Selection & Replacement Standards for Mold Springs Suffering from Fatigue Fracture
Mold springs bear reciprocating compressive loads during long-term production cycles. Excessive opening-closing frequencies, over-compression, and mismatched specifications are the primary causes of fatigue fracture, which can directly trigger mold jamming, mass defective products, and unexpected production shutdowns. To standardize the full-process specifications covering spring replacement judgment, graded model selection, and assembly application, and eliminate fatigue fracture from the source, a set of implementable norms for replacement and type selection has been formulated.
I. Judgment Mandatory Replacement Standards for Fatigue Fracture of Springs
Production must be suspended for immediate spring replacement once any fault feature is observed, and continuous operation with defective springs is strictly prohibited. Visible defects include fractured spring end faces, transverse cracks on spring wires, distorted coils, large-area surface oxidation and rust, and a free length reduction of over 5%. For implicit fatigue identification standards, if ejection jamming, incomplete core pull-back, or offset mold clamping gaps occur within continuous production runs, the spring must be fully replaced after disassembly, even if only subtle fatigue lines are found on the wire without complete breakage.
Mandatory replacement cycles are set according to production shots: black extra-heavy load springs need replacement after 80,000 shots; green heavy load springs after 120,000 shots; red medium load springs after 180,000 shots; blue light load springs after 250,000 shots; yellow extra-light load springs after 320,000 shots. For molds operating under long-term high temperatures (mold temperature over 100°C), the replacement cycle shall be shortened by 40%; for molds forming corrosive raw materials, the cycle shall be shortened by 50%. All springs at identical functional positions on a single mold must be replaced as a complete set. Mixing old and new springs is forbidden, as the difference in elastic force will lead to uneven stress distribution and accelerate fatigue damage of new springs.
II. Graded Selection Matching Standards Based on Spring Load Color Coding
Select springs with corresponding color codes according to mold ejection force, core pulling resistance, and actual compression volume, and strictly follow load matching principles to avoid rapid fatigue caused by low-load springs bearing excessive pressure. Yellow extra-light load springs apply to ejection of small shallow ribs and resetting of small inserts, with the maximum compression limited to 45% of free length. Blue light load springs fit ejector plate reset for ordinary small parts and small sliders, with a maximum compression of 40%. Red medium load springs serve as general standard parts for ejection structures of conventional home appliances and electronic components, with a maximum compression of 35%. Green heavy load springs are adopted for long-stroke ejection, large-size sliders, and core pulling structures of overmolding molds, with a maximum compression of 30%. Black extra-heavy load springs are used for deep-cavity thick-wall molds, large-tonnage sliders, and high-pressure clamping mechanisms, with a maximum compression of only 25%.
A core red line for model selection: the actual working compression volume shall never exceed the upper limit allowed by the corresponding color code. If the structural stroke cannot be reduced, upgrade to a spring with a higher load grade. Springs of uniform specifications and color codes shall be adopted for the same ejector plate or slider assembly to guarantee balanced stress and disperse the bearing load of individual springs, reducing fatigue stress.
III. Supporting Design Standards for Dimension Specification Replacement
When replacing springs, the inner diameter, outer diameter, and free length must match the mold mounting holes and limit posts. Dimensional deviation will result in eccentric load fracture. A gap of 0.8–1.2 mm shall be reserved on each side between the spring outer diameter and mounting hole; an excessively small gap intensifies temperature rise through friction and accelerates fatigue, while an excessively large gap causes eccentricity during operation. The spring inner diameter shall be 0.4–0.6 mm larger than the limit post on each side, and the limit post must run through the full length of the spring to prevent lateral shear force generated by coil tilting during compression.
The calculation formula for free length selection: Free length of spring = Length of limit post + Pre-compression volume + Working stroke + Safety margin. The pre-compression volume is uniformly set as 10%–15% of the free length, with safety margin reserved to avoid full compression of the spring. The solid height of the spring shall not be lower than the mold limit spacing; once fully compressed, permanent fatigue damage will occur after a single use. When replacing springs, the wire diameter and mean diameter parameters shall be consistent with the original parts or increased upwards; specifications with smaller wire diameters that reduce bearing capacity shall not be selected. For multi-spring layout, all springs shall maintain uniform length and specifications to prevent unbalanced stress.
IV. Special Replacement & Selection Specifications for High-Temperature and Corrosive Working Conditions
Special alloy springs with high temperature and corrosion resistance shall be replaced for harsh long-term working conditions, as ordinary chromium-vanadium steel springs cannot adapt to such environments. For high-gloss rapid heating & rapid cooling molds operating at 100–180°C, silicon-chromium alloy high-temperature resistant springs shall be adopted, with a continuous service temperature upper limit of 220°C and elastic force attenuation controlled within 8% under high temperature. For molds forming corrosive raw materials such as PVC and flame-retardant LCP, stainless steel corrosion-resistant springs shall be replaced to prevent stress cracks induced by acid-base flue gas corrosion on spring wires.
Ultra-high strength black alloy heavy load springs with nitrided wear-resistant surface are selected for die-casting and high-speed stamping molds to resist high-frequency impact loads. It is forbidden to use yellow and blue low-load ordinary springs under high-temperature conditions, as high temperature will greatly reduce the fatigue limit of steel and raise fracture risk by more than three times. Special springs for specific working conditions shall not be temporarily replaced by conventional springs; temporary trial molding shall be followed by replacement with matching models.
V. Supporting Standards for Assembly Structure Optimization to Delay Fatigue
Spring replacement shall be accompanied by rectification of assembly structures to reduce stress concentration and extend the service life of new springs. Thickened flat gaskets are installed at both ends of the spring, with higher hardness than spring steel to avoid notch cracks formed by direct friction between spring wire ends and templates. The end face of limit posts is chamfered with R0.5 to eliminate sharp edges that scratch the inner wall of springs. Burrs on the inner wall of mounting holes are removed and polished smooth to avoid metal protrusions scraping spring coils.
The guiding clearance of mold guide pins and ejector plates is tightened to eliminate shaking of ejector plates and eccentric stress on springs. Balance wear blocks are added to slider mechanisms to eliminate unilateral eccentric load. High-temperature grease is regularly applied to springs to reduce heat and wear generated by mutual friction of coils. The compressive load of springs shall be released when the mold is shut down for storage; long-term pre-compression will continuously accumulate fatigue stress and shorten service life.
Conclusion
The replacement and type selection of mold springs for fatigue fracture follow a complete logical chain: confirm replacement timing first, then match load grades, strictly control dimensional adaptation, select special materials for specific harsh working conditions, and optimize assembly structures simultaneously. Clear dual replacement standards based on fracture and production shots eliminate overdue service from the source. Precise model selection by color-coded load and control of compression limits avoid overload fatigue. Unified specifications for inner/outer diameter and length prevent damage from eccentric shear. Special alloy springs are replaced for high-temperature and corrosive environments to adapt to working conditions. Synchronous optimization of gaskets, limit structures, and guiding assemblies reduces stress concentration. Full implementation of this set of standards can drastically cut abnormal spring fracture frequency, shorten mold maintenance downtime, and stabilize long-term continuous production efficiency of injection, die-casting, and other types of molds.
