Laser welding

ADVANTAGES        IN    DESIGN    BY    LASER    WELDING    OF    CO-EXTRUDED THERMOPLASTIC ELASTOMERS

Rolf Klein, Gareth McGrath and Bill Cawley Gentex Corporation, Carbondale PA 18407, USA Bob Donahue Natvar, West Clayton NC 27520, USA

Abstract

Thermoplastic elastomers can be transmission laser welded using near infrared (NIR) absorbing materials as a mechanism to produce heat and localised melting. NIR absorbers generate much less colour than carbon black and so are ideal for medical applications where the use of transparent materials is required.

An example of such an application is laser weldable medical tubes made from TPU with NIR additives. This paper will  discuss  the  advantages of  laser welding  luer connectors to the ends of the tube and will compare the results of laser welding with conventional joining methods like   adhesive   bonding.   Data   will   be   provided   to demonstrate opportunities, such as reduced joint area and welding time, which are created using this unique product development. The quality of laser welding achieved allows optimization of the luer design, while reducing assembly time  significantly.  This  new  technology  eliminates  the need for solvents and adhesives required by most medical in assembly operations currently in the medical market place.

Introduction

Laser  welding  was  first  demonstrated  on thermoplastics in the 1970's, but has only recently found a place in industrial scale situations; this is due to economic reasons and is driven by the reduced costs for laser systems. The technique, suitable for joining both film and moulded thermoplastics, uses a laser beam to melt the plastic in the joint region. Two forms of laser welding exist; CO2  laser welding   and   transmission   laser   welding.   CO2     laser radiation is readily absorbed at the surface of plastics with limited depth of penetration of the beam. This permits only surface melting and restricts the technique to film applications or butt welding where the surface of both components is heated simultaneously and clamped together in a second step, similar to hot-plate welding, but without contact. The radiation produced by diode, Nd:YAG and fibre lasers is less readily absorbed by plastics, so these lasers   are   suitable   for   performing   transmission   laser welding. In this operation, it is necessary for one of the plastics to be transmissive to laser light and the other to absorb the laser energy, to ensure that the beam focuses on the   joint   region.   Alternatively,   an   absorbing   surface coating   may   be   applied   at   the   joint,   to   weld   two transmissive   plastics.   Transmission   laser   welding   is capable of welding thicker parts than CO2  welding, and since the heat affected zone is confined to the joint region no marking of the outer surfaces occurs.

Laser Welding

Laser welding is a high volume production process with the advantage of creating no vibrations and generating minimum weld flash. This highly repeatable, consistent process requires an initial outlay for a laser system. However the benefits of a laser system include; a controllable beam power, reducing the risk of distortion or damage to components; precise focussing of the laser beam allowing accurate joints to be formed; and a non contact process which is both clean and hygienic. Laser welding may be performed in a single-shot or continuous manner, but  the  materials  to  be  joined  require  clamping.  Weld speeds  depend  on  polymer  absorption.  It  is  possible  to create joints in plastics over 1mm thick (with transmission laser welding) at up to at least 20m/min whilst rates of up to 750 m/min are achievable in the CO2  laser welding of films.

Laser welding provides good weld strengths that are comparable or superior to those created in other joining techniques. It accommodates preassembly and high weld speeds,  permits  3-D  contour  joint  lines,  and  facilitates rapid changeover to different products.

Capitalizing on these advantages requires careful attention to six critical factors: laser technology, laser absorbers, substrate materials, part design, clamping, and process parameters. Most thermoplastics are highly transmissive in NIR wavelength range (700 to 2000 nm). To laser-weld them, the top substrate must be transmissive at the laser wavelength, and the bottom substrate must absorb the laser energy. There are thickness limitations for top substrates of  low-NIR-transmissive  plastics,  such  as semi crystalline polymers and polymers containing pigments and fillers. Substrates must be in contact and miscible when heated.

Laser Technology

CO2 Laser Welding

The CO2  laser radiation (10.6µm) is rapidly absorbed in the surface layers of plastics. The plastics will heat up if the laser excites a resonant frequency in the molecule. In practice the absorption coefficients for the CO2  laser with most plastics is very high. Very rapid processing of thin plastic film is therefore possible, even with low or modest laser powers (100 -1000W).

Transmission Laser Welding - Nd:YAG Laser

The Nd:YAG laser is also well established for material processing, and recent developments have led to increases in the power available to above 6kW and reduced the physical size of the laser. The development of diode pumped Nd:YAG lasers increased the beam quality and the energy  efficiency  (from  3%  using  flash  lamps  to  10% using  diode  laser).  In  general,  the  light  from  Nd:YAG lasers is absorbed far less readily in non-pigmented plastics than CO2  laser light. The degree of energy absorption at the Nd:YAG laser wavelength (1.064µm) depends largely on the presence of additives in the plastics. If no fillers or pigments are present in the plastics, the laser will penetrate into the material. The absorption coefficient can be increased by means of additives such as pigments or fillers. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy flexible operation with gantry or robot manipulation.

Transmission laser welding - Diode laser

High power diode lasers (>100W) have been available since 1997. The production of the diode laser light is a far more energy efficient process (30%) than CO2  (10%) or Nd:YAG (3 - 10%) lasers. The interaction with plastics is very similar to that of the Nd:YAG lasers. The diode laser source is small and light enough to be mounted on a gantry or robot for complex processing. Diode lasers are available up to 10,000 W with direct beam or 8,000 W using fibre beam-guiding systems.

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