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Producing a powder coating is a multi-step process. It can be described as semicontinuous because it begins as a batch process (weighing and premixing) but evolves into a continuous process (extrusion and milling). The exciting part of the manufacturing process is taking the abstract concept of the art of powder formulating and making it real and tangible. All the creativity and technology that has been infused into a formula becomes a reality. This Technology Interchange describes the first part of manufacturing powder coatings, namely weigh-up, premixing wand extrusion.
The first process involves weighing the raw materials in ratios prescribed in the master formula. The weigh-up coincides with the amount needed by the customer order as well as the capacity of the premixing vessel. Typical production batches require a number of premix charges to fill a sales order. Multiples of weigh-up charges are therefore necessary. Calculating the overall quantity to weigh at the onset of a powder coating batch must take into account the estimated yield of the production processes. Smaller batches inherently incur lower yields mainly due to "filling the pipeline" of each process. A certain amount of material is lost when establishing a steady process state especially in extrusion and milling.
Relatively small batches (less than 500 lbs.) generate smaller yields than larger batches. A well run large batch process will run between 95 to 99 percent efficient. Smaller batches run between 90 and 95 percent. Experience will dictate capability of processes and guide in determining how much excess raw material is needed per batch size.
Scales need to be sensitive enough to provide consistency in the finished powder coating. In general, large bulk components require less precision than critical additions such as tinting pigments and catalysts. Plus or minus 1 percent is a good rule of thumb for weighing accuracy. Keep in mind that ±1 percent of a small addition can be quite small.
The premixing step homogenizes the raw materials mix so that a consistent feed can be introduced into the extruder. Moreover, large resin flakes are fractured, which helps facilitate a mix that can be fed more easily. A high-intensity mixer can achieve acceptable blending in a relatively short duration (2 to 5 minutes). Adequate mixing can also be accomplished with a slower, less intense process that typically can take 10 to 25 minutes.
Consistency can also be enhanced by the order that raw materials are added to the premixing vessel. "Sandwiching" minor components (tints, additives, etc.) between bulk items (resins, extenders, etc.) helps minimize these minor additives from being flung inside the premixer onto the vessel wall and thereby compromising distribution.
Some mixers employ detachable bowls that offer versatility in managing multiple mixes with the same mixing head. This design also allows for ease in cleaning. Premixing equipment is designed for an optimal range of vessel filling. Charges too small will be poorly mixed because the mixing blades will be incompletely covered. Excessively large charges will be poorly blended due to lack of free space to transport the contents of the vessel. Excessively large charges can also tax the drive mechanism of the mixer to the point of tripping circuit breakers and damaging motors and electrical drives.
With a homogeneous dry blend of raw materials, the extrusion process can commence. The extrusion step can be described as the "paint-making" process. This is where the powder coating formula is compounded. At this point the formula is locked-in, and very little can be altered (with the exception of particle size reduction) after this process.
Two types of mixing are accomplished in successful extrusion. The resins, additives and pigments are intimately distributed to create a consistent blend. This uniformity of mixing affords consistent appearance and performance of the finished coating. Inconsistent mixing typically results in variable gloss, smoothness and possibly film performance.
Powder coating raw materials can be blended in a fixed-bowl, high-intensity mixer in less than 5 minutes.
The distribution facet of the extrusion process entails melting the resinous components and mixing the non-melting constituents throughout the molten mass. This 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. This work increases the ambient temperature of the mixture and extruder mechanism (i.e., barrel and screws). The increased temperature combined with the shear exerted by the extruder screws melts the resinous components, distributes the non-melting constituents and de-agglomerates pigments. This is all accomplished as the material is moving rapidly through the extruder
Compounding powder coatings requires the dispersion of pigment agglomerates. Most pigments are supplied in their natural agglomerated form. The powder coating extrusion process endeavors to disperse these agglomerates. By dispersing the pigment agglomerates, a more consistent and intense color is developed. Incomplete pigment dispersion results in uncontrolled color consistency.
The extrusion process basically consists of a feeding mechanism, a compounding section and cooling. The material is fed into a rotating mechanism, 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 designed to manufacture powder coatings employ a single screw or twin-screw mechanism. Single screw types have flights along the circumference of the screw. These flights convey the mixture into pins that radiate 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 very similar principle; however, they use the shearing action of two co-rotating, intermeshing 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. The shearing action intimately melt mixes raw materials and disperses the pigments.
Extruders are temperature controlled. This is usually achieved either through 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. The initial temperature settings of an extruder should coincide with the melt point of the major resinous components. Temperature is typically set slightly above the melt point of the resin(s). Upon initiating the extrusion process, 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.
Optimal extrusion is achieved by processing a relatively viscous mass of material. Low viscosity causes incomplete filling of the free volume between the screws and the barrel that results in poor compounding. It is therefore important to avoid setting the temperature too high. Moreover, excessive heat can sometimes cause pre-gelation of a thermosetting mixture. This can result in the generation of "seeds," or high molecular grains of chemically reacted material. These seeds invariably cause film defects in the finished coating.
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. Sometimes a formula may contain difficult to disperse components such as carbon black pigment. In these instances, it may require a slower screw speed to achieve adequate dispersion.
Extruders can be cleaned efficiently by feeding a suitable thermoplastic resin after the batch has been compounded. Rigid grades of polyvinyl chloride and polypropylene can be used to purge and clean an extruder. The thermoplastic resin is fed into the extruder and can be reintroduced in its molten state. This action scrubs the interior of the barrel and the kneading blocks or flights of screws. The exit die of the extruder should be removed and cleaned after the purging process because material can hang up in dead spaces associated with the die. Specific materials have been developed for cleaning extruders. These products contain proprietary blends of thermoplastic resins, fillers and wetting agents and are claimed to be more efficient than off-the-shelf grades of PVC or polypropylene.
After extrusion, the formula cannot be easily altered. Modification can be accomplished by re-extruding material with raw components; however, this process is costly and should be avoided.
Extrudate exits the barrel at a relatively high temperature typically ranging between 212 to 284°F (100 to 140°C). This molten material needs to be rapidly cooled to avoid pre-reaction, or "B-staging." The cooling process 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 comminution or milling process.
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