Typical methods for joining plastic parts include screw connections, snap fasteners, press fittings, adhesive bonding, and welding. Welding is an effective method for permanently joining plastic parts. There are various plastic welding processes, including ultrasonic welding, vibration friction welding, laser welding, hot plate welding, and infrared welding.
Vibration friction welding is typically used for welding large parts. For smaller parts, it can be used to weld multiple parts (2-4 pieces) at once, achieving a more economical and faster process. The most important process parameters for vibration friction welding are frequency, amplitude, pressure, time, and weld depth. Optimizing these parameters can achieve very high weld strength. However, the setting of welding parameters depends on the material type, geometry, and cleanliness requirements.
Vibration Friction Welding Machine Process Parameters
1. Frequency
Vibration friction welding machines have two operating frequencies: a high-frequency vibration mode (200-240Hz) and a low-frequency vibration mode (80-120Hz). The choice of frequency depends on the weight and height of the upper mold. Frequency has no significant impact on weld quality.
2. Amplitude
For high-frequency operating modes (e.g., 240Hz), the output amplitude (peak-to-peak) is 0.5-1.8mm. For low-frequency operating modes (e.g., 100Hz), the output amplitude is 2-4mm (see Figure 1). Generally, high-frequency vibration is used when the gap between parts is limited to less than 1.5mm. A larger amplitude results in shorter welding time, but cleanliness will deteriorate.
3. Welding Pressure
The adjustable range is wide, from 0.5 to 20 MPa. The commonly used range is 0.5-2.0 MPa. Higher pressure can reduce welding time. It is worth noting that excessive weld pressure can cause a large amount of melt to flow out of the weld area, bonding with the cold plastic beneath the melt layer to form a cold weld, significantly reducing weld strength.
Generally, weld strength is not very sensitive to vibration frequency and amplitude. For materials containing glass fibers, melt flow should be limited or minimized as much as possible. Lateral flow of the melt can change the orientation of the glass fibers, reducing weld strength. High-viscosity materials can withstand higher welding pressure. However, higher pressure increases the amount of dust in the first stage (solid-state friction stage).
4. Welding Time
Vibration friction welding has two control methods: time control and depth control. Weld depth control is generally more commonly used.
5. Weld Depth
The most important factor determining weld strength is weld depth. When the weld depth exceeds a critical threshold, reaching the minimum depth of the third stage (steady-state flow stage), the weld strength reaches the strength of the base material. When the weld depth is less than this critical threshold, the weld strength decreases. When the weld depth is greater than this critical threshold, welding between the same type of plastic does not increase weld strength. However, welding between different plastics can increase weld strength.
As long as this critical threshold is reached, weld strength is insensitive to welding frequency and amplitude. When at a constant threshold (above the critical threshold), increased pressure reduces weld strength.
The above suggestions can serve as a starting point for parameter settings. Precise parameter settings are also related to material type, geometry, strength requirements, sealing requirements, and cleanliness requirements.
Here are typical parameter settings:
Welding pressure: 1.4 MPa (200 psi)
Frequency: 240 Hz
Amplitude: 1.8 mm
Weld depth: 1.5 mm
Time: 3.5 s
Holding time: 0.5 times the welding time
