Mold Manufacturing: Frequently Asked Questions

Forming Method – Two basic material types can be chosen:

A) Hot-working tool steel, which can withstand the relatively high temperatures of die casting, forging, and extrusion.

B) Cold-working tool steel, used for blanking and shearing, cold forming, cold extrusion, cold forging, and powder molding.

Plastics – Some plastics produce corrosive byproducts, such as PVC.

Corrosion can also occur due to factors such as condensation from prolonged downtime, corrosive gases, acids, cooling/heating, water, or storage conditions.

In these cases, stainless steel mold steel is recommended.

Mold Size – Large molds often use pre-hardened steel. Solid-hardened steel is often used for small molds.

Mold Usage Cycles – Molds intended for long-term use (>1,000,000 cycles) should use high-hardness steel with a hardness of 48-65 HRC. Molds intended for moderate to long-term use (100,000 to 1,000,000 cycles) should be made of pre-hardened steel with a hardness of 30-45 HRC.

Short-term use (Surface roughness – Many plastic mold manufacturers are interested in good surface roughness. While adding sulfur to improve metal machinability, surface quality decreases. Steels with high sulfur content also become more brittle.)

The chemical composition of the steel is important.

The higher the alloy content of the steel, the more difficult it is to machine.

As carbon content increases, metal machinability decreases.

The structure of the steel is also very important to metal machinability.

Different structures include: forged, cast, extruded, rolled, and machined. Forgings and castings have very difficult-to-machine surfaces.

Hardness is an important factor affecting metal machinability. The general rule is that the harder the steel, the more difficult it is to machine. High-speed steel (HSS) can be used to machine materials with a hardness of up to 330-400 HB; High-speed steel with a titanium nitride (TiN) coating can machine materials with a hardness of up to 45 HRC; and for materials with a hardness of 65-70 HRC… For example, cemented carbide, ceramics, cermets, and cubic boron nitride (CBN) must be used.

Non-metallic doping generally has a negative impact on tool life. For instance, Al2O3 (alumina), a pure ceramic, has strong abrasive properties.

Finally, residual stress can cause problems with metal cutting performance. Stress relief is often recommended after roughing.

Roughly speaking, the cost distribution is as follows:

Machining 65%

Workpiece material 20%

Heat treatment 5%

Assembly/adjustment 10%

This clearly demonstrates the importance of good metal cutting performance and excellent overall cutting solutions for the economical production of molds.

Generally speaking, it is: The higher the hardness and strength of cast iron, the lower its metal cutting performance, and the lower the expected life of the inserts and tools.

Cast iron used for metal cutting production generally has good metal cutting performance for most types of applications. The machinability of metals is related to their structure; harder pearlitic cast iron is more difficult to machine.

Flake graphite cast iron and malleable cast iron have excellent machinability, while ductile cast iron is quite poor.

The main types of wear encountered when machining cast iron are abrasion, adhesion, and diffusion wear. Abrasion is mainly caused by carbides, sand inclusions, and a hard casting skin.

Adhesion wear with built-up edges occurs under low cutting temperatures and speeds. The ferrite portion of cast iron is most easily welded to the cutting tool, but this can be overcome by increasing cutting speed and temperature.

On the other hand, diffusion wear is temperature-dependent and occurs at high cutting speeds, especially when using high-strength cast iron grades.

These grades have high resistance to deformation, leading to high temperatures. This wear is related to the interaction between the cast iron and the cutting tool, which necessitates machining some cast irons at high speeds with ceramic or cubic boron nitride (CBN) tools to achieve good tool life and surface finish.

Typical tool properties generally required for machining cast iron are: High thermal hardness and chemical stability are desirable, but these are also dependent on the process, workpiece, and cutting conditions; the cutting edge must possess toughness, resistance to thermal fatigue wear, and edge strength. Satisfaction with cutting cast iron depends on how the cutting edge wear develops: rapid dulling means the formation of thermal cracks and notches leading to premature cutting edge breakage, workpiece damage, poor surface quality, and excessive waviness. Normal flank wear, maintaining a balanced and sharp cutting edge are generally what needs to be achieved.

The cutting process should be divided into at least three types of processes: roughing, semi-finishing, and finishing, and sometimes even super-finishing (mostly for high-speed cutting applications). Residual milling is, of course, done after semi-finishing to prepare for finishing. It is crucial to strive to leave an evenly distributed allowance for the next process in each process. If the toolpath direction and workload rarely change rapidly, tool life can be extended and become more predictable. If possible, finishing processes should be performed on special-purpose machine tools. This improves the geometric accuracy and quality of the mold in a shorter debugging and assembly time.

Roughing: Circular insert end mills, ball end mills, and end mills with large tip radius.

Semi-finishing: Circular insert end mills (circular insert end mills with diameters ranging from 10-25 mm), ball end mills.

Finishing: Circular insert end mills, ball end mills.

Residual material milling: Circular insert end mills, ball end mills, straight end mills.

Optimizing the cutting process by selecting specific tool sizes, flute shapes, and grade combinations, as well as cutting parameters and appropriate milling strategies, is crucial.

One of the most important goals in the cutting process is to create a uniformly distributed machining allowance for each tool in each process.

This means that tools of different diameters (from largest to smallest) must be used, especially in roughing and semi-finishing processes. The primary standard at all times should be to approximate the final shape of the mold as closely as possible in each operation.

Providing a uniformly distributed machining allowance for each tool ensures consistently high productivity and a safe cutting process.

When the ap/ae (axial depth of cut/radial depth of cut) remains constant, the cutting speed and feed rate can also be kept consistently at a high level.

This results in minimal variation in mechanical action and workload on the cutting edge, thus generating less heat and fatigue, thereby increasing tool life.

If subsequent operations are semi-finishing operations, especially all finishing operations, then unmanned or partially unmanned machining can be performed.

Constant machining allowance is also a fundamental standard for high-speed cutting applications.

Another beneficial effect of constant machining allowance is minimal adverse effects on machine tool components—guideways, ball screws, and spindle bearings.

If a square shoulder end mill is used for rough milling of a cavity, a large amount of stepped cutting allowance must be removed in the semi-finishing stage.

This will cause changes in cutting forces, leading to tool bending. The result is uneven machining allowances in finishing, affecting the geometric accuracy of the mold.

Using square-shoulder end mills (with triangular inserts) with weak tip strength produces unpredictable cutting effects.

Triangular or diamond-shaped inserts also generate greater radial cutting forces and are less economical roughing tools due to the fewer cutting edges.

On the other hand, round inserts can mill in various materials and in various directions. Using them results in smoother transitions between adjacent toolpaths and leaves smaller and more uniform machining allowances for semi-finishing.

One characteristic of round inserts is that they produce variable chip thickness. This allows them to use higher feed rates than most other inserts.

The principal cutting edge angle of round inserts varies from almost zero (very shallow cuts) to 90 degrees, resulting in a very smooth cutting action. At the maximum depth of cut, the principal cutting edge angle is 45 degrees, and when contour cutting along a straight wall with an outer circle, it is 90 degrees.

This also explains why round insert tools have high strength—the cutting load increases gradually. For roughing and semi-roughing, round insert end mills should always be used. With proper programming, round insert end mills can largely replace ball end mills.

Round inserts with low runout, combined with finely ground inserts, positive rake angles, and light-cutting fluting profiles, can also be used for semi-finishing and some finishing operations.

In cutting, the basic calculation of the effective cutting speed over the actual or effective diameter is always crucial.

Since the table feed depends on the spindle speed at a given 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 when calculating the cutting speed, the effective or actual cutting speed will be much lower than the calculated speed when the depth of cut is shallow.

This applies to tools such as round inserts (especially in the small diameter range), ball end mills, and end mills with large tip radius. Consequently, the calculated feed rate is also much lower, which severely reduces productivity. More importantly, the cutting conditions of the tool are below its capabilities and recommended application range.

When performing 3D cutting, the cutting diameter varies and is related to the geometry of the mold.

One solution to this problem is to define the steep-walled regions of the mold and the shallow-geometry regions of the part. A good compromise and result can be achieved by developing specialized CAM programs and cutting parameters for each region.