Structural requirements of ultrasonic welding machine for plastic parts
1.1 The structure of plastic parts
Plastic parts must have a certain degree of rigidity and sufficient wall thickness. Too thin wall thickness is dangerous. Ultrasonic welding needs to be pressurized. The general pressure is 2-6kgf/cm2. Therefore, the plastic parts must be guaranteed to be basically not deformed under pressure.
1.2 Pot-shaped or box-shaped plastics, etc., will cause resonance on the surface of the welding head to form some concentrated energy accumulation points, which will cause burns and perforations (as shown in Figure 1). The top of the tank can be designed. Consider the following
○1 Thick plastic parts
○2 Increase ribs
○3 Avoid the middle position of the welding head
1.3 sharp corners
If an injection molded part has a very concentrated stress, such as a sharp corner, it will crack and melt under the action of ultrasound. In this case, consider adding an R angle to the sharp corner. as shown in picture 2.
1.4 Attachments of plastic parts
The protrusions or small parts attached to the inner or outer surface of the injection molded part will break or fall off due to the influence of ultrasonic vibration, such as the fixed pin (as shown in Figure 3). This problem can be minimized or eliminated by the following design:
○1 Add a big R angle or reinforcement at the place where the appendage intersects the main body.
○2 Increase the thickness or diameter of the attachment.
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1.5 Holes and gaps of plastic parts
If the parts contacted by the welding head have holes or other openings, interference and attenuation will occur during the ultrasonic transmission process (as shown in Figure 4). According to the type of material (especially semi-crystalline materials) and the size of the hole, the lower end of the opening There will be a small amount of welding or no melting at all, so try to avoid it.
1.6 Thin and curved transfer structure in plastic parts
In the shape of the plastic part contacted by the welding head, if there is a thin and curved structure, and it needs to be used to transmit the ultrasonic energy from the chamber, especially for semi-crystalline materials, ultrasonic vibration is difficult to transmit to the processing surface (as shown in Figure 5). Show), this design should be avoided as much as possible.
1.7 Short and long distance welding
Short-distance welding means that the welding position is within 6mm from the welding head contact position, and the long-distance welding is greater than 6mm. The energy in the ultrasonic welding machine will be attenuated when the plastic part is transferred. The attenuation is also stronger in low-hard bottom plastics, so special attention must be paid to the design to allow enough energy to be transmitted to the processing area.
Long-distance welding is more suitable for hard glue (such as PS, ABS, AS, PMMA), etc. Some semi-crystalline plastics (such as POM, PETP, PBTB, PA) can also be used for long-distance welding through suitable shape design.
1.8 Design of welding head contact surface of plastic parts
Injection molded parts can be designed in any shape, but ultrasonic welding heads cannot be made at will. The shape and length may affect the welding head frequency, amplitude and other parameters. The design of the welding head requires a reference plane, that is, a reference frequency plane determined according to its working frequency. The reference frequency surface generally occupies more than 70% of the surface of the welding head. Therefore, the surface of the injection molded part should be less than 30% of the entire plastic surface. The surface of a plastic part with a smooth and circular arc transition can be appropriately wider than the standard, and the protruding position should be located in the middle of the plastic part as much as possible or the design should be symmetrical.
The contact surface of the welding head of the plastic part is at least larger than the welding surface, and the welding position should be aligned as much as possible. If the contact surface of the welding head is too small (as shown in Figure 6), it will cause greater damage and deformation, and unsatisfactory welding effect.
In the case where there are damage lines on the surface of the welding head or the shape is slightly different from that of the plastic part, it will leave scars on the surface of the plastic part during welding. The avoidance method is: pad a film (such as PE film, etc.) between the welding head and the surface of the plastic part.