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 faying 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.

Heating

It is crucial to the plastic welding process to form a melt layer at the faying surface to allow intermolecular diffusion for formation of a molecular bond. In the solid state, polymer chains will not flow. Therefore, the joint surface on both of the 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 while semi-crystalline polymers must be heated to above their melting temperature.
In all types of plastic welding, only a thin layer of the parts are melted near the joint. It would be impractical to heat the entire part for several reasons:

• Heating only a small area takes less time, and reduces cooling time
• Limiting the melt also reduces the heat affected zone
• Maintains the molded micro structure of the bulk of the part
• Prevents excess shrinkage or warpage during cooling
• Allows rigid support of the part during welding

Ultrasonic plastic Welding

Ultrasonic plastic welding is the joining or reforming of thermoplastics through the use of heat generated from high-frequency mechanical motion. It is accomplished by converting high-frequency electrical energy into high-frequency mechanical motion. That mechanical motion, along with applied force, creates frictional heat at the plastic components' mating surfaces (joint area) so the plastic material will melt and form a molecular bond between the parts.  The following drawings illustrate the basic principle of ultrasonic welding.