August 2011 Archives

Fig. 1 and Fig. 2 show dies with an integrated cast structure. Since the cast structure allows the preparation of a large shape as a single piece, it can be prepared more easily compared to the preparation using steel plates.

When the material to be formed is a thin plate and the production volume is low, and even in the case of blanking, bending, or drawing, the material frequently used for casting is the gray cast iron FC250 or FC300. This is used for all the die parts such as punches, dies, and pads, etc.

When the thickness of the material to be formed becomes higher, or when the production volume increases, or when the strength is of concern, a spherical graphite nodule cast iron such as FCD600 is used.

In the case of cast iron dies for blanking, SC450 or another material is welded to the cutting edge part. In the case of large volume production of the drawing type or shape forming type, SC450, etc. is used after flame hardening. When there is concern about using cast iron (gray cast iron, spherical graphite nodule cast iron) for the trimming type, etc., a material such as SC450 is used as it is.

The die materials used for the main parts of medium-sized and large-sized dies are described here.
Although there are several types of die structures, explanations are given here are based on the widely used movable stripper structure. The plate configuration of the die shown in Fig. 1 shows the largest configuration of a movable stripper structure.

(1) Holder SS400, S50C (S55C), FC250, SKS3, and A7075

The punch and die holder are not only for fixing the die to the press machine but also for supporting the rigidity of the die. They also have the role of adjusting the die height and providing the space for springs, etc.
Usually, the materials used are SS400 or S50C. There is no big difference between the two. Among cast iron types, FC250 is used. This is the material used when cast iron die sets are used. When high rigidity is required for purposes such as high volume production, etc., SKS3 is used after heat treatment (to a hardness of about 56 HRC). When low weight is required, the aluminum alloy A7075 may be used.

(2) Backing plate SK3, SK5, SKS3, and S50C

Backing plates are used at three locations in a die. They are used for the purpose of backing up so as to prevent components such as small diameter punches, etc., from getting too deep inside the holder due to the force of press operation. Apart from that, backing plates are used for preventing the parts from getting detached (stripper backing plate) and for adjusting the height of the die.
For backing up, an SK material is used after heat treatment (to a hardness of about 56 HRC). The SKS material is used when high rigidity is required. Materials such as S50C, which is used without heat treatment, is used for backing up or preventing the detachment of parts such as a large-sized punch with a large pressure receiving area, or for adjusting the height.

(3) Punch plate SS400, S50C (S55C), SKS3, SKD11, pre-hardened steel

A punch plate is used for the purpose of holding a small punch. Usually, SS400, S50C, etc. are used without heat treatment. In the case of dies for high volume production, pre-hardened steel having a certain amount of hardness, or heat treated SKS3 or SKD11 may also be used. The SK material is rarely used. When the machining tolerance of SKS3 is of concern, the material SKD11 may also be used which is a decision based on the wire cutting characteristics.

(4) Stripper plate S50C, pre-hardened steel, SKS3, SKD11

While the main function of a stripper is to remove scrap, very frequently it is made to have important secondary functions of pressing the material or guiding the tip of the punch (punch guide).
When removing scrap in small volume production is important, materials such as S50C or pre-hardened steel are used which do not require heat treatment. When materials without heat treatment are used, although they are made to have the functions of pressing the material and guiding the punch, the plate may get deformed when there is wrong punching. When a punch guide or a material pressing member is added, a material such as SKS3 or SKD11 is used after heat treatment.

(5) Die plate SK materials, SKS3, SKD11

SK materials or SKS3 is used in dies for small volume production. The standard material used is SKD11. This trend has become established because preparation of dies using electric discharge wire cutting machines has become widespread.

(6) Nested plates (see Fig. 2)

The descriptions given above assume single type plates. In dies, nested type (insert type) plates are used very often. Such plates are used for stripper plates or die plates. In the case of small volume production, materials such as S50C without heat treatment are sometimes used for the main plate. For medium or higher volume production, materials such as SKS3 or SKD11 are used after heat treatment.
The materials SKD11, SKH51, pre-hardened steel, and cemented carbide, etc. are used as the materials for nested plates. The selection of the material is based on ease of maintenance, accuracy, and operating life.

(7) Punch

The materials used for punches are SKS3, SKD11, SKH51, powdered high speed tool steel, cemented carbide, etc.
The standard material used is SKD11. SKS3 is sometimes used in the case of small volume production. SKH51 is used when the size is small, or when toughness is required. Powdered high speed tool steel or cemented carbide are used when wear resistance is required or for large volume production.

Cemented carbides used in press dies are alloys of tungsten carbide (WC) and cobalt (Co). The main constituent of the material is tungsten carbide, and cobalt has the role of a binder (adhesive material). The amount of cobalt is in the range of 5 to 25%.

The hardness of cemented carbide decreases as the amount of cobalt increases.
This material has been stipulated into the types of V10, V20, V30, V40, V50, and V60 in the standard 019 of the Japan Cemented Carbide Tool Manufacturer's Association.
V10 has about 5% cobalt, V30 has about 12%, and V60 about 25%.
The hardness is 89 HRA or more in the case of V10, 87 HRA or more in the case of V30, and 78 HRA or more in the case of V60. By the way, 85 HRA is converted to 67 HRC.

Apart from the amount of cobalt, the hardness is also related to the size of WC particles. The hardness increases as the particle size becomes smaller. The particle size of ordinary cemented carbide is about 2.5 to 1.5 μm. Ultra fine particles have diameters in the range of 0.7 to 0.5 μm.
When the material has ultra fine particles, it is possible to enhance the properties of both wear resistance and brittleness.
Cemented carbide is a hard material but is also brittle. The material property is selected considering the balance between hardness and brittleness according to the intended application. V30 and V40 are about the standard for press dies.

In the case of blanking dies and punches, V30 is used for punches and V40 is used for dies. For bending and drawing, a slightly harder V30 and V20 are used giving priority to wear resistance. A softer V50 and V60 are used in compression forming in order to prevent breakage.
Even if the selection of the material is appropriate, if the surface roughness is bad, it may not be possible to satisfy the expected life even in the case of cemented carbide. Reducing the surface roughness by lapping is a very frequently used countermeasure in the case of cutting blades. However, when machining copper, nickel, and pure iron, the wear of cemented carbide may be fast. The reason for this is that the affinity between the cobalt present in cemented carbide and copper or nickel causes the wearing of the cemented carbide to progress faster.
Although cemented carbide appears to be hard and versatile, caution should be exercised because it may not meet the expectations depending on how it is used.

Although the standards of the Japan Cemented Carbide Tool Manufacturer's Association were used for the above explanations, the code used can be different for different manufacturers. Use the standards of JCCMA to compare and slelct the appropriate material.