Process Toolkit - Plastic Joining

Hot Plate Welding	Hot plate welding is used for larger plastic components. A hot plate is held between the components to be joined, and each component is held against the plate. When the two surfaces have softened, the components are pulled away, the hot plate is removed, and the two components are brought together under mild pressure.
Laser Welding	Laser welding of plastics is a relatively new process. Generally, one component is transparent, and the other is not. The laser beam is focuse through the transparent component at the joint between the two components. The joint is then melted, and the weld created.
Orbitforming or Orbital Riveting	With orbitforming, a metal pin or boss is deformed to create a retention head, similar to a rivet head. The orbitform process is essentially a rotary forming process, where the material is slowly worked over to form a head. Orbitforming can work with a variety of materials, but works best with ductile materials that are easily formed. Less ductile materials require much higher forces, which make automating the process more difficult. Some plastics can also be orbit formed.
Press Fit Assembly	Press-fit assembly uses the interference between male and female features to create a permanent joint. Pressure created by mating the two features results in static friction which can be sufficient to hold the joint together under modest loads. Press-fits can be executed with reasonable forces between plastic/metal and plastic/plastic components. Metal to metal press fits can also be created, but generally require much high forces and are extremely sensitive to small variations in the relative size of parts. When designing a press fit joint, it is important to keep in mind that the plastic components will generally creep under load, and the joint pressure will drop over time. Including barbs or other retentive features often alleviates this condition. Cracking is a common problem during deployment of press-fit joints. Reducing the interference will alleviate this problem, but it may be possible to reduce the problem by increasing the insertion speed during assembly. Press fit joints may be susceptible to chemical induced cracking, depending on the stress and materials involved.
Solvent Bonding	Solvent bonding is a messy and difficult process to control. The solvent is generally very low in viscosity, and is difficult to dispense with any precision. Solvent frequently ends up in locations other than intended.
Spin Welding	Spin welding is a process wherein two plastic components are melted together. Genearlly used on revolute components, the two components are spun relative to one another, and then brought into contact. The high-speed combination generates friction, which causes the plastic at the interface to melt. Features may be added at the interface to promote and control the melting process. Spin welding is often used for low-strength plastics to create sealed containers.
Thermal Impact Staking	Thermal impact staking is a variant of thermal staking, where a cold mandrel is used to form the head. With thermal impact staking, the target boss is heated with a separate hot air or IR source, and then cold-headed. Thermal impact stakers are more complicated devices, requiring the independent movement of various parts of the mechanism. However, because the mandrel is cold, the boss can be clamped during cooling, reducing the relaxation effect seen in typical heat staking.
Thermal Staking	Thermal, or heat, staking is used in plastics assembly to join two pieces together. Typically, the process is used to form a "head" on a plastic boss to retain another component in place. A heated mandrel is used to soften the plastic boss, and then form the head, as pressure is applied from above. The process is affected by speed, dwell time, mandrell temperature, and the material being formed. Excess melting can lead to stringing, where the boss material adheres to the mandrel after the process is complete. Excessive pressure or speed can also lead to buckling or uneven head formation. Thermal staking does not create a tight "clamped" bond. Because the boss is not cool upon release, the joint tends to relax before solidifying. However, thermal staking is quite effective where a small amount of looseness is acceptable.
Ultrasonic Staking	Ultrasonic staking is a plastics joining method, where the ultrasonic process is used to form a boss or other retentive feature, rather than a weld. Ultrasonic staking is similar to thermal staking, but without the relaxation phenomena, as the tool can continue to clamp after energy has been removed from the joint. Ultrasonic staking is typically used where higher joint clamping forces are required, or very precise boss geometries are needed.
Ultasonic Welding	Ultrasonic welding is a common joining technology for plastics. It can be used on many thermoplastics, and can create water-sealed joints between well designed components. In a typical application, two components are mated in an alignment fixture, and then the ultasonic horn is applied to the upper component. The ultrasonic horn, driven by the ultrasonic amplifier, oscillates vertically at high-frequency (20 to 40 kHz), creating heat energy at the welding joint between the two components. An energy director or other feature is usually incorporated into the upper piece to focus the weld energy and create a reliable joint. Other joints are used for more difficult materials.

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