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HOW TO SELECT THE BEST MANUFACTURING EQUIPMENT TO PRODUCE STEEL RULE DIES (literal translation: “Qualities of Cutting Die Rule Material and Selection Criteria”)

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Written by Robert Larson

BY TOKUO SUGITA

DIRECTOR, TSUKATANI HAMONO MFG. CO., INC.

 

Editorial note: This article has been translated from Japanese from a seminar presented at the 1994 Diecutting Symposium in Tokyo, Japa. The technical paper was originally printed in the Symposium Technical Book. All efforts have been made to translate technical aspects of this article.

 

1. CUTTING DIE RULES

 

a. - STEEL USED IN CUTTING DIES

 

Some of the cutting die rules manufactured in Japan are heat-treated while others are not. There are both heat-treated and untreated scoring rules, but cutting rules are mainly heat-treated. Among heat-treated cutting rules, there are three additional types.

 

(1) All-steel: S45C ~ S65C carbon steel is used and heat-treated to Hs45 ~ Hs70, so both the surface layer and core have the same specified hardness. (Fig. 1)

 

(2) Decarbonized steel: After the carbon is removed from the surface layer of the carbon steel in (1) above, the rule is heat-treated. A softer layer is formed within the surface layer while the core remains at the specified hardness. (Fig. 2)

 

(3) Clad steel: A layer of extremely low carbon steel is fused to both sides of a carbon steel core made of the same steel as (1) above, and then entire piece is heat-treated, making the surface layer very soft. At a certain depth, however, the specified hardness is abruptly reached. (Fig. 3)

 

1- Figure 1. All-steel.

2- Figure 2. Decarbonized.

3- Figure 3. Clad-steel.

 

Clad and decarbonized steel rules were developed with the aim of improving the bendability of the all-steel rules which were originally used. A layer of ordinary steel, resistant to hardening, was formed on the surface of the carbon steel. Although the rigidity of the host material was sacrificed somewhat, the surface layer, which has such a large effect on bendability, was softened and the bendability improved.

 

Two types of heat-treatment are also currently used: the quench-and-temper technique for martensite organization and austempering for bainite organization. Steel with martensite organization has high strength and hardness, excellent wear and abrasion resistance and many other outstanding qualities which make it a material appropriate for industrial applications. Although martensite is hard and strong, it is also brittle. To improve this aspect, it is hardened once again and then annealed. With an appropriate degree of annealing, a steel with ductility as well as adequate strength is produced. Steel with bainite organization has high tenacity. Especially in high hardness ranges, this characteristic particularly stands out when compared to martensite steel. The bendability is also high, making it a steel with properties more suited for rules. The results of mechanical property measurements made with tensile and fatigue testing machines on martensite and bainite steel used in rules are shown in Table 1. (Test data from Tsukatani Hamono Mfg. Co., Inc.)

 

Table 1. Differences in mechanical characteristics according to steel type.

 

	Martensite steel			Bainite steel
Tensile test results (kgf/mm2)	Yield point	Tensile strength	Elongation (%)	Yield point	Tensile strength	Elongation (%)
All-steel rule (Hs60)	121.5	132.7	11.4	116.5	131.0	13.9
Clad steel rule (Hs60)	103.9	111.3	12.7	101.1	109.9	12.9
“       (Hs50)	84.2	92.3	13.9	81.7	93.3	13.9
Fatigue test results (repetitions)
All steel rule (Hs60)	295,115			10,000,000 or higher
Clad steel rule (Hs60)	101,692			110,080
“       (Hs50)	89,401			98,434

 

The difference between martensite and bainite organization is more pronounced for all-steel rules than surface-treated ones such as clad steel. In tensile tests, the elongation value for all-steel bainite rules was high, demonstrating bainite steel’s outstanding bendability. Furthermore, in fatigue tests, all-steel bainite rules showed a durability tens of times higher than that of martensite rules.

 

These mechanical characteristics may be the properties which hold the answers to preventing the die rule damage which occurs due to fatigue failure created by the repetitive stress of each cutting cycle--for example, the breakage of corner-cutting portions, cracks in bridges and collapse which may occur along the point where the rule is embedded in the veneer board, or the cracks which may occur on combed parts used in the direction of rotation on rotary dies.

 

b. - RULE SHAPE

 

Rules may be categorized into two main categories: scoring rules and cutting rules.

Scoring rules have a plate width of 0.7~3.0mm and a height of 22.50~23.10mm. There are a variety of shapes (Fig.4, Fig. 5) such as single-curve (1), high-low curve (2), pointed score (3), curved-square (4), V-score (5), U-score (6) and deformed-score (7). For cutting rules (Fig. 6, Fig. 7), shapes include dual-blade (8), pointed dual-blade (9), two-stage blade (10), one-sided blade (11), pointed one-sided blade (12), and one-sided two-stage blade (13). For back portions, there are three basic shapes as shown in Fig. 8: rounded (14), beveled (15) and MG (16). Plate width is 0.45~2.5mm, and height is generally 5.2~100.0mm.

 

1- Scoring rules

2- Scoring rules

3- Figure 4.

4- Figure 5.

 

1- Figure 6.

2- Cutting rules

3- Figure 7.

4- Cutting rules

5- Figure 8.

6- Back shapes

7- Figure 9.

 

Shapes for the cross-sections of the dual blade (8) in Fig. 6, as shown in Fig. 9, include a useful “sewing machine” blade for occasions where intermittently spaced blades for even and uneven pitch cuts are needed. There is also a lead score capable of both cutting and scoring. “New wave” rules with wave-shaped blade edges, as in item (1) Fig. 10, for cutting with a “soft touch,” are also used.

 

1- Figure 10.

2- Figure 11.

3- Veneer board

 

There are two main categories for rules designed specifically for rotary dies: soft type and hard type. Commonly used soft types include serrated cutting rules, such as the one shown in Fig. 10, item (2), which have a 1.42mm plate thickness and a height of 23.80~24.40mm, and scoring rules which have a plate thickness of 0.9~3.0mm and a height of 21.0~22.0mm.

 

Generally used hard types include cutting rules such as the dual blade type, with a plate thickness of 1.07mm and height of 23.1~23.9mm, and the new wave type, with a plate thickness of 0.9~1.07mm and height of 23.60~23.90mm, and scoring rules with a plate thickness of 0.9~3.0mm and height of 21.0~22.85mm.

 

A shape unique to rotary dies can be seen among rules that are used in the direction of rotation. Their back portion has notches cut into it at a specific pitch designed to make the process of curving the rule easier, creating a comb effect. (Fig. 11)

 

c. - MACHINING METHODS FOR RULE TIPS

 

For scoring rules, regardless of whether the rule is heat-treated, almost all rule tips are machined by drawing (shaving with a cutting tool).

 

For cutting rules, in Japan, the majority are shaped in a grinding process using a grindstone. In other countries, some cutting rules are made in the same manner as scoring rules--the tips are shaped by drawing.

 

With rule tips made by drawing, cutting streaks parallel to the rule tip can be seen as shown in Photo 1. These streaks not only produce cutting resistance during the diecutting process, but also promote the creation of dust.

 

With regard to bendability, this is actually something of an advantageous result as streaks run perpendicular to the direction of bending stress, making it more difficult for cracks to develop.

 

In the case of ground rule tips, however, the streaks are essentially perpendicular to the rule tip, as seen in Photo 2. These streaks, unlike with drawing, have almost no effect on the diecutting process. In addition, the edge cuts well since it can be finished to a fine sharpness, and the generation of dust is reduced considerably. During bending, however, the streaks may lead to the development of cracks. It is important to keep the grain of these streaks as fine as possible and maintain a stable pattern during their machining.

 

Photo 3 shows a rule which was machined to produce minimal grinding marks. It is designed to maximize the merits of both methods and is used for special purposes. The surface appears polished like a mirror.

 

1- Photo 1. Rule tip machined by drawing. (x50)

2- Photo 2. Rule tip machined by grinding, 1. (x50)

3- Photo 3. Rule tip machined by grinding, 2. (x50)

 

2. THE MANUFACTURE OF CUTTING DIES AND RULE MACHINING

 

Cutting rules to the proper dimensions, bending, and the machining of bridges are all essential steps in manufacturing a cutting die. To produce precise dies, the machining must be carried out accurately, and care must be taken to prevent the distortion of the rule shape in unnecessary directions.

 

Problem points during rule machining:

- roll over and burrs during cutting

- distortion and burrs during bridge machining

- cracks during bending, flat warp, dimension changes due to recesses (grinding undercut) on outside edge of rule tips, etc.

 

There are many such problems which must be taken into consideration. The skill of experienced workers is required in machining. In some cases, the use of an automatic bender can prevent warping and twisting. In fact, compared with a Kansai-style bender, it is possible to reduce rule tip outside roll-over by 0.04~0.05mm and unwanted changes in height dimension by 0.004mm. As a result, we were able to decrease the use of uneven paper by 60~80% and also reduce set-up time.

 

This may be strange as an introduction of new types of rule material, but the following is a method that has already been adopted in some places. It is referred to as the “flexible anvil die” method, among other names. Two types are in use: 1) mechanically forming cutting and scoring rules directly from steel rolls (a method that may be explained as an extension of the method in which a belt-shaped rule is mounted in a base material such as conventional veneer board), and 2) forming cutting dies by laminating molded cutting and scoring rules to the surface of thin steel plates that have been heat-treated on the outside of a magnetized roll. In both cases, either the method of using the molding roll for only one of the facing rolls or the method of using the molding roll for both rolls may be used. When using these methods, the use of tack sheets or film in the stamping of multiple-shuttle, comparatively thin material can produce effective results.

 

 

 

3. DISTORTION OF RULE TIPS DUE TO FLAT DIECUTTING

 

The tip of rules (cutting rules) contact the paper and facing plate and gradually change shape during flat diecutting. Using photos of cross-sections of used rules and results from test machines, I would like to examine the relationship between those shape changes and rule characteristics.

 

(1) The rules used in the following analysis were all used in actual production and judged to have reached the limits of their intended use. Some were procured based on renewal schedules, while others were obtained after reaching their pre-determined production quotas. Rules which had thus reached retirement are all shown in Table 2 along with information on production conditions. Cross-sections are shown in Photos a~j.

Table 2. Specifications and use conditions of sample rules.

 

Sample No.	a-1~4	b-1~4	c-1~4	d-1~4	e-1~4
Hardness of rule host material (Hs)	50	60	60	50	50
Rule tip hardness (Hs)	70	80	80	70	70
Diecutter name	Bobst 126	Bobst C	Bobst C	Oton	Bobst 126
Diecut paper	Coated cardboard	Coated cardboard	Coated cardboard	Vinyl chloride	Coated cardboard
Pieces cut	146,200	33 (continuos)	30,000	90,190	61,790
Set number	5	3	1	4	2
Sample No.	f-1~3	g-1~2	h-1~2	i-1~2
Hardness of rule host material (Hs)	50	60	50	50
Rule tip hardness (Hs)	70	80	70	70
Diecutter name
Diecut paper	B/F corrugated cardboard	Coated cardboard	Tack sheet	Tack sheet
Pieces cut	10,000	400,000
Set number

[translator’s note: “Oton” may not be the proper spelling.]

 

1- Photo d-4. (Niter??) corrosion (d-3). x100

2- Photo f-3. (Niter??) corrosion (f-2). x100

3- Photo s. Tip cross-section of rule mounted perpendicular to direction of rotation. (x100)

4- Photo r. Tip cross-section of rule mounted parallel to direction of rotation. (x100)

5- Photo t. Difference based on position for tip cross-sections from rules mounted parallel to direction of rotation.

 

(2) Eight types of rules with differing hardness and rule angle were used to test durability with a universal tester. For the tests, a thin plate of SUS 304 was laid on the facing plate. The paper was placed on top and the load at the time of cutting was measured. During the actual testing, the tip of the rule contacted the SUS plate after cutting the paper and the pressure was set to be 470~500Kgf. The contact was repeated continuously. After 1, 500, 1000, 2000, 3000, 4000 and 5000 times, measurements were taken a total of three times each. The average value of those measurements is shown in Table 3. After the 5000th repetition, the rule being tested was cut in half. The shape of the cross-sections are shown in Photos f~k. Photos (a)~(c) and Photo (e) show rules that were used with coated cardboard. Rules (a) and (e) had a host material hardness of Hs50 and rule tip hardness of Hs70. The shape changes shown by the magnified photos of the rule tips seem to show shape changes occurring in the same pattern. The significant difference in cut piece output may be due to differences in diecutting finishing times that arose due to differences in the pieces that were being cut. Comparing the test data with past data, it seems that these two points of rule tip distortion are the average shape change for rules of this hardness. Through direct contact with the facing plate after cutting, or through contact with a stainless steel plate placed on the surface of the facing plate (a method which has recently become popular), the tip of the rule tends to roll over in a sideways direction. As a result, when the flattened surface of the rule tip is used in diecutting, it gradually increases the area of the diecut surface, generates excessive paper dust and decreases the performance of the rule tip. The rules in Photo (b) and Photo (c) had a host material hardness of Hs60 and a rule tip hardness of Hs80. Due to the hardness of the rule tip in particular, the distortion of the rule tip proceeded in the form of simple abrasion, and the widening of the rule tip’s surface area was kept to a minimum. This helped control generation of paper dust and suggests that these types of rules would be suited to paper-cutters where high-quality is required or which are used for large-scale mass-production. This point was also plainly supported by the results of the durability test, shown in Table 3. Comparing sample rules j, l, m, n, o, and p in tests on the same paper, samples j and m showed differences of 60kgf at the first measurement stage. Based on the fact that the data from the first measurement was the same as the average data after three measurements, it is clear that the sharpness of the rules deteriorated steadily between the first and second measurements and the second and third measurements. Furthermore, after the 5000th repetition, the difference had grown to 173kgf, and while sample j had steadily begun to produce paper dust, no changes were observable in the cut of sample h. We also examined changes in sharpness affected by rule angle in this test (samples n, o and q), but while acute angles seem to be more effective than obtuse angles for simple cutting, recycled paper proved to be more complicated. It depends on the percentage of recycled paper content in the paper being cut, but compared to new paper, the pressure needed to diecut recycled paper was very high and the damage from impact on the facing plate (SUS plate) increases geometrically. In some cases, this may suggest that prioritizing rule hardness may take precedence over common-sense arguments for prioritizing rule angle.

 

In addition, recycled paper used in diecutting may contain dyes and cosmetics for enhancing appearance or may be coated with such elements such as aluminum for keeping out moisture or preserving print. It has been confirmed many times by researchers that such factors have a significant effect on die rule abrasion.

 

Table 3. Results of durability tests.

 

Sample No.	j-1~2	k-1~2	1	m-1~2
Hardness of rule host material (Hs)	50	50	50	50
Hardness of rule tip (Hs)	50	50	70	80
Rule angle (degrees)	42	42	42	42
Paper	recycled	new paper	recycled	recycled
Results of durability test	(kgf)	(kgf)	(kgf)	(kgf)
1 time	221	229	197	161
500 times	296	300	244	184
1000 times	306	310	247	187
2000 times	337	315	252	192
3000 times	338	315	256	196
4000 times	356	315	261	196
5000 times	369	322	261	196
Sample No.	n-1~2	o-1~2	p-1~2	‘Q
Hardness of rule host material (Hs)	50	60	60	60
Hardness of rule tip (Hs)	70	75	80	80
Rule angle (degrees)	30	30	42	30
Paper	recycled	recycled	recycled	recycled
Results of durability test	(kfg)	(kfg)	(kfg)	(kfg)
1 time	177	154	159	135
500 times	248	203	186	185
1000 times	260	224	191	189
2000 times	261	240	193	189
3000 times	261	245	194	191
4000 times	261	239	196	195
5000 times	261	244	194	195

 

I have discussed above the relationship between abrasion (distortion) of rule tips and rule materials in flat diecutting, but with regard to rotary die cutting, (hard type), the patterns are almost the same. With rotary dies, however, there is a considerable difference in abrasion (distortion) between rules which are mounted parallel to the direction of cylinder rotation (Photo r) and rules which are mounted perpendicular to the direction of rotation (Photo s). In addition, a considerable amount of variance exists among rules mounted in the direction of rotation, dependent on the rule’s location (Photo t), and the load on those rules is much greater compared to flat diecutting. With soft type rotary dies, the die rules experience wear in the form of simple abrasion through contact with the corrugated cardboard and urethane roll. Therefore, in general, the abrasion occurs evenly. The strength (rigidity) and resistance to wear of the rules are maintained and stable performance obtained.

 

4. GENERAL REMARKS

 

I began this discussion beginning with rules made from three types of steel. Each of these rules were then discussed in terms of processing by one of two heat-treatment methods. These each had specifically categorized hardness levels and some even had rule tips of separate, higher hardnesses. In addition, rules of different plate thickness and height dimensions were used. Naturally this created an incalculable number of rule types. As a result, the number of possible material selection routes also becomes quite large.

 

Nonetheless, the production of these rules was prompted out of necessity during diecutting processes and the manufacture of rotary diecutters. Thus, we may assume that numerous factors were taken into consideration during their initial selection, factors such as the diecutting equipment, the pieces being cut, their shape, production volume, etc. The points I mentioned in items 1, 2 and 3 above are the fundamentals of rule selection and, I’m sure, will prove to be important factors. On the other hand, I hope to continue with further research as the selection of rule materials is an important issue in the progress of rule manufacturing technology and quality.

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