The root cause of thermal fatigue Failure of mold inserts and CNC machining constraints
In injection molding and die-casting molds, inserts, as the core components directly subjected to melt erosion and cooling cycles, their thermal fatigue strength directly determines the service life of the mold. Many engineers often focus on the fit accuracy of geometric dimensions during the initial design stage, while neglecting the evolution law of the material's microstructure under repeated temperature differences, resulting in the appearance of crack patterns or even chipping at the corners of the inserts in the early stage. At this stage, CNC machining is not only a manufacturing method but also a carrier for realizing design intentions. If there is an overly large stress concentration coefficient at the root of the insert, or if the CNC milling trajectory causes residual tensile stress to superimpose thermal stress, the probability of crack initiation will increase exponentially.
Thermal fatigue cracks usually originate from the grain boundaries on the surface or semi-surface of the insert. When the mold is opened, the cold mold comes into contact with the high-temperature melt, and the surface layer expands instantly due to heat, forming compressive stress. During the closing or cooling stage, the surface rapidly cools down and contracts, generating tensile stress due to internal constraints. This alternating tensile and compressive stress cycle is the main driving factor of thermal fatigue. For aluminum alloy mold inserts, the thermal cycling temperature usually fluctuates between -5℃ and 200℃. For steel inserts, the temperature difference is even greater, reaching several hundred degrees Celsius. CNC designers must recognize that the cutting heat, feed rate and tool path during the machining process can significantly alter the residual stress state on the component surface. For instance, if the cutting heat is not controlled in high-speed precision milling, a relatively high tensile stress may be retained under the hardened layer, which becomes a "pre-crack" source for the subsequent initiation of thermal fatigue cracks.
Therefore, the design of modern mold inserts has shifted from a simple "wear-resistant design" to a "heat-force-processing" collaborative design. Designers need to consider the machinability of CNC at the CAD stage and avoid geometric features such as deep holes and narrow slots that are prone to cause stress concentration. At the same time, it is necessary to clarify the influence of the tolerance fit zone on thermal expansion. For instance, if the gap between the insert and the template is too small, the expansion of the insert will be hindered at high temperatures, generating huge contact stress and accelerating surface peeling. Conversely, if the gap is too large, it will lead to an intensification of metal flow erosion. A reasonable gap value (usually recommended to be 0.02mm-0.05mm, depending on the thermal expansion coefficient of the material) is the key to balancing thermal stress and mechanical strength. The CNC machining accuracy should be controlled within ±0.01mm to ensure the consistency of the assembly clearance and prevent local over-tightness.
Material selection strategy: Comparison of thermal fatigue Performance of aluminum Alloy and Steel inserts
Material selection is the cornerstone of crack-resistant design. At present, the mold inserts mainly use pre-hardened aluminum alloys (such as A360, A380, A390) and medium carbon alloy steels (such as 1.2344, 1.2379, or domestic 3Cr2W8V, H13 alternative grades). The thermal fatigue behaviors of the two are completely different, and their applicable scenarios also have their own emphases.
Aluminum alloy has a low density and a high thermal conductivity (approximately 200-250 W/m·K), which means that under the same temperature gradient, the peak thermal stress of the aluminum mold is lower, and it heats up and cools down quickly. It is suitable for thin-walled parts, products that require quick mold changes, or those with a service life requirement of 50,000 to 200,000 injection cycles. However, aluminium has low tensile strength, is prone to deformation at high temperatures, and is sensitive to sulfide corrosion. Steel inserts have a low thermal conductivity (approximately 45-50 W/m·K), large thermal inertia, and although they have a high peak thermal stress, they have a high strength limit. They are suitable for thick-walled, high-filling, or scenarios with a lifespan exceeding 500,000 times, especially in areas requiring polished mirrors or high hardness (such as glass molds). Steel molds are more prone to surface cracks under thermal shock because their low thermal conductivity leads to an extremely fast cooling rate of the surface layer and a steep thermal gradient.
The following table compares the key parameters of two mainstream materials in terms of CNC machining and thermal fatigue performance for reference in design selection:
| Parameters/Characteristics | Pre-hardened aluminum alloy (A360/A380) | Medium carbon alloy steel (1.2344/1.2379) | Design considerations |
|---|---|---|---|
| Thermal conductivity | 200-250 W/m·K | 45-55 W/m·K | Aluminum parts have a fast thermal cycle and low thermal stress. Steel parts need to optimize the waterway layout to shorten the cooling time. |
| Tensile strength (pre-hardened state) | 250-350 MPa | 550-700 MPa | Steel parts have strong resistance to deformation, but the CNC cutting force is large, and the influence of tool wear on surface roughness needs to be considered. |
| Coefficient of thermal expansion | ~24 µm/m·K | ~12-14 µm/m·K | Aluminum parts expand significantly, so a larger allowance needs to be reserved for the assembly gap. The dimensional stability of steel parts is better. |
| Surface hardness | 90-105 HV | 300-400 HV (after quenching) | Steel parts have excellent wear resistance, but they are brittle and have a weaker ability to resist impact crack propagation than aluminum. |
| CNC cutting difficulty | Low (easy to cut, but prone to sticking to the knife) | High (Difficult to cut, requiring a large cutting depth/small feed strategy) | Aluminum parts are suitable for high-speed precision milling. Steel parts need to pay attention to the control of cutting heat to avoid the surface heat-affected zone being too deep. |
It is worth noting that the CNC machining capacity directly affects the release of the material's potential. For instance, 5-axis linkage processing can achieve an inclined arrangement of the internal cooling water channels of the insert, making the water flow direction parallel to the melt filling direction. This greatly enhances the uniformity of cooling, reduces local hot spots, and thereby fundamentally lowers the risk of thermal fatigue. This geometric optimization is difficult to achieve in traditional 3-axis machining, highlighting the value of multi-axis CNC in high-end mold design.
Geometric design optimization: Eliminating Stress Concentration and Thermal gradient Traps
Apart from materials, geometric shape is another core variable that controls the distribution of thermal stress. CNC designers should avoid using right-angle transitions at the contours of inserts. Any sharp inner corner will theoretically generate an infinite stress, but in reality, it will be blunted due to material yield, yet still form a significant stress concentration factor (Kt). It is recommended that all internal corners be made into rounded corners of R2.0-R5.0mm, and the external corners be appropriately increased according to the stiffness requirements. The larger the fillet radius, the lower the stress concentration coefficient and the longer the crack initiation life. When programming CNC, the tool radius should be smaller than the fillet radius to ensure that there are no tool marks on the machined surface. Tool marks are often the source of fatigue under thermal cycling.
The "thermal gradient trap" refers to a "heat island" formed due to poor local heat dissipation caused by sudden geometric changes. For instance, if the reinforcing ribs at the bottom of the insert are too thick and far from the waterway, local high-temperature areas will be formed. When designing, the thickness of the reinforcing ribs should be controlled within 15-20mm, and it should be ensured that there is at least one cooling water channel within every 10mm thickness range. The distance from the center of the waterway to the surface is usually half of the wall thickness (0.5T). When CNC drilling waterways, the hole diameter tolerance should be controlled within H7 grade to ensure that the waterway pipe can be smoothly inserted without generating excessive contact stress. If the bending radius of the water channel is less than three times the hole diameter, boring should be used instead of drilling to avoid the risk of drill bit breakage and surface vibration marks.
For deep cavity inserts, the thermal stress at the cantilever end is the most complex. When designing, the cantilever length should be shortened as much as possible, or the deep cavity should be split into multiple small inserts (modular design) to reduce the overall stiffness and lower the temperature gradient. Modular inserts not only facilitate local maintenance and replacement but also balance the overall stress by matching the thermal expansion coefficients of different modules. When CNC machining modular inserts, it is necessary to ensure the consistency of the positioning datum between each module. It is recommended to use precision pin hole fit, and the tolerance fit band should be selected as H7/f7 to ensure uniform clearance among each module after assembly and avoid local warping.
The influence of CNC machining process on surface integrity and residual stress
CNC machining is not only about removing material, but also a process of introducing surface defects or beneficial compressive stress. For thermal fatigue-sensitive inserts, the roughness (Ra) of the machined surface and the distribution of residual stress are of crucial importance. Research shows that when the surface roughness Ra is greater than 1.6µm, the microscopic protrusions on the surface will become obstacles to the flow of the melt, accelerating local erosion and wear. At the same time, the uneven surface will scatter light, affecting the polishing effect. For precision inserts requiring Ra0.4µm, high-precision CNC finishing must be adopted. Coated carbide ball tools should be selected. The cutting depth ap should be controlled at 0.1-0.2mm, the feed rate f should be controlled at 0.15-0.25mm/tooth, and the spindle speed S should be between 8000 and 12000rpm Specific parameters need to be adjusted according to the hardness of the material.
More importantly, it is the control of residual stress. Rough milling and semi-finish milling usually generate tensile stress, which is inevitable. However, finish milling and subsequent surface treatments (such as vibration aging or low-temperature tempering) can convert it into compressive stress or eliminate it. Tensile stress can reduce the fatigue limit of materials and accelerate the propagation of thermal fatigue cracks. Therefore, the process route should be: Blanking (reserve allowance) → Rough milling (remove 90% of the material and control the cutting force) → semi-finish milling (leave 0.3-0.5mm allowance) → Finish milling (Ra1.6µm or less) → Fitter deburring → vibration aging (eliminate processing stress) → heat treatment (if it is a steel part, low-temperature tempering is required to relieve stress) → Finish grinding/polishing. For aluminum alloys, vibration aging has a remarkable effect and can reduce the residual tensile stress to within ±50MPa. For steel parts, due to their high strength, the effect of vibration aging is limited. Usually, stress relief annealing after heat treatment (holding at 560-620℃ for 2-4 hours) is relied upon.
Tool wear is the main cause of the decline in surface quality of CNC machining. As the fillet of the tool edge increases, the cutting force increases, the plastic deformation of extrusion increases, and the surface roughness deteriorates. Therefore, it is necessary to establish a tool life monitoring system and force tool replacement based on the wear amount (VB value) of the tools. For the key surfaces of the inserts, it is recommended to use brand-new fine grinding tools instead of finishing tools to ensure the consistency of the surface texture. The direction of the surface texture should be consistent with the flow direction of the melt to reduce frictional resistance. When programming CNC, the tool path should be planned so that the smooth surface moves unidirectionally from the entry point to the exit, avoiding the texture interlacing caused by bidirectional movement.
The synergistic effect of heat treatment and surface strengthening technology
Relying solely on CNC machining cannot meet all thermal fatigue requirements; it is necessary to combine heat treatment and surface strengthening technologies. For steel inserts, surface hardening combined with high-frequency induction tempering can enhance the surface hardness and reduce residual tensile stress. The depth of induction heating should be controlled at 1-2mm to avoid insufficient toughness in the core due to a too shallow hardened layer. Among surface treatment technologies, nitriding is a classic method for enhancing wear resistance and corrosion resistance. At a gas nitriding temperature of 560-580℃ for 30-60 hours, a diffusion layer of approximately 0.3-0.5mm can be formed, and the surface hardness can reach 900-1100HV. The high-pressure stress state of the nitrided layer helps resist the initiation of thermal fatigue cracks. However, for steels containing alloying elements such as chromium and molybdenum, the nitrided layer is prone to white layer brittleness, and the composition of the nitriding agent needs to be strictly controlled.
Laser Cladding is a cutting-edge technology in recent years. It can cladding NiCrAl alloy or Stellite 6 alloy on the surface of inserts to form nanocrystalline structures, significantly enhancing the thermal shock resistance. The thickness of the laser cladding layer is usually 0.5-1.5mm, with a dilution rate of less than 10% and high bonding strength. This technology is particularly suitable for repairing failed inserts or preventing critical parts. CNC pretreatment provides an ideal substrate for laser cladding: surface roughness Ra≤1.6µm, free of cracks and porosity. If CNC machining leaves micro-cracks, the high temperature of the laser will cause them to expand, leading to the failure of cladding. Therefore, magnetic particle inspection (MT) must be carried out after processing to ensure that the substrate has no macroscopic defects. For aluminum alloy inserts, anodic oxidation sealing treatment can be adopted to increase the surface hardness to 300-500 HV. However, its thermal stability is poor, and the oxide layer will powderize and fall off at high temperatures. Therefore, it is only suitable for low-temperature molds.
The synergy between surface strengthening and CNC machining needs to take into account dimensional changes. Nitriding will cause a slight expansion (about 0.1-0.2mm), and compensation allowance should be reserved for CNC precision machining dimensions. After laser cladding, if the surface is uneven, a fine grinding allowance of 0.2 to 0.3mm should be reserved. Designers should pre-mark these process allowances in the CAD model to avoid dimensional deviations after processing. Meanwhile, the surface of the inserts after strengthening treatment often appears grayish-white. If a mirror-like effect is required, electrolytic polishing or chemical polishing must be carried out after strengthening. This, in turn, places higher demands on the surface quality of CNC machining - the absence of vibration marks and tool connection marks is a prerequisite for successful polishing.
Engineering Practice of Detection Methods and Acceptance Standards
The effect verification of thermal fatigue crack resistance design relies on rigorous testing and acceptance. In addition to the conventional dimensional tolerance (±0.01mm) and surface roughness (Ra0.4µm) inspections, attention should also be paid to the microstructure and mechanical properties. Metallographic microscope (OM) inspection: Observe the quenched structure, the depth of the nitride layer and whether there is decarburization. Scanning electron microscope (SEM) + energy Dispersive spectrometer (EDS) : Analyze the origin location and elemental distribution of cracks to determine whether the cracks are caused by residual oil stains from CNC machining or material inclusions. Magnetic particle inspection (MT) : Detects surface and near-surface cracks, with a sensitivity of grade 5 (A25/12). Fluorescence penetrant testing (PT) : For non-metallic inclusions or micropores. Three-coordinate measuring machine (CMM) : Verify the overall shape and position tolerance of the insert, especially the flatness and parallelism. The error should be controlled within 0.02mm to prevent excessive local force after assembly.
Functional testing is the ultimate verification method. Simulated thermal cycling test: Place the insert in a thermal shock chamber at -10 ° C to 200 ° C (for aluminum) or -10 ° C to 600 ° C (for steel), cycle 1000 to 5000 times, and then conduct non-destructive testing to compare the crack growth. If the crack length increases by no more than 5% of the original length, the design is qualified. Tensile test: Samples are taken from the non-machined surface of the insert to test the tensile strength and elongation after fracture, ensuring that the mechanical properties of the material meet the grade standards. Fatigue life prediction: Based on the measured crack growth rate (da/dN), combined with the Paris Law, the remaining life is predicted. It is recommended that the acceptance criteria refer to ASTM E399 or GB/T 3075, combined with specific industry norms. For mass-produced inserts, it is recommended to take 10% of each batch for full-size measurement and visual inspection (VT) to check for surface defects such as scratches, rust, and oxidation spots, as these defects are often the starting points of thermal fatigue.
Analysis of Common Failure Modes and Preventive Countermeasures
Thermal fatigue failure is not a single mode. Common ones include surface cracking, deep spalling and root fracture. The surface cracks in a net-like pattern, with a certain depth
