Semi-crystalline plastic is a type of plastic that has a partially crystalline molecular structure. In semi-crystalline plastics, the polymer chains are arranged in both crystalline and amorphous regions, giving the material a unique combination of properties.

Semi-crystalline plastics are typically characterized by their high strength, stiffness, and resistance to creep (deformation over time). They also have a high melting point and can be molded into complex shapes with good dimensional stability.

Examples of semi-crystalline plastics include polyethylene (PE), polypropylene (PP), polyamide (PA or Nylon), and polyethylene terephthalate (PET). These materials are commonly used in a wide range of applications, including automotive components, medical devices, consumer products, and packaging.

The unique properties of semi-crystalline plastics make them well-suited for many different applications, but also present challenges in processing and manufacturing. The crystalline structure of the material can make it difficult to achieve consistent welds using traditional welding methods, such as heat welding. Ultrasonic welding is a popular technique for welding semi-crystalline plastics, as it does not rely on heat to create the weld and can therefore produce strong, consistent welds without damaging the material.