With the rapid development of industrial production, plastics are widely used in various fields of the national economy and people's lives with their advantages of light weight, high specific strength, corrosion resistance, and easy processing. In engineering, plastic welding rods require that the composition of the material is the same as the composition of the material to be welded, but the strength of the material must be one level higher than the strength of the material to be welded, so as to ensure the welding strength. You can blow the disconnected plastic parts directly with a welding torch, and then crimp them. The disadvantage is that the external dimensions will be smaller than the original. So what are the influencing factors of plastic welding, and what kinds of welding methods are there?

1. Factors affecting welding quality

When welding, pressure, time, and heat absorption (melting amount) are the three elements to ensure welding quality.

1. Stress

Appropriate pressure is applied to the welding surface, the welding material will transition from elasticity to plasticity, and it can also promote the mutual diffusion of molecules and squeeze out the residual air in the welding seam, thereby increasing the sealing performance of the welding surface.

2. time

There must be proper melting time and sufficient cooling time. When the heating power is constant, the time is not enough, the welding will occur. If the time is too long, the weldment will be deformed, the slag will overflow, and sometimes there will be hot spots (discoloration) in the non-welded parts. It is necessary to ensure that the welding surface absorbs enough heat to reach a fully molten state to ensure sufficient diffusion and fusion between molecules, and at the same time, sufficient cooling time must be ensured to achieve sufficient strength of the weld.

3. Melting amount

Only when the melting time and heat power are coordinated and adjusted can the most appropriate melting amount be obtained, sufficient intermolecular fusion can be ensured, and the phenomenon of virtual welding can be eliminated. In addition to the welding equipment and the skill level of the operators, various factors from the inside or outside of the plastic have a certain influence on the welding quality and should be paid attention to.

2. Other factors that affect welding quality

1. Hygroscopicity of plastic

If welding wet plastic products, the contained moisture will turn into steam and run out after being heated, and bubbles will appear on the welding surface, which will weaken the sealing performance of the welding surface. Materials with severe moisture absorption include PA, ABS, PMMA, etc. Products made of these materials must be dried before welding.

2. Fillers in plastic

Such as glass fiber, talc, mica, etc., they change the physical properties of the material. The content of filler in the plastic has a great relationship with the weldability and welding quality of the plastic. Plastics with a filler content of less than 20% can be welded normally without special treatment. When the filler content exceeds 30%, due to insufficient surface plastic ratio and insufficient intermolecular fusion, the sealing performance will be reduced.

3. Clean the welding surface

The welding surface must be clean and free of impurities to ensure sufficient welding strength and air tightness.

In addition to selecting the correct welding material and eliminating the unfavorable factors that affect the welding effect, it is also necessary to adopt appropriate welding methods according to the type of material, the shape of the product, and the level of cost.

3. According to the different heating and softening methods used, plastic welding methods can be divided into softening by external heating source, softening by mechanical movement, and softening by electromagnetic action.

(1) There are the following types of welding techniques that are softened by external heating sources:

1. Hot plate welding

It may be the simplest plastic welding technology, but this method is especially suitable for welding large plastic parts that require a large area of welding surface. Generally, a flat electric heating plate melts and softens the two planes to be welded and then quickly removes the electric heating plate to merge the two planes. And add force to cool down. This method has simple welding equipment, high welding strength, and relatively easy design of the shape of the product and the welding part. However, due to the heat generated by the hot plate, the product softens and the cycle is longer; the molten resin will adhere to the hot plate and is difficult to clean (the surface of the hot plate can be coated with F4 to reduce this phenomenon), and the formation of impurities will affect the bonding strength for a long time ; It is necessary to strictly control the pressure and time to ensure the proper melting amount; when different types of resins or metals are joined with the resin, the strength will be insufficient.

2. Hot air welding

When the hot air stream blows directly to the joint area, the filler wire of the same material as the base material in the joint area will melt. The filler material and the welded plastic are melted together to form a weld. This welding method welding equipment is light and easy to carry, but requires relatively high welding skills of the operator.

3. Hot rod and pulse welding

These two technologies are mainly used in the welding of plastic films with smaller thicknesses. And these two methods are similar, they both press two films together and use the instant heat generated by a hot rod or nickel-chromium wire to complete the welding.

(2) The methods of softening and completing welding by mechanical movement are:

1. According to the motion track, it can be divided into linear type and rotary type

The straight type can be used for the welding of straight welds and plane welding, and the rotary type can be used for the welding of circular welds. The frictional heat generated by the two parts under pressure during the friction process melts and softens the plastic in the contact part, and fixes it until it sets firmly.

2. Ultrasonic welding

Ultrasonic welding is the use of high-frequency mechanical energy to soften or melt the thermoplastic at the joint. The connected parts are fixed together under pressure, and then undergo ultrasonic vibration with a frequency of usually 20 or 40 kHz. The transducer converts the high-power vibration signal into corresponding mechanical energy and applies it to the plastic parts to be welded. In contact with the interface, the joints of the weldment are rubbed violently to generate high heat instantly, so that the molecules alternately fuse, thereby achieving the welding effect.

The ultrasonic welding process is very fast, the welding time is less than one second, and it is easy to realize automation. It is widely used in various industries such as electronics, electrical appliances, auto parts, plastic toys, stationery, daily necessities, handicrafts, and cosmetics.

Motion welding is an automatic welding process, which requires special welding equipment. Once the correct welding parameters are determined, the operator can stabilize production. Its advantages are: fast, flexible, short and stable welding, no flux or shielding gas, no harmful gas or slag, and the welding quality of the product is guaranteed.

3. High frequency welding

High-frequency welding is a high-frequency induction heating technology that uses the principle of electromagnetic induction. It penetrates the plastic product to induce induction heating of the inductance or magnetic plastic buried in the plastic part. The welded plastic can generate heat in the fast alternating electric field and require welding. The part quickly softens and melts, and then fills the interface gap, and the perfect mechanical device assists to achieve perfect welding. The most common way to generate high-frequency induction is to use high-frequency current through the coil to obtain a strong high-frequency magnetic field. The induction body (ie heating body) is generally made of iron, aluminum, stainless steel and other materials, but magnetic composite plastics processed by adding magnetic substances are also used. Products made by welding in this way include stationery clips, inflatable items, waterproof clothing and blood bags.

4. Infrared welding

This technology is similar to the welding of an electric heating plate. The two parts that need to be welded are fixed close to the electric heating plate but not in contact with the electric heating plate. Under the action of heat radiation, the connecting part is melted, and then the heat source is removed, and the two parts are butted and pressed together to complete the welding. This method does not produce welding slag, pollution, and has high welding strength. It is mainly used for the connection of PVDF, PP and other pipeline systems with high precision requirements.

5. Laser welding

In the 1970s, lasers began to be applied to plastic welding. Its principle is to focus the beam generated by the laser (usually concentrated strong radiation waves in the infrared region of the electromagnetic spectrum) through the optical path system composed of mirrors, lenses or optical fibers to the area to be welded to form a heat action zone, in the heat action zone The plastic is softened and melted. In the subsequent solidification process, the melted material forms a joint, and the parts to be welded are connected. Usually used for PMMA, PC, ABS, LDPE, HDPE, PVC, PA6, PA66, PS For materials with good light transmittance, add carbon black and other absorbents in the heat action zone to enhance the heat absorption effect. Plastic laser welding has many advantages: fast welding speed and high precision; automation and precision numerical control are easy to realize; the cost is relatively low. Therefore, plastic laser welding technology has been widely used in automobiles, medical equipment, packaging and other fields.

In addition to the welding between plastics and plastics, due to the needs of light structure and economic benefits in the automotive industry, some products also require a hybrid connection of plastics and metals. Plastic hot riveting technology has emerged. This hot riveting technology is simple and easy to implement. , It is safe and reliable, so that the plastic and metal parts are combined in the best way to bear the load, the product quality is reduced, and the economy is improved. In the riveting welding method, a plastic shaft pin (rivet pile) reserved for integral molding can be heated and softened and then stamped and deformed by a cold die to lock the parts together (form a rivet cap. "hot pile"), thus More convenient and economical.