After the raw materials have been successfully mixed into a homogeneous dry blend, the extrusion process can begin. The extrusion step can be described as the "paint-making" process and is where the powder coating formula is compounded. At this point, the formula is locked in; very few alterations can be made (with the exception of particle size reduction) after this step.
The distribution aspect of the extrusion process entails melting the resinous components and mixing the non-melting constituents throughout the molten mass. This mixture is achieved by introducing the homogenized dry ingredients into an extruder. The goal is to shear the mixture against a hard surface until it melts. Mechanical force is put into the mixture to increase the ambient temperature of the mixture and its surroundings. The increased temperature, combined with the mechanical force or shear, melts the resinous components, distributes the non-melting constituents and de-agglomerates the pigments. This is all accomplished as the material is moving rapidly through the extruder.
The extrusion process basically consists of a feeding mechanism, a compounding section and cooling. The material is fed into a rotating mechanism, which is usually a screw device encased by a barrel. The screw is configured with flights or nodes that exert work on the molten mass as it passes through the extruder.
Extruders can be of a single- or twin-screw design. The single-screw types have flights along the circumference of the screw, which convey the mixture into pins that emanate from the interior of the barrel. This creates shearing of the material that, in turn, melts the mass and affects the distribution and dispersion of the components. The molten mass exits the barrel and is then cooled and broken into flakes.
Twin-screw extruders operate on a similar principle; however, they use the shearing action of two co-rotating screws encased in a smooth barrel. Work is exerted into the mixture by the kneading blocks on the screws. The coordinated rotation of the screws pushes the material from screw to screw as it travels the length of the barrel. The heat generated during this process allows the resinous components to melt, and the shearing action affords the distribution raw materials and dispersion of the pigments.
The cooling process typically involves introducing the moving extrudate into a set of chilled rolls, which then convey a ribbon of material onto another cooling surface, such as a continuous belt or rotating drum. After the extrudate is sufficiently cooled, it is broken into flakes that are suitable for feeding into a pulverizing process.
The temperature of extruders is controlled with either electrical resistance heaters or heater/cooling units that circulate fluid media through cavities in the extruder barrel and screw(s). The medium can be oil or a glycol/water mixture. Resistance heated units are used in conjunction with media circulating chillers. When the extrusion process is initiated, the mixture quickly becomes molten and the work put into it establishes a rather consistent temperature. The role of the heater/cooling device becomes more of a cooling process at this point.
Extruder screw speed can be adjusted. Most powder manufacturers run their extruders at full speed to provide the highest output possible. This makes sense in nearly all cases. For formulas that contain difficult-to-disperse components, such as carbon black pigment, a slower screw speed might be needed to achieve adequate dispersion.
After extrusion, the only way to modify the formula is by re-extruding material with raw components. This process is costly and is avoided whenever possible.
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