By Jerry Zybko
As manufacturing has evolved over the years, the methods needed for assembly have expanded. Plastics have certainly gained more widespread acceptance in a variety of industries including automotive, medical, industrial and consumer applications. Depending on part geometry, material/part requirements and financial constraints, a variety of assembly methods are available.
Assembly methods can be broken down into three general categories; mechanical, chemical and heat. Mechanical assembly consists of molded in threads, or snap features, or the use of mechanical fasteners (e.g., screws or rivets). Chemical assembly methods consist of adhesives and solvents. Heated assembly is accomplished either by creating friction (e.g., vibration welding, spin welding or ultrasonic welding) or applying an external heat source.
Welding plastics using external heat sources has been around for decades. It has evolved from directly applying heat (e.g., hot air or radiant) or a heated platen (e.g., hot plate welding) to other sources such as lasers.
The use of lasers for plastics assembly is gaining significant popularity as acceptance has increased and capital equipment prices have fallen. Typical issues experienced by users of other assembly methods have led to a broader acceptance of laser plastic welding for its many benefits. Below is a graphic showing the TTIR (through transmission infrared) welding concept.
LASER PLASTIC WELDING ADDRESSES MANY OF THE COMMON PROBLEMS ASSOCIATED WITH PLASTICS ASSEMBLY:
No complicated joint designs – Laser plastic welding does not rely on melting and flowing plastic material. Energy directors are required in methods that utilize friction to create the heat. Engineering these features could be complicated and add to overall project cost. Lasers require only a flat to flat joint design thereby lowering tooling cost and increasing molding efficiencies.
No particulate development – The parts are presented to the laser in their final assembled position. Since the parts are in intimate contact before and during the weld process, no particulate is developed during the weld cycle.
Minimal heat affected zone – In addition to being vibration free, laser welding minimizes the heat affected zone. Only the localized area is heated to the melting point by laser energy and the rest of the part is not. Sensitive components located in the proximity of the weld seam are not damaged by the weld process.
No material expulsion (flash) – Material flash or expulsion is the release or witness of melted material outside of the desired weld area. It occurs in other weld processes, since mechanical motion is required to generate heat.
Precise weld lines – Laser welding offers a clean, precise and aesthetically acceptable bonding solution. Since the resulting weld is contained within the area affected by the laser energy, the weld lines are always consistent and have a crisp edge definition.
No surface marring – One of the more notable problems with an assembly process where heat is generated due to friction is marring of the visual surface. In laser welding, this issue is addressed by gentle clamping of parts with no relative motion between the parts during the weld cycle. Laser welded parts are processed without scratches or other handling evidence.
No consumables (fasteners or adhesives) – With laser welding there are no consumables. You are simply re-melting the plastic in the weld area and the two components are bonding at that point.
Ability to weld dissimilar thermoplastics – Laser welding process is able to join a variety of plastics, including joining soft to rigid. The main requirement for successful bonding is chemical material compatibility and overlapping melting temperatures.
REQUIREMENTS FOR SUCCESSFUL LASER WELDING:
Transparent top layer and light absorbent bottom layer – The laser welding process is made possible by transmission and absorption properties of the two materials being joined. The top layer needs to be transmissive, allowing the laser light to pass through. The bottom component must absorb the laser energy, creating heat. Sufficient heat is then transferred to the top, transmissive piece, and a successful weld is achieved.
Effective nesting or fixturing – To assure a consistent and strong bond between laser transparent and laser absorptive surfaces a clamping force must be applied. Since the top layer is transparent to the laser, it can be heated up only through conduction, by touching the bottom piece. It is essential for both components to maintain intimate contact during the welding process.
Sufficient clamping – The most common approach is to place the parts into a clamping device. The components are placed in a fixture, or nest, and they are pressed up against a glass frame. In some cases, a metal frame can be utilized to clamp down on the top piece right next to the weld area. This method is often chosen when extreme clamp forces are needed. A patented Globo welding method can be used to apply localized pressure.
The last and most important element is the introduction of laser light to the assembly. The most common laser sources for plastic welding are Diode, Fiber or Nd:YAG. An important equilibrium must be achieved by balancing the proper amount of laser energy with the proper exposure duration. The beam is then positioned or aimed at the work-piece and, depending on the beam-shaping, is either fixed or moved throughout the areas of the assembly that need to be joined. Pictured below are a variety of beam shaping techniques:
Bringing the laser process into the production environment can be accomplished in two ways; self-contained standalone workstations or individual laser component sets for direct integration into automated production lines.
Lasers continue to gain popularity as a method for assembling plastic components. Thousands of applications using lasers for plastic assembly are in production and this number grows each year. New laser sources, beam shaping methods, materials (films, fabrics) and plastic combinations continue to push lasers as an acceptable technique for plastic assembly.
Jerry Zybko is the General Manager at Leister Technologies.