1. Density - based Plastic Sorting Machines

• Working Principle
  Density - based plastic sorting machines operate on the principle that different types of plastics have distinct densities. For example, high - density polyethylene (HDPE) has a density ranging from about 0.941 - 0.965 g/cm³, while low - density polyethylene (LDPE) has a density of 0.910 - 0.940 g/cm³. These machines often use a liquid medium with a specific density. Plastics are placed in this medium, and those with a density lower than the liquid will float, while those with a higher density will sink.
• Features
  They are relatively simple in structure and cost - effective for basic plastic sorting. However, the accuracy may be affected by factors such as the presence of impurities in plastics and the stability of the liquid medium density.
• Applications
  Widely used in small - to - medium - scale plastic recycling plants for the initial separation of common plastics like HDPE, LDPE, and polypropylene (PP), which have significant density differences.

2. Near - Infrared (NIR) Sorting Machines

• Working Principle
  Near - infrared sorting machines utilize the fact that different plastics absorb and reflect near - infrared light in unique patterns. Each type of plastic has a characteristic near - infrared spectrum due to its molecular structure. The machine emits near - infrared light onto the plastic materials, and sensors detect the reflected light. By comparing the detected spectra with a pre - stored database of plastic spectra, the machine can accurately identify and sort different plastics.
• Features
  Highly accurate, capable of sorting a wide range of plastics, including those with similar densities. It can handle complex mixtures of plastics and is suitable for modern, high - volume recycling operations. However, it requires regular calibration and is relatively expensive to purchase and maintain.
• Applications
  Commonly used in large - scale recycling facilities and high - end plastic recycling industries, where the separation of various plastic types, such as separating polyvinyl chloride (PVC) from other plastics in electronic waste plastic mixtures, is crucial.

3. Electrostatic Sorting Machines

• Working Principle
  Electrostatic sorting machines rely on the different electrostatic properties of plastics. When plastics are subjected to an electrostatic field, they acquire different charges. For instance, some plastics will become positively charged, while others will be negatively charged. The charged plastics then move differently in an electric field, allowing for separation. This can be achieved through methods such as triboelectric charging, where plastics are rubbed against each other or a specific material to generate charges.
• Features
  Can effectively separate plastics with different electrostatic behaviors. It is suitable for separating plastics that are difficult to distinguish by other methods. However, the performance can be affected by humidity and the surface condition of the plastics.
• Applications
  Used in recycling plants that deal with mixed plastic waste, especially when separating plastics with similar densities but different electrostatic properties, like separating certain types of engineering plastics from general - purpose plastics.

4. Optical Sorting Machines (Based on Color and Shape)

• Working Principle
  These machines use cameras and image - recognition software to analyze the color and shape of plastic materials. Different types of plastics may have distinct colors or shapes after being processed into a certain form. For example, PET bottles are often transparent or have a characteristic blue - tinted transparency, and their shape is typically cylindrical with a specific neck design. The image - recognition system compares the captured images of plastics with a database of known plastic color and shape characteristics to identify and sort them.
• Features
  Simple to operate and can quickly sort plastics based on visual characteristics. However, it may be less effective when plastics have been discolored due to aging or contamination, or when the shapes are irregular due to improper pre - processing.
• Applications
  Applied in recycling facilities for the initial screening of plastics, such as separating colored plastics from transparent ones or sorting plastics with distinct shapes, like separating plastic containers from plastic films in household waste plastic collections.