Laser transmission welding method column
 [Table of Contents]
 - Introduction
 - Effects on welding quality compared through two types of energy distribution
 - About the laser scanning method - robot control/galvano scanning
 - Impact of energy distribution and directivity on welding
 - Diversity of welding method by means of galvano scanning - welding utilizing thermal behavior
 - Conclusion
 For laser resin welding, it is said that a laser with top hat distribution like LD laser is suitable. However, even with a fiber laser having a beam with Gaussian distribution shape, welding is possible. Regardless of whether the laser is converging light or parallel light, welding is possible. The welding method should be selected according to the shape or size of the actual workpiece, resin type, and demanded welding quality. It is important to optimize the absorbing power and set conditions according to the material, demanded quality, and laser characteristics. Such welding methods that use high directivity and galvano scanning also exist. This section explains a welding case example in which our LAW material is used to control the material absorption value (calorific value) by means of material and galvano-type fiber laser used, and its utilization method.
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 Effects on welding quality compared through two types of energy distribution
 This section gives an explanation using semiconductor laser and fiber laser, which have two different energy distributions. The differences between them in terms of principle are explained in detail in various materials related to laser and manufacturers’ websites. Therefore, those differences are omitted here. This section eyes only the energy distribution that affects laser welding behavior.
 - Energy distribution of laser beam (Schematic diagram)
 The distribution in the left diagram is called top hat distribution, in which almost-uniform energy is radiated to the irradiation area and causes uniform heat generation in the spot diameter. For example, LD laser is a representative laser with such a distribution. On the other hand, the distribution in the right diagram is called Gaussian distribution, in which energy tends to increase toward the center of the irradiation area, causing a difference in calorific value between the spot center and periphery. In other words, the temperature at the center is higher compared to that at the periphery. For example, many fiber lasers have an energy distribution like this.
 Before the introduction of data related to welding, the method to operate laser (scanning) is briefly explained here.
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 About the laser scanning method – robot control/galvano scanning
 <Source of image: Panasonic Industrial Devices SUNX Co., Ltd.>
 As shown in the figure above, laser scanning is roughly divided into two types in general. One type is the robot-driving or stage-driving scanning method, in which the workpiece or laser head is moved to operate the laser. This method is subject to the influence of the workpiece or laser head on the tracking accuracy depending on the weight or driving speed of the workpiece or laser head. In the case of the welding of the workpiece (part) corner, for example, accuracy also depends on the corner shape. This method is commonly employed in LD lasers, and the upper limit of the normal scanning speed that enables stable welding is said to be around 100 mm/s. On the other hand, galvano scanning performs scanning by reflecting a laser by controlling the galvano mirror. Therefore, it exhibits excellence in tracking accuracy and high-speed scanning as fast as over 5,000 mm/s. This system is employed in lasers with high directivity such as fiber lasers and is capable of operation for parts with complicated shape, but it has the disadvantage of difficulty in laser irradiation on shadows when a protrusion is present. For details, contact a laser equipment manufacturer. In jig design, it is very important to realize a pressured state in consideration of the influence of shrinkage and warping of the formed product. When a gap is formed between parts to weld, heat conduction is not performed in laser transmission welding and a welding defect is caused.
 Next, an actual welding case example is introduced.
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 Impact of energy distribution and directivity on welding
 Differences between LD and fiber laser and between top hat distribution and Gaussian distribution have been explained. This section explains how those differences affect welding. Please take a look at the following data.
 - Welding strength and its stability
 The figure above shows an example of a comparison between welding behaviors caused by different laser energy distributions when we used forming plates that were made of PBT resin GF30% from company A and added with our eBIND LAW-9810 as an absorber, and whose absorbing powers had been adjusted to respective laser wavelengths (940 nm/1070 nm) of LD and FAYb lasers. In terms of both strength and stability, the results showed differences from LD. This indicates that the heat generation behavior of FAYb laser with Gaussian distribution were different. Due to the difference in temperature distribution between the center and periphery, the range where a favorable energy amount is obtained is narrow in both portions compared to that in laser with top hat distribution and it is considered that strength is likely to vary. Also, as excessive heating at the center is likely to cause strength degradation and decomposition of resin, the welding strength, which is strength per unit area, is also low. Regarding heat generation behavior, the calorific value at the center tends to be large even with a laser of top hat distribution, but the figure indicates that the degree of heat generation also differs depending on the difference in energy distribution. As there are differences in welding behavior between LD and FAYb as shown above, it is crucial to select welding methods suitable for respective lasers. The welding method considered to be suitable for FAYb is multi-scan welding as explained below.
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 Diversity of welding method by means of galvano scanning welding utilizing thermal behavior
 It has been explained earlier that by using galvano scanning, high speed scanning is possible. It has also been explained that the large energy of fiber laser at the center affects welding behavior. As a method to utilize galvano scanning, this section introduces the method to alleviate the disadvantageous influence caused by energy distribution. In this section, its welding method is referred to as multi-scan. This method requires more time than single scanning, but the welding quality will be stabilized.
 The above figures show examples of the irradiation method. “Single scan” refers to an irradiation method in which with a single laser irradiation, welding is completed. On the other hand, “multi-scan” refers to a method in which laser is radiated multiple times at high speed on the same track to gradually perform heating, melting, and welding. If the track is a circle, laser is radiated like drawing many circles with the same track.
 The multi-scan and single scan are compared with each other, and their advantages and tendencies are summarized in the table below.
 The effect of limiting bead width shown in the table above is expected to be particularly remarkable when laser with Gaussian distribution is used. Utilization of multi-scan can alleviate disadvantageous phenomena related to energy distribution in a Gaussian distribution shape. However, there are the following two disadvantages. The first one is bad energy efficiency, which means it takes more time in processing than a single scan. Another disadvantage is that the influence of thermal factors (thermal conductivity of resin, cooling effect of jig and the like, thickness, etc.) is great. Control of thermal factors is very important for achieving stable welding.
 In the next column, we will introduce data related to actual welding behavior (strength/air tightness evaluation).
 The material is PBT resin MBS230H03L (to product website) for automobile use from Automotive & Industrial Systems Company, Panasonic Corporation.
 (As of today, as the PBT resin material of the same series that enables black x black welding, laser transmission black PBT resin MBS230H93L is released from Automotive & Industrial Systems Company, Panasonic Corporation.)
 As the laser welder, the VL-W1 series (to product website) from Panasonic Industrial Devices SUNX Co., Ltd. was used.
 These are data obtained due to the cooperation from the above two companies.
 As the workpieces, we used a box-shaped forming product like this photograph for evaluation.
 Example of coloring of box-shaped forming product by eBIND® - PBT -
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 In this column, we have explained that condition setting according to use and environment including top hat distribution and Gaussian distribution as well as galvano scanning and multi-scanning is important for stable production using laser resin welding. In the column we are going to publish next time, we are planning to introduce data from the viewpoint of “airtightness,” which is a very important theme for laser resin welding.
 - eBIND, LTW, and LAW are registered trademarks of Orient Chemical Industries, Ltd.

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