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Laser Polymer Welding: Designing for Success
Laser polymer welding offers several advantages over other joining methods, but implementing it right requires an understanding of the technology, and often benefits from discussions with a knowledgeable equipment supplier early on in the product development cycle.
Polymers offer several unique characteristics and advantages over other materials. These include a high strength-to-weight ratio, mechanical flexibility, corrosion resistance, biocompatibility, electrical and thermal insulating ability, and even optical transparency in some cases. In terms of manufacturing, polymer parts can often be produced using various molding techniques. These methods offer high production throughput and low unit cost.
All this has led to greater use of polymers in areas as diverse as packaging, automotive production, microelectronics, and medical devices. A common requirement across many of these applications is for joining of two or more polymer parts during product assembly. For applications involving sophisticated products, such as medical implants and electronic sensors, this joining must be accomplished with high mechanical precision, minimal particulate debris production, and excellent bond strength.
For volume production, this is usually done using some kind of welding, rather than simply gluing. This is because welding can usually be performed much faster and more accurately than adhesive bonding, and makes a stronger and more reliable connection.
"The increasing use of polymer parts, especially in high precision products, is driving manufacturers to look for joining technologies that offer better welds, increased production throughput and reduced costs."
There are numerous different polymer welding methods in use. Usually these involve selectively melting the material using applied heat, through friction or vibration, or even by the use of chemical solvents. Each of these techniques has its advantages and uses.
Laser polymer welding has become increasingly popular for the most demanding applications because it delivers a unique combination of advantages. These include:
| Precision | Highly localized application of laser energy produces little or no part distortion, delivers tight dimensional tolerances, and can be used with complex shaped parts |
| Repeatability | The laser process is inherently highly consistent and can be closely regulated with process monitoring equipment |
| Weld Quality | Weld seams are narrow and cosmetically excellent (no flash), and post processing is rarely required |
| Weld Strength | Laser welding delivers a strong weld, which is free of gaps, and can provide hermetic sealing |
| Low Contamination | Laser welding doesn’t use filler materials, and produces virtually no debris |
| Speed | The process is fast and well and lends itself to automation |
Laser Polymer Welding Basics
Laser polymer welding utilizes a laser as the heat source to melt the material. There are many different ways this can be implemented depending upon the materials being joined, the specific requirements of the application, and various production considerations such as cost or speed.
One of the most useful and commonly employed techniques is called “through transmission laser welding” (TTLW). This method involves joining one part made from transparent plastic to another which is opaque. In this instance, “transparent” and “opaque” specifically refer to whether or not the parts absorb or transmit the wavelength of the laser being used, as opposed to being visually transparent or opaque.
Figure 1: In TTLW, a laser beam passes through a clear plastic part and is absorbed by an opaque part beneath it. This heats the bottom part and melts it to weld the parts together.
There are several different ways that TTLW can be performed depending upon the part size and shape, the required throughput speed, the desired weld quality and characteristics, as well as other factors. One of the most useful and versatile of these methods is called quasi-simultaneous welding.
In quasi-simultaneous welding, the two parts are either clamped together or brought into direct contact, with the clear part on top. The laser is focused in through the clear part, and down towards the opaque one. The opaque polymer absorbs the laser light, heats up and melts. The heat from it also melts some of the clear part.
The laser beam is rapidly scanned to trace out the pattern of the desired weld seam. Typically it is scanned over the pattern numerous times, and has the effect of melting the entire weld path simultaneously (hence the name). After the entire weld path is molten, the laser is turned off, and the melted material quickly resolidifies to form the weld joint.
Quasi-simultaneous TTLW is a fast, versatile method that provides excellent bonds and high production throughput. It is most useful for welds seams that are entirely in a single plane (flat), or have minor height changes.
Figure 2: In quasi-simultaneous welding, the laser beam is scanned rapidly over the entire weld path in order to melt it all at once.
Collapse Rib Method
One particular part configuration frequently used for quasi-simultaneous TTLW is called the “collapse rib” method. Here the bottom part has a thin protruding rib which mates into a corresponding groove in the top part. However, the groove is a bit wider than the rib
The bottom rib is partially melted by the laser during welding while clamps actively press the two parts together. Some of the bottom rib becomes molten, and this material flows and fills some of the gap between the top and bottom parts. This then resolidifies to make the weld joint.
Figure 3: Schematic of the major steps in the “collapse rib” method of quasi-simultaneous TTLW.
This particular embodiment of TTLW is especially useful because it delivers a good weld joint even if the parts aren’t perfectly flat or tightly toleranced. Plus, the “collapse height” – that is, the amount that the top part moves down during the welding process – can be monitored and used for closed loop process control. This enables very consistent results in volume production, even in the presence of part-to-part variations in dimensions or material absorption of laser energy. It can even compensate for changes in laser output power or focused laser spot characteristics..
Steps for Success
Of course, there are many nuances and considerations in bringing TTLW polymer welding into production. So, what is the best way to get the technology implemented? There are really three key things to consider before starting production, and maybe even right in the product design cycle.
The first of these is material selection. It is essential that there be some temperature range over which both polymers (clear and opaque) will remain molten (but not decompose) for the method to work. The greater this overlap, the wider the process window. And, a wider process window makes production easier and more robust. The chart summarizes which common polymer combinations are compatible with laser welding.
Figure 4: Material combinations which are compatible with TTLW.
"Coherent produces laser polymer welding systems that are readily integrated into production environments."
The next consideration is “design for manufacturing” issues. For example, implementing the collapse rib method requires a part design having sufficient space in the appropriate location for the clamps to engage during welding, while also permitting unobstructed access to the entire weld path for the laser beam.
The dimensions and shapes of the rib and groove must also be chosen to provide enough material for the welding process and to accommodate the melt flash that is produced. Plus, it’s necessary to design the parts to allow for a sufficient collapse height. For high precision applications, alignment features, such as locating pins, may have to be incorporated 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 remove flash.
Finally, there are all the issues surrounding process development. That is, picking the right laser source for the polymer materials to begin with, determining the optimum laser operating parameters, and identifying what process variables must be monitored or controlled to achieve the desired yields. There may also be various practical issues in terms of part handling, the mechanical and software interface of the polymer welding system with other production equipment, and, of course, cost of ownership.
The simplest way to address all these factors is to partner with a supplier who can provide applications development assistance. Specifically, this means finding a vendor who can run tests to determine what system configuration will yield best results, and perhaps even help identify the optimum laser parameters for the production process. Coherent Labs provides precisely that service, and Coherent also produces laser polymer welding systems that are readily integrated into production environments.
Summary
In conclusion, laser welding enables precision joining of polymer parts, and is a cost-effective method over a wide range of production volumes. It can help deliver on the promise of polymers to lower cost, save weight, and provide advanced functionality in a wide range of products. Unless an organization already possesses expertise in polymer welding, effective implementation is aided by working with a knowledgeable equipment supplier from the very outset.