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Today, the society requires environmental protection and energy conservation for lighting, LED lighting penetration rate is more and more wide, and secondary lens as an important part of LED lighting devices, has also received great attention and development. The selection of lens materials is restricted by the forming difficulty, transmittance, cost and stability. Generally, glass and transparent plastic are selected. However, silica gel, as a new material with high transmittance, high stability and safety, aging resistance and easy forming, can not be directly applied to the large-scale production of secondary lens due to technical reasons.
At present, the developed silica gel lens has broken the confinement of this technology and expanded the new field of silica gel application.
1、 The characteristics and application market orientation of silica gel lens are in the lighting industry. The main factors for choosing lens materials are light transmittance, high temperature resistance and aging resistance. Optical grade plastic lenses generally use optical grade PMMA, optical grade PC and other materials. The advantages of optical grade plastic are: small quality, easy to form complex surfaces, low cost, easy to achieve large-scale production. The disadvantages are: low resistance to high temperature and aging, such as PMMA temperature not more than 80 ℃ (thermal deformation temperature 92 ℃), PC temperature not more than 120 ℃ (thermal deformation temperature 135 ℃).
The glass lens is just the opposite of the plastic lens. The glass lens has high transmittance, good high temperature resistance, but high density. Due to the special state of glass body after glass softening, the fluidity is very poor, it can not form complex curved surface, and it is not easy to realize large-scale production. In addition, the mold cost is high, the processing cycle is long, the precision is not easy to control, and it is brittle, especially for the lens with large thickness change, the unexpected bursting is not available Law anticipates and solves. With the improved technology that can be realized now, the strength of glass can only be improved by coating or tempering treatment, but after these treatment, the light transmittance of glass lens will be reduced and the cost will be increased.
Compared with traditional glass lens and plastic lens, silica gel lens has excellent characteristics in light transmittance, molding difficulty, aging resistance and so on. Take the automobile headlamp lens developed by Dongguan Xinghui vacuum coating plastic products Co., Ltd. and trial produced with optical grade silica gel material of Dow Corning company as an example, its performance is as follows:
(1) Temperature resistance range: - 40 ~ 250 ℃, and resistance to sudden cold and heat shock.
(2) Flexible texture, not easy to burst, excellent explosion-proof performance.
(3) Non toxic, low density, stable performance, light transmittance up to 92%, excellent weather resistance.
(4) There is almost no residual stress and good scalability, which ensures the stability of optical properties.
However, silica gel lens has the following disadvantages:
(1) The cost of raw materials is 1.5-3 times that of traditional PC lens.
(2) The immaturity of the existing lens injection molding technology further increases the cost of silica gel lens.
Therefore, the market positioning of silica gel lens is mainly as follows:
(1) Aerospace lighting lens.
(2) Miner's lamp or special lamp lens under inflammable and explosive conditions.
(3) Automobile, outdoor lamp.
(4) Conventional LED indoor lighting fixtures.
In the automotive headlights, because of the excellent optical and safety performance of silica gel lens, it can completely replace the traditional glass lens, and the quality is smaller, the surface can be more complex, which opens up a broad development space for the future upgrading of automotive headlights. In the special lighting industry, such as aerospace equipment, in the sudden cold and hot environment, silica gel lens shows excellent safety and good optical performance. In addition, in the dangerous environment such as mining industry and nuclear industry, because silica gel itself is an explosion-proof material, the lighting lamps made of silica gel lens have dual functions of explosion-proof and secondary optics, which can not only achieve the effect of secondary optics, but also be not afraid of impact and achieve two goals with one stone. Due to the excellent stability of silica gel, it can work stably under the premise of maintaining the optical effect in the harsh environment such as chemical industry or corrosive gas. Silica gel lens combines the advantages of existing lens materials, will occupy an irreplaceable position in the field of special lighting, and further expand to other lighting fields.
2、 Design of injection mold for silica gel lens
Optical principle of lens
The lens design shall follow the following basic rules, and the schematic diagram of its optical propagation principle is shown in Figure 1.
(1) Independent propagation, different light cross will not affect each other, but continue to emit in the original direction.
(2) In the same transparent material, light travels in a straight line.
(3) Reflection and refraction: when light travels from one transparent material to the critical point of another, some light will be reflected back, and some light will continue to travel after changing direction.
(4) Total reflection, when the incident angle of the light is less than the refraction angle, when the light enters from one transparent material to another different transparent material, only reflection occurs, without refraction.
(a) Reflection and refraction of light
(b) Optical path diagram of convex lens
Fig. 1 Schematic diagram of optical propagation principle of lens
Lens design
Because of its strong plasticity, silica gel can form complex curved surface, which is conducive to the realization of optical structure. In the forming structure, it can design complex curved surface, so that the lens can be used in various lighting occasions. The design of the lens requires the highest light efficiency utilization, and achieves different spot designs in the form of flat convex, TIR reflecting surface, cylinder and so on. The two-dimensional drawing of the lens product of a general Q5 lamp on the market is shown in Figure 2.
Fig. 2 Q5 silica gel lens product of a general automobile in the market
The silica gel lens can completely replace the existing glass lens, and because of the use of silica gel material, the problems of sealing and shockproof can be completely avoided, that is, the original halogen lamp source and xenon lamp source can be used, which can be completely universal without any change. At the same time, in the design, the wide angle of the lamp lighting, different light intensity in different range, special ship facula and other factors are taken into account. At the same time, because of these excellent properties, silica gel lens has a broad prospect in the future development direction of lamps, such as LED lamps, LED matrix headlights and so on.
At present, the silicone lens produced by the company has been used in indoor lighting, road lighting, wall washing lamp, industrial and mining lighting, lamp lighting and other special lighting. The lens design has a high degree of authenticity. Professional optical simulation software (such as lightools, etc.) and mechanical modeling software (such as Pro / E, UG, etc.) are used for scene simulation design. The data simulated by the software, such as light angle, light intensity, illumination, etc., are analyzed and processed to optimize the lens structure, so that the lens products meet the design requirements. The application scenario simulation of silica gel lens is shown in Figure 3.
(a) Road scene simulation
(b) Stadium scene simulation
Fig. 3 application scenario simulation of silica gel lens
Mould material
Through a series of experiments on die materials, practical die design experience is summed up.
First of all, the heat shield should be made of non alloy steel. As the formwork is to be used at a high temperature of 170-210 ℃, pre tempered steel is used to improve the stamping resistance. The mold of liquid silica gel injection molding will wear in the actual production, which will affect the accuracy of the mold. Therefore, special inserts or other replaceable components should be used in the design of the mold, so that when the components are worn, they can be replaced separately without replacing the whole mold components. The designed die adopts a common runner system, and must have a proper isolation on the cold and hot interface. Titanium alloy like tial6v4 has poor thermal conductivity compared with other steel materials, which is a good material for cold and hot isolation. For the whole mold heating system, an insulating layer should be placed between the mold and the mold of the injection molding machine to minimize the heat loss.
The most precise part of the silicone lens mold is the optical mold core. The die core is made of special steel (such as S136 mirror steel in Sweden) after heat treatment to 55HRC, and the rough embryo is finished. On the basis of the rough embryo, it is nickel plated, and then it is processed with ultra precision curved surface.
mould design
When designing silica gel injection molding mold, first of all, the position of parting line should be considered, because some channels need to be set inside the parting line, and these channels should be used for exhaust. The exhaust hole must be set at the end of the mold where the injection material finally reaches. Considering the above factors in advance will help to avoid air entrainment and the loss of weld line strength. Due to the low viscosity of silica gel, the accuracy of parting line must be ensured to avoid flash.
Although the compression of liquid silica gel in the injection molding process is small, due to the high thermal expansion coefficient of silica gel, there is usually 2% ~ 3% shrinkage after cooling and demoulding. The mold designer should consider some factors affecting the shrinkage in advance, which mainly include forming temperature, demoulding temperature and forming pressure.
When the mold is closed, the air stays in the mold cavity. With the injection of liquid silica gel, the air is compressed and gradually discharged from the mold cavity. Due to the low viscosity of silica gel, the mold cavity is filled quickly. In the process of rapid filling, if the air can not be completely discharged from the cavity, it will be entrained in the liquid silica gel (usually a circle of white edge along the periphery of the product or a smooth small bubble inside the product). The best way of exhausting air is to use the method of vacuumizing to extract the air from the cavity in every injection cycle. Therefore, when designing the parting line, make sure that the mold is closed, and use vacuum pump to vacuumize the mold cavity. Once the vacuum degree reaches the predetermined requirements, start the injection immediately.
Due to the high expansion coefficient of liquid silica gel, it will expand greatly when heated, but it will only contract slightly when cooled, so that the products can not maintain the accurate side size in the mold. The liquid silica gel is molded with ordinary flow channel to keep the low temperature and fluidity, and the small injection point of liquid silica gel is used to directly inject to the edge of products or even products, so as to achieve no waste . no flash molding, increase product output, reduce waste products and reduce labor cost.
In the process of injection, each channel of the closed system uses "sealing needle" or "needle valve" to control the precise injection quantity of silica gel material. When using the common runner system, the temperature of the common runner should be low enough. In order to avoid leakage of glue, the needle valve is installed on the surface of the mold parts. After injection, the needle valve immediately closes the nozzle.
The injection molding of silica gel with common runner system can save the elimination of gate trace, thus avoiding the labor-intensive production process and the waste of a large number of materials. In many cases, no runner design will also reduce cycle time.
Compared with the production and molding technology of optical grade plastics, the production technology of ordinary flow channel of silica gel lens adopted is quite different, as shown in Figure 4.
(a) Plastic injection molding
(b) Silicone injection molding
Figure 4 Schematic diagram of plastic injection molding and silica gel injection molding
It is necessary to set up an effective isolation between the hot cavity and the common runner by using the common runner system. If the temperature of the shunt channel is too high, the material will start to solidify before injection, and if the cooling is too fast, the material will absorb too much heat from the mold valve area, preventing the completion of the solidification. The perforation size of the needle valve of the closed system is generally designed as the diameter of the needle valve? (0.5-0.8) mm to ensure the needle