In the thermal field system of a photovoltaic single crystal furnace, Φ300-500mm、 The isostatic pressed graphite guide tube with a length of 400-600mm serves the dual functions of guiding silicon melt and thermal field insulation. Its inner wall roundness (required to be ≤ 0.025mm), length to diameter ratio (2.5-4:1) adaptability, and surface roughness (Ra ≤ 0.8 μ m) directly affect the continuity of silicon single crystal growth. If the inner wall roundness exceeds the standard, it can easily lead to uneven flow of silicon material and cause single crystal edge breakage; The characteristics of graphite, such as low hardness (Mohs 1-2), high brittleness, and easy accumulation of dust, combined with the problem of processing deflection caused by a large aspect ratio, make it difficult for traditional processes to balance accuracy and efficiency. A leading domestic photovoltaic graphite component enterprise is facing three core bottlenecks when processing high-purity isostatic pressed graphite guide tubes (made of Toyo carbon TFG-85, with a bulk density ≥ 1.82g/cm ³):
1. Accuracy loss of control: Traditionally, it requires four processes: "horizontal lathe rough outer wall → vertical boring machine fine boring inner wall → manual correction of straightness → high-pressure airflow dust removal". Multiple clamping results in the coaxiality deviation between the inner wall and the flange reference exceeding 0.04-0.06mm, and the length to diameter ratio is too large, resulting in an inner wall straightness error of 0.08-0.1mm. The scrap rate caused by deflection deformation is 16%, and the final product qualification rate is only 78%;
2. Dust problem: Graphite dust accumulates in the deep cavity of the inner wall of the guide tube. After high-pressure airflow cleaning, there is still 8% residual dust in the product, which requires additional ultrasonic cleaning and increases manufacturing costs by 20%; At the same time, dust intruded into the spindle bearings and guide rails of the equipment, resulting in an average of 11 machine tool failures per month, and the precision degradation cycle of the guide rails was shortened to 4 months;
3. Low efficiency: The processing cycle of a single guide tube is as long as 42 minutes, of which manual correction and dust cleaning account for 35%, and require the cooperation of 2 operators. The labor cost for a single product exceeds 32 yuan; Diamond cutting tools are prone to breakage due to cutting vibration, with a lifespan of only 30 pieces per blade and a single product tool cost of 26 yuan.
Customer usage scenarios
To overcome the difficulties, the company introduced a customized Taiwan Cloud KD500 graphite specific CNC vertical lathe and built a dedicated processing system for the guide tube with "length to diameter ratio adaptation+dust-proof precision dual control":
Customized design of equipment core:
1. Rigidity and deflection control: Natural granite bed body (thermal expansion coefficient ≤ 0.4 × 10 ⁻⁶/℃) is used, and internal stress is eliminated after 20 months of natural aging. Combined with a three-point positioning structure of "spindle+2 sets of adjustable auxiliary supports", the support spacing is optimized through finite element analysis, and the deflection of the guide tube with a length to diameter ratio of 4:1 is controlled within ≤ 0.01mm. Equipped with Fanuc 31i-B5 CNC system and grating ruler full closed-loop control (resolution 0.01 μ m), it achieves ± 0.005mm positioning accuracy and accurately matches the H7 level tolerance requirements of the inner wall.
2.Comprehensive dust control:Integrated with a "spindle built-in cyclone separator (dust removal efficiency of 93%)+annular side suction dust collection hood (air volume of 1800m ³/h)+HEPA 13 level high-efficiency filter (filtration accuracy of 0.3 μ m)" three-level dust prevention system, the dust concentration in the working area is stable ≤ 1.2mg/m ³, far below the 4mg/m ³ limit specified in the national standard GBZ 2.1-2019; At the same time, dust-proof cable drag chains and oil-free lubrication guides are used to prevent dust and lubricant from mixing and contaminating workpieces and equipment.
Exclusive configuration of cutting system:
Using a Φ 500mm porous vacuum suction cup (with an adsorption force of 0.11MPa and an adsorption area coverage rate of 90%), deformation of the graphite guide tube clamping is avoided through uniform negative pressure; Configure a diamond coated inner hole turning tool (with a rounded edge of 0.01mm and a tip radius of 0.02mm) and a -18 ℃ dry ice cold air cooling system to control the cutting zone temperature below 22 ℃ and suppress inner wall cracking caused by graphite thermal stress; The spindle adopts a low vibration design (vibration acceleration ≤ 0.07g), which is suitable for the high brittleness processing requirements of graphite.
In terms of technological innovation, a breakthrough has been achieved in the precision machining of photovoltaic graphite guide tubes through a single clamping process
1. Process integration: The equipment integrates a vacuum suction cup, an 8-station servo turret (with a tool change time of 1.4 seconds), and a dedicated long blade holder for the inner wall. It can complete the precision turning of the outer wall of the guide tube (roundness ≤ 0.018mm), the precision boring of the inner wall (cylindricity ≤ 0.01mm), the precision turning of the flange end face (flatness ≤ 0.025mm), and the drilling of the bottom guide hole (positional accuracy ≤ 0.12mm) in one go, directly eliminating manual correction and ultrasonic cleaning processes, shortening the process by 50%;
2. Deflection and brittleness control: Innovative "segmented progressive cutting method" - for the difference in length to diameter ratio, the guide tube is divided into 3-4 sections for processing, each section adopts the parameters of "low speed (700-900r/min)+slow feed (40-70mm/min)+micro back cutting tool (≤ 0.05mm)", combined with Renishaw in machine measurement probe (measurement accuracy ± 0.0008mm) to collect real-time inner wall straightness data, dynamically adjust the auxiliary support force (compensation accuracy 0.002mm), so that the inner wall straightness error is controlled within ≤ 0.05mm, and the edge breakage amount is ≤ 0.006mm;
3. Optimization of changeover efficiency: Equipped with 22 sets of photovoltaic graphite guide tubes (suitable for 12-18 inch single crystal furnaces) process parameter templates, changeover only requires retrieving the corresponding program and replacing the dedicated long blade cutting tool. The changeover time is shortened from traditional 3 hours to 15 minutes, meeting the needs of multi specification batch production.
The implementation results fully comply with the strict standards of the photovoltaic industry:
Accuracy and qualification rate: The roundness stability of the inner wall of the graphite guide tube is ≤ 0.02mm, the coaxiality is ≤ 0.03mm, and the surface roughness reaches Ra0.6 μ m, fully meeting the technical requirements of GB/T 3074.1-2019 "Graphite Electrode" and photovoltaic single crystal furnace thermal field components. The product qualification rate has increased from 78% to 99.3%, and the deflection deformation scrap rate has decreased to 0.7%;
Efficiency and Cost: The processing time for a single piece has been reduced from 42 minutes to 20 minutes, and the daily production capacity has increased from 90 pieces to 210 pieces; Cancel manual correction and ultrasonic cleaning processes, reducing labor costs by 32 yuan per product; The lifespan of diamond cutting tools has been extended by 80% due to optimized cutting parameters (from 30 pieces/blade to 54 pieces/blade), and the cost of single product cutting tools has been reduced to 14 yuan;
Equipment operation and maintenance: The three-level dust prevention system has reduced the monthly average number of equipment failures from 11 to 0.7, extended the precision calibration cycle of the guide rail to 2.5 years, increased the comprehensive utilization rate of the equipment from 65% to 94%, reduced annual downtime by 680 hours, and saved maintenance costs of over 220000 yuan annually.
Customer processing scenarios
The Taiwan Cloud Precision Machinery KD500 has solved the industry pain points of graphite guide tubes, such as large aspect ratio, easy deformation, difficult dust cleaning, and low efficiency. ”The production director of the company stated, "Currently, our guide tube has been certified by JA Solar and Trina Solar, and has been successfully applied to an 18 inch high-efficiency single crystal furnace production line. It meets the requirement of 'thermal field insulation performance attenuation ≤ 5% after 15 months of continuous use', which provides core support for us to occupy 38% of the market share in the high-end graphite components of photovoltaics." This case confirms that the CNC vertical lathe has become a key equipment for breaking through the "triple bottleneck of precision, efficiency, and operation and maintenance" in the manufacturing of photovoltaic grade isostatic pressed graphite guide tubes through the deep synergy of "customized design of length to diameter ratio processing+graphite brittleness adaptation process+dust integrated treatment".
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