Laser sealing of plastics for medical devices

Medical Plastics '98 - 12th International Conference, 7-10 September, Gothenburg and Addendum July 2001

Ian Jones and Nicki Taylor from TWI, Cambridge, UK review the latest developments in the use of lasers for plastics processing, and describe methods which can be applied to plastics film, sheet and mouldings for medical devices.

Joining requirements for plastics in medical devices

Plastics' joining is used widely in medical applications for products of ever increasing diversity. The wide range of applications, including welding of sheet, mouldings and soft-touch plastic fabrics for blood and ostomy products, micro joining expertise for catheters and large scale welding for mattress fabrics and moulded devices call for a variety of welding and joining methods. The joining processes currently used in medical applications include:

• for film and sheet welding

- heat sealing
- adhesive bonding
- impulse welding
- dielectric welding
- ultrasonic welding

• for welding of thick sheet and mouldings

- hot gas welding
- hot plate welding
- vibration welding
- ultrasonic welding
- adhesive bonding
 

The qualities demanded by the medical industry, such as high and consistent joint integrity, high precision and often high production rates increasingly call for improvements to the current methods of joining or the development of new joining processes. In certain applications, the use of lasers for joining can assist in meeting some of these demanding requirements.

Introduction to laser welding of plastics

Lasers are a very attractive tool for materials processing in many industrial areas. Their features allow for a precise delivery of a controlled amount of energy exactly to the point where it is required. The beam from a laser may be focused to a small spot giving very high power densities, capable of vaporising any known material or expanded to carry out a broad surface treatment. Since the construction of the first laser in 1962, these special features have been put to use in an ever-increasing variety of applications ranging from CD players to ship construction. 

In application to plastics processing, lasers may be used to deliver energy for welding cutting, drilling or surface treatment. Process advantages include:

• high speed
• no contact with a heated tool
• highly automated and robotically manipulated
• controlled heating for low thermal damage or distortion

In addition, lasers are available with outputs covering a range of wavelengths, which has a large bearing on the interaction of the light with the plastic material. An understanding of these absorption characteristics has led to the development of other novel applications in plastics processing.

In this paper we review the laser types which are available for plastics processing in medical applications and compare them in terms of their beam properties, cost of ownership and interaction with plastics materials. The application capabilities are discussed with reference to the alternative methods available for plastics processing culminating in a description of the recent developments in transmission laser welding.

Laser types and their interaction with plastics

The main commercially available laser types of interest in plastics processing are listed in Table 1. The different applications possible with each laser type are dependent mostly on the wavelength of light produced, which dictates the form of energy absorption in the plastic. However, for a given laser type, the processing behaviour also varies with the material type being used. The table also compares the cost of ownership of the lasers and the beam output available. Infrared lamps are included because the range of wavelengths available from them overlaps with the lasers and they therefore serve as a cost-effective alternative in some applications where a large beam diameter can be used. In contrast to lasers the infrared lamp emits a broad band of wavelengths of light which cannot be focused to small spot sizes (<3mm) and high power densities.

Table 1 Comparison of commercially available laser and infrared lamp sources

* - approximate figures for general case
Laser/Infrared Lamp Type	Wavelength µm	Max. Power W	Beam Transmission	Raw Beam Profile	Capital Cost* £k	Running Cost* £/hr	Interaction With Plastics*
CO 2	10.6	45,000	Refection off mirrors	Circular	100W - £20k 1000W - £50k	100W - £0.2-0.5 1000W - £2-4	Complete absorption at surface in <0.5mm
Nd:YAG	1.06	4,000	Fibre optic and mirrors	Circular	100W - £40k 1000W - £80k	100W - £0.1 1000W - £3-5	Transmission and bulk heating for 0.1-10mm
Diode	0.8-1.0	2,000	Fibre optic and mirrors	Rectangular	100W - £10k 1000W - £50k	100W - £0.1-0.2 1000W - £1-2	Transmission and bulk heating for 0.1-10mm
Excimer	0.15-0.35	1,000	Refection off mirrors	Rectangular	100W - £80k	100W - £5-10	Chemical bond breaking action at surface <0.01mm
Infrared lamp (tungsten filament)	0.7-2.5	3,000	Limited use of mirrors	Radiative	1000W - £2k	1000W - £0.2	Transmission and bulk heating for 0.1-10mm

The CO ₂ laser is a well-established materials processing tool, available in power output up to 45kW, and most commonly used for metal cutting. The CO ₂ laser radiation (10.6µm wavelength) is rapidly absorbed in the surface layers of plastics. Absorption at these photon energies (0.12eV) is based on the vibration of molecular bonds. The plastics will heat up if the laser excites a resonant frequency in the molecule. In practice the absorption coefficients for the CO ₂ laser with most plastics is very high. Very rapid processing of thin plastic film is therefore possible, even with fairly modest laser powers (<1000W). The CO ₂ laser beam cannot be transmitted down a silica fibre optic, but can be manipulated around a complex process path using mirrors and either gantry or robotic movement.

The Nd:YAG laser is also well established for material processing, and recent developments have led to increases in the power available to above 4kW and reduced the laser size. In general, the light from Nd:YAG lasers is absorbed far less readily in plastics than CO ₂ laser light. The degree of energy absorption at the Nd:YAG laser wavelength (1.064µm, 1.2eV photon energy) depends largely on the presence of additives in the plastics. If no fillers or pigments are present in the plastic, the laser will penetrate a few millimetres into the material. The absorption coefficient can be increased by means of additives such as pigments or fillers, which absorb and resonate directly at this photon energy or scatter the radiation for more effective bulk absorption. ^[1] The Nd:YAG laser may therefore be used for heating plastics to depths of a few millimetres or for heating a more highly absorbent medium (either metal or a plastic containing suitable additives) through or within a transmissive plastic part. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy flexible operation with gantry or robot manipulation.

High power diode lasers (>100W) have been available since early 1997. They are now available up to 2kW and are competitively priced compared to CO ₂ and Nd:YAG lasers. The production of the diode laser light is a far more energy efficient process (30%) than CO ₂ (10%), Nd:YAG (3%) or excimer (<1%) lasers. The interaction with plastics is very similar to that of the Nd:YAG lasers, and applications overlap. The beam from a diode laser is rectangular in shape, which, while being preferential for some applications, limits the minimum spot size and maximum power density available. The diode laser source is small and light enough to be mounted on a gantry or robot for complex processing. ^[2]

Excimer lasers were first operated in 1975, some years after the CO ₂ and Nd:YAG lasers. They are available with average powers up to about 1kW, but with pulse powers greater than 108W, and focusable to very high power densities. There is a family of wavelengths available by exciting different gases within the laser. These are all in the ultraviolet (0.15-0.35µm, 3.5-7.9eV photon energy), and lie in a photon energy range capable of breaking chemical bonds and splitting molecules. The C-H bond, for example, has a bond energy of 3.5eV. This effect leads to many precision machining and surface treatment applications, which are unique to excimer lasers. Excimer laser light is absorbed by molecules in the surface of plastics (<10µm depth), and rapidly breaks the molecular bonds within the polymer structure. This leads to a rapid increase in pressure and expulsion of material over a very precise region defined by the laser beam size. The excimer laser beam is transmitted by mirrors and often focused through masks to give the required features on the material surface. ^[3]

Infrared lamps operate by the electrical resistance heating of a wire filament. In contrast to lasers this leads to the emission of photons with a range of wavelengths, with the peak emission being at a wavelength dependent on the temperature of the filament. ^[4] Tungsten filament lamps emit most of their power at 0.7-2.5µm, which gives them absorption characteristics in plastics similar to the Nd:YAG and diode lasers. Absorption coefficients tend to be higher than for lasers, however, because there are more wavelengths of light available to couple into the molecular vibrations of the materials. Infrared lamps are relatively cheap and lightweight heat sources which are generally applied using an elliptical focusing mirror but without transmission of the light over long distances.

Review of processing capabilities and applications

A summary of the processing capabilities of the laser types is shown in Table 2. 

Fig.1 Lap weld in 0.1mm thick polyethylene made with a 900W CO ₂ laser at 100m/min.

CO ₂ laser welding of thin film is possible at very high speeds as shown in Fig.1. ^[5] Welding has been demonstrated with a range of plastic films at speeds of up to 500m/min. Clamping to keep the films in contact at the joint line is the most important feature of a system designed to carry out laser welding. This technique may be applied as an alternative to ultrasonic, dielectric or impulse welding where a fast, clean, fully automated joint is required. A simultaneous cut/seal may also be carried out for packaging or bag making purposes by controlling the laser beam power distribution to cut two films in contact whilst leaving a welded region at the edge of the cut. CO ₂ laser welding of plastics greater than 0.5mm thick is not possible at high speeds unless the joint surfaces are melted directly with the laser and then butted together. ^[6] This is a variation on hot plate welding in which the joint surfaces are heated against a hot plate before butting together. Focused infrared lamps have also been used as a non-contact heat source for this form of welding, reducing the chance of weld contamination. Nd:YAG and diode lasers may also be used to weld films if the pigments or fillers are present to absorb the beam or if an absorbent substrate is present. ^[7]

Table 2 Summary of application capabilities for the laser and infrared lamp types

Application	Material or configuration	Process Depth mm	Suitable Laser	Power W	Speed	Process Mechanism	Alternative processes
Welding	Film, lap Sheet, lap	0.1 2	CO 2 Diode, Nd:YAG	1000 500	100m/min 5m/min	Thermal absorption Interface absorption	Hot bar, ultrasonic, RF Hot plate, vibration
Cutting	Film, cut/seal Sheet	0.1 2 10	CO 2 CO 2 CO 2	1000 500 500	100m/min 8m/min 1m/min	Thermal absorption Thermal absorption Thermal absorption	Ultrasonic, die Die Saw
Drilling/machining	Through holes Blind holes	2 0.1	CO 2 Excimer	100 100	50msec 1msec	Thermal absorption Bond breaking	Hot needle Chemical etch
Surface treatment	Marking Enhanced adhesion	0.01 0.01	Excimer Excimer	100 100	N/A N/A	Bond breaking Bond breaking	Ink jet, dot matrix Plasma, chemical etch
Bulk curing	Paint, adhesive	0.1-10	CO 2 , Excimer	100	N/A	Photochemical and thermal initiation	UV, visible light, Electron Beam

CO 2 lasers are widely used for high speed cutting and drilling of plastics. The rapid heating and thermal decomposition of the plastic enables a 500W CO ₂ laser to carry out cutting of sheet up to at least 10mm thick. The cut may be carried out using a compressed gas (usually air) jet flowing coaxially with the laser. The gas jet removes the molten or vaporised material to leave a cut of 0.1-0.6mm width. ^[8] The process is non-contact, so soft materials such as foams are readily cut without deformation of the edge. Laser cutting or drilling may be used as an alternative to ultrasonic, die, saw or other mechanical cutting methods.

CO ₂ lasers can drill holes of 0.15-1mm diameter in 1-50msecs per hole. ^[9] Even higher precision drilling and micromachining is carried out using excimer lasers. Features with dimensional accuracy of as good as 1µm are possible in most plastics. Such control over removal of material allows for a range of applications, such as wire stripping, sensor nozzle drilling, and surface marking or coding, to be carried out. ^[3] Some of these tasks are not possible using other methods.

Excimer lasers have also been used to modify polymer surfaces prior to adhesive bonding. The photochemical reaction during the laser treatment can improve the shear strength of the lap joints compared to similar control joints made with abraded/degreased/abraded and primed surfaces. ^[10]

Transmission laser welding using Nd:YAG and diode lasers

Nd:YAG and diode lasers (and infrared lamps) can be used to heat surfaces which are in contact by transmitting energy through one side of the joint, given that one of the plastics is transmissive to the laser and the other absorbs. This process of transmission laser welding is shown in Fig.2. An example of polypropylene welded using this technique is shown in Fig.3. The process provides a means by which plastic parts with different absorption characteristics may be welded with no melt flash, no marking of the outer surfaces and only a small heat affected zone. The welding is carried out with either a Nd:YAG laser or a diode laser, and narrow welds can be made at up to 10m/min even with lasers of 100W or less. Infrared lamps can also be used to carry out similar joints but the broader wavelength band gives less selectivity between absorption in the two materials to be welded. In many cases the infrared lamp may offer a cheaper alternative to lasers for large joint areas.

Fig.2 Diagram of transmission laser welding.

Fig.3 Laser transmission weld in 4mm thick polypropylene using a 100W Nd:YAG laser at 1.6m/min. The weld is at the interface between the light and dark materials.

The process requires two materials that transmit the laser to different degrees. This can be arranged either by using filler or pigment in one side of the joint. Successful welds can be made with as little as 0.05% carbon black in the absorbing layer, [11] and dyes which absorb in visible light do not necessarily absorb in near infrared light, ^[12] so it is possible to weld two similar coloured materials with different dye types. Other requirements for successful application of the process include intimate contact at the weld interface and a smooth interface surface. Positive pressure is required over the joint area, though this is assisted by the thermal expansion at the interface.

Current areas of development in transmission laser welding

The transmission laser welding process has already reached production in a non-medical application for the manufacture of keyless entry cases by a German company - Marquardt. ^[12] Ten 30W diode lasers are used to make a 76µm wide weld at 5m/min. Transmission laser welding may also be used to heat and seal metal parts within a plastic, or to weld elastomers to plastics, where ultrasonic or vibration welding would not be possible. Other advantages include the ease of automation, process monitoring and control of quality, easier change of tooling than for vibration welding and reduced weld surface contamination compared to hot plate welding. Developments relating to this process are still underway. A greater understanding of the effect of fillers and pigments and other additives in materials and the effect of different laser types on the welding process is required. There is also a need for short and long term testing to be carried out before the process can be applied to structural components.

Summary

It is expected that laser processing will be used increasingly in applications requiring the rapid and precise application of heat to produce a joint, without thermal damage to surrounding areas. The variety of laser types and wavelengths available also allow equipment to be selected to match the precise absorption qualities of the material being processed and the action that is required on that material. Lasers have even been considered for joining living tissue as well as for microsurgery.

Biography of Speaker

Ian Jones is a Senior Research Engineer in the Plastics Group at TWI, Cambridge, UK. He has worked in the development of laser materials processing techniques since 1989, when he joined TWI after graduating from the University of Cambridge Department of Materials Science and Metallurgy. His work has included high power laser welding of steels and aluminium with the Laser Centre at TWI, and more recently laser welding of plastics with the Plastics Group of the Advanced Materials and Processes department.

References

N°	Author	Title
1.	Seredenko MM,	'Determining Spectral Characteristics of Pigment Absorption and Scattering in the Middle IR Spectral Range', Optics and Spectroscopy vol.76 no.3 1994 pp418-420.
2.	Haug M and Rudloff T,	'Assessment of Different High Power Diode Lasers for Material Processing', SPIE Vol. 3097 P5 and 3 Lasers in Materials Processing (Munich) June 1997.
3.	Gower MC,	'Excimer lasers: principles of operation and equipment, and current and future application in industry and medicine', from Laser Processing in Manufacturing edited by Crafer RC and Oakley PJ, pp163-261, Chapman and Hall, 1993.
4.	Readdy A F,	'Plastics Fabrication by Ultraviolet, Infrared, Induction, Dielectric and Microwave Radiation Methods'. Plastics Technical Evaluation Centre 1973.
5.	Jones IA and Taylor NS,	'High speed welding of plastics using lasers', ANTEC '94 conference proceedings, 1-5 May 1994, San Francisco, USA.
6.	Potente H, Heil M and Korte J,	'Welding of plastics using CO 2 lasers', IIW commission XVI document, XVI-681-95, 1995.
7.	Hardy R and Jones I,	'Welding of plastic films using infrared lamp and laser technologies', TWI members report, 1998.
8.	Powell J,	'CO 2 laser cutting', pp 91-115, Springer-Verlag, 1993.
9.	Coherent Inc,	'Lasers - operation, equipment application and design', pp55-71 Halliday Lithography, 1980.
10.	Tavakoli SM and Riches ST,	'Laser surface modification of polymers to enhance adhesion, part 1- polyolefins', proceedings of ANTEC '96, 5-10 May 1996, Indianapolis, USA.
11.	Potente H, Korte J and Stutz R,	'Laser-transmission welding of PE-HD', Kunstoffe 87 (1997) 3, pp.348-350.
12.	Puetz H et al,	'Laser welding offers array of assembly advantages', Modern Plastics International, Sept 1997.

Addendum (July 2001)

Further developments have now been carried out which enable the laser welding of clear to clear plastics (or a wide range of coloured plastics) using the transmission laser welding technique. Named Clearweld®, the process utilises a consumable absorber for the laser beam, placed at the joint interface. The resulting welds are clear and almost colourless and of high integrity. Melt occurs only at the joint interface and not throughout the component and the process is applicable to sheet and moulded parts, films and fabrics in a wide range of plastic types.

Additional information can be found on the Clearweld® website - www.clearweld.com.