Answers to some of the most frequently asked questions in the field of mold manufacturing


1) What is the most important and decisive factor in choosing a die steel?

Forming Method - Choose from two basic material types.
A) Hot machined tool steel, which can withstand relatively high temperatures during die casting, forging and extrusion.
B) Cold worked tool steel, which is used for underfeeding and shearing, cold forming, cold extrusion, cold forging and powder press forming.

Plastics - Some plastics produce corrosive by-products, such as PVC plastic. Condensation, corrosive gases, acids, cooling/heating, water or storage conditions caused by prolonged shutdowns can also produce corrosion. In these cases, it is recommended to use die steel of stainless steel material.
Die Size - Pre-hardened steel is often used for large size dies. Overall hardened steel is often used in small size molds.
Number of Die Uses - Long term (> 1 000 000 times) dies should use high hardness steel with a hardness of 48-65 HRC. Medium term (100 000 to 1 000 000 times) dies should use pre-hard steel with a hardness of 30-45 HRC. Short term (Surface Roughness - Many plastic molders are interested in good surface roughness. When sulfur is added to improve metal cutting performance, surface quality decreases as a result. Steels with high sulfur content also become more brittle.

2) What are the primary factors that affect the cut ability of the material?

The chemical composition of steel is important. The higher the alloy composition of steel, the more difficult it is to process. As the carbon content increases, the metal's cutting performance decreases.
The structure of the steel is also very important for metal cutting performance. The different structures include: forged, cast, pressed, rolled and machined. Forgings and castings have very difficult surfaces to machine.

Hardness is an important factor that affects metal cutting performance. The general rule is that the harder the steel, the more difficult it is to work. High Speed Steel (HSS) can be used for hardnesses up to 330-400 HB; HSS + titanium nitride (TiN) coated for hardnesses up to 45 HRC; and for hardnesses 65-70 HRC, carbide, ceramic, metallic ceramic and cubic boron nitride (CBN) must be used.
Non-metallic involvement generally has an adverse effect on tool life. For example, Al2O3 (aluminum oxide), which is pure ceramic, has a strong abrasive property.
The last one is residual stress, which can cause problems with metal cutting performance. It is often recommended that the stress release process be carried out after rough machining.

3) What are the components of the production cost of mold manufacturing?

Roughly speaking, the distribution of costs is as follows.
Cutting 65%
Workpiece material 20%
Heat treatment 5%
Assembly/adjustment 10%
It is also a very clear indication of the importance of good metal cutting performance and an excellent overall cutting solution for the economic production of molds.

4) What are the cutting characteristics of cast iron?

In general, it is.
The higher the hardness and strength of cast iron, the lower the metal's cutting performance and the lower the life expectancy from the insert and tool. Cast iron used in metal cutting production has generally good metal cutting performance for most types. The cutting performance of the metal is related to its structure, and the harder pearlite cast iron is more difficult to process. Flake graphite cast iron and malleable cast iron have excellent cutting properties, while ductile iron is quite bad.

The main types of wear encountered when working with cast iron are: abrasion, bonding and diffuse wear. Abrasion is mainly caused by carbide, sand inclusions and hard cast epidermis. Adhesive abrasion with phyllodesis occurs at low cutting temperature and cutting speed conditions. The ferrite part of cast iron is easiest to weld to the blade, but this can be overcome by increasing cutting speed and temperature.
On the other hand, diffuse wear is temperature-dependent and occurs at high cutting speeds, especially when using high strength cast iron grades. These grades have a high resistance to mold change, resulting in high temperatures. This wear is related to the interaction between the cast iron and the tool, which causes some cast iron to be machined with ceramic or cubic boron nitride (CBN) tools at high speeds to achieve good tool life and surface quality.

The typical tool properties required for machining cast iron are: high thermal hardness and chemical stability, but also related to the process, workpiece and cutting conditions; toughness of the cutting edge, resistance to heat and fatigue wear and edge strength are required. The degree of satisfaction in cutting cast iron depends on how the wear of the cutting edge develops: rapid dulling means thermal cracks and notches that cause the cutting edge to break prematurely, workpiece breakage, poor surface quality, excessive rippling, etc. Normal rear blade wear, balance and a sharp cutting edge are exactly what you generally have to work towards.

5) What are the major, common processing processes in mold making?

The cutting process should be divided into at least three 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 a semi-finishing process. It is important to strive to leave an evenly distributed margin in each process for the next. If there are few rapid changes in the direction of the tool path and work load, the tool life may be longer and more predictable. If possible, the finishing process should be performed on a dedicated tool machine. This will improve the geometric accuracy and quality of the mold in a shorter commissioning and assembly time.

6) What are the main tools to be used in these different processes?

Roughing process: Round insert end mills, ball end mills and end mills with large tip radius.
Semi-finishing processes: Round insert mills (round insert mills with diameters from 10 to 25 mm), ball end mills.
Finishing processes: round insert end mills, ball end mills.
Residual quantity milling processes: round insert mills, ball end mills, upright mills.
It is important to optimize the cutting process by selecting a specialized combination of tool size, groove and grade, as well as cutting parameters and an appropriate milling strategy.

7) Is there a single most important factor in the cutting process?

One of the most important goals in the cutting process is to create an evenly distributed machining margin for each type of tool in each process. This means that tools of different diameters (from large to small) must be used, especially in roughing and semi-finishing processes. The main criterion at all times should be to be as close as possible to the final shape of the mould in each process.

A uniformly distributed machining margin for each tool ensures a constant high productivity and safe cutting process. When the AP/AE (axial depth of cut/radial depth of cut) is constant, the cutting speed and feed rate can also be kept constant at a higher level. As a result, there is less mechanical action and less variation in the working load on the cutting edge, and therefore less heat and fatigue are generated, thus increasing tool life. If the latter processes are some semi-finishing processes, especially all finishing processes, then either unmanned or partially unmanned processes can be performed. A constant amount of material remaining for machining is also the basic standard for high-speed cutting applications.

Another favorable effect of a constant machining margin is a small adverse effect on the tooling machine - the guide rails, ball screws and spindle bearings.

8) Why are round insert mills most often used as the first choice for tooling roughing?

If rough milling of the cavity is done with a square shoulder milling tool, a large amount of table-like cutting residue is removed in the semi-finishing process. This will cause the cutting force to change and cause the tool to bend. The result is to leave an uneven processing margin for finishing, which affects the geometric accuracy of the mold. If a square shoulder miter (with a triangular insert) with a weak tip strength is used, it will have an unpredictable cutting effect. Triangular or diamond-shaped inserts also produce greater radial cutting forces, and they are less economical roughing tools because of the smaller number of cutting edges in the inserts.

On the other hand, the round insert can be milled in a variety of materials and in all directions, and if used, the transition between adjacent toolpaths is smoother and can also leave a smaller and more uniform machining margin for semi-finishing. One of the characteristics of round blades is that they produce variable chip thickness. This allows them to use a higher feed rate than most other blades. The main deflection of the round insert changes from almost zero (very shallow cutting) to 90 degrees, and the cutting action is very smooth. At the maximum depth of the cut, the main deflection angle is 45 degrees, and when cutting along a straight wall imitation with an outer circle, the main deflection angle is 90 degrees. This also explains why the strength of the round insert tool is high - the cutting load is gradually increasing. Roughing and semi-roughing should always be done with round insert end mills, and by using good programming, round insert end mills can largely replace ball end mills. Round inserts with low runout combined with finely ground, positive front corners and light-cutting grooves can also be used for semi-finishing and some finishing processes.

9) What is effective cutting speed (VE) and why is it important for high productivity?

In cutting, the basic calculation of the effective cutting speed at the actual or effective diameter is always very important. Since the table feed depends on the rotational speed at a certain cutting speed, if the effective speed is not calculated, the table feed will be incorrectly calculated.
If the nominal diameter value (Dc) of the tool is used in the calculation of the cutting speed, the effective or actual cutting speed is much lower when the depth of cut is shallow. Tools such as the round insert (especially in the small diameter range), ball head end mills, large-tip arc radius end mills, and end mills. As a result, the calculated feed rate is also much lower, which severely reduces the production rate. More importantly, the cutting conditions of the tool are below its capabilities and recommended range of applications.
When cutting in 3D, the diameter of the cut changes, which is related to the geometry of the die. One solution to this problem is to define the steep wall area of the mold and the geometrically shallow part area. Good compromises and results can be achieved if specialized CAM procedures and cutting parameters are prepared for each area.