Welding

Welding is an effective method of permanently joining plastic components. There are various welding methods.

Welding works on the principle of a phase change from solid to liquid (melt or in solution) followed by a solidification phase at the interfaces to be joined. In several welding processes, some material will be squeezed out of the weld by the pressure on the mating surfaces. The velocity of the out flowing material has a parabolic profile over the width and increases towards the edges of the part as a consequence of accumulating melt flowing from the centre to the edges. See the figure.

The flow direction of the polymer melt is perpendicular to the direction of injection moulding: after the polymer solidifies this unfavourable orientation remains in the weld zone, which is the reason for the reduced strength of the weld compared to the bulk strength of the material.

Welding is aesthetically clean, and forms a very strong bond which is more-or-less permanent.

Welding of thermoplastic parts is based on interdiffusion of molecular chains.It requires elevated temperatures, pressure and time to achieve a good mechanical bond.

Molecular interdiffusion and entanglement during welding

Welding techniques

There are a variety of welding techniques. Generally these techniques can be distinguished into two basic types.

In friction welding the required heat energy is generated by friction between the two parts due to relative motion, as in vibration welding, spin welding, and ultrasonic welding.

An external heat source is used in case of hot plate welding, laser welding, radio frequency welding (or Dielectric or High Frequency Welding), induction welding (or Electromagnetic Welding), resistance welding, hot gas welding.

The major advantages and disadvantages of the various welding methods are given in the table below.

Advantages and disadvantages of various welding techniques

Staking is not a welding process, but there are some similarities. In this process one part is provided with studs, which protrude through holes in the other part. The studs are then deformed through the cold flow or melting of the plastic to form a head which mechanically locks the two components together. Staking is specially suited to connect parts made from dissimilar materials (e.g. plastic to metal).

Costs

One of the decisive factors in the selection of the optimal welding technique is the welding cost per part. This cost depend on the batch sizes, as shown in the figure below. The effect of the batch size on the cost per part is not the same for every welding technique. This means that the most cost-effective process also depends on the batch size.