorable appearance.
Pindisc mills are usually used to crush powder paints. They include an internal classifier to control the maximum particle size, and the coarse particles out of size are returned to the mill. A high-volume fan draws air into the mill, which causes heat loss and also transfers powder particles. Particle size distribution can be controlled by controlling the residence time of the particle inside the mill and the speed of the classifier.
The collection of transported powders is done by moving the air by the cyclone. Particles above 10 microns and a small amount of smaller particles depending on the efficiency of the cyclone are removed and collected as a product.
The powders coming out of the cyclone are passed through a continuous sieve before entering the packaging area, so that the particles above the required size (over site) are separated from it. Direct collection of all powders from the filter is also possible.