subject: The Best Assembly Techniques [print this page] Welding technology - In addition to the assembly techniques described in article 7 of
this series, many different welding methods can be used to join plastic parts. To ensure
low-cost, functionally efficient designs, it is necessary to select a suitable welding
method and give careful thought to the required joint geometry at an early stage in the
design process.
Welded joints are assemblies for permanently connecting plastics parts without additional assembly elements. The choice of welding method depends on several criteria: the geometry of the moulded part and on the materials used, on cost-effectiveness, suitability for integration into the overall production cycle and the mechanical and aesthetic quality requirements for the assembly zone.
Different welding methods
There are many different, cost-effective welding methods suitable for industrial mass production. The methods most frequently used for plastics engineering components are:
- hot-tool welding
- spin welding
- vibration welding
- ultrasonic welding.
Other methods worth mentioning include:
- high-frequency welding
- induction welding
- hot-gas welding.
New methods are also being developed (e.g. laser welding), but these are not yet widely used in industry.
In all these methods, the assembly operation is carried out by applying heat (melting the surfaces to be joined) and pressure. Heat can be generated directly by contact or radiation, or indirectly by internal or external friction, or by electrical effects.
Choosing the right method
To achieve good, reproducible weld quality, it is necessary to choose a suitable welding method, optimize welding parameters and ensure that the parts to be assembled are correctly designed for the welding method being used. Welding machinery manufacturers supply not only standard equipment but also various special welding units to cater for a wide variety of welding tasks. Before deciding on a welding method of the plastic molding, it is advisable to consult the machinery manufacturers or resin suppliers.
Different welding properties
Theoretically, all thermoplastics are weldable, but the welding behaviour of plastics differs considerably in some cases. Amorphous and semi-crystalline polymers cannot be welded together. Plastics that absorb water (e.g. nylon) need to be pre-dried, since moisture leads to poor-quality welds. For best results, nylon parts should either be welded immediately after injection-moulding or kept in a dry state before welding. Resin additives such as glass fibres and stabilizers can also influence welding results. Welded assemblies of unreinforced plastics can attain weld factors close to the strength of the parent material, given suitable process parameters and part design. With glass-fibre-reinforced plastics, loss of strength due to fibre separation or reorientation in the welding zone must be taken into account.
