Selecting a stamping die pressure system, Part I
This article is part one of a two-part series that focuses on the different stamping die pressure systems available, as well as the advantages and disadvantages of each. The article also discusses some of the controlling factors that contribute to system selection.
Selecting the stamping die's pressure system can be a critical decision. Many questions must be answered to determine what type of pressure system best suits your application.
1. Pressure requirements
Simple conventional metal cutting dies, such as those used in blanking and piercing operations, often do not require a high-force pressure system. In most cases, coil spring pressures are sufficient. However, specialty-cutting operations such as fine blanking and Grip flow®, may require forces unobtainable with a conventional coil spring. Also, metal forming operations, such as drawing, flanging, and bending often require higher forces. The type of metal being formed and cut also affects the pressure requirements. Higher strength material, such as that used in the automotive industry, usually requires higher holding, forming, and stripping forces.
2. Die height and shut height requirements
The finished height of the die, otherwise known as its shut height, must be manufactured to a set of given parameters that are controlled primarily by the amount that the press can be adjusted. In other words, the die must be manufactured to meet a certain thickness tolerance when fully closed in order to fit and operate in a given press.
Items, such as coil spring, have limited deflection rates. The deflection rate of each type of spring varies with respect to the strength of the spring. Most medium-strength coil springs have a deflection rate of approximately 30 percent of their free length. For example, if you needed 3 inches of travel on a spring, the spring would have to be at least 10 inches long. (10 in. time 30 percent = 3 in.) This additional thickness adds to the thickness of the die and can add additional die cost. Figure 1 shows this example.
3. Volume requirements and tooling cost
Justifying the cost of a long-life pressure system for a die used to produce a very high volume of parts for a long period of time is very easy. However, low-cost, low-production tools rarely justify the high cost of such a system. For this reason, a less expensive system often will be used with low-production or prototype tools.
4. Press capacity
Keep in mind that a high-pressure hydraulic or gas system can exert a force on the press ram that exceeds its rated tonnage at that particular point during its stroke. With the exception of specialty servo and hydraulic presses, most mechanical crank presses don't achieve their rated tonnage capacity until the ram is near the bottom of the stroke. Be careful to not overload your press at any point during the stroke.
5. Pressure distribution and pressure rise
During sheet metal drawing, pressure on the blank holder must be distributed as evenly as possible throughout the entire pressure pad surface. Coil springs have a tendency to fatigue and lose pressure. This can result in poor pressure distribution. In addition, stand-alone gas cylinders have a great pressure rise on the down stroke. This dramatic increase in pressure can cause excessive pressure on the blank causing the drawn part to fracture if pad equalizers are not being used. This problem can be overcome easily by using a manifold or by adding a surge tank to the pressure system.
Pressure Systems Available
1. Urethane (rubber) springs
Urethane springs are very popular in the manufacture of both low- and high-production tooling. Available in various sizes and shapes as well as different hardness grades, urethane springs provide sufficient force necessary for stripping pierce punches, as well as other pressure pad requirements. Urethane punch strippers also are easy to replace.
Urethane springs also provide suitable pressure for special stamping operations that require a non-marking pressure system, such as using pre-painted material. The springs are very reasonably priced, and tremendous improvements have been made regarding their strength and life. Urethane springs also can be custom molded for specialty operations. Best applications: Punch stripping, pressure pads, prototype work, low production tools. Figure 2 shows urethane strippers and various available shapes.
2. Coil springs
Coil springs have been used longer than any other pressure system. They come in numerous diameters, lengths, and strengths. They cost much less than gas springs. They are suitable for both stripping and holding pressure. Readily available and in stock from a number of distributors, they are best-suited for stripping and holding during bending and flanging, but are poor candidates for drawing dies. Figure 3 shows coil springs used in the manufacture of stamping dies.
One of the disadvantages of coil springs is that they fatigue and break differently from spring to spring. Also, if a coil spring breaks and the breakage is not confined within a spring can or cage, flying debris can cause die damage or personal injury. A coil spring's pressure cannot be monitored or regulated. Coil springs have a poor stroke to length ratio and poor pressure distribution.
An advantage of coil springs is that they are relatively inexpensive. They can provide adequate force requirements and are easy to maintain, install, and replace.
3. Stand-alone nitrogen and gas springs
Gas springs are becoming more and more popular in stamping die manufacturing. Stand-alone springs are springs that work individually. They have many advantages over conventional coil springs. They are available in many diameters, stroke lengths, and pressures. Because there are many providers of gas cylinders, availability is high.
Gas springs can provide the force necessary to function as the pressure system for almost any stamping die application, with the exception of specialty applications in which extremely high holding forces are necessary. Unlike coil springs, gas spring force can be regulated and controlled by changing nitrogen gas pressures. Figure 4 shows a stand-alone nitrogen spring.
Nitrogen gas spring advantages include a high contact force; a good stroke to free length ratio (as much as 40 percent of the free length), which allows the dies to be manufactured with a smaller shut height; and they are less likely to break than coil springs. These long-life springs can achieve pressure unobtainable with coil springs. Fewer springs can be used in the tool and they can be plumbed together to create a system with even pressure distribution. Also, with gas springs, pressure can be monitored during production.
Disadvantages of the gas springs include the fact that they are more expensive than coil springs. They cannot and should not be preloaded and excessive pressure will occur if they are not used with a surge tank. Gas springs must be mounted square to the die plate surface and take longer to install than coil springs. And gas springs can leak.
Part II of this series will discuss the use of gas manifolds, plumbed gas systems, both hydraulic and air press cushions, and gas stock lifter springs.