An Introduction of Resin Laser Welding Equipment ML-5220A
Kouji KAWAMURA, Shinichi NAKAYAMA
There are some problems related the environment in typical joining method such as adhering for plastic parts. Plastic laser welding is a non-contact welding well suited to overlap joints. This technology is already used for automobile industry and packaging. Recently, fiber coupled diode lasers in power range of several 10W are compact and more cost efficient. We have developed a unique diode laser equipment ML-5220A for plastic welding. In this report, we introduce a ML-5220A in detail and show some examples of plastic welding.
Key words: laser, diode laser, plastic, welding, thermoplastic
1. Introduction
The present typical methods to joint resin are: screwing, welding by ultrasonic vibration (friction heat), welding by heated plate, and bonding by adhesives. Though these jointing are less expensive, they cause a lot of troubles as shown in Table 1. Recently, laser welding of resin attracts a great deal of attention because it solves environmental issues (chemicals, recycling); furthermore, it lessens heat effect to surroundings of welded portion and reduces vibration exerted to adjacent parts.
Since most of the resin presents high absorption in infrared region, application of CO2 laser to resin is limited to a thinner sheet (film) of resin. On the contrary, such as Nd:YAG laser and diode laser can weld thicker resin plate because resin has high transmittance in near infrared region. Welding utilizing high-power laser diode is expected to be a new welding technology because high-power laser diode has come to be well accepted, resulting in miniaturization and cost reduction of laser equipment.
We will introduce our newly developed resin diode laser welding equipment ML-5220A.
Table 1. Conventional jointing methods and problems
| Jointing method | Problem | Resin laser welding |
| Ultrasonic welding | Influence of vibration on electronic parts on resin | Not affected |
| Hot plate jointing | Heat influence in wide range; so local welding impossible. | Heating time is several seconds. |
| Bonding | Chemicals are an environmental issue. Gas affects parts.Hardening time, Yield | Gas (very small amount) generated only in welding |
| Screwing | Scrapping time for recycling | Scrapped directly |
2. Principle of resin welding (1)
2.1 Principle of resin welding
Principle of laser welding is shown in Figure 1. A laser-absorbing resin plate and laser-transmitting one are contacted and pressed to a certain extent. When a laser light is projected by such as scanner onto a transparent resin plate, it transmits the resin plate naturally; then it reaches the laser-absorbing resin plate. The lower plate is heated by the absorbed energy and begins to melt, conducting the heat to the upper transparent plate. As the result of this thermal process - absorbing, heating, and thermal conduction - both upper and lower plates melt to be welded.
2.2 Influence of process parameter
Major processing conditions of resin laser welding are as follows:
(1) Density of projected laser (W/cm2)
(2) Beam-operating speed (mm/s)
(3) Optical absorbing characteristics of resin material
2.2.1 Density of projected laser and beam-operating speed
When density of projected laser is increased and beam-operating speed is decreased; namely, when projection time is extended, melting energy in unit volume increases. As the result, width and depth of the weld increases because heat diffusion continues for a longer period of time.
2.2.2 Optical characteristics of material
In order to change the absorption characteristics of resin material, additives to coloring matter (such as carbon) is added to the material. As shown in Figure 2, the shape of weld receives an influence of absorption characteristics. In high-absorbing resin, whose density of additives is high, absorption length of laser is shortened and shape of weld becomes thinner (shallower) and wider. On the contrary, in low-absorbing resin, absorption length is elongated and heat diffusion into transmitting resin decreases; resultingly, shapes of welds in transmitting resin and absorbing resin become asymmetrical. As explained above, weld shape and symmetry in each material change, depending on absorption characteristics of absorbing resin. Therefore, it is the common practice that material of longer absorption length is used for butt joint, and material of shorter one, for lap joint.
Figure 2. Explanation on difference of weld shapes
between high-absorbing material and low-absorbing one
(a) Penetration in absorbing resin comes to be deep as the length of absorption increases and heat diffusion into transmitting resin decreases. The shape of weld is asymmetrical.
(b) If length of absorption is short, only surface absorption of absorbing resin occurs, resulting in larger weld area. The shape of weld is nearly symmetrical.
3. Introduction of resin laser welding equipment
3.1 Diode laser equipment ML-5220A
The light source of which resin welding system is composed is ML-5220A shown in Figure 3. Standard ML-5220A is composed of laser diode of 30 W output and optical fiber (Type SI) of φ0.6 mm core dia. which transmits laser beam. A fiber (Type SI) of φ0.4 mm core dia. is available (optional). Laser wave length is 810 nm ± 10 nm.
Features of ML-5220A are as follows:
(1) Small sized and light weighted. Suited for floor space saving.
(2) Owing to high-efficiency laser diode, full air cooling and low power consumption are realized even for output of 30 W.
(3) Spot-outputting optics and beam-scanning optics are selectable.
Spot-outputting optics (image formation ratio = 1:1) is excellent in price-to-performance, and beam-scanning optics can weld complex figures. What is more, data furnished by commercial CAD software can be used.
Basic specifications of ML-5220A are shown in Table 2. Figure 4 shows an example application of resin laser welding. As seen, laser-transmitting resin which is opaque in visible light region and transparent in near infrared region has been developed. Though scanning speed of beam in laser welding depends on material, it ranges 10 mm/s to 100 mm/s.
Table 2. Specifications of ML-5220A
| Equipment specification |
Input power supply | Single-phase 90|130 VAC / 180|260 VAC Automatically switched, 50/60 Hz |
| Power consumption | 600 W | |
| Heat exchanging | Forced air-cooled electronic cooling | |
| Operation | Dedicated operation panel, Personal computer | |
| External I/F | 6 inputs, 7 outputs | |
| External communication | RS-232C | |
| Laser specification |
Oscillation wave length | 810 nm ± 10 nm |
| Maximum output | 30 W (fiber output) | |
| Oscillation form | CW oscillation |
Figure 3. Resin laser welding equipment ML-5220A
 |
(a) Example welding of acryl resin (transparent) and ABS (black) |
(b) Example welding of polypropylene (PP) resin (white and black) |
|
(c) Example welding of laser-transmitting PP resin (black) and laser- absorbing PP (black) |
Figure 4. Example resin welding by ML-5220A
3.2 Two-dimensional resin laser welding equipment
As the major application to electronic parts is hermetic sealing of package, demand for two-dimensional beam operation has been increasing. ML-5220A employs beam-scanning output optics to meet such a demand. Processing ranges are φ35 cm in diameter and φ75 cm, which are selectable. When an optical fiber ofφ0.6 mm diameter is used, its beam diameter is approximately φ1 mm and φ2 mm, respectively.
Figure 5 shows an example welding of transparent polycarbonate (PC) of 1 mm thick and black PC of 1 mm thick with welding conditions: scanner-operating speed of 15 mm/s, and laser output of 2 W. Weld width was approximately 1 mm. As shown in this example welding, two-dimensional resin laser welding equipment is suitable for hermetic sealing of such as package, sensor; moreover, this equipment easily responds to complex welding shape.
Figure 5. Example welding using scanner unit
Resin: Transparent and black polycarbonates
Laser output: 2 W
Beam-operating speed: 15 mm/s
Figure 6. Three-dimensional system using robots
3.3 Three-dimensional resin welding system
Figure 6 shows three-dimensional system utilizing robot. This system is suited to weld a curved surface such as a tail lamp of automobile (2). This is composed of spot-outputting laser ML-5220A as a light source, type SI optical fiber of φ0.6 mm core diameter, output unit, and three-dimensional robot.
Figure 7 shows example welding of transparent acryl cylinder and black semicircular ABS resin. Welded surface was very clean and enough strength was obtained. The transparent acryl is 1 mm thick; weld width, 2.5 mm; laser output, 10 W; beam-operating speed, 20 mm/s.
Figure 7. Example resin welding using three-dimensional system
Resin material: Acrylic resin (transparent), ABS resin (black)
Laser output: 10 W Beam-scanning speed: 20 mm/s
Conclusion
The resin laser welding as the jointing method which solves environmental issue (chemicals, recycling), and brings product to higher quality level, and the diode laser welding equipment ML-5220A were introduced. ML-5220A has two kinds of output units; namely, spot-outputting and beam-scanning output units, which are selectable. These make user's system design flexible and, in addition, ML-5220A can be applied to soldering and brazing joints.
The laser welding effectively joints most of the thermoplastic resins and its applications will expand more broadly in the field of industry. Meanwhile, these systems must be improved in welding speed and finer weld width; additionally, these have to be more user-directed ones, requiring a less maintenance cost.
* References
(1) S. Abed et al: "Diode Laser Welding of Polymers: Microstructures of the Welded Zones for Polypropylene", ICALEO'2001, Section C
(2) http://www.clt.fraunhofer.com/service/InfoSheets/plastic01.pdf
