Glass Filled Polymer Injection Tool Wear Mechanisms
Glass filled polymer injection tool wear stems from fiber micro-ploughing and matrix erosion, requiring hard powder metallurgy steels or duplex PVD coatings.
Thermal Fatigue Micro Cracking
Meaning
Failure mechanism involves the formation of microscopic cracks on a material surface due to repeated cycles of heating and cooling. In the high-speed production of smart device housings, thermal fatigue micro cracking affects the mold surfaces that experience rapid temperature fluctuations. These cracks eventually propagate and lead to visible defects on the surface of the molded part.
Thermal Stress
Rapid cooling of the tool after the injection of hot molten plastic creates a temperature gradient. This thermal fatigue micro cracking results from the constrained expansion and contraction of the tool steel near the cooling lines. Over thousands of cycles, the resulting stresses exceed the yield strength of the material.
Material Performance
Selection of steels with high thermal conductivity and a low coefficient of thermal expansion helps to mitigate the stress. While thermal fatigue micro cracking is inevitable in long-run tools, proper management of the cooling water temperature can slow its progression. Surface treatments that increase ductility can also prevent the initial formation of these cracks.
Failure Detection
Regular dye penetrant testing or high-magnification visual inspection identifies the presence of these cracks before they cause structural failure. Once the thermal fatigue micro cracking becomes deep enough to trap plastic, the tool must be removed for resurfacing. This maintenance ensures that the parts produced maintain their required dimensional and aesthetic standards.
A documented history of these inspections allows for a better understanding of the tool’s remaining useful life and informs the procurement of future molds.