Ultrasonic Welding

Topics Covered

How Does It Work?

Advantages Of Using Ultrasonic Welding

Welding Systems

Driving Force for Development

Reactive Welding Systems

Removing Variability

Joint Design

Welding of Metals

How Does It Work?

UItrasonic welding is a method of joining two parts by converting electrical energy into heat energy by high frequency mechanical vibration and is suitable for plastics and metals. One part of the assembly is set in motion in order to cause intense friction between it and the other part that remains static. This movement is provided by a vibrating component called a sonotrode which is applied at right angles to the surface of the part to be welded.

Friction is localised at the interface of the assembly. The resultant heat quickly melts the plastic which flows and amalgamates. After it has cooled, a solid homogeneous weld between the two components results. The vibration frequency of the sonotrode is 20 to 40 kHz which is outside the perception limit of the human ear.

Advantages Of Using Ultrasonic Welding

Ultrasonic plastics welding has advantages over joining methods using solvents or adhesives. Production cycle times are usually improved with speeds in the region of 2000 parts an hour being achieved in the most favourable cases. Additional advantages include clean exteriors of welded parts, potential manpower savings, absence of drying time in the jig, gas tight and completely stable assemblies, possibility of welding in the presence of foreign bodies such as powders and liquids, and of welding materials which are incompatible using any other conventional assembly processes.

Welding Systems

The basic components of an ultrasonic plastics welding system are an electric generator which transforms mains electricity of 50 cycles into a controlled high frequency electrical energy of 20,000 to 40,000 cycles with 200 to 3000 Watts of electrical power available. A transducer converts electrical frequency into mechanical vibration and a booster is used to increase or decrease or simply couple the mechanical vibration to the sonotrode. The sonotrode transmits the vibration to the area being joined. It is usually made from an aluminium alloy or an acoustic titanium material and is tuned to the relevant frequency. The mechanical vibration is then transmitted to the part to be welded with a controlled weld pressure and accurate weld time. The function of the press is to deliver this vibration under consistently rigid and controlled conditions. The jig must support and locate the part or parts to be welded with complete stability.

Methods and techniques for joining plastics, and the quality of the joints have advanced and developed dramatically in recent years. Plastics component manufacturers and parts assembly specialists require highly sophisticated plants to furnish an ever increasing market demand. Because of the inherent complexity of plastics welding, whether it is plastic to plastic or plastic to metal, industry requires expert guidance in order to determine their actual plant.

Driving Force for Development

One of the major instigators for the advancements that have been made, within Europe, in ultrasonics plastics welding is the automotive industry. As plastics have continued to supersede metal, leather and wooden finishes in cars, the assembly and jointing capability demanded has increased enormously. Not only has the automotive industry dictated investment in the continued development of ultrasonics plastics welding techniques but also that the control of these new techniques provides the highest finished quality. Industry has benefited from these developments.

Recent ultrasonic plastics welding machines have a high level of sophistication which allows them not only to be completely automatic but also able to control, adjust and protect themselves. This enables the user to aim at total weld quality control.

Reactive Welding System

The Forward Technology Industry Group's reactive welding system (RWS) ensures that the plastic parts are precisely assembled regardless of their dimensional tolerances. RWS is an operating mode which enables the machine to identify the ultrasonic disconnection time corresponding to a precisely welded part. Once the joint has been correctly welded, the ultrasonics are instantly switched off. Dependent on the joint tolerances, the sonotrode will determine the ultrasonics required to perfect the weld.

The reactive welding system ensures that quality welded joints can be achieved even though the tolerances of the components may vary considerably throughout an entire production programme. This allows all production units real control over their individual welding process and provides much more flexibility with regard to component tolerances, resulting in a more efficient and effective assembly production.

Removing Variability

One of the difficulties with this application is the variable dimensions of the plastic components to be welded such as extruded materials which have high dimensional tolerances. More complex machines are able to compensate for these factors and produce repeatable products based on the variability on feed materials. Such machines not only perform the weld but monitor the things like the original length to ensure they are all finally welded to the same depth using a linear encoder.

More recent ultrasonic plastics welding machines are very technically advanced. For example, a standard model is simple to operate; no manual adjustment is needed and automatic tuning is provided. Adjustment is achieved by memorising the height of the part to be welded. This ensures that inaccuracies in dimensional tolerance of the component are instantly allowed for. All welding parameters are automatically controlled and adjusted which protects the acoustic stack assembly and ensures accuracy of all welds.

An example of precision jointing is the assembly of the new metric spirit dispensers. The spirit measure or optic unit is assembled by two separate ultrasonic welding operations that provide an exact metric measure as well as a hermetically sealed dispenser. The liquid inlet is ultrasonically welded to the spirit dispenser's body and then the transparent viewing glass is welded on to the front of the unit. Both of these welds must be completely leak proof and provide a joint with finite tolerances that will ensure the volume of the liquid contents is reliably accurate.

A microprocessor welder is capable of this assembly because of its precision and repeatability. It monitors the depth of closure of the joints, an essential requirement in providing the accuracy of weld, which in turn ensures accuracy of the liquids dispensed. The welded joints also ensure that the finished dispenser is tamper-proof.

Joint Design

Joint design is a critical part of plastics welding, whether close or distant welding procedures are being performed. Close or near field welding is when the vibrating sonotrode is applied very close to the assembly joint. This method is necessary for welding plastic materials that are bad conductors of ultrasonic waves. This is the case with plastics that have a low modulus of rigidity meaning they have little capability for transmitting vibrations. Plastics that require close welding are nylons, polypropylenes and acetates.

The technique of close welding requires that the sonotrode adopt, almost exactly, the contour of the joint to be welded. Conversely, distant or far field welding is when the sonotrode is not applied in the immediate vicinity of the welding point. The success of this technique depends essentially on the ability of the plastics materials to propagate vibrations without damping them. Excellent results can be obtained with general purpose polystyrene, acrylonitrile butadiene styrene, and styrene acrylonitrile, as these are rigid materials, with a high rigidity modulus. The method is not limited, as it permits the welding of objects of very complex design using a sonotrode which is very simple in form. There is obviously a critical distance for each plastics material beyond which homogeneous welding is not possible. Distant welding permits welding of objects of large dimensions by applying several carefully placed sonotrodes whilst taking into account the critical distances.

Welding of Metals

Metal welding can also be performed using ultrasonics. The principle of the process is based on the interaction of weld pressure and relative movement of the components to be welded. The basic design of the ultrasonic metal welding machines differs from that of the plastics machines in the arrangement of the welding heads and consequently the transmission of the welding energy.

The welding head is arranged horizontally and therefore transmits transverse oscillations to the parts to be welded. Machines are equipped to weld continuous welds using rotating welding heads or for spot welding. Metal welders are available that allow the programming of welding parameters which include constant time, constant height and constant travel modes. They achieve rapid weld adjustment potential by memorising the height of the part to be welded, as well as automatic adjustment of the welding programme and the cooling time.

Modern ultrasonic welders provide the user with the skills to perform accurate and repeatable jointing programmes, speedily and constantly.

 

Primary author: Gordon Franklin

Source: Abstracted from Materials World, vol. 3, pp. 279-80, 1995.

 

For more information on this source please visit The Institute of Materials

 

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