Cold heading is a core technology in fastener manufacturing, widely used in the mass production of bolts, nuts, rivets, etc., due to its high efficiency, material saving, and excellent product performance. Ningbo Yijun Technology Co., Ltd. will systematically analyze how to improve the manufacturing precision and quality of fasteners from six aspects: process characteristics, factors affecting precision, equipment selection, raw material requirements, lubricant selection, and mold technology support. Combined with technical practice, the company will provide practical solutions for the industry.
I. Core Characteristics of Cold Heading Process
Cold heading is a forging method that uses molds to upset metal bars at room temperature. Its core advantages include:
① Precision forming: Based on the theory of metal plastic deformation, a predetermined shape and size are achieved through pressure. Surface precision can reach IT7-IT8 level, and dimensional tolerances are controlled within ±0.05mm.
② High Efficiency and Automation: Multi-station cold heading machines can continuously complete processes such as cutting, forming, chamfering, and thread rolling, achieving a production efficiency of 60-600 pieces per minute and an equipment utilization rate exceeding 90%.
③ Performance Improvement: The work-hardening effect increases the tensile strength of the workpiece by 15%-30%, improves the continuity of metal fibers compared to machining, and significantly extends fatigue life.
④ Economic Benefits: Material utilization is as high as 85%-95%, reducing metal loss by 60%-70% compared to traditional machining, and lowering the cost per piece by more than 40%.
II. Six Core Factors Affecting Cold Heading Accuracy
Achieving high-precision cold heading requires strict control of the following variables:
① Chemical Composition: Pure metals have better plasticity than alloys, while impurities (such as sulfur and phosphorus) easily lead to brittleness. For example, for every 0.1% increase in carbon content in steel, the ultimate tensile strength increases by 6-8 kg/mm², but plasticity decreases by 5%-10%.
② Metallographic Structure: Grain size, segregation, and inclusions (bubbles, porosity) directly affect plasticity. Spheroidizing annealing can transform the microstructure into spherical pearlite, significantly improving cold deformation capability.
③ Deformation Temperature: At room temperature, plasticity fluctuates non-linearly with temperature. A mold temperature control system is needed to stabilize the workpiece temperature between 20-50℃.
④ Strain Rate: Excessive strain rate can easily cause adiabatic heating, leading to localized softening; excessively slow rate increases frictional losses. Typical strain rates are controlled between 0.1-10 s⁻¹.
⑤ Mechanical Conditions: Triaxial compressive stress is conducive to plasticity. Mold design needs to optimize stress distribution, for example, by using a combined parting structure to reduce lateral stress.
⑥ Deformation Continuity: Intermittent deformation can improve the forming performance of low-plasticity metals, such as reducing the amount of deformation per pass through multi-station step-by-step upsetting.
III. Selection Criteria for High-Performance Cold Heading Machines
High-quality equipment is the foundation for ensuring precision and must possess the following characteristics:
① Structural Strength: The machine body is made of alloy ductile iron casting with a tensile strength ≥450MPa; high wear-resistant alloy copper bearings are used at the crankshaft connection, extending service life by more than 3 times.
② Power System: Two-stage gear transmission combined with pneumatic clutch and brake increases transmission efficiency to 92% and reduces energy consumption by 15%.
③ Operational Stability: The guide plate drives the cutter rod to ensure linear transmission of cutting force, with dynamic balance accuracy ≤0.1mm; equipped with fault detection and safety protection devices, it can automatically stop the machine to protect the mold.
④ Feeding Accuracy: The feeding box has a thrust device and supports variable frequency stepless speed regulation, with a feeding error ≤0.02mm.
⑤ Intelligent Monitoring: Integrated sensor and PLC control system monitors parameters such as pressure and temperature in real time, achieving closed-loop process control.
Typical Case: Ningbo Yijun Technology introduced the TXS308 multi-station high-speed automatic cold heading machine. Through its horizontally arranged forging stations, it increased screw production efficiency to 800 pieces/minute, with a product qualification rate consistently above 99.2%.
IV. Strict Selection Standards for Cold Heading Raw Materials
Material quality directly determines the upper limit of finished product performance:
① Condition Treatment: Spheroidizing annealing is required. The microstructure should be spheroidal pearlite, with a hardness of 75-85 HRB. The surface should exhibit a "hard surface, soft core" characteristic, with a core hardness ≤60 HRB to optimize cutting quality.
② Hardness and Plasticity: The hardness of cold-drawn material should be ≤85 HRB, and the elongation ≥25% to reduce the risk of cracking and protect the mold.
③ Surface Requirements: The lubricating film should be uniformly dark in color, free of scratches, cracks, rust, or oxide scale; surface roughness Ra ≤0.8μm.
④ Decarburization Control: The total thickness of the radial decarburized layer must not exceed 1% of the diameter; otherwise, surface cracks are easily triggered.
⑤ Hardening Sensitivity: Raw materials are evaluated through cold heading tests; lower sensitivity facilitates forming. For example, 35CrMoA steel requires pretreatment to a hardness ≤80HRB before cold heading.
V. Key Roles of Cold Heading Oil
Lubrication is not only about reducing friction but also a crucial guarantee for the forming process:
① Extreme Pressure Lubrication: Cold heading oils containing sulfurized lard and sulfurized fatty acid esters can form a chemical reaction film under high pressure, preventing mold sintering, especially suitable for difficult-to-machine materials such as stainless steel.
② Heat Dissipation and Environmental Protection: Thermal conductivity ≥0.15W/(m·K), avoiding localized overheating; low smoke and low odor design improves the operating environment and complies with RoHS standards.
③ Protective Performance: Reduces sludge formation, provides inter-process rust prevention, and extends mold life by more than 50%.
④ Wide Applicability: Meets the processing needs of products with large deformation, such as hollow rivets and high-strength bolts, with deformation reaching over 60%.
Application Example: After adopting specialized cold heading oil, Ningbo Yijun Technology extended its mold replacement cycle from once per shift to once every three days, reducing lubrication costs per part by 0.02 yuan.
VI. Core Support of Mold Technology
As a leading mold supplier in the industry, Ningbo Yijun Technology's technological practices provide key support for improving cold heading precision:
① Mold Design: Employs integrated CAD/CAE/CAM design to simulate metal flow paths, optimize mold cavity structure, and reduce flash generation.
② Material Selection: The mold working surface uses DC53 mold steel with a hardness of 58-62 HRC, offering twice the wear resistance of Cr12MoV.
③ Surface Treatment: Through TD coating or PVD plating technology, the mold surface hardness reaches 1200-1500 HV, improving anti-adhesion by three times.
④ Precision Control: The mold manufacturing precision reaches ±0.005mm, and with the help of an online detection system, dynamic adjustment of product dimensions is achieved.
Improving fastener precision is a systematic project involving multiple aspects such as equipment debugging, material selection, lubrication environment optimization, and mold technology support. Chongqing Heng Hui Precision Mold Co., Ltd. has successfully controlled its product defect rate to below 0.8% by introducing advanced equipment, optimizing process parameters, and strictly controlling quality, setting a benchmark for the industry. Strict adherence to process specifications not only ensures product quality but also reduces costs and increases efficiency, driving the upgrading of China's fastener manufacturing industry towards high-end and intelligent directions.
