Forming method - Choose from two basic material types.
A) Hot work tool steel, which can withstand relatively high temperatures of die casting, forging and extrusion.
B) Cold working tool steel, which is used for blanking and shearing, cold forming, cold extrusion, cold forging and powder press forming.
Plastics - Some plastics produce corrosive by-products, such as PVC plastic. Corrosion can also occur due to condensation, corrosive gases, acids, cooling/heating, water or storage conditions caused by prolonged downtime. In these cases, stainless steel mold steels are recommended.
Mold size - Large size molds often use pre-hardened steel. Solidly hardened steel is often used for small size molds.

 

Mold usage times - Molds that are used for a long time (>1,000,000 times) should use high-hardness steel with a hardness of 48-65HRC. Molds that are used for medium and long periods of time (100,000 to 1,000,000 times) should use pre-hardened steel with a hardness of 30-45HRC. Molds used for short periods of time (<100,000 times) should use mild steel with a hardness of 160-250HB.
Surface Roughness–Many plastic mold manufacturers are interested in good surface roughness. When sulfur is added to improve metal cutting properties, surface quality is degraded. Steel with high sulfur content also becomes more brittle.

Chemical composition of steel is important. The higher alloy content of steel, the more difficult it is to machine. When carbon content increases, metal cutting performance decreases.
Structure of steel is also very important to metal cutting performance. Different constructions include: forged, cast, extruded, rolled and machined. Forgings and castings have surfaces that are very difficult to machine.
Hardness is an important factor affecting metal cutting performance. General rule is that the harder steel, the more difficult it is to work. High-speed steel (HSS) can be used to process materials with a maximum hardness of 330-400HB; high-speed steel + titanium nitride (TiN) coating can process materials with a maximum hardness of 45HRC; for materials with a hardness of 65-70HRC, carbide, ceramics, cermets and cubic boron nitride (CBN) must be used .
Non-metallic inclusions generally have a negative impact on tool life. For example, Al2O3 (aluminum oxide) is a pure ceramic and is highly abrasive.
Last one is residual stress, which can cause metal cutting performance problems. A stress relief process is often recommended after rough machining.

Roughly speaking, distribution of costs looks like this:
Cut 65%
Workpiece material 20%
Heat treatment 5%
Assembly/Adjustment 10%
This also shows very clearly importance of good metal cutting performance and an excellent overall cutting solution for economical production of molds.

Generally it is:
The higher hardness and strength of cast iron, the lower metal cutting properties and the lower life you can expect from inserts and cutting tools. Cast iron used in metal cutting production generally has good metal cutting properties for most types of metal cutting. Metal cutting performance is related to structure, and processing of harder pearlitic cast iron is also more difficult. Flake graphite cast iron and malleable cast iron have excellent cutting properties, while ductile iron has rather poor machining properties.
Main types of wear encountered when machining cast iron are: abrasive, adhesive and diffusion wear. Abrasion is mainly caused by carbides, sand inclusions and hard casting skin. Adhesive wear with built-up edge occurs at low cutting temperatures and cutting speeds. Ferrite portion of cast iron is easiest to weld to insert, but this can be overcome by increasing cutting speed and temperature.

 

Diffusion wear, on the other hand, is temperature-dependent and occurs at high cutting speeds, especially when using high-strength cast iron grades. These grades have a high resistance to deformation, resulting in high temperatures. This wear is related to interaction between cast iron and tool, which makes some cast irons need to be machined at high speeds with ceramic or cubic boron nitride (CBN) tools to obtain good tool life and surface quality.
Typical tool properties generally required for machining cast iron are: high thermal hardness and chemical stability, but they are also related to process, workpiece and cutting conditions; cutting edge toughness, thermal fatigue wear resistance and edge strength are required. Degree of satisfaction when cutting cast iron depends on how the wear of cutting edge develops: rapid dulling means premature breakage of cutting edge due to thermal cracks and chips, breakage of workpiece, poor surface quality, excessive waviness, etc.
Normal flank wear, maintaining a balanced and sharp cutting edge is what you generally strive for.

Cutting process should be divided into at least 3 process types:
Roughing, semi-finishing and finishing, and sometimes even super-finishing (mostly high-speed cutting applications). Residual milling is of course prepared for finishing after semi-finishing process. It is important to strive at each step to leave evenly distributed margins for next step.
If tool path direction and workload rarely change rapidly, tool life is likely to be longer and more predictable. If possible, finishing operations should be performed on dedicated machine tools. This increases geometric accuracy and quality of molds with shorter commissioning and assembly times.

Roughing process: round blade end mills, ball end mills and end mills with large tip arc radius.
Semi-finishing process: round blade milling cutter (round blade milling cutter with a diameter range of 10-25mm), ball end mill.
Finishing process: round blade milling cutter, ball end milling cutter.
Residual milling process: round blade milling cutter, ball end milling cutter, vertical milling cutter.
It is important to optimize cutting process by selecting a specific combination of tool size, geometry and grade, as well as cutting parameters and a suitable milling strategy.
For high productivity tools that can be used, see Mold Making Catalog C-1102:1

One of the most important goals in cutting process is to create evenly distributed machining allowances for each tool in each operation. This means that tools of different diameters (from large to small) must be used, especially in roughing and semi-finishing operations. Main criterion at all times should be to get as close as possible to final shape of mold in each operation.
Providing evenly distributed machining allowances for each tool ensures constant and high productivity and safe cutting processes. When ap/ae (axial cutting depth/radial cutting depth) remains unchanged, cutting speed and feed rate can also be maintained at a constant high level. This results in less mechanical action and workload changes on cutting edge, resulting in less heat and fatigue, thus increasing tool life. If following processes are some semi-finishing processes, especially all finishing processes, unmanned processing or partially unmanned processing can be performed. A constant material allowance is also an essential criterion for high-speed cutting applications.
Another beneficial effect of constant machining stock is low negative impact on machine tool - guide rails, ball screws and spindle bearings.