Reasons and Solutions for Hydraulic Press Mold Failure

This article mainly introduces the reasons for the failure of hydraulic press molds and solutions.

1. Mold material

Mold steel belongs to alloy steel. There are defects such as non-metallic inclusions, carbide segregation, central pores and white spots in its structure, which greatly reduce the strength, toughness and thermal fatigue resistance of the mold. Generally, it is divided into ordinary and high-quality molds according to quality. Due to advanced production technology, high-quality molds are pure in quality, uniform in structure, small in segregation, and have high toughness and thermal fatigue performance.

Solution: Forging ordinary molds to break large non-metallic inclusions, eliminate carbide segregation, refine carbides, and make the structure uniform to achieve the effect of high-quality molds.

2. Molds design

When designing the mold, the outer dimensions of the module should be determined according to the material and geometric dimensions of the formed part to ensure the strength of the mold. In addition, during the heat treatment and use of the mold, due to the small radius of the fillet, the wide thin-wall section, the large wall thickness difference, and the inappropriate position of the hole and slot, it is easy to cause excessive stress concentration and crack initiation. The mold design should avoid Sharp corners as much as possible, and the hole and slot positions should be arranged reasonably.

3. Manufacturing process

1) Forging process

The mold contains many alloy elements, has a large deformation resistance during forging, poor thermal conductivity, and a low eutectic temperature. If you don’t pay attention, it will cause mold failure. It should be preheated at 800-900℃ and then heated to 1065-1175℃. To remove large non-metallic inclusions, eliminate carbide segregation, and refine carbides, upsetting and drawing should be repeated during the forging process with uniform organization. During the cooling process after forging, quenching cracks tend to be produced. It is easy to produce transverse cracks in the center. Slow cooling after forging can avoid this problem.

2) Cutting

The surface roughness of the cutting process greatly influences the mold’s thermal fatigue performance. The surface roughness of the mold cavity is low, and there are no defects such as knife marks, scratches, and burrs, which will cause stress concentration and cause thermal fatigue cracks to initiate.
Solution: When processing the mold, prevent knife marks from being left on the radius of the corners of complex parts. And grind off the burrs on the holes, groove edges and roots.

3) Grinding

During the grinding process, local friction heat can easily cause defects such as burns and cracks and produce residual tensile stress on the grinding surface, leading to premature failure of the mold. The burns caused by grinding heat can temper the mold surface until tempered martensite is formed. The brittle and untempered martensite layer will greatly reduce the thermal fatigue performance of the mold. When the local temperature rise of the grinding surface exceeds 800℃, and the cooling is insufficient, the surface material will be re-austenitized and quenched into martensite. The mold surface will produce higher structural stress. The rise in temperature of the mold surface will produce thermal stress during the grinding process, and the superposition of structural and thermal stress can easily cause grinding cracks in the mold.

4) Electrospark machining

Electrospark machining is an indispensable finishing method in the modern mold manufacturing process. When the spark discharge occurs, the local instantaneous temperature exceeds 1000℃, so the metal at the discharge point melts and vaporizes. There is a thin layer of melted and resolidified metal on the surface of the electrospark machining. There are many microcracks in it. This thin layer of metal is bright white. Under the load of the mold, these micro-cracks are easy to develop into macro cracks, resulting in early fracture and wear of the mold.
Solution: After EDM processes, the mold is tempered to eliminate internal stress. However, the tempering temperature must not exceed the maximum tempering temperature before EDM.

5) Heat treatment process

A reasonable heat treatment process can enable the mold to obtain the required mechanical properties and improve its service life. If the heat treatment process design or operation is improper and causes the mold to fail, it will seriously damage the bearing capacity of the mold, resulting in early failure and shortening the service life. Heat treatment defects include overheating, overburning, decarburization, cracking, uneven hardening layer, insufficient hardness, etc. After a period of use, when the accumulated internal stress reaches the dangerous limit, stress relief and tempering should be performed. Otherwise, the mold will crack due to internal stress when it continues to be used.

4. Use of molds

1) Preheating of molds

The mold has a high alloy element content and poor thermal conductivity. It should be fully preheated before work. If the mold temperature is too high during use, the strength will decrease, and plastic deformation will easily occur, resulting in mold surface collapse. When the preheating temperature is too low, the instantaneous surface temperature changes greatly when the mold starts to be used, the thermal stress is large, and it is easy to crack.
Solution: The mold’s preheating temperature is determined to be 250-300℃. This can not only reduce the die forging temperature difference and avoid excessive thermal stress on the mold surface but also effectively reduce the plastic deformation on the mold surface.

2) Mold cooling and lubrication

To reduce the mold’s heat load and avoid high temperatures, the mold is usually forced to cool during the mold interval. Periodic heating and cooling of the mold will cause thermal fatigue cracks. The mold should be cooled slowly after use; otherwise, thermal stress will occur, resulting in mold cracking and failure.
Solution: When the mold is working, water-based graphite with a 12% graphite content can be used for lubrication to reduce the forming force, ensure the normal flow of metal in the cavity and smooth the release of the forging. Graphite lubricant also has a heat dissipation effect, which can reduce the mold’s operating temperature.

The above are all the reasons and solutions for hydraulic press mold failure. Zhengxi is a manufacturer specializing in hydraulic press equipment. If you need anything, please get in touch with us.