Why is the ultrasonic welding result unstable under the same parameters?

Variations and Defects in Injection Molded Parts

There are fluctuations in the injection molding process, such as short injection time/insufficient shot size, warpage, partial shrinkage, and differences in mold cavities. When the injection time is short / the injection volume is insufficient, the plastic does not completely fill the cavity. In this case, there is a risk that the weld surface will be incomplete, possibly resulting in reduced weld strength and insufficient sealing. Short injection times can also lead to voids in the material, where cracks can easily lead to product failure.

Warpage and shrinkage of parts will directly affect the size and shape of product welds. If the warpage is small, the welding seam of the upper and lower parts can be fully contacted by pressing the product, which helps to normalize the welding surface. But if the warp is too large, more material needs to be melted to achieve full melt of the weld. This will lead to differences in weld strength across welds on the part, as well as cosmetic issues.

Differences and wear between different cavities. If the weld results are erratic and only occur on parts produced in one mold, then the mold may have worn out. For parts with an external dimension of about 80mmX80mm, the size difference of parts produced by different mold cavities may also be around 0.2mm. Usually on the customer side, the quality control of part size is weak.

Fixture defects

Loss of fixtures. For ultrasonic plastic welding, horn wear can cause differences in weld performance. For contact plastic welding such as hot plate and hot riveting. Material residues can build up on the hot plate and cause differences in solder performance. Residues may build up over time due to molten material adhering to the hot plate. Usually, when a hot plate is run at high temperature and coated with a high temperature non-stick coating, the residue will burn quickly without causing any problems. However, in lower temperature applications, this residue buildup can allow degraded material to enter the weld and hinder direct contact and heat transfer between the part and the hot plate.

Support and positioning. Check whether the lower jig has no support for the part, or if there is a poorly supported part, and ensure that the upper and lower jigs are relatively level and centered. Support and positioning are critical for high-strength welding and precise assembly. Uneven weld strength, false welds, leaks, usually due to insufficient support below the weld. Therefore, whether it is ultrasonic, hot plate, vibration, infrared, hot air and laser plastic welding processes, it is necessary to repeatedly confirm that there is strong support under all welds. Sometimes due to part features, a gap needs to be left under the welding rib, and the size of the gap should be less than 3mm. When the gap is large, test verification is required. This clearance should not have a significant impact on the weld.

Examining and evaluating the above potential problems can help you identify possible causes of differences in weld performance