Selecting a stamping die pressure system, Part II
Selecting a 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. This article is Part II of a two-part series that focuses on the different 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.
Part I discussed some of the controlling factors and a few available pressure systems. Part II focuses on gas manifolds, plumbed gas systems, press cushions, and specialty electronic shimming systems. With the exception of the section entitled "Specialty Pressure Cylinders," this article will discuss the pressure systems as they pertain to drawing.
Gas and Nitrogen Manifolds
Manifolds are perhaps one of the most popular types of pressure systems used in deep drawing. They essentially are large blocks of steel or aluminum checked for porosity and inserted with a series of individual gas springs. A series of large ports and gas channels are gun-drilled into the manifold. These gas channels connect each of the cylinders and unify the force exerted by each cylinder.
In addition, each cylinder can operate independently. Independent suspension is important to reduce binder deflection and accurately control metal flow. Uniform pressure and distribution characteristics can be achieved with a nitrogen manifold. Figure 1 shows a typical nitrogen manifold.
- Gas manifolds are easy to install. They are a self-contained single block that can be designed quickly and mounted in a die set or cast foundation.
- They require little maintenance. Unlike plumbed gas systems, there are no hoses to deal with. Because there are no hoses, leaks are less likely to occur.
- A manifold can serve as the lower die set, reducing tooling cost. With a little preplanning, guide pins can be installed in the manifold.
- Manifolds have a low-pressure rise. Because of the large volume of nitrogen held in the manifold, the force or pressure rise is limited to about 15 percent when the cylinders are fully compressed, eliminating the need for a surge tank.
- The force is fully adjustable. Adding or removing nitrogen gas easily adjusts pressures.
- Cylinder heights can be staggered to reduce blank holder surface shock loading.
- Cylinders can be replaced easily with quick-connect devices.
- When the manifold is part of the die, die change time is reduced.
- Gas manifolds typically cost more than a plumbed system.
- Manifolds usually have to be custom-made for each application.
- The force exerted by the manifold must be included in press bolster deflection calculations.
- No method of preacceleration is available to reduce blank holder shocking.
Numerous press cushions are available today, including air and hydraulic. Some types can be preaccelerated to reduce blank holder shock loading, and some can adjust the force exerted on the blank holder during the press stroke.
Hydraulic cushions can exert the tremendous blank holder forces needed for stretch drawing parts such as automobile hoods and outer door panels. Unlike manifolds, press cushions supply the force necessary to control metal flow by transferring the needed pressure through an air or cushion pin. Typically made of low-carbon steel, the pins are uniform in length. The press cushion is flat, clean and well-maintained and distributes even pressure throughout the entire blank holder surface. However, if the air pins are damaged, bent, or uneven, binder deflection can occur, causing a poor fit between the die face and the blank holder that may result in a loss of metal control. In addition, if the press cushion is damaged, dented, or dirty, regardless of how accurate the air pins are machined, binder deflection can occur. It also is important to make sure that the bottom surface of the blank holder is flat and free of dents and dirt. Provided they are well maintained, press cushions are excellent force suppliers. Figure 2 shows a typical press cushion. Figure 3 shows some of the problems that can occur using a press cushion.
- Press cushions are versatile and can be used for a variety of stamping operations.
- If properly maintained, they are good candidates for deep drawing.
- High-holding pressures can be achieved.
- Some cushions can reduce or increase holding pressure during the press stroke.
- Some offer blank holder preacceleration.
- Pressure rise is minimal.
- Some cushions allow quick die change.
- The use of cushions can reduce die cost by eliminating the need for springs, gas cylinders, and manifolds.
- Some cushions can go into dwell at the bottom of the press stroke. This allows for a shorter press stroke and a possible rate increase.
- The initial press cost is higher.
- Cushions require precision-machined cushion pins.
- If the cushions aren't properly maintained, drawing problems may occur.
- Because of natural deflection, it is hard to achieve even pressure distribution.
Specialty Cushions and Electronic Shimming
Electronic shimming systems provide the most flexibility in blank and metal flow control for deep-drawing operations. Difficult-geometry parts, such as double-bowl sinks, are drawn using this specialty system. Electronic shimming allows for blank holder pressure adjustments to be made anywhere around the perimeter of the drawn shape at any point during the press stroke. Because the system uses hydraulic pressure, high holding forces can be achieved.
In addition, this system can compensate for changes in metal thicknesses that occur during the drawing process, eliminating the need for a running spot on the blank holder. Electronic shimming systems can be fully programmed to draw many geometric shapes. They control metal flow with special membranes that deflect a special blank holding plate. Figure 4 shows an electronic shimming blank holder. (Image courtesy of Hydraulico)
- Electronic shimming systems provide the ultimate blank control.
- They are fully adjustable.
- They can be used to make deeper draws and difficult part geometries.
- They provide product design flexibility.
- The system can be integrated into any standard hydraulic press.
- Electronic shimming systems are expensive.
- They are slower than conventional crank-drive press drawing.
Specialty Pressure Cylinders
Many types of specialty cylinders are commercially available. Stock lifter cylinders are becoming popular for designing and building progressive dies. These ready-to-install gas springs can adsorb more side thrust and abuse than conventional cylinders and come with pretapped holes in the piston for mounting stock rails. These springs make short work of what is often a time-consuming job, helping diemakers build dies faster. Figure 5 shows a commercial stock lifter.
Pressure systems other than those covered in this two-part series are available. Always choose a system that best meets your needs. In other words, don't buy an expensive electronic shimming system if a simple plate and urethane spring will suffice. On the other hand, don't expect to deep-draw a double sink with your urethane pressure system.