Key Factors Influencing Fatigue Life in Mold Components

Mold parts often come in complex and unusual shapes—featuring corners, notches, drilled holes, threads, engravings, and various other design characteristics. Additionally, their surfaces are affected by machining and grinding marks, heat treatment stresses, scratches, and similar imperfections. All of these factors can significantly influence the fatigue life of the component.

Material Selection

Harder materials generally exhibit lower fatigue life relative to their yield strength when compared to softer materials. However, a hard surface is often necessary to meet other mold requirements, such as wear resistance, abrasion resistance, and prevention of indentation. To keep stress levels below the fatigue strength, designers often need to increase the cross-sectional area of the steel used.

Finishing Processes

Finishing operations such as EDM (Electrical Discharge Machining) and plating can negatively affect both the surface quality and the fatigue life of mold parts. Since these processes are often unavoidable, designers should aim to reduce stress levels in the part—commonly by increasing its thickness.

Product Shape

In most cases, designers cannot alter the product shape, as it is typically specified by the customer. However, if certain features—such as sharp corners or a rough surface finish—are likely to seriously impact mold life, designers may propose modifications to the product design. Customers often understand this concern and are willing to accept suggestions that enhance mold durability.

Stress Analysis in Design

It is crucial for designers to understand the stress state of each mold component—including its magnitude, direction, and loading frequency—in order to predict how loads will affect the part. Many mold components, such as mold plates, do not require special consideration. They are usually large enough to experience only mild stresses, are often not hardened, and therefore offer good fatigue performance.

However, if a plate includes cross-drilled holes or cavities, its fatigue life must be checked to prevent failure. All parts subjected to cyclic loading—especially those that are hardened, such as cavities, cores, inserts, hot runner components, ejector rings, and similar items—should be carefully evaluated for fatigue resistance.

Stress Concentration

All mold components are susceptible to stress concentrations or “stress raisers.” Most fractures in mold parts are related to such concentrated stresses. Designers should be able to identify where stress concentrations may occur and ensure they are either avoided or minimized in severity.

Even when a designer selects a stress level considered safe (below the fatigue limit), the part can still be seriously affected by stress concentrations. It is important to remember that fatigue data is usually derived from tests on smooth, polished specimens and may not fully represent real mold parts—which always include corners, notches, holes, and other features. Therefore, extra attention is required when designing for longer mold life.