Thermoplastic materials, such as PP, can be easily welded at the corresponding temperature. Temperature, pressure and time are crucial for the quality of welding for all welding methods. Methods: Thermal pulse, heat contact, separating seam, ultrasonic and high frequency. Corona pretreatment and having too much colouring or additives can affect the weld strength. Sealing means binding film surfaces using "seal seams" or "seal surfaces," usually by applying higher temperatures. The word "sealing" actually applies to film that cannot be welded, e.g. aluminium foil or cellophane which are made suitable for welding by applying sealing layer. Sealing is used in many methods of film processing, particularly for manufacturing and sealing bags or sacks, for attaching cover films to cups or bowls, for shaping, filling and sealing methods, and for manufacturing blister packs. Heat sealing is most common today, cold sealing is limited to special applications.