TechArchive

Original article date: March 1999

Ultrasonic welding is a versatile process which can be used to join a wide range of thermoplastics. Jerfurth’s BRIAN SMITH explains how to avoid some of the more common pitfalls.

The process of ultrasonic welding is not just about joining long sections of rigid thermoplastics- It can be used directly and indirectly to fasten injection mouldings extrusions and thermoformed products together either continuously or in discrete patches in a method similar to spot welding.

Thermoplastics are rate-dependent materials. What this means is that the faster a vibration is applied to a material the stiffer it is. Because ultrasonic welding relies on very high frequency vibration it is applicable to a wide range of thermoplastics not just those which are normally considered to be “engineering grades”.

In general terms the welding time should be kept as shod as possible. Ideally it should be less than 15s. Any longer than this and there is a real risk that the moulding will be damaged. If a satisfactory weld cannot be made within this time then a more powerful welder should be used. It is also important not to move the pad too soon after the ultrasonic energy is switched off or distortion will occur between the welded pads.

A second point to ensure is that the contact force between the plastic itself and the sonotrode – the device which puts the ultrasonic welding energy into the plastic – is not too great. If it is this could damage delicate pads.

There are four steps to getting the best out of ultrasonic welding and a methodical approach will pay dividends.

The sonotrode’s amplitude is the foundation of the process- While the best practical values fur a range of materials is shown in the accompanying table in general terms with amorphous plastics the sonotrode should be set to vibrate between 10 and 30 while with semi-crystalline materials a wider range of vibrations is the rule. Typically this should be between 25 and 50 cm.

Secondly the application force must be right. Lower amplitude vibrations need higher sonotrode contact pressures and vice versa. It is best to increase contact force gradually and reduce the amplitude of vibration so that the lowest level of energy to produce a strong weld is found. Using the wrong amplitude is inefficient and energy will be wasted in the welding process if the sonotrode is not applied with enough force to the pads. Moreover gradually increasing the contact pressure will give welders the chance to see if there is likely to be any danger of low melt viscosity polymer leaving the welding area.

For simple welding the generator should be switched on after the right contact force has been reached In flange-welding and riveting the vibrating sonotrode should be placed directly on the moulding. This melts the plastic very quickly and helps ensure a strong join. Similarly when using ultrasound to embed metal parts the sonotrode must be vibrating when the metal pads contact. This prevents the parts being embedded either cold or in incompletely fused holes.

Impact velocity is the last key. It should be set between 0.5 and 50mm/s. This has a decisive influence on welding quality. As it has to be found by trial and error working with an ultrasonic welding company can help to save time in this area.

If welders have followed this procedure and still unsatisfactory joins are being produced then the welder should look to the condition of the component. Porous welding seams and excessively long weld times point to pads which contain too much moisture. Ultrasonic welding works best in rigid thermoplastics. The rigidity of the pads depends on temperature and in some cases on moisture content.

Moisture content should be as low as possible except for polyamide parts — where properly conditioned parts which have absorbed the correct level of moisture give better welds than unconditioned parts. Components made in hygroscopic materials should be thoroughly dried before welding and kept in polyethylene bags for as long as possible.

Pads where welds show obvious flow seams shrinkage cavities and surface defects such as sink marks and poor welded surface appearance are probably being welded with a badly designed tool or a sonotrode designed for a different material.

Warpage and cracking after welding in semi-crystalline polymers are symptoms of a different problem – of being welded too soon after injection moulding. This is because re-melting a small portion of the polymers crystalline matrix causes unbalanced stresses to be developed within the rest of the part. Semi-crystalline polymers should therefore be stored for a day between injection moulding and welding. This phenomenon does not occur with amorphous polymers and these may be welded almost as soon as they have been thermoformed or injection moulded.

High levels of re-grind can also cause problems if care is not taken to find the melt properties of the material before welding. Virgin grades usually have a higher melt viscosity than re-granulated products and this can lead to problems of sink marks and surface marking if the same level of force is applied to the surface as would be used to weld virgin.

If the parts weld badly and there is little adhesion between the welded surfaces this could be due to either mould release agents or a dirty welding surface. Washing the welding faces will help here in conjunction with ensuring that the best welding conditions have been fund using the checklist.

• Herfurth UK
• Brian Smith
• Tel: 01582436000

March 1999