Materials and processing technology of needle barrels for circular knitting machines

The material selection and processing technology of the needle cylinder for circular knitting machines: The needle cylinder is the core component of the circular knitting machine. Its material selection and processing technology directly determine the operational accuracy, weaving efficiency and fabric quality of the circular knitting machine. The following is a detailed explanation from three aspects: material properties, processing flow, and key process control. I. Material Selection and Characteristics of Syringes (1) Common Material Types 1. High carbon chromium bearing steel (GCr15) is the mainstream material for syringes. GCr15 features high hardness, high wear resistance, and good dimensional stability. After heat treatment, it can meet the wear resistance requirements under high-speed operation conditions and is suitable for the manufacturing of syringes in medium and high-end circular knitting machines. 2. High-quality alloy structural steel (20CrMnTi) This material can enhance its surface properties through carburizing and quenching, combining high surface hardness with core strength and toughness. It has excellent impact resistance and is suitable for weaving heavy fabrics and high-speed rotating heavy-duty circular knitting machine syringes. 3. Stainless steel (304/316) has excellent corrosion resistance and is suitable for weaving environments that are humid and contain many chemical auxiliaries, such as the production equipment for spandex elastic fabrics and sea island silk fabrics. However, its hardness is slightly lower than that of bearing steel and requires surface hardening treatment to enhance its wear resistance. (2) Requirements for raw material pretreatment: Raw materials must undergo normalizing and spheroidizing annealing treatment to eliminate casting or forging stress, refine grain structure, and ensure low tool wear and high surface finish during subsequent cutting processes. After pretreatment, the hardness of the material should be controlled at HB 180-220 to ensure the compatibility of the cutting performance with the heat treatment process. Ii. Core Processing Technology Flow of the syringe 1. Blank forging: The die forging process is adopted to form the syringe blank, ensuring that the structure is dense, free of metallurgical defects such as pores and sand holes, and the wall thickness of the cylinder body is uniform, avoiding stress deformation in subsequent processing. 2. Rough machining: The inner diameter, outer diameter, end face and wall thickness of the syringe are machined by turning, with a allowance of 0.5-1mm reserved for fine machining. At the same time, the prototype of the keyway and other connection structures is machined. 3. Heat treatment strengthening: Select carburizing and quenching (20CrMnTi) or integral quenching (GCr15) based on the material type, and combine it with low-temperature tempering to eliminate internal stress and stabilize dimensional accuracy. 4. Finishing: The inner hole, outer circle and end face are machined by CNC turning and grinding processes to ensure dimensional tolerance and positional accuracy. The needle slots are processed by a dedicated CNC milling machine to control the cumulative error of the tooth pitch. 5. Surface protection treatment: Select blackening, phosphating, hard chromium plating or nitriding treatment based on the usage environment to enhance the rust resistance and wear resistance of the syringe surface. 6. Precision inspection and trial assembly: Full-dimensional inspection is completed through a three-coordinate measuring machine, hardness tester, and dynamic balancing machine. Trial assembly of knitting needles is carried out to verify the smoothness of sliding. Iii. Key Control Points of Key Processing Techniques (1) Precision Processing Technology of Needle Slots The needle slots are the core functional part of the syringe and directly affect the movement trajectory of the knitting needles. When processing, a CNC dedicated slot milling machine should be used, equipped with diamond tools, to ensure that the slot width tolerance is ≤0.02mm and the slot depth deviation is ≤0.03mm. The cumulative error of the pitch of adjacent needle slots is ≤0.01mm/100mm, and the surface roughness of the slot side is ≤Ra 0.8μm. After processing, burrs in the groove need to be cleaned to prevent the knitting needles from getting stuck. (2) Control of heat treatment process parameters 1. Carburizing and quenching (20CrMnTi syringe) : Carburizing temperature 920-940℃, holding time adjusted according to the depth of the carburizing layer (0.8-1.2mm); Quenching temperature: 840-860℃, oil cooling or staged cooling is adopted. The low-temperature tempering temperature is 180-200℃, and the holding time is 2-3 hours. The final surface hardness reaches HRC 58-62, and the core hardness is HRC 30-35. 2. Integral quenching (GCr15 syringe) : Quenching temperature 820-840℃, water cooling or oil cooling; The low-temperature tempering temperature is 160-180℃, with a holding time of 3-4 hours. The surface hardness is HRC 60-64, ensuring high wear resistance. 3. Aging treatment: After heat treatment, artificial aging is required at a temperature of 120-150℃ for 12-24 hours to further eliminate internal stress and prevent the syringe from deforming during use. (3) Dynamic balancing processing and detection: When the syringe operates at high speed (with a rotational speed of over 3000r/min), excessive unbalance can cause equipment vibration, noise, and even damage components. After fine processing, it needs to be inspected and corrected on a dynamic balance testing machine. The balance is adjusted by milling the end face allowance, and the final unbalance quantity is ≤5g·mm/kg, meeting the requirements for smooth operation under 1.2 times the rated speed. Iv. Quality Inspection Standards 1. Dimensional Accuracy Inspection: Inner hole tolerance H7 grade, outer circle tolerance h6 grade, full-length straightness of the cylinder body ≤0.05mm/m, perpendicularity of the end face to the axis ≤0.02mm. 2. Surface performance inspection: The surface roughness of the inner hole and outer circle should be ≤Ra 0.4μm. The coating should be uniform without peeling. The anti-rust performance must pass the neutral salt spray test (no rust for 48 hours). 3. Assembly fit inspection: The fit clearance with the knitting needle should be controlled within 0.01-0.03mm, and there should be no jamming or abnormal noise during the trial run. The keyway connection with the transmission component complies with the GB/T 1095-2003 standard for circular knitting machines