Author: Shenye Plastic TechnologyDate：2021-10-12Pageviews：365
In the precision-driven world of injection mold engineering, the stability of the steel is just as important as the accuracy of the CNC machine. Before a mold is quenched or put into production, it often undergoes annealing—a heat treatment process used to soften the metal, relieve internal stresses, and refine the grain structure.
However, "annealing" is not a single process. Depending on the steel grade and the desired outcome, different methods are employed. Here is a breakdown of the primary annealing treatments used in professional mold making.
During heavy machining, grinding, or welding, mold steel accumulates significant internal "residual stress." If these stresses aren't removed, the mold will likely warp or crack during the subsequent quenching phase.
The Process: The steel is heated to a temperature below its critical point (usually between 550°C and 650°C), held for several hours, and then cooled very slowly in the furnace.
Why it's used: It ensures dimensional stability. At Sunye Plastic, we prioritize stress relief annealing after rough machining to ensure that the final tool maintains its tight tolerances over millions of cycles.
Full annealing is primarily used for hyper-eutectoid steels or steel that has become too hard to machine comfortably.
The Process: The steel is heated to about 30°C to 50°C above its upper critical temperature (Ac3). Once fully austenitized, it is cooled at a controlled, extremely slow rate.
Why it's used: This process results in a coarse pearlitic structure, which provides the maximum softness and ductility, making the steel easy to drill, mill, and turn.
This method is specifically designed for high-carbon tool steels (like SKD11 or certain stainless mold steels).
The Process: The steel is heated to a temperature just around or slightly below the lower critical point (Ac1) and held for an extended period to cause the carbides to form small, spherical shapes (spheroids).
Why it's used: Spheroidized steel has superior machinability compared to pearlitic steel. It also provides a better foundation for the final hardening process, reducing the risk of quenching cracks.
When time is a factor but quality cannot be sacrificed, isothermal annealing is the preferred choice for many high-alloy mold steels.
The Process: The steel is heated to an austenitizing temperature and then quickly transferred to another furnace (or cooled rapidly) to a specific intermediate temperature. It is held at this constant temperature until the transformation is complete, followed by air cooling.
Why it's used: It produces a more uniform grain structure than traditional full annealing and significantly reduces the total processing time.
This is a "deep" annealing process typically performed at the ingot or large block stage.
The Process: The steel is heated to very high temperatures (1100°C to 1200°C) for a long duration (up to 15 hours).
Why it's used: It is designed to eliminate chemical segregation and ensure that the alloying elements are distributed evenly throughout the entire steel block. This is vital for large, complex molds where material consistency is paramount.
Annealing is not just a technical step; it is a strategic insurance policy for your tooling investment. At Sunye Plastic, our engineering team evaluates each project's steel grade—from P20 and 718 to high-end S136—to determine the optimal annealing sequence.
By correctly matching the annealing method to the mold's geometry and material, we ensure that every mold we produce is built on a foundation of structural integrity and long-term stability.
Whether it's the precision of stress relief or the chemical uniformity of diffusion annealing, mastering these methods is what separates high-quality mold manufacturers from the rest. Understanding these treatments allows OEMs to make informed decisions that result in lower maintenance costs and higher production efficiency.
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