Laser processing in the mold industry

With the development of science and technology and the diversification of social needs, the competition for products has become increasingly fierce and the cycle of replacement has become shorter and shorter. For this reason, it is required not only to design new products as quickly as possible according to the requirements of the market, but also to produce prototypes in as short a time as possible so as to carry out performance tests and modifications, and ultimately to form products. In the traditional manufacturing system, a large amount of mold design, manufacturing, and debugging are required. The high cost and long cycle can no longer adapt to the ever-changing market changes. In order to improve the speed of research and development and production, to quickly and accurately produce high-quality, low-cost molds and products, able to respond quickly to changes in the market, people have done a lot of research and exploration work. With the ever-decreasing prices of industrial lasers and the growing sophistication of industrial laser processing technology, major changes have been brought about in mold manufacturing and product production processes. This article first introduced the industrial processing lasers, and then introduced and analyzed several aspects such as laser manufacturing of molds, laser enhancement and replacement of mold surfaces, laser repair of molds, and laser cleaning of molds.

Industrial lasers

At present, there are two main types of industrial lasers used for laser processing: solid-state lasers and gas lasers. Among them, the solid laser is represented by a Nd:YAG laser, while the gas laser is represented by a CO2 laser. With the development of laser technology, people have begun to use high-power fiber lasers and high-power semiconductor lasers in certain processing applications.

1) Nd:YAG laser

The laser working substance of the Nd:YAG laser is a solid Nd:YAG rod with a laser wavelength of 1.06 μm. Due to the low laser conversion efficiency of the laser, and the limitation of the volume and thermal conductivity of the YAG rod, the laser output average power is not high. However, because the Nd:YAG laser can compress the pulse width of the laser output through the Q-switch, a very high peak power (108W) can be obtained when working in pulse mode, which is suitable for laser processing applications requiring high peak power; another great advantage is that It can be transmitted through optical fibers, avoiding the design and manufacture of complex transmission light paths and is very useful in three-dimensional processing. In addition, the laser wavelength can be converted to 355 nm (ultraviolet) by the triple frequency technique, which is applied in the laser three-dimensional shape forming technology.

2) CO2 laser

The laser working substance of the CO2 laser is a CO2 mixed gas, and its main application laser wavelength is 10.6 [mu]m. Due to the high laser conversion efficiency of the laser, the heat generated by the laser operation can be quickly transferred to the laser gain area through convection or diffusion, and the average laser output power can be achieved at a very high level (over 10,000 watts), satisfying High-power laser processing requirements.

The high-power CO2 lasers used for laser processing at home and abroad are mainly cross-flow and axial-flow lasers. 1 Cross-flow laser: The beam quality of a cross-flow laser is not very good. It is a multi-mode output and is mainly used for heat treatment and welding. At present, China has been able to produce a variety of high-power cross-flow CO2 laser series, which can meet the domestic laser heat treatment and welding requirements. 2 Axial flow lasers: Axial lasers have good beam quality and are fundamental or quasi-matrix output. They are mainly used for laser cutting and welding. The laser cutting equipment market in China is dominated by foreign axial lasers. Although domestic laser manufacturers have done a lot of work on axial lasers abroad, due to the need for major accessories to be imported, the price of the products is difficult to reduce significantly and the penetration rate is low.

Wuhan Bolai Technology Development Co., Ltd. has developed a swirl CO2 laser. As shown in Fig. 1, with the new rotating gas flow method, the swirl CO2 laser has a good beam quality of the axial CO2 laser and the cost of the cross-flow CO2 laser. Low, small size advantages. The popularization and application of this kind of industrial processing laser will play a positive role in promoting the development and popularization of China's laser processing industry.

Figure 1. Wuhan Bolai Technology Development Co., Ltd.
500W Swirling CO2 Laser

Mould laser manufacturing

1) Indirect laser molding process

1 The Stereo Lithography Apparatus (SLA) process uses a UV laser beam to scan the light-curing adhesive layer by layer to form a three-dimensional solid workpiece. In 1986, 3D Systems of the United States introduced a commercial prototype SLA-1. The highest processing accuracy of the SLA process can reach 0.05mm. 2Laminated Object Manufacturing (LOM) process using thin materials, such as paper, plastic film, etc., developed by the United States Helisys company in 1986. Through repeated CO2 laser cutting and material sticking, a layered and manufactured solid workpiece is obtained. The LOM process is characterized by its suitability for manufacturing large workpieces with an accuracy of 0.1 mm. 3 The Selective Laser Sintering (SLS) process is formed using powdered materials and was successfully developed by the University of Texas at Austin in 1989. It is selectively used layer by layer with a high-intensity CO2 laser. Scanning sintered powders to form three-dimensional workpieces, the biggest advantage of the SLS process is the wide selection of materials.

The three laser rapid prototyping technologies mentioned above have been widely used at home and abroad due to their long development time and relatively mature technologies. However, the three-dimensional workpiece formed by the above method cannot be directly used as a mold and needs to be processed in a subsequent manner, so it is referred to as a laser indirect molding process. The main processing methods are: 1 After the rapid prototyping of the workpiece is used as a mold. Paper molds made by LOM are directly replaced with sand casting wood molds by surface treatment; or paper molds made by LOM are directly used as low-melting-point alloy molds and injection molds for surface treatment; or wax molds for lost wax casting. After the workpiece made of SLS was infiltrated with copper, it was used as a metal mold. (2) Use a quick-formed part as a master mold to cast silicone rubber, epoxy resin, polyurethane, etc. to make a soft mold. 3 Use a quick-formed part to make a hard mold. One is to directly make a paper-based mold with LOM, and then conduct metal arc spraying and polishing to develop a metal mold; the other is a metal surface hardback mold. The above hard molds can be used for sand casting, lost foam molding, injection molding, and simple non-steel tensile dies.

Using the above laser indirect molding process to make the mold not only avoids the complex mechanical cutting processing, but also guarantees the precision of the mold, and can also greatly shorten the molding time and save the cost of molding. For the complex shape of the precision mold, its advantages are especially prominent. However, there are still shortcomings in the relatively short mold life, so the above-mentioned laser indirect forming mold is more suitable for small batch production.

2) Laser Direct Molding Process

Selective Laser Melting (SLM) technology has been developed based on Selective Laser Sintering (SLS) technology. The characteristics of SLM are: (1) using a high power density, small spot laser beam to process metal, making metal parts with a dimensional accuracy of 0.1 mm; (2) parts made of molten metal have a metallurgical bonding entity, the relative density can be almost Up to 100%, greatly improved the performance of metal parts; (3) Because the laser spot diameter is very small, it can melt high-melting metal at a lower power, making it possible to manufacture parts with a single component metal powder. Figure 2 shows the all-metal parts manufactured by the German company EOS GmbH using a selective laser melting (SLM) process.

Fig. 2 Selective laser melting with EOS GmbH, Germany
(SLM) Full Metal Parts Made by Process

Laser multilayer (or three-dimensional/three-dimensional) cladding direct rapid prototyping technology is a high-tech manufacturing technology developed on the basis of rapid prototyping technology combined with synchronous feed laser cladding technology. Its essence is three-dimensional laser under computer control. Cladding. Due to the rapid solidification characteristics of the laser cladding, the manufactured metal parts have uniform fine dendrite structure and excellent quality, and their density and performance are comparable to those of conventional metal parts. Laser multilayer cladding has developed a variety of methods, the most representative of which is the metal called Laser Engineered Net Shaping (LENS) developed by the Sandia National Laboratories in the United States. Rapid prototyping technology. Using this method, stainless steel, maraging steel, nickel-base superalloy, tool steel, titanium alloy, magnetic material and nickel-aluminum intermetallic compound workpieces have been successfully manufactured, and the density of parts has reached nearly 100%. Figure 3 shows the metal mold made by LENS technology at Sandia National Laboratories, USA.

Figure 3 All metal molds made by Lendia National Laboratories in the USA using a laser engineered net shaping process (LENS)

Selective laser melting (SLM) technology and laser engineered net forming (LENS) technology have been used in industry and academy because of their compactness, high metallurgical bonding, high precision, and long tool life. The general emphasis of the industry has been the introduction of a variety of equipment prototypes in foreign countries, and some have even begun commercialization; while the domestic current research and application is still in its infancy.

In addition, there is a layered manufacturing technology (LOM) based on fine laser cutting of metal parts, which has the characteristics of rapid, low-cost manufacturing of large-scale, complex-shaped molds. As early as the 80s, Japan's Nakagawa Weixiong Research Laboratory applied metal sheet LOM technology to achieve rapid delamination of metal molds. After development, the metal sheet LOM technology has been gradually applied to the manufacture of large-scale interior and exterior trim molds such as automobiles and injection molding molds with complex flow paths.

Mold surface laser modification

Mold surface treatment has always been a matter of importance in the field of machining. With the development of new technologies and new processes, there are many traditional methods of processing that are not yet applicable. For a mold with a complex shape, the most ideal surface treatment method is to use a laser. It is hardly deformed, the surface hardness is higher than that of a conventional treatment, and it is more wear resistant and has a longer service life.

1) Laser transformation hardening

Laser transformation hardening is also called laser quenching. Since the cooling rate during laser quenching far exceeds that of conventional quenching, a very fine martensitic structure can be obtained. The advantages of laser transformation hardening are that the hardness is higher than that of conventional quenching, the deformation is small, the surface layer and partial quenching can be achieved, and the mechanical properties of the substrate are not affected.

2) Laser shock reinforcement

Laser shock reinforcement is a technique that changes the physical and mechanical properties of a material surface by a strong shock wave generated by the interaction of a high power density, short pulse laser beam and a substance. In the laser shock process, the laser-induced peak shock wave stress is greater than the dynamic yield stress of the material, so that the material produces a dense, uniform and stable dislocation structure, so that the metal surface plastic deformation, and the formation of a deep residual compressive stress, In order to improve the strength, wear resistance, corrosion resistance and fatigue life of metal parts. Its main advantages are: high impact pressure, enhanced depth of 4 to 8 times the depth of the traditional shot peening; able to process parts of the traditional process can not be processed, such as small slots, holes and contours and the like; laser shock strengthened metal The surface is free of distortion and mechanical damage, no thermal stress damage, no phase change, etc.

3) Laser alloying and laser cladding

Laser alloying and laser cladding are to apply a layer of material with a certain property that is different from the matrix composition of the mold to the mold substrate, and at the same time, irradiate the coated area with a high-energy laser beam. Laser alloying makes the coating material melt together with part of the matrix by adjusting the output power of the laser and the alloying process takes place; whereas laser cladding is the rapid fusion of the coating layer with the surface of the substrate by laser fusion, and it is alloyed with the laser. The main difference is that the chemical composition of the coating does not change substantially after the action of the laser, and the composition of the matrix does not substantially enter the coating. The new technology of laser surface alloying and laser cladding surface modification based on the synthesis and preparation of rapid solidification new materials is one of the most effective methods to improve the wear resistance and corrosion resistance and other high temperature performance of mold materials at high temperatures.

Die laser repair

The failure of the mold is in fact scrapped due to the wear of the surface material on the surface, and the metal mold has a long processing cycle and high processing costs. The service life of the mold depends on its ability to resist wear and mechanical damage. Once it is excessively worn or mechanically damaged, it must be repaired before it can be used again. Currently used maintenance techniques include electroplating, surfacing, and thermal spraying. The electroplating layer is thin, and it is poorly combined with the matrix, and the shape damage part is difficult to repair. When the surfacing welding and spraying are performed, the heat injection is large and the heat affected zone of the die is large. The use of lasers for mold repairs, due to the near adiabatic rapid heating due to the high energy density of the laser beam, has a small thermal effect on the substrate and the resulting distortion can be ignored. There are two main methods that can be used for laser repair of molds:

1) Laser Cladding Die Repair

The use of laser cladding method to achieve the repair of the mold. The high-power CO2 laser beam is incident on the mold surface at the same time as the metal powder flow at a constant power. The metal melts to form a molten pool, which then rapidly solidifies to form a metallurgically bonded coating. This method generally uses a high-power CO2 laser as a heat source, and is suitable for the repair of large-size, large-area molds, and the repair of large-sized workpieces such as steel rolls.

2) Laser Deposition Welding Die Repair

The laser deposition welding die repair uses medium and small power pulsed Nd:YAG lasers. The defects of the die are filled with laser beam and filamentous filler material. The laser beam melts the surface of the wire and the workpiece at the same time, and the height of the deposit is achieved by a multi-layer welding method; after the welding is completed, the mold part is further processed into the final size. This method is suitable for small precision molds. The StarWeld welding machine manufactured by Rofin-Sinar is a representative of such equipment, as shown in Figure 4.

Figure 4 StarWeld laser welding machine manufactured by Rofin-Sinar

Mold laser cleaning

The use of high-power laser pulses to remove surface contamination from molds during use is yet another use of laser technology in the mold industry. There are two cleaning mechanisms: one is to directly use the laser to heat the dirt so that it vaporizes and evaporates, or it instantaneously expands by heat and is carried away from the surface of the mold base by the steam; and there is a high-energy density, high-frequency pulsed laser. The splitting stress is generated in the dirt layer and is separated from the mold matrix. Compared with traditional sandblast cleaning methods, laser cleaning has the advantages of fast cleaning speed, no damage to the mold surface, and on-line cleaning (which can save a lot of time for disassembly, installation, and debugging). At present, the laser cleaning equipment produced by German JET Laser Systems is relatively advanced.

Conclusion

The high brightness, high directivity, and high monochromatism of the laser make the laser beam focused through the lens and can generate thousands of degrees or even tens of thousands of degrees of heat near the focal point, making it capable of processing almost any material. Laser processing has been widely used in various manufacturing fields and industries abroad; in China, it has also invested a lot of manpower and material resources in laser processing equipment and laser processing technology for research and development. Mould is one of the important industrial equipments for industrial product molding. It largely determines the resilience of enterprises in market competition. Mould forming has become an important means and process development direction of modern industrial products. Laser processing has great advantages in mold making and replacing molds in certain applications (such as laser cutting to replace punching molds in sheet metal parts, and laser marking instead of stamping). How to apply laser processing technology in actual production to shorten the mold manufacturing cycle (T), ensure the mold manufacturing quality (Q) and reduce the mold manufacturing cost (C) requires continuous in-depth research and exploration.

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