Job Knowledge 62
One of the key factors to successful ultrasonic welding is good component design, in particular the shape of the surfaces to be joined ie the joint design. There are a variety of joint designs possible, each with specific features and advantages. The choice of design depends on the following factors:
- Type of thermoplastic
- Part geometry
- Weld requirements
One of the basic requirements of any joint design for ultrasonic welding is a small, uniform initial contact area. This can be achieved using a projection joint or a shear joint.
The basic projection or energy director joint is shown in Fig.1. The joint consists of a small triangular section moulded into the component and typically running the length of the joint perimeter. The purpose of the energy director is to focus the ultrasonic energy at the apex, resulting in a rapid build up of heat. This causes the triangular section to melt and flow across the joint interface, forming a weld.
The type of thermoplastic to be welded determines the form of the triangular energy director. Amorphous materials require a right-angled triangle with the 90° angle at the apex. For semi-crystalline materials, a 60° equilateral projection is used. Typical heights for energy directors are between 0.2 to 1.0mm, depending on the material.
The projection joint is favoured for use with amorphous materials such as PC, ABS and PS where a hermetic seal is not required.
Figure 2 shows a variation of the projection joint, in the form of a tongue and groove design. The advantage of this design is that the weld flash is hidden and the parts to be joined are self-locating. However, joint strength is relatively weak since the weld is only about half the width of the joint.
For some applications, a projection joint may not provide sufficient strength. In such cases, a shear joint can be used.
A basic shear joint design is shown in Fig.3. The joint allows one component to shear inside the other, providing self-location. Welding is accomplished by first melting the small initial contact area and then continuing to melt with a controlled interference along the vertical walls as the parts telescope together. The smearing action of the two melt surfaces at the weld interface is beneficial for two reasons.
Firstly it eliminates leaks and voids, so a strong, hermetic weld is produced. Secondly, it eliminates exposure to air, preventing premature solidification. This is particularly important for semi-crystalline materials, which rapidly change from a molten state to a solid state. As such, semi-crystalline materials should only be ultrasonically welded with a shear joint.
The vertical dimension of the weld, typically between 1.0 and 1.5mm, controls the strength of the joint and can be adjusted to suit the requirements of the application.
A design consideration with this type of joint is the wall thickness of the lower part, which should be sufficient to prevent outward movement during welding. Side-wall support from a fixture jig should also be provided.
Other design considerations
Aside from the joint design, other aspects of the moulded component must be considered if ultrasonic welding is to be effective.
The distance between the joint line and the contact surface where the welding horn meets the component can be critical. Far field welding, as shown in Fig.4, is where the distance is greater than 6mm. This arrangement is best suited to rigid amorphous materials such as PS, ABS and PMMA, which have good ultrasound transmission properties. Many semi-crystalline materials, such as PP are poor transmitters of ultrasonic energy, requiring the joint to be as close as possible to the welding horn area. This is termed near field welding, as shown in Fig.5.
For all materials, the use of near field welding is preferable, since it tends to require shorter weld times and lower pressures.
Sharp corners on the moulding should be avoided, as these can localise stress, possibly leading to fracture under the action of the ultrasonic vibratory energy. Minimum radii of 0.2 to 0.5mm are suggested.
There are a number of parameters that must be selected correctly in order to achieve good ultrasonic welds. These include vibration amplitude, welding mode, downspeed, trigger pressure, weld time, hold time. For this article, the amplitude and welding modes are considered.
Successful welding depends on the proper amplitude of vibration occurring at the tip of the welding horn. For any booster/horn combination, the amplitude is fixed. Amplitude selection is based on the thermoplastic being welded such that the proper degree of melting is achieved. In general, semi-crystalline materials require more energy and, therefore, more horn tip amplitude compared to amorphous materials.
Process control on modern ultrasonic welding machines can allow the amplitude to be profiled. High amplitude may be used to initiate melting, followed by a lower amplitude to control the viscosity of the molten material.
Welding by time is termed an open-loop process. The components to be welded are assembled in the tooling fixture before the welding horn descends and makes contact. The ultrasound is then applied to the assembly for a fixed duration of time, typically between 0.2 to 1.0 seconds. This process gives no indication of successful welding.
It works on the assumption that a fixed weld time will result in a fixed amount of energy being applied to the joint, giving a controlled amount of melt. In reality, the power drawn to maintain amplitude is never the same from one cycle to the next. This is due to factors such as the fit between the components.
Therefore, since energy is a function of power and time, and time is fixed, the energy applied will vary from one component to the next. For mass production, where consistency is important, this is undesirable.
Welding by energy is a closed loop process, giving feedback control. The ultrasonic machine software measures the power being drawn and adjusts the exposure time so that the desired energy input to the joint is delivered.
The assumption with this process is that if the energy consumed is the same for every weld, the quantity of molten material in the joint is the same each time. However, in reality there are energy losses, within the welding stack and especially at the interface between the welding horn and the component.
As a result, some components may receive more energy than others, with the possibility of inconsistent weld strengths.
Welding by distance allows components to be joined by a specific weld depth. This mode operates independently of time, energy or power drawn and compensates for any tolerance variation in the moulded components, thus giving the best guarantee that the same amount of material in the joint is melted each time. Limits can be set on the amount of energy used or the time taken to make the weld, for the purposes of quality control.
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