What are the factors for successful laser welding of plastics (what to consider in plastic laser welding)

Industrial lasers are widely used in various processing of engineering plastics, serving many industry markets. Some common plastic processing applications include laser marking, laser cutting and laser welding. In the automotive and medical device fields, laser welding is mainly used in automobiles and medical devices.

Why choose laser welding for plastics?

Compared with traditional contact welding, laser plastic welding has obvious advantages. First, laser welding is contactless welding; second, the weld point is usually located at the interface of two overlapping parts, the welding area is hidden, it looks beautiful, and will not pollute the surface of the welded parts. Therefore, from the appearance, it is impossible to judge whether the two contact parts are welded together from top to bottom.

Traditional plastic welding technologies such as ultrasonic or hot stamping will inevitably affect the outer surface. Laser plastic welding uses the partial transmission, reflection, scattering and absorption of laser in the polymer chain to achieve welding. By selecting appropriate laser parameters, sufficient heat can be generated at the selected position of the plastic to achieve melting and bonding.

Laser welding technology should be established in the early stages of product development. Some plastic products are not specially developed for laser welding and can sometimes be welded with lasers; however, if laser welding technology is considered in the early stages of plastic product development, some problems that may occur in laser welding will be greatly reduced.

Figure 1: Design Considerations for Laser Plastic Welding

Plastics have excellent mechanical, geometric, thermodynamic, and optical properties (see Figure 1), so the laser weldability of plastics is determined by the following factors:

Is the selected material suitable? Compatibility refers to whether the two plastics being welded are suitable for welding together in terms of melting temperature, chemical properties, mechanical properties, geometry, and optical properties.

In the weld area, can the laser beam be effectively transferred through the upper plastic to the interface, and can the lower plastic absorb the laser energy to generate heat?

During the weld, can the parts fit together effectively and can the applied force be controlled? Can the part geometry fit well without creating gaps?

Can the laser energy be effectively transferred to the plastic part and generate heat while the geometry is fixed?

Thermodynamics consistent with chemical properties

The melting point of plastics is much lower than that of metals. The melting point of engineering plastics is about 250掳C. Some plastics have higher melting points, such as polyetheretherketone (PEEK), which has a melting point between 350 and 400 掳C. When the melting point temperatures of two plastics match, it helps to mix the molten pool and improve the mechanical strength during resolidification. Two plastics with matching melting temperatures are particularly suitable for laser welding.

In addition, the chemical composition of the plastic is also an important influencing factor. For example, although high-density polyethylene (HDPE) and low-density polypropylene (PP) belong to the same family of materials, these two plastics cannot be welded together. Low-density polyethylene (LDPE) can be welded with polypropylene. Therefore, laser plastic welding must consider the combination of welding materials.

Optical Property Matching

The lasers used in material processing are monochromatic or correlated focused beams with very narrow bandwidths, with wavelengths of 800 nm~2渭m near-infrared and infrared lasers are widely used in plastic welding. These wavelengths are longer than the 532nm green light and 635nm red light visible to the human eye. The laser wavelength is 800~2000nm. Within this range, the welded plastic must have a certain transmittance and absorption.

Plastics are generally semi-crystalline structures, including crystalline phases and amorphous phases. When the laser irradiates the plastic, the difference in refractive index between the amorphous and the crystalline will not only cause transmission and absorption, but also scattering and reflection of the laser. The effect of the semi-crystalline structure on welding depends mainly on the scattering and reflection intensity of the laser. Therefore, through effective optical design, the laser can pass through the upper plastic and absorb the lower plastic (see Figure 2). Sometimes additives need to be added to the plastic to improve the absorption capacity of the laser. In the design stage of the plastic product, it should be considered whether the additive is suitable - for example, is the additive suitable for medical device FDA testing?

Figure 2: Ideal laser transmission and absorption model in plastic welding

Some plastics contain glass fibers (such as polyamide PA-66, commonly known as nylon). The content of glass fibers will affect the transmittance of the laser. The higher the concentration of glass fibers, the lower the laser transmittance.

A common question in plastic welding is: what colors of plastics can be welded together? There is no uniform answer to this question. In fact, many color combinations of plastics can be welded together.

In addition, laser welding between plastics of the same color, such as transparent plastic, white plastic, and black plastic, can also be achieved through carefully designed plastic composition. Visible light generally cannot pass through colored plastics, but lasers with monochromatic wavelengths may pass through.

Assembling and fixing parts

Plastic parts should ensure that the plastic forms a certain geometric shape, and through the good fit of the assembled parts, make it suitable for laser plastic welding, and ensure that the joints can fit fully. Because lasers do not easily transfer heat across air gaps, the welded parts of the parts should maintain full contact during the laser welding process. Lap welding is a good example. In some cases, butt welding can also achieve good weld quality, which depends largely on how the laser beam acts on the weld and the part tolerances produced by the plastic molding machine. Welding a lid to a container is a good example of part mating.

Achieving downward force during welding is essential for welding complex parts, especially for large parts that are difficult to assemble naturally at their boundaries. The clamping force can be driven by a servo or generated by pneumatic clamping. Plastics have collapse forces when welded, which determines how much force the plastic can withstand before it melts and deforms due to heat, and how much force is required to push the parts together during melting. Typically, during laser plastic welding, the force applied to the parts during welding is monitored and controlled by an integrated force-displacement sensor.

Heat is effectively controlled

For laser welding, there are many ways to transmit the laser to the workpiece. Here are a few methods briefly introduced. First, keep the laser welding head fixed, and the XYZR worktable moves compared to the laser head. Because the motion platform needs to accelerate when starting, stopping, and changing direction, large parts may not be heated evenly during welding. However, this welding method is very flexible because the laser path can be generated from CAD data.

The second method is to use a high-speed vibrating mirror to move the laser beam at 10. m/s. The high-power laser beam moves at a very high speed, which can make the entire weld almost instantly heated from one end to the other, and the heating is relatively synchronous and uniform.

The third method is to illuminate the part with a laser through a mask. This method requires the aperture of the mask to be consistent with the shape of the weld. Every time the shape of the weld changes, the mask must be remade, so this method is not flexible.

The fourth method is less commonly used. It uses a special laser lens to produce a linear focus and sew the weld in a specified direction. The laser power may need to be controlled during the welding process, and the workpiece temperature during the welding process can be measured by a pyrometer, and this information can be fed back to the power control circuit of the laser controller.

Summary

This article introduces some basic considerations in the laser plastic welding process. Most importantly, the plastic product itself must be designed for laser welding - this requires the R&D team to work with customers in the early stages of product design to fully understand the key points of laser welding. The mold manufacturing cost of complex plastic products is high, and mold designers must be aware of the mold design and manufacturing issues that exist in laser welding in advance. By solving the mold problems, it can be ensured that the geometry of the plastic part coming off the mold can be directly laser welded.

Laser plastic welding is increasingly used in industry for the manufacture of a variety of high-volume products, from automotive light assemblies to inkjet printer cartridges.