- Pressing – the application of pressure is important in both the heating and cooling stages since it is used to keep the part in proper orientation and to improve melt flow across the surface
- Heating – allows intermolecular diffusion from one part to the other across the faying surface
- Cooling – is necessary to solidify the newly formed bond which can have a significant effect on weld strength
The importance of using pressure in Plastic Welding
In Plastic Welding, pressure used to be applied through the application of pneumatic presses but lately, servo motors are being employed for at least a few of the common processes. Pneumatic presses are considered more economical and suitable for simple applications while servo motors offers greater control and precision which is required for more difficult applications or when the equipment is used for a variety of applications.
The use of pressure during Plastic Welding process provides multiple purposes;
- It flattens the surface asperities to increase part contact at a joint
- It maintains orientation of a part
- It compresses melt layer to encourage intermolecular diffusion between the parts
- It prevents the formation of voids from part shrinkage during the cooling stage
Methods of Plastic Welding
Ultrasonic welding – a welding technique that makes use of high frequency ultrasonic acoustic vibrations that are locally applied to the workpieces that are being bonded together under pressure to create a solid state weld. Ultrasonic plastic welding is accomplished by converting high-frequency electrical energy into high frequency mechanical motion which along with applied force will create frictional heat at the plastic component’s mating surfaces so that the plastic material will melt and form a molecular bond between the parts.
Vibration welding – is a frictional welding technique capable of producing strong, air-tight welds in thermoplastic parts through the use vibration. In the process, vibration occurs through transverse reciprocating motion that is controlled electro-magnetically by a swing frame assembly containing precision springs, electromagnets and an electromagnetic drive assembly which controls both amplitude and frequency of the vibrating head.
Spin welding – is a technique of welding thermoplastic parts using a circular axis joint that has high bond strength/hermetic requirements. During spin welding, one part is held stationary in a holding fixture while the other is rotated against it under pressure at speeds up to 16,000 rounds per minute (rpm). The resulting friction causes the joining surfaces to melt and fuse together to produce a strong hermetic weld.
Laser/infrared welding – heating and melting during radiation welding of plastic through broadband infrared emitters are based upon the absorption of non-reflected radiation energy of the polymer matrix and the fillers therein. This technology provides for more accurate part temperature control and eliminates plastic sticking to tooling.