Plastic welding is the process of creating a molecular bond between two compatible thermoplastics.
Different Stages of Plastic Welding
- Heating: In the plastic welding process it is crucial to form a melt layer at the faying surface to allow intermolecular diffusion to form a molecular bond. In the solid state polymer chains will not flow. Therefore, the joint surface on both parts must be melted to allow the plastic molecules to diffuse across the interface and bond with molecules of the other part. The hotter the melt is, the more molecular movement is achieved, and a weld can be made in a shorter cycle time. Amorphous polymers must be heated to above their glass transition temperature and semi-crystalline polymers must be heated to above their melting temperature.
- Force: Force is applied throughout the welding process. The use of force during the weld flattens surface asperities to increase part contact at the joint. Applying force improves melt flow across the interface and compresses melt layers to encourage intermolecular diffusion between the two parts. It prevents formation of voids from part shrinkage during cooling and maintains part orientation. Historically, force has been applied using pneumatic presses. Recently, servo motors are used for common processes. Servo motors offer precise control and consistency to the process.
- Cooling: Cooling is important to solidify the newly formed bond; the execution of this stage has a significant effect on weld strength. For semi-crystalline materials, the cooling stage or “hold” phase, provides the opportunity for the polymer to re-crystallize. The rate of cooling will affect the final microstructure. For amorphous polymers, the cooling stage solidifies the microstructure into the orientation created by the melt flow. The force applied during this stage, the time allotted for it prior to putting the part under stress, and the rate of cooling all have significant effects on the final weld strength.