A new manufacturing technology of radio telescope panel 1

Transcription

1 A new manufacturing technology of radio telescope panel 1 Pengzhi LI *a,b,c, Yi CHEN a,b a. National Astronomical Observatories / Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing , China; b. Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing , China; c. University of Chinese Academy of Sciences, Beijing , China ABSTRACT Through a new manufacturing technology, achieved the machining of high precision composite radio panel. It adopts foil sticking in a vacuum, metal bonding, rubber precision compensation, stress release and other technology. Through designing the manufacturing process, and adopting a large number of experiment scheme to improve, finally summarized a technical route of high feasibility and accurate process parameters. At last, by the measurement of PGI Dimension red outside precision detector (the surface detection accuracy of which can reach 2 um) for surface precision of experimental panels, we verified the feasibility and reliability of this technique and provided the reliable data to support for practice application of this technology. Key words: radio telescope panel;mold design; foil sticking in a vacuum; metal bonding; precision compensation; stress release 1. INTRODUCTION Precision measurement of millimeter wave and submillimeter wave radiation spectrum and distribution of space is one of the major experimental subjects in modern cosmology. The reflector panel is important to radio telescope, as optical primary mirror to an optical telescope. It determines working band, astronomy function and precision, measurement precision and working band of austerity tightening field static area. The processing precision of panel mainly depends on the processing technology and detection precision. In China, the representative radio telescope is Shanghai 65 meters radio telescope. The reflective surface area of the telescope is 3780 square meters, assembled by 1008 high precision panels. The accuracy of each panel is 0.1 mm [1]. Figure 1. Shanghai sixty five meters radio telescope 1 National Natural Science Foundation of China, No :Reseach of replication technology of the submillimeter wave radio telescope unit panel. * pzli@niaot.ac.cn; phone ; fax ; AOPC 2015: Micro/Nano Optical Manufacturing Technologies; and Laser Processing and Rapid Prototyping Techniques, edited by Lin Li, Minghui Hong, Lan Jiang, Proc. of SPIE Vol. 9673, 96730N 2015 SPIE CCC code: X/15/$18 doi: / Proc. of SPIE Vol N-1

2 Common production methods of high precision panel are as follows. 1. The aluminum machined panel; 2. The honeycomb structure panel: honeycomb aluminum is the substrate which is adhered by adhesive with aluminum skin. It forms a sandwich layer. It is used in various antennas reflecting surface [2]. 3. The rigid reinforcing and aluminum skin structure panel; 4. The carbon fiber submillimeter panel: Its precision can reach micron grade. It can also adopt the method of vacuum aluminum plating. Such panel structure is used in many space projects. For example, it is used in the antennas of ALMA telescope array made by Japan. In addition to the above methods, research institutions are also actively studying the new ways, such as pressure molding technology and technology of using memory alloy as skeleton [3] [4] [5].These techniques are very expensive. Some of them are very complicated. The task of this new technology is to overcome the shortcomings of existing technologies. Its advantages are the simple production process, short cycle, low-cost. 2. PROCESS DESIGN 2.1 Design of mold and substrate The panel is designed according to the type of the inner ring surface panel of 5m THZ telescope for the South Pole. The mold of which the surface roughness is Ra0.4 cooperates with bottom plate by square groove, then uses sealant to seal. Mold inside is vacuum chamber, and there are many air pores and grooves evenly distributed on mold surface. As shown in figure 2. Bottom plate is fixed on the workbench with bolts. At last, the surface type of the panel substrate should match the mold. Figure 2.The mold 2.2 Selection of coating and the metal foil Due to two bonding surfaces are metal, it is important to choose a special high strength metal adhesive for bonding. So epoxy resin and acrylic acid reactive adhesive are very suited to this research. Metal foil needs certain ductility and strength. Stainless steel foil, aluminum foil and copper foil are appropriate under the consideration of panel strength and foil pressing process. Through the finite element simulation analysis, the thickness of metal foil in 0.1 mm or so is best [6].By investigation, there are three kinds of choosing for metal thickness in fact.they are 0.75 mm, 0.1 mm, 0.15 mm. 2.3 Technological process For the purpose of high precision, high stiffness ratio, low cost,we design a complete manufacturing process which is one of advanced flexible forming techniques. One of its core characteristics is that using high precision surface mold, cementing agent s liquidity and negative pressure to improve surface precision of substrate, the other one is using good gelatinizing effect of metal foil to enhance surface rigidity and strength of panel. Proc. of SPIE Vol N-2

3 1 The experiment process is as follows. Mold design - matrix prefabrication - adhesive collocation - surface treatment - vacuum - pasting metal foil - gelatinizing - placing substrate - curing at constant temperature - aftertreatment. As shown in figure a f Figure 4. The manufacturing process diagrammatic drawing 1. metal foil, 2. substrate, 3.vacuum hole, 4.adhesive layer, 5.certain weight, 6.mold, 7. bottom plate, 8.Workbench, 9.vacuum tube, 10.pressure control valve 3. KEY TECHNIQUES EXECUTE SOLUTION 3.1 Design of vacuum holes and shallow grooves The local deformation of foil material is mainly affected by the distribution of adsorption force in the process. So it needs to adopt surface slot to release local stress. To make the mold surface fit with metal foil closely and release local deformation, the location of vacuum holes and shallow grooves is the key to the vacuum mold. Vacuum holes with uniform distribution can also cause the local stress concentration, but shallow grooves through vacuum holes can effectively disperse local suction caused by vacuum, eliminate the local deformation of metal film and ensure the surface accuracy. Specific aperture and shallow groove width can be confirmed according to the thickness of metal foil and the area of the panel, and air exhaust speed should match with the number of holes. extraction rate can't keep up with, will the adsorption effect of the foil material. If the number of holes is too many, the exhaust speed can't keep up, and it will impact adsorption effect of the foil material. 3.2 Selection of foil thickness and cementing agent According to the main indicators of panel surface accuracy, it needs a very high requirements for thickness of metal foil and the surface roughness. If thickness is too big, it is difficult for foil to deform, and mold joint effect will be very poor. In addition, precision compensation effects of adhesive layer can t be reflected. In reverse, If thickness is too small, it will lead to a low cementing strength, it is easy to cause bumps and creases in the operation process. Common metal adhesive has three series. They are epoxy resin two-component reactive adhesive, polyurethane adhesive and acrylic acid reactive adhesive. The selection of cementing agent in this technology needs to consider its performance characteristics, work environment and curing conditions. Performance characteristics include liquidity, viscosity and adjustability before curing, also include brittleness and impact resistance after curing, etc. Panels work in the outdoor mostly, it is Proc. of SPIE Vol N-3

4 unavoidable that they will suffer from the rain or hail weather, so they need high surface strength. At last, curing conditions include curing temperature, preloading load and cure time. With the comprehensive consideration of these factors, we select epoxy resin two-component reactive adhesive and acrylic acid reactive adhesive as cementing agent of the technology. 3.3 Solution of surface treatment and gelatinizing Surface treatment is an important link in the process of cementing, it directly affects the bonding strength and comprehensive performance. On the object surface, especially the metal surface, there are many rusts, oxide layer, contaminated spots and scars except the oil scale. In order to obtain satisfactory bonding strength, not only need to remove the rust on the surface, should also elaborate a smooth surface with no scars but certain roughness. This requirement can be meet through sand blast, sand paper and wire brush. They are all feasible method for surface pretreatment to enhance the bonding strength between coating and matrix. In these methods, Uniformity and smoothness of the surface through sand blast is the best, and polishing process through sandpaper or wire brush is very time-consuming. Therefore, fine sand blasting process is adopted in substrate surface mechanical processing. Cementing surface of metal film should be burnished by sand paper first, then do some decontamination treatment and pickling for it. The effect of bonding can be improved effectively with these solutions. The adhesive used in this technology is a kind of two-component adhesive, need to be prepared in proper ratio before use. Correct calculation and accuracy weighing are two important points for the preparation of precise ratio adhesive. Although the ratio of glue may be accurate, but adhesive is not necessarily very uniform everywhere after mixing. Resin is sticky. It is difficult to stir its two-component evenly. No enough stirring will lead to a phenomenon that there is only resin in some places, and curing agent is overmuch in other places. So it is necessary to join the right amount of acetone solvent for attenuation. To control the thickness of coating which is replaced by h between 0.1mm and 0.3mm, and the volume of the glue is replaced by V. and V=S h. The area can be calculated according to the following formula. 8π F S = x 2 x1 + F + F 4F 4F Where x1 and x2 denote the inner radius and external radius of the parabolic torus, and F denotes the focal distance. 3.4 Control of curing pressure, temperature and time Curing conditions mainly include curing pressure, temperature and time. These factors are mainly related to the properties of adhesive. Curing pressure directly affects the uniformity and strength of coating. Quantitative calculation is necessary. For example, curing pressure is replaced by P, m1 denotes the weight of matrix,so the weight need to increase can be calculated according to the following formula: m= ( PS PS 0 mg 1 )/g. Where S denotes the area of substrate, P 0 denotes the pressure of vacuum and g denotes the acceleration of gravity. For example, curing pressure of epoxy resin is commonly between 0.1 to 0.4MPa [7]. Curing time and temperature of reactive adhesive depend on his composition. For example, there are three kinds initial curing time of common two-component epoxy resin. They are 5 minutes, 10 minutes and 30 minutes. Also, the three kinds initial curing temperature of it are the normal temperature, degrees Celsius and degrees Celsius. After the initial curing, putting the panels in a constant temperature box is a good way to accelerate curing and shorten the manufacturing cycle. 3.5 Technology of surface measurement Measuring instruments for this technology is British Taylor PGI Dimension Contourgraph which has high automation data acquisition function. And its detection accuracy can reach 2 um, its function of automatic alignment can save much time of measurement. As shown in Figure 5. Proc. of SPIE Vol N-4

5 Figure 5. PGI Dimension Contourgraph The method of measurement is as follows. Pick out an annulus in the middle of the panel for measurement area, compare the actual theoretical value with measured data, then calculate the RMS values of surface precision. Through changing parameters, introducing new technology and workmanship and optimizing the panel manufacturing process step by step, to get the best technological process. 4. EXPERIMENT AND RESEARCH Figure 6 shows the results of planar substrate bonding experiment with different cementing agent and metal foil. Tests shows that aluminum foil has low strength, it is easy to produce deformation because of contraction of the cementing agent in curing process, so it's not suitable for panel materials. As shown in figure a. Also acrylic acid reactive adhesive will produce a lot of glue bumps after glue mixing, it leads to poor flatness after curing. As shown in figure b. On the contrary, the epoxy resin reactive adhesive which has good liquidity and suitable strength produces good flatness. The process of glue preparation will generate certain reaction heat, initial curing temperature should not be too high, because it is difficult to control the deformation of metal foil at high temperature. From experiments of stainless steel foil and copper foil, it can be seen that the two flatness and deformation can both satisfy experiment requirement. But, because the stainless steel bonding strength is poorer, it is easy to detach from the matrix, copper foil with epoxy resin is the best combination for cementing. (a) Aluminum foil with epoxy resin (b) Aluminum foil with acrylic acid (c) Stainless steel foil with epoxy resin (d) Copper foil with epoxy resin Figure 6. Real figure of panel with planar substrate Furthermore,from experiments for the copper foil thickness, it can be seen that copper foil of 0.15mm is too thick, it is difficult for it to deform in the negative pressure. In reverse, the strength of mm copper foil is too low for bonding, Proc. of SPIE Vol N-5

6 lots of wrinkles will be generated during operation, which directly leads to poor surface precision. So the comprehensive property of 0.1 mm copper foil is the best. As shown in figure 7, it is the panel made by 0.1mm copper foil. Figure 7. The real figure of cambered panel Table 1 shows the results of optimization experiment for cambered panel manufacturing process. Tests show that cementing effect of panels with sand blast is better than the ones with antioxidant cleaning. Adding acetone solvent in the process of mixing is good for controlling liquidity of adhesive which helps adhesive to mix better and be gelatinized more evenly. So it is easy to get good coating of high smoothness. Surface treatment of copper foil which can directly affect the bonding strength is also an important link. Through research, the good solution for surface treatment is as follows. Firstly, polish the copper foil with alumina abrasive cloth to improve the roughness of bonding surface, then do some pickling for antioxidant processing, finally clean it with alcohol and acetone, the process will get the best bonding property. Table 1. Optimization experiment for cambered panel manufacturing process Surface treatment for substrate Surface treatment for copper foil Solvent Bonding strength Surface precision (/um) Cleaning with Cleaning with alcohol No acetone Low antioxidant Cleaning with Cleaning with alcohol Acetone Low 6-10 antioxidant Sand blast Polishing with alumina abrasive cloth and cleaning with alcohol Acetone High 3-6 Sand blast Polishing with alumina abrasive Acetone Higher 1-3 cloth, pickling, cleaning with alcohol and acetone Initial curing time and temperature directly influence surface precision of glue joint panel. Because reaction process of epoxy resin will continue to produce heat of reaction, it is unfavorable to choose a high initial curing temperature. Otherwise the coating is prone to generate bubbles or stress concentration. Through research with various curing conditions for surface accuracy, the epoxy resin with an initial curing time of 30 minutes and an ordinary initial curing temperature which is Convenient to prepare and gelatinize adhesive is a good choice. After initial curing, put the panel in a constant temperature cabinet with 80 degrees Celsius, let stand for 3 hours. Proc. of SPIE Vol N-6

7 In addition, tests show that joining the solvent in the adhesive can improve the precision of panel obviously. Mainly because good liquidity, under the effect of curing pressure, can make the metal foil and substrate fit more closely which can lead to a good surface smoothness. In addition, the effect of pickling is also very obvious, which can make the cleaning surface microporous and strengthen the bonding strength. In a word, by measuring, the panel precision of this technology can satisfy the requirement of the radio telescope panel manufacturing. 5. Conclusions The method of film forming manufacturing for radio telescope panel proposed by this paper is a new technology. It uses membrane s high laminating and plastic flow to ensure that the accuracy of the panel is in accordance with the mold highly. The process needs relatively low cost. And for a parabolic reflector, what we just need to do is to designate one certain torus and make one mold for each torus. we can replicate all the panels of the torus. Because of no cutting amount of the surface, this method can maximally save raw materials and reduce production costs. The success of this experiment means that there is another kind of high precision panel production method created for the telescope industry. 6. REFERENCES [1]Shen Zhiqiang, "Shanghai 65 m radio telescope," science. 65(3), 15-20(2013). [2]M.Doro, D.Bastieri, etc. "The reflective surface of the MAGIC telescope," Nuclear Instruments and Methods in Physics Research, 595(1), (2008). [3]Ho Chang, Tsing-Tshih Tsung, Liang-Chia Chen, Lee-Long Han. "The manufacture of microwave plate reflector antennas by employing the improved superplasticity hydraulic forming method," Journal of Materials Processing Technology, 160, (2005). [4]S. K. Golushko, A. V. Yurchenko. "Effect Of Structural And Mechanical Characteristics Of The Composite Material On The Deformation Of A Reflector Antenna," Journal of Applied Mechanics and Technical Physics, 43(2), (2002). [5]M.R. Hassana, F. Scarpa b,m. Ruzzene c,n.a. Mohammed. "Smart shape memory alloy chiral honeycomb," Materials Science and Engineering, 481, (2008). [6]Lei Yan,Yi Chen. "Finite Element Simulation in Fabrication of High Precision Reflector Panels[C]," SPIE,8450, 84505A1-10,2012. [7]Zhou Jianfang, Li An, Rao Baolin, "The influence of surface treatment and cementing process for metal workpiece s bonding shear strength," Chemistry and Adhesion, 29(1), 30-32(2007). Proc. of SPIE Vol N-7