SHINHOP laser

When it comes to welding, the first thing that comes to mind may be the connection of metal parts. So how are the various plastic products we see in daily life welded? Next, the editor will popularize plastic welding from the aspects of plastic welding process methods, plastic laser welding process, applications in the plastic laser welding industry, and development trends, divided into two parts.
1. Can all plastic materials be welded?

▲Thermosetting plastics　　　▲Thermoplastic plastics
Is it true that any type of plastic material can be welded? Plastics are classified based on their physical and chemical properties into thermosetting plastics and thermoplastic plastics.
Thermosetting plastics have a chemical molecular structure and are generally formed in one go. As shown in the left image, the switch panel and the bowl for children are made of thermosetting plastics. They have high-temperature resistance, are hard, difficult to melt, and do not soften after heating; excessive heating leads to direct decomposition.
Another type of thermoplastic plastics has distinctly different characteristics. As shown in the right image, thermoplastic plastic parts soften or melt when heated, can flow and form, and can be reheated multiple times. Therefore, not all plastic materials can be welded.
2. What categories of plastic welding processes are there?

▲Categories of plastic welding processes
Currently, the commonly used plastic welding process methods mainly include four categories:
(1) Hot plate welding - welding performed after heating with a hot plate, as shown in the schematic diagram, the product is melted and then the hot plate is removed, and the product is pressed to achieve welding;
(2) Ultrasonic welding - using high-frequency vibration waves transmitted to the surface of the object to be welded, under pressure, friction generates heat to weld the product;
(3) Vibration friction welding - two plastic components come into contact and rub against each other under certain pressure, amplitude, and frequency. The heat generated by friction melts the material at the weld interface. Under pressure, the molten plastic flows out of the weld area, forming overflow material. After the vibration stops, the molten plastic layer solidifies, creating a strong joint;
(4) Laser welding - laser welding technology uses the heat generated by the laser beam to melt the contact surface of the plastic, thereby bonding thermoplastic sheets, films, or molded parts together.
The advantages and disadvantages of these four welding methods are shown in the diagram below, among which the advantages of plastic laser welding are particularly obvious. It can process complex parts and has characteristics such as short welding cycles, online monitoring, and low equipment maintenance costs.

▲Comparison of advantages and disadvantages of welding methods
Plastic laser welding has so many advantages; what is its principle?
3. The principle of plastic laser welding
The laser plastic welding method requires that the upper and lower layers of the materials to be welded exhibit good transmittance and good absorbance for a certain wavelength of laser. Based on this, plastic laser welding is also called laser transmission welding, and the principle of laser welding is shown in the diagram.

Principle of laser welding
During welding, the laser passes through the upper layer material and is absorbed by the lower layer material at the interface between the upper and lower materials, generating heat. The heat is transferred between the upper and lower materials, melting the interface. The molten plastic molecules diffuse and entangle under the pressure of the welding fixture and thermal expansion, generating van der Waals forces to achieve welding.

Transmittance of different materials to wavelength         Weldability of commonly used plastics
The left image shows the transmittance of several transparent plastic materials to wavelength. It can be seen that the transmittance of plastic materials is very high for wavelengths below 1200nm, while the absorption rate increases for wavelengths above 1200nm. Generally speaking, when the laser transmittance of the upper layer material is higher than 50% and that of the lower layer material is lower than 20%, a better laser welding effect can be achieved. The chart on the right shows the weldability of commonly used plastic materials.
Since the plastic welding process requires applying pressure, the welding of plastic parts has corresponding structural requirements. As shown in the diagram below, plastic part welding generally has an overlapping structure, and for pipe parts, interference fit pipe welding can be achieved.

▲Welding structure of plastic parts
4. Types of plastic welding methods
Plastic welding methods are generally divided into four categories:
1. Contour welding: The laser moves along the contour line of the plastic welding layer and melts it, gradually bonding the plastic together. It is mainly used for welding parts with regular shapes and speed requirements;
2. Synchronous welding: Using a laser emitter with multiple laser ports, the direction and shape of the laser beam are adjusted through optical devices; the laser beam is guided by a program to weld along the contour line of the welding layer, melting and bonding the entire contour line simultaneously. It is mainly used for welding parts with large contact areas and high consistency requirements;
3. Scanning welding: Also known as quasi-synchronous welding, it combines the above two welding technologies. It uses optical devices to generate high-speed laser beams that move along the area to be welded, gradually heating and fusing the entire welding area together. This method has higher requirements for laser emission equipment;
4. Mask welding: This technique involves using a pre-made template to shield the laser beam from ineffective areas, exposing only the precise welding zone and melting solely that area to complete the welding process. This welding technology boasts high precision and is currently one of the most widely adopted laser welding methods. The equipment is versatile, the templates are easy to produce, and the costs are low, enabling high-precision welding down to 10μm.

▲Classification of process methods
With the continuous decrease in the cost of equipment required for domestic laser technology and the gradual increase in the requirements for plastic welding processes in various fields, laser plastic welding technology is becoming increasingly popular. So how should a plastic laser welding device and tooling be configured? What does the welding process include? What defects may occur during the welding process, and how can they be avoided? We will continue to reveal in the next article!