Plastic welding

Plastic welding is the process of creating a molecular bond between two compatible thermoplastics. Welding offers superior strength, and often drastically reduced cycle times, to mechanical joining (snap fits, screws) and chemical bonding (adhesives). There are three main steps to any weld: pressing, heating, and cooling. The application of pressure, which is often used throughout both the heating and cooling stages, is used to keep the parts in the proper orientation and to improve melt flow across the interface. The purpose of the heating stage is to allow intermolecular diffusion from one part to the other across the surface (melt mixing). 

Cooling is necessary to solidify the newly formed bond; the execution of this stage can have a significant effect on weld strength.There are several possible methods of plastic welding: Ultrasonics, Vibration, Spin, Hot Plate, Laser / Infrared, Radio Frequency, and Implant are the most common. These plastic welding processes are primarily differentiated by their heating methods. The application of pressure and allowances for cooling are mechanical considerations may vary from machine to machine within the general process category.

Pressure:
The use of pressure during the weld serves multiple purposes:

• Flattens surface asperities to increase part contact at joint.
• Maintains orientation of part .
• Compresses melt layer to encourage intermolecular diffusion between the two parts
• Prevents formation of voids from part shrinkage during cooling.
• Historically, pressure has been applied for plastic welding through the use of pneumatic presses. 

Recently, however, servo motors have been employed for at least a few of the common processes. Pneumatic welders are economical and well-suited to most simple applications. The precision of servo motion, however, offers greater control and precision which is desirable for more difficult applications or when the equipment is used for a wide variety of applications.