In the processing of copper capillary assemblies, the formation of internal wall scratches directly affects their performance and lifespan. This is especially true in fields with extremely high requirements for fluid transmission precision, such as precision refrigeration and medical equipment, where scratches can lead to leaks, flow deviations, and other problems. Therefore, a comprehensive approach is needed, addressing aspects such as mold design, process control, lubrication, materials, operation, inspection, and environment, to achieve precise control over the smoothness of the internal wall.
The condition of the copper capillary assembly mold is a direct cause of scratches. As a critical forming component, if the internal threaded mandrel has cracks, uneven wear, or edge burrs, it will rub violently against the inner wall of the copper tube during drawing, forming longitudinal or spiral scratches. Furthermore, if copper shavings, foreign objects, or insufficient surface roughness remain in the sizing die, they will scrape the tube wall like sandpaper. Therefore, it is necessary to regularly polish the working area of the mold to ensure rounded edges; clean the mold cavity with high-pressure gas or a special brush before each production run to prevent impurities from embedding; and replace molds with excessive wear promptly to prevent uncontrolled metal flow due to excessive clearance.
The rationality of process parameters is crucial to the quality of the internal wall. When the drawing speed is too high, the copper tube material cannot deform uniformly, easily leading to localized stress concentration and the formation of microcracks or scratches. Fluctuations in drawing force can cause periodic changes in tube diameter, resulting in "bamboo-like" defects on the inner wall. For example, in multi-pass drawing, if the reduction in a certain pass is too large, it may cause tearing of the material surface. Therefore, it is necessary to control the reduction in a single pass within a reasonable range and ensure stable drawing force; a constant tension control device should be used to avoid tube wall abrasion caused by tension fluctuations.
Lubrication conditions are key to reducing friction. Lubricating oil not only reduces the coefficient of friction between the die and the copper tube but also carries away the heat generated during processing, preventing the material from sticking to the die due to overheating. Insufficient lubricating oil flow or oil contamination will create dry friction, exacerbating scratch formation. Therefore, it is necessary to select a special lubricating oil with suitable viscosity and good extreme pressure properties, and ensure unobstructed oil passages; regularly replace the filter screen to prevent impurities from entering the die cavity; for critical parts such as the internal thread mandrel, spray lubrication or oil bath lubrication can be used to ensure complete lubrication film coverage.
Material quality is the source of scratch control. If the inner wall of the copper tube has dents, flattening, or oil stains, these defects will be magnified during drawing, causing the mandrel to directly contact the tube wall and form scratches. Furthermore, coarse grains or compositional segregation in the tube will reduce the material's plasticity, resulting in a rough processed surface. Therefore, strict quality control of the tube is necessary to ensure a smooth, defect-free inner wall. For materials with excessive grain size, annealing can refine the grains; for tubes with compositional segregation, compositional homogenization treatment is required.
Operational procedures directly affect processing stability. If the cutter enters too deeply or the cutting direction is off-center during tube cutting, excessive burrs will appear at the tube end. These burrs will then fall off and scratch the inner wall during drawing. In addition, improper clamping or equipment vibration may cause slight displacement of the copper tube during processing, resulting in rubbing against the mold. Therefore, a specialized cutter must be used, the feed rate controlled, and burrs removed with a reamer after cutting; during clamping, ensure the copper tube is concentric with the mold to avoid eccentric drawing; regularly check equipment vibration and replace aging parts promptly.
The processing environment for copper capillary assembly must be kept clean and orderly. If dust, metal shavings, or other impurities fall into the mold cavity or adhere to the surface of the copper tube, they will embed themselves into the inner wall during the drawing process, causing scratches. Therefore, processing must be carried out in a cleanroom equipped with dust covers and air filtration devices; operators must wear dust-proof clothing to avoid contamination from hair, dander, etc.; the copper tube should be wiped with a clean cloth before and after processing to ensure that the surface is free of impurities.
