Common Failure Modes and Countermeasures of Die Casting Molds
Aluminum and magnesium alloy die-casting molds operate under extremely harsh conditions (high-temperature molten metal, high pressure, high-speed impact, rapid heating and cooling), resulting in diverse and complex failure modes. Understanding these failure modes and taking corresponding countermeasures is key to extending mold life, improving production efficiency, and reducing costs. This article shares some common mold failure modes and countermeasures.
The following are common failure modes of die casting molds and corresponding countermeasures:
I. Major Failure Modes
1 Thermal fatigue cracks (crazing)
Repeated heating (molten metal injection) and cooling (demolding, spraying) on the surface of the mold cavity generate alternating thermal stress, leading to the formation and gradual propagation of network cracks (crazing). This is the most common failure mode of die-casting molds.
High-risk areas: near the gate, ingate, core, areas of abrupt changes in wall thickness, and areas of stress concentration (sharp corners, small rounded corners).
Impact: Crack propagation leads to scratches, burrs and dimensional deviations on the surface of the casting, ultimately resulting in the scrapping of the mold.
2 Erosion/Abrasion
The high-speed, high-pressure flow of molten metal exerts strong mechanical scouring and friction on the mold cavity surface (especially the gating system,ingate, and core flow-facing surface), causing the material to be gradually "shaved off" and lost.
High-risk areas: sprue sleeve, inner gate, bends in the runner, and the front surface of the core.
Impacts: Changes in cavity dimensions, increased surface roughness, and deformation of the gating system lead to dimensional deviations in castings, decreased surface quality and poor filling.
3 Chemical corrosion/mold sticking
Aluminum alloys: Molten aluminum reacts with mold steel (iron) at high temperatures to form a brittle Fe-Al intermetallic compound layer. This layer is easily torn during demolding, causing the mold surface material to stick and be removed ("welded"). Mold release agent residues may also form corrosive compounds at high temperatures.
Magnesium alloys: Although magnesium has a low affinity for iron and is less prone to sticking to the mold, magnesium alloys are highly susceptible to oxidation, and oxide particles can exacerbate mold wear. Molten magnesium alloys may produce corrosive gases (such as HCl) in humid environments (due to moisture in the release agent).
High-risk areas: Cavity surfaces, especially areas with higher temperatures.
Impact: Increased surface roughness of the mold, difficulty in demolding, scratches and adhesion on the surface of the casting, requiring frequent mold cleaning, and accelerated thermal fatigue.
4 Overall plastic deformation
Under sustained high temperature (approaching or exceeding the yield strength of the mold material) and high pressure, permanent plastic deformation (collapse, bulging) occurs on the surface or weak points of the mold cavity.
High-risk areas: cores, bosses, and areas with thin walls.
Impact: Dimensional deviations in castings, difficulty in demolding.
5 Mechanical stress cracking
Brittle or tough fractures can be caused by unreasonable mold design (stress concentration), manufacturing defects (machining marks, residual stress), improper operation (premature mold closing, unbalanced ejection), or accidental overload (excessive injection pressure, flash jamming).
High-risk areas: core root, slider wedge block, ejector pin hole edge, and weak points in the mold.
Impact: The mold cracked and had to be scrapped, resulting in huge losses.
6 Wear (non-erosion)
Friction and wear are caused by the relative movement between moving parts (slider, angled ejector, ejector pin) and guide sleeve and cavity.
High-risk areas: slider guide surface, angled ejector rod, ejector pin, and reset rod.
Impacts: Jamming of moving parts, decreased precision, flash, and difficulty in demolding.
II. Main Countermeasures
1 Optimize mold material selection and heat treatment
For materials, it is recommended to select high-quality hot work die steel with high thermal fatigue resistance, high toughness, good thermal conductivity, high red hardness, and resistance to tempering softening. H13 (4Cr5MoSiV1) is the most commonly used and cost-effective choice. For applications with higher requirements, higher performance steel grades (such as DIEVAR, QRO90, DAC, H11 Modified, high vanadium H13, etc.) should be selected.
Heat treatment requires strict control of quenching and tempering processes to ensure the acquisition of a uniform and refined microstructure (tempered martensite) and to achieve the target hardness (usually in the range of 44-52 HRC, which can be adjusted according to the specific location and failure mode).
2 Optimize mold design
Mold design must ensure that the mold as a whole and key parts (core, slider) have sufficient strength and rigidity to avoid stress concentration (using large rounded corners for transition).
The mold's gating system is designed with a reasonable gate location, shape, and size to guide the molten metal to fill smoothly and reduce direct high-speed impact on the cavity and core. The overflow groove and venting groove design are also optimized.
The design of the cooling system is of paramount importance! The cooling system should be designed with efficient and uniform cooling water channels (conformal cooling) to ensure a uniform and controllable temperature field in the mold, avoiding localized overheating or overcooling. Priority should be given to point cooling and conformal cooling for areas that are difficult to cool, such as the core.
Moving parts: Ensure the fitting accuracy and guide length of moving parts such as sliders, angled ejectors, and ejector pins, and set up a reasonable wear compensation structure (wear-resistant blocks).
3 Surface treatment
Nitriding: Gas nitriding, ion nitriding (PVD), and salt bath nitriding (QPQ) are among the most widely used surface strengthening methods that improve surface hardness, wear resistance, anti-sticking properties, and certain corrosion resistance.
Physical vapor deposition: hard coatings such as TiN, TiAlN, CrN, AlCrN, and DLC. Significantly improves surface hardness, wear resistance, erosion resistance, and anti-sticking properties, while reducing the coefficient of friction. Particularly effective in solving aluminum sticking issues.
Surface modification: such as laser cladding and electron beam surface alloying, can reduce scratches and thermal cracks by cladding high wear-resistant and heat-resistant materials in specific areas.
Carburizing/carbonitriding: Used for moving parts requiring extremely high surface hardness and wear resistance. Commonly used in components such as ejector pins and sliders.
Polishing/Mirror Polishing: Improves the surface finish of the mold cavity (Ra < 0.2μm or even higher), reduces the adhesion of molten metal, improves demolding properties, and delays crack initiation. Therefore, polishing time is often compressed to meet deadlines. Sufficient time should be allocated in the mold development cycle to ensure thorough polishing.
4 Strictly control die-casting process parameters
Mold temperature control is the core control point! Use a mold temperature controller to precisely control the mold's operating temperature within a reasonable range (typically 150-250°C for aluminum alloys and 180-300°C for magnesium alloys), ensuring uniform temperature across all areas and keeping the mold in a relatively stable thermal equilibrium field. Ensure sufficient preheating to avoid cold mold injection. Identify key areas based on product structure characteristics, refine temperature ranges, and monitor them daily.
While ensuring good fluidity and filling properties, use the lowest possible pouring temperature (650-720°C for aluminum alloys, 640-680°C for magnesium alloys).
Regarding injection parameters, optimize slow injection speed, fast injection speed and transition point, avoid excessive impact speed and pressure while ensuring filling quality, and reasonably set boost pressure.
Select a high-efficiency, stable mold release agent, and optimize spraying time, location, pressure, and spray volume. Ensure uniform and effective spraying, which can both release the mold and cool it, avoiding excessive spraying that can cause localized overcooling. Regularly clean any residue from the mold surface.
While ensuring the solidification of the casting and the cooling of the mold, the cycle time should be shortened as much as possible to improve efficiency, but the mold temperature should be avoided from getting too high.
5 Standardized operation and maintenance
(1) Thoroughly clean the mold parting surface, cavity, venting groove, ejector pin hole, and cooling water channel of residues (aluminum/magnesium slag, release agent carbon deposits).
(2) Inspect and repair minor surface damage (such as small scratches or minor cracks) to prevent them from spreading.
(3) Inspect and lubricate all moving parts (guide post, guide sleeve, slider, ejector pin).
(4) Check whether the cooling water circuit is unobstructed and whether the flow rate and water temperature are normal. Strictly follow the procedures and avoid rough operation (such as forceful hammering, premature forced mold closing/opening).
(5) Perform regular maintenance (per shift, daily, weekly).
(6) When shutting down, thoroughly clean, apply anti-rust oil, and store in a dry environment. For vulnerable areas (gate, core), use replaceable inserts to facilitate maintenance and replacement, and reduce the overall mold cost.
III. Emphasis on Failure Characteristics of Aluminum Alloy vs. Magnesium Alloy Molds
A Aluminum alloy
(1) The temperature of the aluminum liquid is relatively high (~660°C), which puts a greater heat load on the mold, and thermal fatigue is the primary problem.
(2) Aluminum has a strong affinity for iron, and the problems of sticking to the mold/chemical corrosion are very prominent, which is the second most important cause of failure after thermal fatigue.
(3) Erosion problems are also quite significant.
(4) Countermeasures focus on: selecting excellent heat fatigue resistant materials, strengthening cooling (especially the gate area), efficient surface treatment processes (such as anti-stick aluminum coating TiAlN, CrN), strictly controlling mold temperature and aluminum liquid temperature, and optimizing the spraying process.
M Magnesium alloy
(1) The temperature of magnesium liquid is relatively low (~650°C), the heat load is relatively small, but the die casting speed is usually higher (due to good fluidity).
(2) Magnesium has a weak affinity for iron, so the problem of sticking to the mold is relatively mild.
(3) It is easily oxidized, and the abrasive effect of oxides aggravates erosion and wear.
(4) There is a potential risk of corrosion in a humid environment (moisture in the release agent).
(5) Countermeasures focus on: good wear resistance (against erosion), good thermal conductivity (rapid heat dissipation), efficient cooling system, oxidation control (melting protection, reducing air entrapment), ensuring the release agent is dry or using low moisture/no release agent technology, and paying attention to corrosion prevention.
