In the production of precision discs, improving the surface flatness of the disc base is crucial for ensuring data storage stability and readout accuracy. As the physical foundation of the disc, its surface flatness directly impacts laser focusing accuracy, which in turn determines the signal readout error rate and storage lifespan. Therefore, every step of optimization, from material selection to processing, must focus on reducing surface roughness and eliminating microscopic defects.
Material selection is the primary prerequisite for improving flatness. High-purity polycarbonate (PC) has become the mainstream material for disc bases due to its low water absorption, high light transmittance, and excellent dimensional stability. During the manufacturing process, the raw materials must be rigorously filtered to remove impurities and bubbles, thereby preventing surface irregularities caused by internal defects. Furthermore, the addition of nano-scale fillers or modifiers can further improve the material's fluidity and thermal stability, minimizing shrinkage and deformation during the molding process.
Injection molding is the core step in disc base processing, and its process parameters directly impact surface quality. Mold design requires the use of high-precision steel molds with hard chrome or diamond-like carbon coatings to reduce friction and improve wear resistance. During injection molding, the injection speed and pressure of the molten plastic must be controlled to avoid surface flow marks or internal stresses caused by underfilling or over-holding. Furthermore, employing a multi-stage holding pressure and slow cooling process ensures uniform material shrinkage within the mold, reducing warping and deformation. Precise mold temperature control is also crucial. Excessively high temperatures can cause material decomposition, while excessively low temperatures can cause cold spots, both of which can compromise surface smoothness.
Heat treatment is a key step in eliminating internal stresses and stabilizing dimensions. Injection-molded disc substrates undergo annealing, which involves holding the material at a specific temperature for several hours to fully release internal stresses. The annealing temperature is typically close to the material's glass transition temperature, but should be kept below a certain temperature to avoid deformation. Furthermore, controlling the heating and cooling rates can prevent surface waviness caused by uneven thermal expansion and contraction. Some high-end processes also employ isothermal annealing to further eliminate residual stresses and enhance long-term dimensional stability.
Machining is a direct means of improving smoothness. During the roughing stage, high-speed milling or fly cutting is used to remove burrs from the substrate edge, providing a reference surface for subsequent finishing. Finishing utilizes ultra-precision turning or grinding techniques, using diamond tools or nano-scale abrasive media, to achieve surface roughness within the nanometer range. Tool wear must be continuously monitored and replaced promptly during the process to prevent surface waviness caused by varying cutting forces. Furthermore, an online measurement system provides real-time feedback on flatness data, dynamically adjusting machining parameters to ensure consistency.
Chemical mechanical polishing (CMP) is a core technology for achieving ultra-smooth surfaces. This process combines chemical etching and mechanical grinding. Chemical reagents in the polishing slurry react with the material surface to form an easily removable softened layer, which is then removed by mechanical friction. The polishing pad must be selected to balance hardness and elasticity to accommodate the curvature of the substrate and distribute pressure evenly. The abrasive particle size and concentration in the polishing slurry must be precisely controlled to avoid scratching the surface and uneven polishing. A multi-step polishing process can further enhance results, for example, initially using large-size abrasives to quickly remove surface defects, followed by fine-finishing with smaller-size abrasives.
Cleaning and inspection are the final lines of defense for quality control. The substrate undergoes multi-stage ultrasonic cleaning to remove residual polishing fluid, metal particles, and organic matter from the processing process. The cleaning fluid must possess high detergency and low surface tension to prevent residual liquid. Inspection uses a laser interferometer or atomic force microscope to perform full-scale scans of surface flatness to ensure compliance with design requirements. Defective products are traced back to the specific process steps through a feedback mechanism, allowing for continuous optimization of process parameters.
