Laser material processing of polymers

Po~merTes~ng3 (1983) 303-310

LASER

MATERIAL

PROCESSING

OF

POLYMERS

S. E. NIELSEN

Laboratory of Process and Production Engineering, The Technical University of Denmark, 2800 L yngby, Denmark

SUMMARY

Since the first laser was developed in 1960, laser technology has been growing and now covers a wide range of laser types and applications. One of the most important applications of lasers is the use of a high power laser beam for material processing. One very interesting material group in which lasers can be used for processing is polymers. In these materials the lasers can be used for drilling and cutting, as well for welding. Lasers can produce holes at very high speeds and in dimensions which in practice are unobtainable by other processing methods. Contour cutting with high speeds and narrow tolerances is also possible. This paper will briefly describe the laser processing mechanisms involved in laser drilling, cutting, welding and engraving, and will give some typical examples of laser processing with special attention to processing of polymers.

1.

LASERS FOR MATERIAL PROCESSING

T h e most applicable laser types for material processing are the CO2-1aser, the N D - Y A G - l a s e r and the ND-GLAS-laser. The CO2-1aser is a gas laser and the other two mentioned are solid-state lasers. The CO2-1aser is the most powerful and is able to operate in both a continuous and pulsing mode. The average power is from 50 to 20 000 W. The N D - Y A G - l a s e r is also available in both continuous and pulsing modes with average powers up to 400 W. The NDGLAS-laser is only able to work in a pulsing mode but it is, on the other hand, able to give the most powerful single pulses with typical peak powers of about 1MW. 303 Polymer

Testing 0142-9418/83/$03.00 (~ Applied Printed in Northern Ireland

Science Publishers Ltd,

England,

1983.

304

s.E. NIELSEN

A special difference between the CO2-1aser and the solid-state lasers is the wavelength of the emitted light. The CO2-1aser emits light at 10.6 ~m and the solid-state lasers at 1.06 ~m. A lot of materials have different optical properties at these wavelengths which in some cases, gives the two types of lasers complementary applications. Transparent types of polymers will not be affected very much by the solid-state lasers, because they only transmit the light through the material. However, the CO2-1aser can be used instead because most polymers absorb light at this wavelength.

2.

CHARACTERISTICS OF LASER MATERIAL PROCESSING

The laser material processes are characterised by high beam intensities and short pulse widths. These characteristics and the fact that the laser output can be controlled very precisely gives the possibility of: ---controlling the heat input to get either the hardening, melting, drilling or cutting process; --heating up a very small, well-defined area (down to 0.0001 mm2); ----controlling the total heat input so that it is very small and thereby eliminating the risk of heat damage. Laser material processing normally requires a small beam diameter to get a geometrically bounded process, which means narrow cuts and narrow welding grooves. The irradiated area can be as little as 0.0001 mm 2. It is, however, necessary to focus the beam with a lens to get a beam diameter of this size. A nozzle is placed in connection with the lens so that a gas can be sent to the point of processing (Fig. 1). Depending on the kind of processing, this gas serves different purposes: --it shields the lens against damaging vapours or drops from the processing; --it removes vapour and drops from the light path to avoid light diffusion; --it participates in the process, either as a shielding gas for welding or, if oxygen, by assisting the cutting process. Besides the advantages of quickly, precisely and locally placed heating, laser material processing also offers: - - n o contact between material and tool; - - t h e possibility of placing processing materials behind transparent materials; - - t h e possibility of processing taking place under atmospheric pressure and in nearly all kinds of gases; - - a very suitable method for automatic production.

LASER MATERIAL PROCESSING OF POLYMERS

305

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FINE FOCUS 'ADJUST

GAS ~ INLET

~

t ~'iJH

AXIAL / NOZZLE ADJUSTMENT

Fig. 1.

GASIE T

~_.-.----~NOZZLE

Lens/nozzle system for laser material processing, t

Some types of lasers are very flexible and are able to shift processing parameters very quickly. It is then possible to use only one laser in a production including both drilling and welding by quick, automatic shifts during the process.

3.

LASER PROCESSING OF POLYMERS

The material group, polymers, contains a number of materials which are industrially processed by lasers. These include acrylics, polytetrafluoroethylene, polystyrene, polycarbonates, poly(vinyl chloride) and different kinds of rubber materials. They absorb energy at 10.6 ~m, the wavelength of light emitted by a CO2-1aser. T h e main types of processing are drilling, cutting, welding and engraving. In the following these processes will be described and illustrated with some industrial applications.

3.1. Drilling Laser drilling requires a laser working in a pulsing mode. The energy in the pulses is often of the order of 0.1 to 30 J. The pulse widths are typically of the order of 0.1 to 2 ms. The mean power used in the process is calculated by

306

s.E. NIELSEN

multiplying the pulse energy by the repetition rate. This means that lasers with a few watts of m e a n p o w e r are able to drill without requiting any special repetition rate. T h e diameter of the drilled hole is a function of: - - t h e wavelength of the light; - - t h e power distribution in the laser b e a m ; - - t h e pulse length and the repetition rate when m o r e than one pulse per hole is needed. If the p o w e r distribution in the laser b e a m is optimal the m i n i m u m hole d i a m e t e r drilled with a laser is 3 to 4 times the wavelength of the light. T h e m a x i m u m hole diameter is limited by the laser power and the intensity necessary to e v a p o r a t e the material. T h e typical m a x i m u m hole size formed by drilling with a CO2-1aser is about 0.4 mm. For a N D - G L A S - l a s e r it can be m o r e than 1 mm. Larger holes must be cut out. T h e depth of a laser-drilled hole can be m o r e than 10 times the hole diameter. If the hole is relatively deep with a small diameter and the heat-affected zone is required to be minimized, m o r e than one pulse is necessary to drill the hole. If the g e o m e t r y of the hole does not need to be very fine, only one pulse may be necessary. Laser drilling gives the possibility of processing areas with difficult access and at quite high production rates (up to 1000 holes/s). Figure 2 illustrates an element m a d e of polystyrene, which is a part of a h e a r t - l u n g machine. In the outer shell of the element, which separates the

Fig. 2.

Laser-drilled holes in polystyrene.

LASER M A T E R I A L PROCESSING OF POLYMERS

Fig. 3.

307

Laser-drilled polyurethane vein.

blood from the oxygen 125 holes are drilled. Each hole has a diameter of 0.23 mm and is drilled with very high precision because a very precise flow through the holes is required. In the production the element is placed in a jig which is able to rotate. The holes are drilled and the laser is activated to give 125 pulses per rotation. The element rotates 15 times, which means that every hole is processed 15 times before full penetration of all the material, which is 3 mm thick, is made. Figure 3 illustrates a vein made of polyurethane which is placed in the human body by surgical operation. The vein which is 8 mm in outer diameter and 1 mm in material thickness has been perforated with holes drilled in the surface. The laser beam has been divided into two beams and directed to the surface of the vein to make a double hole drilling at the same time. The two holes are positioned in such a way that the bottoms of the holes just meet in the middle of the material. In this way a passage through the surface and out on the same side is made. The perforation pattern is made by letting the vein pass the optical system in a spiral curve while the laser is pulsing at a precise rate. These holes enable the vein to grow together with the tissue of the human body. 3.2. Cutting Lasers with both continuous and pulsing modes are used for laser cutting. Generally, a laser beam with a good power distribution is needed and a better cutting quality is obtained with increasing cutting speed. The cutting rate is nearly proportional to the mean power in the laser beam and only lasers with mean powers over 200 W are normally used for cutting

308

s.E. NIELSEN

applications. T h e most frequently used lasers for cutting are CO2-lasers with mean powers between 250 and 5 0 0 W . For some applications where an especially narrow cut is required ND-YAG-lasers are used. As mentioned earlier a gas is normally used in connection with laser material processing. For the cutting process these gases are important to achieve a good cut quality and a high cutting rate. T h e nozzle system is normally placed approximately I m m from the surface of the material. Laser cutting is used for: ---contour cutting in metals and polymers, where the production volume is small and where blanking for this reason is uneconomical; --cutting complicated materials, e.g. stainless steel; --cutting thin materials. Generally the penetration depth for laser cutting is limited and the best quality is obtained in thin materials. Laser cutting gives very narrow cuts typically about 0.3 mm up to 2 mm in material thickness, and the heat-affected zone of the material is approximately 0.1 to 0.3 mm. Figure 4(a) shows a 13 mm-thick piece of acrylic which was cut with a 250 W beam using a 5 in. lens. At this power level it is possible to cut such a material at a speed of 70 to 90 mm/s. The cutting process is able to give fire polished edges in contrast with other cutting methods which give rough, non-transparent edges. Acrylic cutting rates are given in Fig. 4(b). As can be seen, cutting rates increase as the material gets thinner. Also note that 11 cttts

lO

CW

9

"0 ffJ

m4 .c =3 U

(a)

2

(b)

Thickness-inches

Fig. 4. Laser-cut acrylic (a) and acryliccutting rates (b).2

LASER MATERIAL PROCESSING OF POLYMERS

309

doubling the beam power does not exactly double the cutting speed, an effect which becomes more pronounced as materials get thicker. Laser cutting of woven polypropylene web is a very good application of laser processing. The process gives at high cutting speeds a sealed edge which prevents unravelling of the material. For the same reason the numerically controlled laser cutting system has been successfully applied in the textile industry. It is possible to cut many layers of fabric at one time with the pattern programmed into the machine. Applications, such as cutting nylon seat belts, foam rubber and many other kinds of polymers, are also being processed with lasers. 3.3. Welding The thermal properties of polymeric materials are characterized by low heat conduction and the narrow temperature intervals in which the polymers are in the melting phase. For some polymers this interval only amounts to 10 degrees, which makes it extremely difficult to process. Under these conditions, it is only possible to weld thin polymer materials and very precise control of the beampower and power distribution in the beam is required. Figure 5 illustrates that thin layers of polyethylene can be laser lap-welded together. The sheet, which is 0.075 mm in thickness, was welded from both sides (by splitting the beam into two) to obtain a symmetric and uniform power distribution. 3.4. Engraving Selective rubber removal is a technique employed by the printing industry. When a laser is used for this process the photographs or halftones of the pattern to be reproduced are placed on a drum; this rotates under an optical reader that uses a H e - N e laser tracking beam. The beam is focussed to a small

Fig. 5.

Seam-welded thin sheets of polyethylene. 3

310

S.E. NIELSEN

Fig. 6. Rubberplate engraving.2 spot and the scattered light from it is read by photodetectors which indicate black or white tones. This information is fed back to the laser, which turns on or off according to the pattern being read. The linear surface speeds for this process are 0.7-0.8 m/s and the material is translated at a rate of 0.05-0.1 mm/revolution. Figure 6(a) shows the laser machining the rubber plate (on the far end of the drum). The optical reader and laser essentially scan at the same rate, so whatever the reader reads, the laser will reproduce in the rubber film. This technique results in seamless printing rollers--a significant advantage over conventional techniques. A finished rubber plate is shown in Fig. 6(b).

REFERENCES 1. OLSEN, F. O. (1982). New Constructive Possibilities for Laser Material Processing, AMT.

AP-report 82-18. (In Danish). 2. CormREr~rRADIATION,INC. Lasers, Operation, Exluipment, Application and Design. 3. Corm~m RADIATION,INC. Industrial Laser Application Notes.