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When is a Clearweld Coating or Resin Needed?

Transmission laser welding plastics uses a beam of high-density, near-infrared (NIR) energy that passes through an IR-transmissive substrate until it reaches an IR-absorbing substrate. Here the laser light is absorbed and localized heating occurs at the joint interface, producing strong, hermetically sealed welds with minimal thermal and mechanical stress.

Laser welding offers a number of advantages compared with other plastics joining techniques. Because it does not involve vibration, it does not cause mechanical damage to a part or generate particulates. Moreover, it yields bond strengths that are comparable or superior to other joining techniques, such as hot - plate and ultrasonic welding. It accommodates preassembly and high weld speeds, permits 3-D contour joint lines and facilitates rapid changeover to different products. In addition, process parameters can be controlled precisely, resulting in low heat input which reduces the risk of thermal distortion.

The process requires near-infrared-absorbing materials to convert laser energy to heat. Traditionally carbon black dispersed throughout the lower substrate material has served this purpose. The major disadvantage of to using carbon is that it results in a black or dark - colored opaque substrate. More recently, laser-absorptive coatings and resins have been developed to provide greater color and design flexibility in laser welding clear and opaque plastics.

The use of carbon black for laser welding dates back to the mid-1980s, when it was used to join under-the-hood automotive components such as manifolds, fluid containers, electronic housings and others. It is still used primarily for these type of applications, where it serves as an efficient, inexpensive absorber that is thermally stable. However, it imparts color to the substrate, a distinct disadvantage in applications where appearance is important. In addition, it requires high loading in the absorptive material, up to 0.50% to 1.00%, to be effective.

 Clearweld® coatings are designed to absorb laser energy in the 940nm to 1,064nm range using either diode or Nd:YAG lasers. Available in a variety of absorption levels, the appropriate coating is determined by the application requirements.

Clearweld coatings are available in 50ml to 1-liter containers.

When a coating is applied to plastic, a thin, uniform layer of an IR-absorbing material is deposited on the surface. Since most plastics do not absorb IR energy, only the areas where the coating has been applied and a laser is introduced will melt and form a weld. The coatings are low-viscosity, solvent-based liquids that are applied by liquid dispensing systems. The solvent serves as a carrier and evaporates quickly, leaving a film of absorbing material at the joint interface.

Clearweld coatings may be applied to substrates by various methods, including needle-tip, microsolenoid or spray dispensing. The method of application is determined by process speed, surface geometry and the area to be coated. Needle-tip dispensing is the best method for applying a thin line of coating to defined areas such as a channels, ledges or edges that can contain the coating. It does not work as well on perfectly flat surfaces, where the coating tends to spread out. A non-contact method, microsolenoid dispensing accommodates surface irregularities. It also works better than needle-tip dispensing on perfectly flat surfaces. Spray dispensing is the best method for coating large, flat surfaces and tubings.

Clearweld coatings produce colorless welds for medical devices.

Because they produce colorless welds, coatings are ideal for laser welding colorless and transparent materials for medical components such as filters, containers, fittings, tube sets and microfluidic devices. They can also be used to weld opaque materials. They offer limitless color flexibility and are compatible with the widest range of thermoplastics of all the laser welding techniques. They have passed USP Class VI biocompatibility and cytotoxicity testing. In addition, they permit selective welding without the need for masking and provide the capability to weld multiple layers simultaneously. Parts can be pre-coated off-line and inventoried for subsequent welding. The use of coatings in laser welding plastics is limited only by the fact that they require a separate dispensing/application step and their shelf life, even under proper storage conditions, is six months.

In addition to coatings, Clearweld offers additives for compounding or masterbatching into resins designed to absorb specific wavelengths in the 800nm to 1,064nm range. The type of additive used is determined by polymer compatibility, color requirements and laser welding wavelength. These additives result in parts with a high level of visible transmission, but impart a slight tint to the substrate , which can be color corrected. Resins containing Clearweld absorbers or carbon black can also be used to extrude films that can be die cut and insert molded or used an intermediate layer for laser welding.

Clearweld resins eliminate the need for a separate dispensing operation.

Laser-absorptive resins are well suited to molding colored transparent and light-colored opaque substrates for electronic housings, medical devices, fluid containers and other relatively small components with large weld areas. They are highly transparent and can be matched to a broad spectrum of colors. And unlike coatings, they do not require a secondary dispensing step. However, they do impart slight coloration to the substrate and are subject to thermal degradation if not used properly.

Summary

Carbon black is inexpensive and effective, but poses aesthetic issues. Coatings produce colorless welds, but require a secondary dispensing/application step. Resins eliminate this step, but impart color and may not be cost-effective for large parts with relatively small weld areas.

Ultimately the decision as to which laser-absorptive medium to use will be application-specific. Each has its pros and cons (see table below), but all offer the advantages of laser welding compared with other plastics joining methods.

RequirementCarbonClearweld CoatingClearweld Resin Additive
Colorless weld
x
+
x
Highly transparent part
x
+
+
Biocompatible
+
+
+
Processable in most polymers
+
n/a
o
Color Flexibility
o
+
+
Multiple layer welding
x
+
x
No secondary processing step allowed
+
x
+

 

+ can achieve requirement
o limited success in achieving requirement
x cannot achieve requirement

For more details related to your specific application contact Clearweld.