Laser welding of plastics and working with polymers - Part 2

Some time ago we published on our website members about laser welding of plastics and working with polymers. The article deals with the basic principles of laser welding, the choice of suitable material and its thermal properties. Today we would like to focus more on quasi-simultaneous welding and welding with the "collapsed rib" method. But let's start by talking about plastic welding in a bit of a general way.

Plastics have several unique properties and advantages over other materials. These include a high strength-to-weight ratio, mechanical flexibility, corrosion resistance, biocompatibility, electrical and thermal insulation and, in some cases, optical transparency. In terms of manufacturing, plastic parts can often be produced by various moulding techniques. These methods offer high production capacity and low unit costs.

All of this has led to increased use of plastics in areas as diverse as packaging, automotive manufacturing, microelectronics and medical devices. A common requirement in many of these applications is the joining of two or more plastic parts in the assembly of a product. For applications involving sophisticated products such as medical implants and electronic sensors, this joining must be done with high mechanical precision, minimal particle debris formation and excellent bond strength.

In high-volume production, some form of welding is usually used rather than simple bonding. This is because welding can usually be done much faster and more accurately than bonding and produces a stronger and more reliable joint.

Many different methods of welding plastics are used in industrial manufacturing. Typically, these involve selectively fusing the material by heat, friction or vibration, or by using chemical solvents. Each of these techniques has its own advantages and applications. Laser welding of polymers is becoming increasingly popular for the most demanding applications as it brings a unique combination of benefits. These include:

The basics of laser welding of plastics

Laser welding of plastics uses a laser as a heat source to melt the material. There are many different ways this can be done depending on the materials being joined, the specific requirements of the application and various production aspects such as cost or speed.

One of the most useful and commonly used techniques is called "transmission laser welding" (TTLW). This method consists of joining one part made of transparent plastic to another that is opaque. In this case, the terms 'transparent' and 'opaque' refer specifically to whether the parts absorb or transmit the wavelength of the laser used, not whether they are visually transparent or opaque to the human eye.

TTLW can be performed in several different ways depending on the size and shape of the part, the desired pass rate, the desired quality and characteristics of the weld, and other factors. One of the most useful and versatile methods is called quasi-simultaneous welding.

In quasi-simultaneous welding, two parts are either clamped together or come into direct contact with the free part on top. The laser is directed inwards through the transparent part and downwards towards the opaque part. The opaque polymer absorbs the laser light, heats up and melts. The heat from it also melts part of the clear part. The laser beam is quickly and scanned to render the pattern of the desired weld. Repeated rapid passage of the beam over the material results in simultaneous melting of the entire weld path (hence the name). Once the entire weld path has been melted, the laser is switched off and the molten material is rapidly solidified to form the weld joint. Quasi-simultaneous TTLW is a fast, versatile method that provides excellent joints and high production capacity. It is most useful for welds that are completely flush (flat) or have little elevation change.

Collapsed rib method

One particular part configuration that is often used for quasi-simultaneous TTLW is called the "collapsed rib" method. In this case, the lower part has a thin protruding rib that fits into a corresponding groove in the upper part. However, the groove is slightly wider than the rib. The lower rib is partially laser melted during welding, while the clamps actively press the two parts together. Part of the lower rib is melted and this material flows to fill part of the gap between the upper and lower parts. This then re-solidifies to form the welded joint.

This particular version of TTLW is particularly useful because it provides a good weld joint even when the parts are not perfectly straight or with tight tolerances. In addition, the "collapse height", i.e. the amount by which the top part moves down during welding, can be monitored and used to control the closed-loop process. This allows very consistent results to be achieved in batch production, even when individual parts vary in dimensions or in the absorption of laser energy by the material. It is even possible to compensate for changes in laser output power or laser focus point characteristics.

How to achieve success

There are, of course, many subtleties and factors involved in introducing TTLW polymer welding into production. So what is the best way to introduce this technology? In fact, there are three key things to consider before production begins, and perhaps even right away in the product design cycle.

The first is material selection. For the method to work, it is essential that there is a certain temperature range in which both polymers (clear and opaque) will remain molten (but not disintegrate). The larger this overlap, the wider the process window. And a wider process window makes manufacturing easier and more robust. The table summarizes which common polymer combinations are compatible with laser welding.

Another aspect is the issue of "design for manufacturing". For example, implementing the collapsed rib method requires that the part design has sufficient space in a suitable location for clamp engagement during welding, while still allowing the laser beam free access to the entire weld path. The dimensions and shapes of the rib and groove must also be chosen to provide sufficient material for the welding process and to accommodate the resulting melt. In addition, the parts must be designed to allow sufficient collapse height. For high precision applications, it may be necessary to incorporate alignment features such as positioning pins into the part design. The goal is to achieve a strong weld and good weld cosmetics while eliminating the need for post-processing to trim or deburr. Last but not least, there are all the issues related to process development. This means selecting the right laser source for the polymer materials, determining the optimal laser operating parameters, and determining what process variables need to be monitored or controlled to achieve the desired yields. There may also be various practical issues regarding parts handling, mechanical and software interfacing of the polymer welding system with other production equipment and, of course, operating costs.

Conclusion

Laser welding enables precision joining of plastic parts and is a cost-effective method in a wide range of production volumes. It can help deliver on the promise of plastics to reduce costs, save weight and provide advanced functionality across a wide range of products. And if you don't already have experience implementing laser welding, contact us. Working with Coherent's application labs, we'll design the right laser and welding method and help you implement it from the start.