Plastic welding

From Plastics Wiki, free encyclopedia

Plastics welding is the process of joining two pieces of thermoplastics at heated state and under a pressure as a result of cross-linking of their polymer molecules. The work pieces are fused together with or without filler material. The joint forms when the parts are cooled below the Glass transition temperature (for amorphous polymers) or below the melting temperature (for crystalline polymers).

Thermosets (thermosetting resines) in cured condition cannot be welded, since cross-linking of their molecules has completed.

1 Types of plastic welding

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Types of plastic welding

Plastics welding processes:

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Hot gas welding

Hot Gas Welding is a plastics welding process, utilizing heat of hot gas stream. The gas (usually air) is heated by electric heating elements mounted within the welding gun.

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Hot plate welding

Hot Plate Welding is a plastics welding process, utilizing heat of hot plate placed between the surfaces to be joined.

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Ultrasonic welding

Ultrasonic Welding is a plastics welding process, in which two work pieces are bonded as a result of a pressure exerted to the welded parts combined with application of high frequency acoustic vibration (ultrasonic).

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Spin welding

Spin Welding is a plastics welding process, in which two cylindrical parts are brought in contact by a friction pressure when one of them rotates. Friction between the parts results in heating their ends. After a predetermined time the rotation stops and the molten regions of the work pieces are fused together under an axial pressure applied until the joint is cooled down.

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Vibration welding

Vibration Welding is plastics welding process, in which two work pieces are vibrated at certain frequency and ampltude. The parts rubb each other under a pressure causing a friction between their surfaces, which generates heat. The heat results in melting polymer in the joint region. The work pieces are fused together and after a predetermined time the vibration stops. The polymer molecules are cross-linked when the work pieces cool down, forming a strong joint.

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Contact welding

This is the same as spot welding except that heat is supplied with convection of the pincher tips, instead of electrical conduction. Two plastic parts are brought together and heated tips pinch them, melting and joining the parts in the process.

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High frequency welding

Certain plastics with chemical dipoles, such as polyvinyl chloride (PVC), polyamides (PA) and acetates can be heated with high frequency electromagnetic waves. High frequency welding uses this property to melt the plastics for joining. The heating can be localized and the process can be continuous.

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Laser welding

This technique requires one part to be translucent to the Laser and the other one to be opaque. The two parts are put under pressure while the laser beam moves along the joining line. The beam passes through the first part and is absorbed by the other one generating enough heat to melt the interface. Once the laser beam is removed, the interface solidifies creating a permanent bond.

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Solvent Welding

In solvent welding, a solvent is applied which can temporarily dissolve the polymer at room temperature. When this occurs, the polymer chains are free to move in the liquid and can entangle with other similarly dissolved chains in the other component. Given sufficient time, the solvent will permeate through the polymer and out into the environment, so that the chains lose their mobility. This leaves a solid mass of entangled polymer chains which constitutes a solvent weld.

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