What factors contribute to weld quality?

What factors contribute to weld quality?

 

Joint quality can mean a variety of different things depending on who is asking and for what application. A quality weld to one engineer could mean a strong hermetic seal and to another it could refer to a clean and consistent weld for aesthetic reasons.

This article will focus on weld quality in reference to strength and seal.

There are three categories of factors that play into the quality of a weld:

  1. Materials
  2. Part/geometry
  3. Process
Before we dive in to each of these categories it is important to point out that a large variety of factors that play into the quality of a joint make it difficult to create universal statements on what makes a good weld or a bad weld. All of these factors should be considered, but do not necessarily all need to be applied.

Material

  • Melting temperature – it is very important that the two pieces of plastic to be joined have similar melting temperatures. If the melting/softening ranges are too dissimilar the plastic with the lower melt temperature will begin burning/degrading before the other joining partner has even begun to soften. 
  • Chemical compatibility – the plastics to be joined must also be of similar chemical compatibility. This is mainly a function of the plastics weight or chain length. Please see the Material Compatibility Chart for more information.

*It should be noted that the most common thermoplastics are weldable. Also, laser plastic welding has introduced other combination possibilities such as hard-to-soft material welds.

  • Fills – fills such as glass or fiber will affect the welding process by changing the way the laser interacts with the plastic. The upper layer requires transmission of the laser radiation. Most thermoplastics are naturally laser transmissive, but fiber fills will change the transmission rates of the plastics. Fills will either cause the laser to be absorbed more or will tend to scatter the radiation. In both cases less radiation makes its way to the weld interface. Less radiation is not necessarily a bad thing, in some cases it can be beneficial, however, it is important to understand this interaction and how it affects the process.
  • Amorphous v. semi-crystalline – similar to how fiber fills tend to scatter radiation, semi-crystalline plastics will scatter radiation more than its amorphous counterparts. This scattering will cause a less focused radiation field to the joint and also create more absorption within the upper, transmissive layer.
  • Additives – flame retardants, UV stabilizers and heat stabilizers will all affect the transmission properties of the plastics as well, typically causing a reduction in transmission; to what extent depends on the amount of additive and the types of plastic.
  • Colors – colors additives and pigmentation in almost all cases will add absorbing effects to the plastic. Similar to the visual light spectrum, darker colors will absorb more radiation than lighter colors. However, it is very important to realize that optical transparency and laser transparency are not one in the same. Opaque dies that allow full laser transmission do exist. For more details on how color affects the weld process please see our last blog post, “How does color affect the weld process?

Part/Geometry

  • Upper layer thickness – the thickness of the upper layer plays a major role in the transmission capabilities of the plastic. The thicker the part the less radiation will be transmitted. Ideally, this part should be about 3mm in thickness, however, depending on other factors this can be much thicker or thinner.
  • Collapse rib – two flat plates are capable of being welded to one another, however, it is recommended to add a weld rib on the lower layer. This rib will allow for collapse of the two parts into one another under clamping pressure. This collapse distance will help ensure part tolerances are overcome and the joint is consistent and sealed.
  • Part tolerances – parts should be designed to fit well together and injection molders should be encouraged to use precise molding methods. Poorly fitting parts often have gaps at the joint line. Gaps are undesirable as they result in poor conduction of the heat energy from the lower layer to the upper layer at this point. Conduction is paramount in achieving adequate melt of both parts.
  • Beam access – considerations should be made for access of the beam to the joint line. Aspects to consider are how the clamp tooling will interact with the part, ensuring it does not block the laser at any point. Also, part features such as channels or molding gates should be paced outside of the joint area. Changes in plastic thickness, cavities or plastic densities will affect the amount of applied energy and the consistency of the weld.

Process

  • Process type – there are 4 major process types: contour welding, simultaneous welding, quasi-simultaneous welding and hybrid welding. Please see the image below for clarification on these processes.
  • Radiation wavelength – thermoplastics are transmissive to laser radiation between 880 to 1070 nano meters, with variances between types of plastics. LPKF, in particular, employs lasers with wavelengths of 808 or 980 nano meters.
  • Laser power/energy – the amount of energy that is absorbed by the lower layer plays a big role in the process. Too much and the plastic can be degraded, whereas, too little and melt is not reached. Energy can be controlled by laser power input (typically 100 to 300 watts) and laser time on part or travel speed.
  • Clamp tooling – with laser plastic welding the initial concern is almost entirely focused on the laser interacting with the plastic, however, the majority of quality issues seen in laser welding are of mechanical nature. Proper clamping is paramount to the process. A variety of tooling can be used to achieve clamping based on the needs of particular applications. The goal is to get solid clamp pressure consistently around the entire joint. Flat parts often use a simple acrylic block to provide force to the entire surface. More complex and 3D parts use metal clamp tooling specifically designed to fit the curves of the part.
  • Nesting – the nest is often just a common work piece carrier. The idea is to provide support for the entire length of the joint from below.

Conclusion

These are the most common factors affecting weld quality. Each of the above factors will affect and change the process and considerations for the rest of the factors. It is, therefore, difficult to make universal statements and all of these considerations should be considered nothing more than guidelines.

For a more in depth look at these factors and other consideration we encourage you to research our Design Guidelines, posted here in both document and webinar formats.

Have questions or comments? Join the discussion at the Laser Plastic Welding LinkedIn group!

 

// Zoho