# An Automatic Foreign Matter Detection and Sorting System for PVC Powder
## 1 Introduction  
Polyvinyl chloride (PVC) is a polymer material obtained by the additional polymerization of vinyl chloride. It is the third most widely synthetic polymer after polyethylene and polypropylene [1]. PVC is synthesized with the addition of pure water, liquefied vinyl chloride monomer (VCM), and dispersant to the reactor following the addition of the initiator and other auxiliary agents in the suspension polymerization process. The PVC powder is generated as the VCM undergoes radical polymerization when a high temperature of the reactor is reached. During the formation process, continuous stirring is required to obtain a uniform particle size suspended in water. After removal of residual VCM through a stripper, PVC slurry is subjected to a drying process, followed by sieving before being loaded into packages. A flow diagram of the PVC production process is presented in Figure 1.  
However, some issues remain in the PVC manufacturing process. For examples, the presence of choking occurs due to local high temperature, or rust in the piping system during the manufacturing process. If these faults are not excluded, the yield of related polymer products could be affected. Usually, two-step quality inspections, magnetic and visual selections, are performed before packing and shipping. After the magnetic selection of rust (black color) is carried out by pouring a certain amount of PVC powder into a funnel and passing it through a magnetic bar (as shown in Figure 2a), non-magnetic slags (dark pink color) are inspected with visual observation by pouring the same powder onto a white paper and viewing through a magnifying glass with a brush and puffed air under strong backlight. Non-magnetic slags are picked out with a marker on white paper as an inspection report, as shown in Figure 2b. Because it is difficult for human eyes to identify non-magnetic slags in PVC powder with a particle size ranging from 120 to 150 \(\upmu\)m, the inspection time required is about 30 min for a pack of 500 g of samples. In addition, the inspectors will be troubled by persistence of vision (POV) due to constant staring at similar background textures, which may cause eye injuries in the long term. In the manufacturing process of PVC powder mentioned above, a solution that helps to reduce the overall cost is to change the inspection with human eyes to machine vision inspection before packaging and shipment, so that there will be an opportunity to improve the quality of inspection and reduce work injuries and other related problems. Root cause analysis can be conducted by tracing the time of occurrence of foreign matter and analyzing its characteristics. These results can be utilized as feedback to the processing personnel for machine parameter adjustment. Figure 1: Flow diagram of suspension PVC production. Figure 2: The manual foreign matter inspection method. (**a**) Scenario of magnetic inspection; (**b**) visual inspection report. Studies of foreign matter detection in powder can be found from over 30 years ago [2; 3; 4]. However, there were few related investigations after those patents had been issued. For the last decade, research on the detection of foreign matter in powders has been widely conducted in the field of crushed materials such as minerals, plastics, metals, and dry powders. These investigations were probably performed due to the expiration of the patent rights for these early inventions and the conversion of related knowledge into public property. In these studies, the powders vary in size and color. Nevertheless, the main objective is to detect foreign particles or pollutants during the manufacturing process. In addition, food powder inspection is another issue that is a concern of food safety. The purpose of food powder inspection is to identify environmental hormones or additives harmful to the human body mixed into the food.