Molds and dies are tools for shaping materials and they are very important elements in product manufacturing. Molds are usually used in injection molding to shape melted resin or casting molten metal. Dies are mainly used to cut and/or shape metal to to a desired shape or profile.
Materials for dies and molds include tool steels with carbon or chromium elements, as well as die steel, high-speed steel, and cemented carbide. Recently, ceramics have also been adopted as a tool material. Materials used for dies and molds are mostly hard and difficult to cut.
Steels selection for molds and dies
#1 Consider working conditions
When the blank is plastically deformed in the mold cavity, it both flows and slides along the surface of the cavity, causing severe friction between the surface of the cavity and the blank, resulting in the failure of the mold due to wear. Therefore, the wear resistance of the material is one of the most basic and important properties of the mold.
Hardness is the main factor affecting wear resistance. In general, the higher the hardness of the mold parts, the smaller the amount of wear and the better the wear resistance. In addition, wear resistance is also related to the type, quantity, shape, size and distribution of carbides in the material.
Most of the working conditions of the mold are very bad, and some often bear a large impact load, which leads to brittle fractures. In order to prevent sudden brittle fracture of mold parts during operation, the mold must have high strength and toughness.
The toughness of the mold mainly depends on the carbon content, grain size and organizational state of the material.
Fatigue fracture resistance
During the working process of the mold, fatigue fracture is often caused under the long-term action of cyclic stress. Its forms include small-energy, multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture and bending fatigue fracture.
The fatigue fracture performance of the mold mainly depends on its strength, toughness, hardness, and the content of inclusions in the material.
High temperature performance
When the working temperature of the mold is high, the hardness and strength will decrease, resulting in early wear of the mold or plastic deformation and failure. Because the mold material should have high anti-tempering stability, to ensure that the mold has high hardness and strength at working temperature.
Thermal fatigue resistance
Some molds are in a state of repeated heating and cooling during the working process, which causes the surface of the cavity to be subjected to tension, pressure and stress, causing surface cracking and peeling, increasing friction, hindering plastic deformation, and reducing dimensional accuracy, resulting in Mold failure. Hot and cold fatigue is one of the main forms of failure of hot work dies, and these dies should have high resistance to cold and heat fatigue.
When some molds such as plastic molds are working, due to the presence of chlorine, fluorine and other elements in the plastic, after heating, strong corrosive gases such as hcl and hf are decomposed, which erode the surface of the mold cavity, increase its surface roughness, and aggravate wear and failure.
#2 Consider process and performance requirements
The manufacture of molds generally has to go through several processes such as forging, cutting, and heat treatment. In order to ensure the manufacturing quality of the tools and reduce the production cost, the material should have good forge ability, machinability, harden ability and grind ability; it should also have small oxidation, decarburization sensitivity and quenching. Deformation and cracking tendency.
It has low hot forging deformation resistance, good plasticity, wide forging temperature range, low tendency for forging cracking and cold cracking and precipitation of network carbides.
Good for annealing
That means the spheroidizing annealing temperature range should be wide enough; the annealing hardness is low and the fluctuation range is small enough, and the spheroidization rate is high enough.
Easy to cut
The cutting amount is large, the tool loss is low, and the machined surface roughness is low.
Oxidation and decarburization sensitivity
When heated at high temperature, it has good anti-oxidative energy, slow decarburization speed, is insensitive to heating medium, and small tendency to produce pitting.
High hardness after quenching
After quenching, it has a uniform and high surface hardness.
High harden ability
After quenching, a deep hardened layer can be obtained, which can be hardened by using a mild quenching medium.
No cracking during quenching
The volume change of conventional quenching is small. The shape is warped, the distortion is slight, and the abnormal deformation tendency is low. Conventional quenching has low cracking sensitivity and is not sensitive to quenching temperature and workpiece shape.
The relative loss of the grinding wheel is small, the limit grinding amount without burn is large, it is not sensitive to the quality of the grinding wheel and the cooling conditions, and it is not easy to cause abrasion and grinding cracks.
General properties, processing, and service characteristics of tool steels
|AISI designation||Wear resistance||Toughness||Working hardness (HRC)||Machinability||Amount of distortion||Resistance to cracking|
|Molybdenum high-speed steels|
|Tungsten high-speed steel|
|Intermediate high-speed steel|
|Chromium hot-work steel|
|Tungsten hot-work steel|
|Molybdenum hot-work steel|
|Air-hardening, medium-alloy, cold-work steel|
|Oil-hardening cold-work steels|
|Low-alloy special-purpose steels|
|Low-carbon mold steels|
#3 Consider the cost and availability of the material
When selecting materials for molds, the cost and availability of the material must be considered. If the materials meet the performance requirements, we choose the one with lower price. For example, some of the carbon steel can be used instead of the alloy steel, because they have similar properties, and the base material price of carbon steel is a little lower than alloy steel.
In addition, the production and supply situation of the market should also be considered when selecting tool steel materials. The types of steel selected should be as few as possible and should be easily obtained from the steel market.
To sum up, The materials used to make the molds and dies are required to have properties such as high hardness, high strength, high wear resistance, appropriate toughness, high hardenability, no deformation (or less deformation) during heat treatment, and no cracking during quenching. For molds with different purposes, we should never neglect their working conditions, stress conditions, the properties of the material, and the performance during processing.