Powder coating production

The production of powder coating is a multistage polymer processing operation.

Pre-mixing and dry blending

The first stage is to accurately weigh the required dry powders and then mix them together. This produces the pre-mix.

Compounding

The second stage is described as compounding and is commonly carried out in an extruder.

The pre-mix is fed into the extruder where it is compacted and heated until it melts. Shear forces then break down the pigment aggregates to form a homogenous dispersion which is subsequently discharged from the extruder.

Figure 1: Extruder processing zones

Cooling/Flaking

The next stage converts the hot melt into a cool, hard, brittle strip by passing it through cooled rollers. Depending on the size of the extruder and its production rate, the extrudate may be further cooled on a cooling band. The final operation in the cooling stage is to break the cold, brittle extrudate into small flakes around 5 to 15mm (called kibble) in diameter, using rotating hammers fitted at the end of the cooling stage. Toner powders are also sometimes further pulverized down to between 50 to 250µm to give a product called kneaded toner.

Milling/Classification

The final stages of the manufacturing process are milling or grinding, and classification. These processes convert the kibble or kneaded toner into a fine powder with a specified particle size range. In the coatings industry micropulverizers are most commonly used for this application. Toner processes tend to use impact or jet milling.

Kibble or kneaded toner is transferred from a feed hopper into the mill/pulverizer by a screw feed. This screw further reduces the product size as it feeds the mill. The product is carried in an air stream into the milling stage(s) where it is reduced to a fine powder by a rotating disc fitted with metal pins or, in a jet mill, by being fired directly at ceramic collision plates, or at another material stream. The milling or pulverizing is caused by the product particles impacting with ceramic plates, each other, or with the rotating pins depending upon the mill design.

The milled powder is transferred from the mill/pulverizer on an air stream designed to ensure that oversized particles drop down for return to the milling chamber. Particle size distribution is effectively controlled using a combination of rate of feed and mill parameters such as speed of rotating pin disc and/or air velocity.

In toner applications either cyclonic or mechanical wheel separation is used to separate the oversize particles for further milling. Most toner manufacturing plants have between two to four classification stages to deliver the final particle distribution needed. Powder transport between each classification stage is done by vacuum transfer. The transport air is separated from the powder using either cyclonic or mechanical wheel separation. The extracted air is filtered to remove any unwanted fine powder.

Collection

The milled/classified powder is then transferred to a collection chamber. This may be a cyclone in which powder falls to the bottom whilst air is exhausted from the top, filtered and then exhausted to atmosphere, or a bag filter system. In toner applications the powder is generally collected in a weighing vessel. This allows batch processing with other additives (such as silicates) which are required for the copy process. These additives are fed by separate screw feed system and the batches are mixed together mechanically then discharged to the packing stage.

Finished powder generally undergoes a final sieving process to remove any possible contaminants before being packed ready for sale or use.