Locking and heat-melting of plastics and laser welding

Plastic Locking and Laser Welding

Laser-based welding of plastics offers several key advantages over traditional contact methods in a wide range of applications, including medical devices and automotive parts.

Industrial lasers are used to process engineered plastics in many markets. Examples of some common processes include laser marking, cutting, and welding. Recognized markets for laser plastic welding include automotive and medical devices.

The Basics of Laser Plastic Welding

Laser-based welding of plastics has several important advantages over conventional contact methods. Laser welding is a non-contact process at the weld point, which typically occurs at the interface of overlapping parts. As a result, the weld zone is encapsulated. This produces an aesthetically pleasing weld that is sterile and does not contaminate the surfaces of the parts being joined. It may seem a little counterintuitive that parts that are already in contact with each other can be welded together from the top to the bottom without disturbing their outer surfaces. With conventional plastic welding techniques, such as ultrasonic or hot stamping, contact with the outer surface of the plastic parts being welded together is inevitable. Non-contact laser plastic welding works on the principle of partial transmission, reflection, scattering and absorption of laser light within the polymer chains being joined. By carefully selecting the optical properties of the plastic and the laser, sufficient heat can be generated at the target location to melt and fuse the materials together.

The design of the laser welding process should be established early in product development. Attempts to retrofit the laser welding process to plastic products that were not originally designed for laser welding sometimes work. However, designing for laser manufacturing principles in the early stages of plastic product development will greatly reduce manufacturability issues later.

Design Considerations for Laser Plastic Welding

Plastic products have mechanical, geometric, thermal, and optical properties (Figure 1). Fundamentally, the laser weldability of plastic components can be roughly determined by the following factors:

Are the selected materials compatible? Compatibility refers to the melting temperature and chemical, mechanical, geometric, and optical properties of the two plastic parts to be welded.

Can the laser beam effectively penetrate the top material to the joining interface, and can the bottom material absorb the laser beam to generate heat where it is needed?

Can the parts be held together properly during the welding process and can the applied forces be controlled? Does the part geometry result in a good fit with no gaps between the parts?

Given the geometry, can the laser beam delivered to the part be distributed and controlled in an effective manner?

Thermal and Chemical Compatibility

Plastics melt and decompose at much lower temperatures than metals. Typical melting temperatures for engineering plastics are around 250掳C. Some plastics melt at much higher temperatures, such as polyetheretherketone (PEEK), which is in the 350掳 to 400掳C range. Close compatibility of melt temperatures will aid in mixing of the melt pool and improve mechanical strength upon resolidification. Certain combinations of plastics that are relatively closely matched in melt temperature are good candidates for plastic welding. The chemical composition of the plastics is also a factor. For example, trying to weld high-density polyethylene (HDPE) to polypropylene (PP) will not be successful, but it is possible to weld low-density polyethylene (LDPE) to polypropylene (PP), even though the polyethylenes are in the same family.

Matching Optical Properties

Lasers in material processing typically emit a beam of one wavelength or a very narrow bandwidth of wavelengths. Unlike natural light, laser beams are coherent and focusable. Near-infrared and infrared wavelengths are most widely used for plastic welding from 800nm to 2渭m, typically using high-power diode lasers. These wavelengths are longer than those visible to the human eye, such as 532nm for green and 635nm for red in the visible spectrum. With wavelengths between 800 and 2000nm, the plastic to be welded must exhibit some degree of transmission and absorption within this range. Plastics are somewhat semicrystalline in structure and have both amorphous and crystalline phases. When the laser beam strikes the difference in refractive index between the amorphous and crystalline phases within the plastic, light scattering and reflection can occur in addition to the necessary transmission and absorption. This can be a benefit or a hindrance to laser welding, depending on the extent of these effects. A design combination of these properties helps achieve transmission of the laser beam through the top plastic part and absorption in the lower part (Figure 2). Sometimes, additives are added to the masterbatch to make the polymer absorb the laser. Whether additives are acceptable in the product should be considered during the design phase鈥攆or example, will a medical device made with this plastic be FDA approved? This can be a benefit or a hindrance to laser welding, depending on the extent of these effects. A design combination of these properties helps achieve transmission of the laser beam through the top plastic part and absorption in the lower part (Figure 2). Sometimes, additives are added to the masterbatch to make the polymer absorb the laser. Whether additives are acceptable in the product should be considered during the design phase鈥攆or example, will a medical device made with this plastic be FDA approved? This can be a benefit or a hindrance to laser welding, depending on the extent of these effects. Whether additives are acceptable in a product should be considered during the design phase鈥攆or example, would a medical device made from this plastic be approved by the FDA? This can be a benefit or a hindrance to laser welding, depending on the extent of these effects. A designed combination of these properties helps achieve both transmission of the laser beam through the top plastic part and absorption in the lower part (Figure 2). Sometimes, additives are added to the masterbatch to make the polymer absorb the laser. Whether additives are acceptable in a product should be considered during the design phase鈥攆or example, would a medical device made from this plastic be approved by the FDA?

The glass fiber content in some plastics, such as polyamide (PA-66), commonly known as nylon, can affect the transmission of light through them, especially at higher glass fiber concentrations and lower transmittance. A common question about plastics is, which colors can be welded together? There is no simple answer: Many combinations are possible, and even materials of the same color, such as clear to clear, white to white, and black to black, can be combined through carefully designed composition. While visible light is not transparent through colored plastics, the opposite may be true for a single laser wavelength.

Adequate part fit and retention forces along part ends

Designers must always try to ensure that part geometry allows for laser plastic welding with a good fit and accessible joints for assembly components. Laser welding is not good at transferring heat through air gaps and it is important that the components of the joint are in contact with each other. The lap weld configuration achieves this. In some cases, butt welds are possible and much depends on how the laser beam is applied to the weld and the part tolerances of the molding machine producing the plastic part. Welding a lid to a container is a good example of part fit.

Downward force during welding is essential for certain complex parts, especially large parts that are difficult to naturally achieve a good fit around their boundaries. The clamping force can be generated by a servo drive or by pneumatic clamping. Plastic welding has a collapse force that determines how much force the hot, molten plastic can withstand before it begins to deform significantly, and how much force is required to push the parts together when melted. Force displacement sensors are often integrated into laser plastic welding tools to monitor and control the forces applied to the components during the welding cycle.

Effectively Distribute and Control Laser-Generated Heat

In laser welding, there are several methods for delivering the laser beam to the workpiece. An example is the use of a Cartesian axis system, where there is relative motion between a fixed laser plastic welding head and an XYZR table motion system. These devices may not result in uniform heating of large parts due to the accelerations required at the start and end points and changes in direction. However, this approach is flexible because the laser path can be generated from CAD data. The use of high-speed galvanometer scanning heads allows the laser beam to be moved extremely quickly at speeds of up to 10m/s. Scanning around the weld at very high speeds and with sufficient power can heat the entire weld from one end to the other almost instantaneously. This reduces the effects of simultaneous heating and cooling produced by Cartesian axis systems.

Another, less flexible approach is to laser the part through a mask. In this case, the mask holes take the shape of the desired weld, but must be made each time the design is changed. Sometimes, specialized laser lenses are used that produce a line focus that provides a seam weld along a defined length. It may be necessary to control the optical power during this process, which can also be achieved by a pyrometer measuring the workpiece temperature during welding and feeding this information back to a power control loop in the laser controller.

In Summary

Laser plastic welding has many interdependencies and has important engineering implications in designing solutions for complex parts and exotic materials. The applications of laser plastic welding in industry are impressive, with high-volume products ranging from automotive light assemblies to inkjet printer cartridges routinely utilizing laser plastic welding.

This article aims to explain some of the basic considerations that must be taken into account when laser welding plastics. Most importantly, the product itself must be designed with laser welding in mind 鈥?which is why product design and process development teams should adopt a collaborative approach with customers early in the product design and development phase, recognizing the points mentioned above. Molds used to produce complex plastic products are very expensive to manufacture, and mold designers must understand the design for manufacturing issues surrounding laser welding before finalizing the mold design. This will ensure that the part geometry that exits the mold can subsequently be laser welded.