Mold and machine components are used while under load. The load can be a static load with constant strength, or a fluctuating load that varies. Generally, cases involving fluctuating loads are more common. Furthermore, it is known that due to the accumulation of repeated loads, a phenomenon called fatigue occurs in machine parts, causing them to fracture at a stress level far lower than the fracture stress caused by a static load.
This type of failure caused by the accumulation of repeated loads is called fatigue fracture. Generally, there is statistical data showing that the majority of mold parts and machine parts fracture within 107107 (10,000,000) repeated stress cycles. The lower limit stress that causes fatigue fracture is called the fatigue limit, which is influenced by the following factors. By paying attention to ensure these factors do not become unfavorable, the fatigue limit can be increased.
In the design of mold parts with short molding cycles and a high number of shots, taking measures to increase the fatigue limit can lead to a reduction in maintenance costs.
Factors Affecting Fatigue Limit
- Tensile strength
- Notch effect
- Size effect
- Surface roughness
- Corrosion
- Stress applied by other parts
- Ambient temperature during machining
- Service environment temperature
- Repeated load speed
- Combined load
- Repeated load
- Fluctuating load
- Load pause
- Fatigue deformation
Common examples of fatigue fracture in plastic injection mold parts include the breakage of core pins, destruction of cavities, buckling failure of ejector pins, and breakage of coil springs.
Mold designers must always keep fatigue fracture in mind, even for small loads, if they are applied repeatedly.