Common defects and treatment methods of ultrasonic welding
1. The strength cannot reach the required standard
First of all, we must understand that the strength of the ultrasonic welding operation can never reach the strength of one-piece molding, it can only be said that it is close to the strength of one-piece molding, and the standard for welding strength must rely on multiple cooperation. What are these collaborations? what?
Plastic materials: The result of the fusion of ABS and ABS is definitely stronger than the fusion of ABS and PC, because the melting points of the two different materials are different, and the strength of the fusion cannot be the same. Although we are discussing whether ABS and PC can be welded to each other, the answer is yes, but is the strength after welding what we want? unnecessary!
On the other hand, what if ABS melts with PP and PE? If the ultrasound immediately emits 150 degrees of heat, although the ABS material has melted, the PVC, PP and PE have just softened. We continue to heat to above 270 degrees. At this time, PVC, PP and PE have reached the ultrasonic welding temperature, but the ABS material has been decomposed into another molecular structure.
Three conclusions can be drawn from the above discussion:
1. Plastics with the same melting point have higher welding strength.
2. The greater the melting point difference of the plastic material, the smaller the welding strength.
3. The higher the density of the plastic material (hard), the higher the welding strength and the lower the density (high toughness).
2.There are scars or cracks on the surface of the product
In ultrasonic welding operations, scars are usually left on the surface of the product, and fractures or cracks are generated at the joint.
Because there are two situations in the ultrasound operation:
1. High heat energy directly contacts the surface of plastic products.
2. Vibration transmission. Therefore, when ultrasonic vibration acts on a plastic product, the surface of the product is easy to burn, and the plastic column or hole with a thickness of less than 1m/m is also easy to break. This is a prerequisite for ultrasound operation. avoid.
On the other hand, due to the lack of ultrasonic output energy (expansion stage and HORN upper die), it takes a long time to convert the vibration friction energy into heat energy for welding to compensate for the accumulation of heat energy. Lack of output power. This welding method cannot obtain vibration and frictional heat immediately, but requires welding time to accumulate heat. Even if the melting point of the plastic product reaches the welding effect, the heat stays on the surface of the product for too long, and the accumulated temperature and pressure can cause the product to burn, shatter or crack. Therefore, power output (number of segments), welding time, dynamic pressure and other coordination factors must be considered at this time to overcome this lack of operation.
1. Reduce stress;
2. Reduce delay time (early vibration);
3. Reduce welding time;
4. The coverage of the cited media (such as PE bags);
5. Surface treatment of mold fixture (hardened or chrome-plated);
6. Reduce the number of machines or reduce the magnification of the upper mold;
7. For products that are easy to crack or break, the fixture should be cushioned with soft resin or other materials or covered with cork (this item does not affect the welding strength);
8. Add R angle to products that are easy to break.
3. Distortion of the product
There are three reasons for this deformation:
1. The body and the object to be welded or the cover cannot match each other due to the angle or arc;
2. The thickness of the product is thin (within 2m/m) and the length exceeds 60m/m;
3. The product is deformed and distorted due to conditions such as injection molding pressure.
Therefore, when our products are deformed by ultrasonic operation, it seems to be the cause of ultrasonic welding on the surface. However, this is only a result. Any factor before the plastic product is welded, what kind of result will be formed after welding. If there is no discussion on the main cause, it will take a lot of time to deal with the problem of incorrect prescription. Moreover, in the ultrasonic indirect conduction welding operation (non-direct melting), the pressure below 6kg cannot change the bristness and inertia of the plastic.
So don't try to use strong pressure to change the deformation before welding (the maximum pressure of the welding machine is 6kg), including forced extrusion with mold fixtures. Perhaps we will also fall into a blind spot, which is to discuss the cause of deformation from the surface, that is, it is not visible to the naked eye before welding, but after the ultrasonic welding is completed, the deformation is obvious. The reason is that before the product is welded, it is difficult to find the cumulative error of the product itself in various angles, radians and residual materials due to the existence of the fuse. But after the ultrasonic welding is completed, it appears deformed that can be seen with the naked eye.
1. Reduce the pressure (the pressure is best below 2kg);
2. Reduce ultrasonic welding time (reduce strength standard);
3. Increase the hardening time (at least 0.8 seconds or more);
4. Analyze whether the ultrasonic upper and lower modes can be adjusted locally (when not necessary).
5. Analyze the main causes of product deformation and make improvements.