With the rapid development of imaging technology, people can record what is happening, scenery and people around them in their cameras or mobile phones anytime and anywhere. The core components of these high-tech products are precisely the high-precision optical components. In the past, this kind of optical lens used glass as the main material, but glass inevitably has disadvantages such as high quality, high brittleness, and high price. industry, and the information industry. The key to mass production is injection molding.
As we all know, injection molding is widely used in mass production of plastic parts, but traditional injection molding technology is difficult to achieve the precision of optical components. To achieve the required dimensional tolerances and surface quality, the entire process chain must be optimized. After years of research, precision optical components with more functions and reasonable prices can now be made through precision injection molding technology to meet the needs of the market.
Studying the injection molding process, it can be found that precision injection molding has six obvious differences compared with traditional injection molding.
1. Product Structure Design
In order to obtain the best surface quality and the smallest dimensional tolerances, product structure design is very important. The product design also indicates the dimensional tolerances of the plastic parts. From some experience, the common design principles are as follows: avoid local wall thickness of plastic parts, resulting in shrinkage cavities; control the size of the minimum wall thickness (determined by the material); there should be no holes, slots, etc. Form a weld line; the wall thickness should not change too much, choose a smooth transition; keep the wall thickness of the plastic part uniform.
Since plastic is less stable than glass, the accuracy of the refractive index of plastic lenses is lower than that of glass lenses. Generally speaking, under standard environmental conditions, the range of change in the refractive index of plastic lenses is greater than 1%, and changes in the refractive index will cause changes in the focal length of the lens. It can be known from physical experiments that the focal length of a common spherical lens is determined by the refractive index n, lens thickness T and spherical radius R, and these three parameters have different effects on the focal length, among which the refractive index n has the greatest influence. In order to reduce the change of the refractive index, the geometric tolerance and processing accuracy of the lens must be strictly marked during the design.
2. Tool design
Tool design is as important as product design, and the cutting effect will be directly reflected on the surface of the plastic part. When the precision of plastic parts reaches the micron level (μm), the dimensional tolerance of the tool must be lower than 1 μm. Although this is not an easy task for tool design, there are many tool units to choose from. It is worth noting that dimensionally stable knives require high-strength materials that can accommodate various heat treatments, the importance of which is often overlooked. Experiments have proved that if the transformation process of the metallographic structure of hardened steel from austenite to martensite is not completely completed, the microstructure of the material will change, causing macroscopic dimensional changes, even in the absence of load. A deformation of 0.01 to 0.001mm will occur.
3. Injection molding equipment
Injection molding equipment is an important part of the entire process chain. Injection molding equipment melts, plasticizes polymers, injects them into molds, and circulates continuously. It requires precise control of every process parameter, such as injection temperature, injection volume, injection rate, cavity pressure, etc. The accuracy of injection molding equipment determines the molding accuracy of plastic parts.
Precision injection molding equipment is a closed loop, and its operation is completely controlled by those parameters. During injection molding, every mechanical action must be accurate (such as the parallelism of two mold mounting plates when moving), and all parts on the equipment require a high degree of stability. Since the driving unit of the molding equipment is driven by electricity, it has obvious advantages in accuracy and reproducibility, and is suitable for precision injection molding.
4. The processing capacity of the mold workshop
Apart from design elements, precision machining is also a very important part of injection molding. Mold processing must go through precise machining and closely matched assembly process. If this part of the dimensional tolerance is not well controlled, it will be difficult to repair the dimensional tolerance of the plastic part in the later injection molding process, or the range of injection molding parameters that can be adjusted is narrower. With the development of high-speed cutting, it can be predicted that precision high-speed multi-axis milling will gradually replace EDM (discharge machining).
In order to make the mold insert meet the quality requirements, single crystal diamond can be used as the grain of the machine tool for turning. The biggest disadvantage of diamond turning is that it cannot directly cut ferrous metals, such as steel, because iron will wear out the diamond rather quickly. At present, some enterprises have done some research on the heat treatment process, which is to achieve the effect of single crystal diamond turning by improving the cutting performance of alloy tool steel. Early results look very promising. Of course, we must also pay attention to the turning or milling tool itself, because the cutting edge of the cemented carbide turning tool will wear out after high-speed turning, so it is necessary to use a precision sharpening machine to re-sharpen the tip of the cutting edge. We pay great attention to the cutting plane and cutting edge of these tools, even the smallest flaws on the cutting edge will be reflected in the formed product.
5. Injection molding process
The injection molding process can be divided into two types: traditional injection molding and injection compression molding. In traditional injection molding, internal stress will be generated during the cooling process of the plastic, which will change the performance of the plastic part and cause lens polarization. In order to overcome this potential internal stress, one of the treatment methods is annealing of plastic parts, but this method will cause deformation of plastic parts, which is not suitable. Injection compression molding can now be used. Injection compression molding is often used to form products with fine structures, such as plastic lenses with diffraction functions. It differs from the traditional injection molding process in several obvious ways. The scope of its molding process parameters is summarized as follows:
Injection pressure (holding pressure): greater than 100MPa (depending on plastic parts or materials); injection rate: depending on molds, plastic parts and materials; plasticizing temperature: 200-320°C; mold temperature: 100-150°C; Molding cycle: more than 0.5min.
Because precision injection molding is a new type of injection molding method, there is no experience to learn from its molding parameters. In order to obtain suitable molding parameters, the following methods can be used to try. First, design and manufacture a set of injection molds (without considering the shrinkage rate), and in the second step, select one of the injection molding parameters, divide it into several differentials, and perform injection molding optimization one by one. Then detect the size of the molded plastic part, and modify the shape and size of the injection mold according to the plastic part. The process parameters obtained by this method often have high stability and accuracy. Of course, sophisticated measuring equipment (coordinate measuring machine), advanced mold shop (multi-axis milling center) and mathematical capabilities of the design part (simulation analysis) are necessary to implement this solution.
6. Ability of technicians
In order to achieve tight dimensional tolerances for plastic parts, precision injection molding must be considered from the very beginning. Consider various factors such as optical design, product structure design, molding process parameters and molding equipment, and consider these interacting factors as a whole, and no one can be ignored. Therefore, it is necessary to hire some high-tech and experienced design engineers who can complete tasks such as optical design, product structure design, tool design, finite element analysis and mold flow analysis. On the other hand, although most of the operations in the injection molding process can be controlled by computers to realize fully automated production, some highly educated and high-tech talents are still needed in the workshop. Because the control of the precision injection molding process is the most cutting-edge technology in the field of injection molding. Its typical feature is that the injection molding machine has an advanced control interface, which requires someone to continuously monitor and adjust key process parameters in time, so the human factor is very important.
With precision injection molding, polymer optics can be produced in high volumes and with high precision. Of course, this is just the beginning. Precision injection molding is still not perfect in some aspects, such as: research and development of polymer materials, design of injection molding equipment, detection of mold status, precision measurement of plastic parts, and application of molding simulation analysis software. These researches will surely provide people with better plastic optical lenses.
