Mold as the mother machine of industrial molding, product appearance texture, molding yield, production cycle and mold service life, are directly related to the cavity surface quality, fine structure accuracy. As consumer electronics, optics, automotive, medical and other products continue to move towards lightweight, miniaturization and refinement, the shortcomings of traditional CNC milling, EDM, chemical etching, and nanosecond laser processing are becoming more prominent. CNC is limited by the minimum size of the tool, so it is difficult to process extremely small internal angles and submicron structures. EDM is easy to leave recast layer and microcracks, and the subsequent polishing workload is huge. Chemical etching depends on masks, which has problems such as side etching, environmental protection control and surface processing difficulties. Ordinary long pulse laser processing has obvious thermal effect, which is easy to cause cavity edge melting and substrate annealing deformation. Relying on the unique cold ablation mechanism, femtosecond laser has become an important process scheme for high-end precision mold development, texture re-engraving and microstructure molding with non-contact, no thermal damage and high-precision processing characteristics.
The femtosecond laser pulse duration is extremely short, the energy completes the material peeling in an instant, and the heat is too late to conduct and diffuse to the surrounding substrate, forming a cold working effect that is almost free of heat-affected zone. The processing area will not produce slag, recast layer and heat-induced microcracks, the edge of the incision is clean and sharp, and a large number of post-polishing processes can be omitted after the formation of most fine structures. At the same time, it does not have the problem of tool and electrode wear. Whether it is quenching die steel, hard alloy, ceramic or special coating surface, it can realize controllable etching at micron or even sub-micron scale. Plane, inclined surface, curved surface and roller circumferential surface can be adapted to meet the manufacturing requirements of multi-material, complex cavity and high consistency of mold.
The processing of dermatoglycan and bionic texture on the surface of the mold is the most widely used scene in the field of femtosecond laser. In the past, the leather patterns of household appliances, automobile interiors and consumer plastic shells mostly relied on chemical etching, with long cycle and slow pattern iteration. It was difficult to control the texture uniformity of curved surface and deep cavity position, and waste liquid treatment also needed to bear higher environmental protection costs. Femtosecond laser can directly carve any texture layer by layer on the surface of the mold cavity, from delicate litchi grain, frosted grain and wire drawing texture to customized geometric pattern and bionic texture. After the design document is adjusted, it can be quickly proofed, thus greatly shortening the texture development cycle. For rotary master molds such as embossing roller, coating roller and optical microstructure roller, femtosecond laser combined with rotating shaft linkage system can batch write micro grooves, micro bumps and prism arrays with consistent cycle on cylindrical curved surface, ensure stable texture depth and morphology at all parts of the roller surface, and provide high-precision master support for light guide plate, optical film and film embossing products.
The micro-structure cavity and micro-feature forming fully release the accuracy advantage of femtosecond laser. Precision molds corresponding to optical lenses, microfluidic chips, microneedles, microconnectors and other products contain a large number of tiny grooves, array microholes and acute angle microstructures inside the cavity. Many feature sizes are only a few microns to tens of microns. Traditional EDM requires splitting a large number of electrodes. Programming, clamping, compensation and debugging take a long time, and the internal angles are difficult to be sharp and clear. Femtosecond laser maskless direct writing can form high-aspect ratio microholes, thin-walled microcavities, complex three-dimensional microstructures at one time, and the straight wall profile is regular, effectively avoiding the size drift problem caused by electrode loss. The dense exhaust holes in the injection mold and the cooling microholes on the micro inserts are also suitable for femtosecond processing. The hole wall is smooth and free of burrs, and the exhaust is smooth, thus reducing defects such as material shortage, trapped gas and burning in injection mold and improving product yield.
Mold surface functional modification is the direction of rapid development in recent years. By femtosecond laser, micro-pits and micro-groove arrays can be prepared on the surface of the cavity, bionic lubrication texture can be constructed, release agent or lubricating oil can be stored in the molding process, the adhesion between melt and mold surface can be reduced, the demoulding resistance can be reduced, the problems of die sticking and strain in the process of die casting and stamping molding can be alleviated, and the service life of the mold can be significantly prolonged. This kind of micro-nano texture can also control the flow state of the plastic melt, improve the uniformity of filling, and even give the mold surface hydrophobic and lyophobic characteristics, reduce the melt residue, reduce the frequency of downtime cleaning. For the mold surface that has been coated with PVD, femtosecond cold processing will not destroy the binding force of the coating substrate, and the functional microstructure can be processed directly on the coating, taking into account the wear protection and surface control effect.
In the mold repair, marking and traceability management, femtosecond laser also has practical value. Precision molds often need to locally fine-tune texture and modify micro-cavity features during the mold test stage. Traditional processing is easy to damage the finished high-glossy surface around them. Femtosecond laser can selectively etch at fixed points, the processing range is controllable, large-scale thermal damage will not be brought, and the mold repair is more precise and safe. For the traceability identification of core components such as mold kernel and insert, femtosecond laser can carry out internal carving marking inside the material, the identification is buried inside the matrix, does not damage the cavity working surface, and does not wear and blur after long-term repeated molding friction, thus meeting the requirements of mold life cycle traceability and version management.
Of course, the popularity of femtosecond laser in the mold industry is still restricted. Equipment input cost is high, large area, large amount of material removal processing speed is not as traditional milling, more suitable for microstructure, texture, micro-hole such fine features processing, not suitable for large mold cavity rough machining. Different grades of die steel, hard alloy, ceramic materials processing threshold difference is large, process parameters debugging needs to be combined with molding test repeated optimization, surface, deep cavity processing also need to match the five-axis linkage platform, the equipment and environmental stability requirements are higher. With the continuous iteration of high-frequency femtosecond light source, spatial light shaping and multi-focus parallel processing technology, the comprehensive cost of equipment is gradually declining, and more and more precision mold factories begin to incorporate femtosecond laser into the process chain, which is complementary to CNC, EDM and polishing processes.
Facing the development trend of precision molding industry, femtosecond laser is not only a processing method to replace traditional etching and micro-discharge, but also broadens the boundary of mold surface design, making many micro-nano functional structures that were difficult to realize in the past to mass production. In the future, femtosecond laser will be deeply integrated with mold digital design, on-line visual inspection and automatic processing system, and will continue to penetrate into subdivided fields such as optical imprinting mold, medical micro-molding mold and high-end automobile interior texture mold, promoting mold manufacturing to upgrade from simple re-engraving shape to controllable surface function and high-precision micro-nano molding.
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