Influence of Mold Steel Heat Treatment Process on Injection Mold Service Life

1. Core Function of Mold Steel Heat Treatment on Injection Mold

Mold steel heat treatment mainly includes quenching, tempering, quenching and tempering, nitriding, vacuum heat treatment and other processes. The core purpose is to refine the internal metallographic structure of steel, uniformly precipitate carbides, and reasonably match hardness and toughness. The injection mold cavity is in a high-temperature plastic environment of 180～300℃ for a long time, and also bears mechanical impact of frequent mold opening and closing, plastic melt erosion and wear, and demolding friction corrosion. Mold steel without formal heat treatment has coarse internal structure and insufficient hardness, which is prone to cavity wear and strain, collapse deformation, edge chipping, cracking and water leakage. Standardized heat treatment can stably improve the surface wear resistance, overall impact toughness, high-temperature tempering stability and corrosion resistance of the mold, fundamentally extend the mass production life of the mold, and reduce mold repair and replacement costs.

2. Influence of Quenching Process on Injection Mold Service Life

Quenching is the most critical process of heat treatment. Process parameters include quenching temperature, holding time and cooling rate. Any parameter deviation will directly damage the mold life. Excessively high quenching temperature leads to coarse steel grain, high mold hardness but greatly reduced toughness, prone to cavity cracking, edge chipping and insert fracture in use; too low quenching temperature leads to insufficient dissolution of carbides, failure to reach design hardness, rapid mold wear, easy pitting and pulling after polishing.

3. Key Influence of Tempering Process on Mold Service Life

Quenching must be matched with multiple tempering to eliminate residual internal stress generated by quenching, stabilize metallographic structure, and balance hardness and toughness. Many molds have short service life and later deformation and cracking, the root cause is inadequate tempering, few tempering times and mismatched tempering temperature. Excessively high tempering temperature reduces mold hardness, easy cavity wear, shrinkage collapse and dimensional deviation; too low tempering temperature cannot release internal stress. After cold and hot cycle of mass production, fine cracks appear and gradually expand, eventually causing mold insert cracking and scrapping.

Conventional plastic mold steel needs two to three segmented tempering to fully release internal stress, avoid deformation during finishing, electrical discharge machining and polishing, and also prevent fatigue cracks caused by cold and hot cycle in injection molding production. Molds with inadequate tempering work normally in the early stage, and will have cracking of thimble holes, deformation of parting surface and distortion of cavity after tens of thousands of mold productions, which directly shortens the service life of the mold.

4. Improvement and Control of Surface Heat Treatment such as Nitriding on Mold Life

For corrosive plastics such as ABS, PC, glass fiber reinforced and PVC, nitriding surface treatment is often carried out on the mold cavity. Qualified nitriding layer has high hardness, low friction coefficient, acid and alkali corrosion resistance and melt erosion resistance, which can significantly improve demolding smoothness, reduce flash and pulling, and extend the mirror polishing maintenance cycle of the mold. However, improper control of nitriding process will also damage the mold life in reverse: too thin nitriding layer leads to poor wear and corrosion resistance; too thick nitriding layer has high brittleness and peeling and chipping under slight impact; stress relief is not complete before nitriding, and tool marks and cracks remain on the surface, cracks will extend inward after nitriding, causing local cavity falling off. Reasonably control nitriding depth, hardness gradient, nitriding temperature and time to maximize the service life of the mold without sacrificing toughness.

5. Common Failure Modes of Mold Caused by Heat Treatment Process Defects

Unqualified heat treatment process will lead to several typical premature aging problems of injection molds: insufficient hardness causes cavity wear and polishing failure, products become rougher and rougher; insufficient toughness causes edge chipping, insert fracture and mold insert cracking; residual internal stress leads to mold finishing deformation, warping displacement in mass production, and permanent flash caused by loose mold closing of parting surface; surface decarburization and oxidation cause easy rusting of molds, poor corrosion resistance, and pitting corrosion pulling products after corrosion. For glass fiber reinforced plastics and corrosive plastic molds, if heat treatment and nitriding are not in place, the wear and corrosion speed will increase exponentially, and the frequency of mold maintenance will rise greatly.

Mold steel heat treatment process is the decisive factor affecting the service life of injection molds, far exceeding the influence of mold steel grade itself. Quenching determines the basic hardness and metallographic structure of the mold; tempering eliminates internal stress and balances toughness and hardness; vacuum heat treatment avoids surface decarburization and oxidation; nitriding surface treatment strengthens wear and corrosion resistance. Only strictly control quenching temperature, holding and cooling rhythm, standard multiple tempering, accurately control nitriding parameters, and eliminate process defects such as coarse grain, residual internal stress, uneven hardness and surface decarburization. It can make the mold have comprehensive properties of high wear resistance, high toughness, crack resistance, corrosion resistance and cold and hot fatigue resistance, greatly reduce mold failure problems such as cavity wear, cracking, deformation and pulling, and effectively extend the mass production life of injection molds, reducing the comprehensive cost of enterprise mold repair and new mold development.