Plastic joining - staking
by Anita Buxton

Summary
In the staking process, a stud protruding from one component fits into a hole in the second component. The stud is then deformed through the cold flow or melting of the plastic to form a head which mechanically locks the two components together. It is a versatile technique benefiting from being quick, economical and consistent. Unlike welding techniques, staking has the capacity to join plastics to other materials (e.g. metal) in addition to joining like or dissimilar plastics and it has the advantage over other mechanical joining methods in eliminating the need for consumables such as rivets and screws.

Scope
There are four methods of staking:
cold staking
heat staking
thermostaking
ultrasonic staking

The choice of process is dependent upon the materials to be joined, the loads to which the assembly will be subjected and the required cosmetic appearance. In each case, accurate alignment of the components is of utmost importance and the common design parameters include stud diameter, stud height and stud geometry. Due to the deformation of previously formed parts, staking is restricted to thermoplastic materials.

Cold staking
In cold staking, the stud is deformed through the application of high pressures. Cold flow subjects the stud region to high stresses and consequently it is only suitable for use with the more malleable plastics. The high pressures also render this technique unsuitable for the formation of tight assemblies.

Heat staking
In heat staking, the compression probe is heated so that less pressure is required to form a head on the stud. This widens the application of staking to a broader spectrum of thermoplastic materials than is possible with cold staking, including glass-filled materials. The quality of the joint is dependent on control of the processing parameters: temperature, pressure and time - a typical cycle time being between 1 and 5 seconds. Heat staking has the advantage that parts can be disassembled. It also has the flexibility to allow the simultaneous formation of a large number of studs and to accommodate a variety of stud head designs.

Thermostaking
In thermostaking or hot air staking, heat is applied to the stud by means of a stream of superheated air, delivered through a tube which surrounds the stud. A separate cold probe then lowers to compress the stud head. A variety of stud head designs are feasible by changing the probe tip design. Thermostaking is a high speed, low cost process and due to its highly controllable nature, it is especially advantageous in applications involving decorative surface finishes.

Ultrasonic staking
In ultrasonic staking, the stud is melted using ultrasonic energy supplied through a welding horn. During the continued pressure of the horn, the melted stud material flows into the cavity within the boss tip to form the required head design. Cycle times are typically less than two seconds and welds may be performed with a hand-held welding head.

Applications
Staking is widely used across a range of industries including, automotive (e.g. for attaching parts to door liner panels), telecommunications, electronics, medical and the consumer appliance market. It is becoming extensively used for assembling components on printed circuit boards and for fixing those circuit boards within their housing.

*Information courtesy of The Welding Institute World Center for Materials Joining Technology; UK