Prior to deciding on the joint design for a particular material/component, it is necessary to have an appreciation of the factors which determine the specific type of equipment to be used.
What then are the parameters that need to be considered when proposing ultrasonic welding for an assembly?1. Frequency
2. Amplitude
3. Acoustic property of material
4. Design of joint
5. Equipment capability
6. Sonotrode design
Taking each point in turn, all are important.
Frequency
The lower the operating frequency the better the energy will conduct through the component, so 20 kHz is generally better for welding of a crystalline material such as, polypropylene, as more of the ultrasonic energy will reach the weld area. However, if surface use of an ultrasonic process is being considered such as, spot welding or riveting, then 40 kHz would be a better choice. This frequency could also be beneficial in preventing unwanted ultrasonic activity in other parts of the assembly.The additional advantages of 40 kHz equipment are - it produces less noise, it offers part protection and the tooling is small. It is ideal for hand application or multi-head machines for the assembly of less robust parts.
Amplitude
The amplitude is determined by the equipment selected and the booster and sonotrode ratios discussed earlier. This feature is usually determined by the equipment manufacturer and cannot usually be varied.Acoustic Property Of The Material
This is quite simply the ability of a material to transmit the vibratory energy. If a material does not have this ability, then it cannot be welded using ultrasonics, but could be joined other means. Materials with low acoustic properties should not be totally ruled out when considering this application.Design Of Joint
It is not a good idea to simply try to excite the complete surface area of the parts to be joined. This would just lead to dissipation of the energy and although a join might be achieved, it would not be a very good one either in strength or looks. The ideal joint design must try to focus the vibrational energy and maximise the acoustic property of the material in the joint area.The choice of energy director type, i.e. triangular or shear joint, again depends on a number of factors including the material, size of joint area, whether a seal is required, moulding capabilities etc.
For a triangular type of energy director of suitable dimensions there is certain formula for guidance of which the rule of thumb instruction is the basis. To obtain the height of the energy director, divide the width of the joint by 8 and to find the base width, divide the same measurement by 4.
Note that when ultrasonic energy is transmitted through a component it can cause stress cracking at sharp corners, consequently it is recommended that all internal corners have a radius of minimum 0.2mm.
Equipment Capability
The size of the machine will be initially determined by the size and material of the assembly to be welded. In general the amorphous materials conduct ultrasonics extremely well and hence, do not consume too much power. In contrast, the more crystalline the materials, the greater the power required to effect a weld and the closer the sonotrode contact face has to be to the joint area.The following standard machine sizes are available in the Xfurth range of ultrasonic welders:
- 400 watts 40 kHz
- 800 watts 40 kHz
- 800 watts 35 kHz
- 1000 watts 30 kHz
- 2000 watts 25 kHz
- 1500 watts 20 kHz
- 2000 watts 20 kHz
- 3000 watts 20 kHz
The component to be welded is the yardstick that will determine the frequency to be used. The component can be quite large in area but the actual weld point or points may be quite small and fragile. As already mentioned when discussing frequency, it is important to decide on the correct frequency before any other factors are considered.
Sonotrode Design
This is, after the frequency choice has been made, the area in which success or failure of the weld will be determined. As it is quite a specialised area, a complete and more comprehensive section of the course has been allotted to it. This will explain tool design in considerable detail and expand on the points already discussed.
The following pages outline basic information regarding material properties and joint design. It is well to remember that experience in the design for ultrasonic welding can often greatly assist in obtaining an optimum design for a particular application. With the wealth of experience Xfurth Ltd have built up over the years, we would once again stress the assistance we are able to offer with all types of applications and offer this service free of charge to our customers.
