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1.Introduction

Introduction to Laser Welding of Thermoplastics is an overview tutorial for using laser in plastics assembly and covers the following topics:

• Common Problems in Plastics Welding that can be addressed by a variety of Laser Welding techniques.
• Fundamental Concepts of laser welding and conditions required for making it possible
• Various Methods of Laser Welding which reviews commonly used and proprietary techniques for shaping and delivering laser energy.
• Equipment options and automation covers laser systems and possible configurations available from Liester

2. Common Problems in plastic assembly

Laser Welding of thermoplastics offers reliable solutions to many common bonding problems.

a. Leaking assembly – Laser welding is capable of achieving a hermetic seal, which is commonly required in many industries. Therefore laser is often used in air and water tight applications.

b. Contaminated Assemblies - Laser welding does not require vibration or friction to create heat. Consequently, there is no particulate development during the weld process.

c. Damage of sensitive components – 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.

d. Material Flash – Material Flash or Expulsion occurs when mechanical motion is required to generate heat. Alternatively 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.

e. Consistent Processing – Laser Welding of plastics is a very repeatable process thanks to the equal distribution of energy generated during every cycle and the elimination of mechanical motion between parts. Additionally, in laser welding there is minimal collapse between parts. Finally with clean, precise and reproducible weld lines, scrap rates are substantially reduced.

f. Assembling Small parts –Leister’s proprietary Mask Welding process allows for small part assembly. This technique realizes complicated welding patterns with minimum of 100 micron feature size. Mask Welding is one of the fastest laser welding methods. Mask Welding is covered in greater details in a later section.

g. Marring of visual surface – 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.

h. Assembling Different Materials – 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. This chart identifies the common combinations of materials. Please contact Leister to discuss your specific material requirements.

i. Consumables –Laser welding is a consumable free process while other methods may be more expensive and cumbersome to maintain. Laser welding does not require any consumables including:

• Horns
• Transducers
• heating elements
• tips
• o-rings
• seals
• tapes
• adhesives
• screws
• rivets
• glue

j. Complex Joint Design – 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. Laser requires only a flat to flat joint design thereby lowering tooling cost and increasing molding efficiencies.

k. Achieving a Strong Weld –Laser welding provides exceptional bond strength. Typically, the weld strength will exceed parent material strength and this is evidenced by material transfer. This assembly was broken apart in a burst test (with a hammer and a screw driver) revealing a successful strong weld as evidenced by material transfer on both sides and no break in the weld plane.

3. Fundamental Concepts

a. Transmission and Absorption requirements - 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.

b. Materials - The popular choice for the top, laser transparent component is natural, or virgin material. The bottom, laser absorptive component, is often a material that contains ½ to 1% of carbon black. While the natural to black pairing is often used in laser welding, not all assemblies can afford this restriction in color choice. The top component need not be clear to the eye – it need only be transmissive to the laser. Thus, one can add pigment or die to the top component to obtain the desired color. Conversely, even though carbon black is the most economical absorber for the laser, other colors, such as dark blue, green or brown, may also absorb a sufficient amount of the laser.
In some cases when two laser transparent pieces are welded together, third party absorbers such as Gentex Clearweld™ can be applied to the mating surface or molded into the bottom component to create the necessary heat conversion at the interface.

c. Fixturing and Clamping - To assure a consistent and strong bond between laser transparent and laser absorptive surfaces one must apply a clamping force. 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. 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. A number of different clamp configurations can be adopted, based on application requirements. 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

d. Laser Energy Delivery - With the parts resting in a proper clamping device the final step is to provide the necessary amount of energy for a specific amount of time throughout the weld path. The two common processes are moving the laser across the weld area or having the laser applied simultaneously across the complete weld area. With either method there is an important equilibrium that must be achieved by balancing the proper amount of laser energy with the proper exposure duration. Applying too much energy or moving too slow will result in excess heat. Conversely providing insufficient energy or moving to quickly will result in inadequate heating. Laser light can be shaped to a Spot, Line, Circle or some other custom shape. In the following section we will cover different laser welding techniques that utilize these laser light formations.

4. Various Methods of Laser Welding

In this section we will review six common laser welding methods:

a. Contour - Contour welding is one of the original laser welding methods. In this method the laser beam is shaped in a spot. The laser light is delivered from the source through a fiber-optic cable to the optics head responsible for shaping the light. The light emits from the aperture in a conical shape until reaching the desired spot size. A typical working spot size ranges between 0.6mm and 3mm. The laser spot is then traced around the predefined contour creating the desired weld pattern.

b. Simultaneous - Simultaneous welding also utilizes a fiber optic delivery method. However instead of focusing the beam in a conical fashion, it is formed into a specific and fixed pattern. The shaping of the laser light is realized by custom made diffractive optics. The typical weld area for a predefined pattern is within 50mm2

c. Radial - The radial method uses a simultaneous ring-shaped laser pattern, which is projected onto a conical mirror directing the light around a tube shaped assembly. The components to be welded are press-fit together and inserted into the Radial Head. The end result is a simultaneous circumferential weld around the outside the tube assembly

d. Globo - As we discussed in the previous section it is essential to maintain intimate contact throughout the weld path. However not every assembly is flat, allowing for uniform force distribution when clamped against a pane of glass or metal frame.
The Globo welding method developed by Leister overcomes this obstacle by incorporating the clamp function into the laser optic head. The clamping is achieved by pressing the glass ball lens at the end of the Globo head to the current weld area. The glass ball lens is free floating, enabling the Globo head to roll along a two or three dimensional weld path.
In addition to three dimensional assemblies, large components that are difficult to clamp can be welded utilizing the Globo Method.

e. Mask - The Mask Welding Method was developed by Leister to realize small and intricate weld patterns. In this method the laser beam is directly shaped into a line. The line is then moved across a mask made to allow laser light to pass through where welding is required and block laser light in areas that need to be protected. Most welding masks are a piece of glass with a chrome layer etched away in certain areas according to weld pattern requirements.(show a mask)
The Mask process is especially beneficial in applications containing channels for delivery of fluids or gases. For these reasons mask welding is commonly used in medical industry for assembling micro-fluidic devices.

f. Continuous Mask - The Continuous mask method presses a hollow cylindrical glass mask against a continuous feed of two plastic films. The laser line is transmitted from the center of the mask enabling a repeated continuous pattern welding of the two films. Accurate and precise welding of disposables or other products can be accomplished at speeds up to 200 feet per minute.

5. Equipment options and automation

Each of the six methods described in the previous section can be configured in a number of ways:

a. Novolas WS - Novolas WS is a turnkey laser workstation that includes it’s own Process and Motion Control Systems available via the Novolas Software. The software allows for quick adjustments to the weld path and other process parameters. Upon arrival, Novolas WS requires only power and compressed air connections prior to operation.
A standard WS comes complete with:

• Laser module
• Clamp device
• XYZ motion system
• Class I laser enclosure
• PC Controlled Software

Optional features include:

• Pyrometer (for Temperature detection and weld validation)
• Vision system (for ease of setup)
• On board power measurement device
• Two station rotary table

b. Basic AT - Novolas Basic AT laser system was developed for quick and easy integration into a new or existing production line or any other custom setup. The Basic AT unit takes care of safety, communication and all other internal operating conditions, leaving the integrator with simple interface for Process Control.

c. Basic AT Compact - Novolas Basic AT Compact is an air cooled laser system, that it is the smallest and most cost effective solution for applications under 40W. Basic AT Compact can be used with all methods that utilize fiber-coupled optics. This tabletop unit is equipped with a front display of the processing state and system diagnostics. Novolas Basic AT Compact is an excellent choice for low power applications.

d. Conclusion - In conclusion we hope you enjoyed Leister’s laser plastics Welding Tutorial. Leister is a provider of unique techniques for laser welding thermoplastics with sale and service centers around the world. Please contact one of our labs to discuss your particular application and to receive complimentary material compatibility testing.

Thank you for your interest in laser welding of thermoplastics!

© Leister Technologies LLC 2007.