Tooling Article

Conventional wisdom has it that metalformers must cut costs to survive. That may be true to a point, but here’s a more accurate nugget: Metalformers must spend more wisely than ever before to survive. A prime example is the relatively recent trend of tool designers and builders, for in-die pressure needs, choosing individual hosed gas springs linked via a control panel to replace manifold assemblies for purely cost reasons. There’s a problem with that, explains Mike Sanchez, manager of engineered products at Hyson Products, Brecksville, OH, a supplier of nitrogen-gas and hydraulic systems.

Low-Pressure-Rise Manifolds Ideal for Drawing

“Manifold systems often have found a home in first-station drawing

Manifold systems supply low, even pressure without spikes, making them ideal for deep-draw applications where sudden pressure rises can tear material.

where a consistent, balanced force is needed,” he says. “Because all of the manifold cylinders locate in one block, they tie to one volume of nitrogen, just as with hosed gas springs. That provides balance—equal pressure at every pressure point. A benefit with the manifold system: You don’t need to link to a gas-volume tank, as the volume is an integral element of the block itself and pressure rise can be as much as 45 percent lower. Manifold systems typically provide a pressure range of 10 to 15 percent, so a system charged to 1000 psi, at the bottom of a stroke, will provide only 1100 psi and lends itself to high part quality. The low pressure rise is important in deep drawing where rapid pressure spikes can cause material tearing or excessive press wear.”

Protect Piston Rods to Prevent Leaks

Something else to remember about manifold systems—they all feature bore-seal cylinders that protect piston rods. Grinding particles, trim steels and shavings all can locate around cylinders in a typical stamping environment.

“With cylinders in manifold systems,” explains Sanchez, “the condition of the piston rod is not an issue because you are sealing on the bore. With a standard rod-seal gas spring, the main seal surface is the piston rod itself, so any imperfections on the surface of the piston rod will cause leaks. A rule of thumb: If you run your fingernail across the piston-rod surface and it snags, a leak is guaranteed.”

On each stroke, as the imperfection passes by the seal, some nitrogen will escape. Over the course of several strokes, the system will continue to deplete its nitrogen volume.

“In a harsh environment where metal shavings or other abrasive contaminants find their way to the piston-rod scraper surface,” offers Sanchez, “you’ll want to employ manifolds, not rod-seal gas springs, in the lower die. A rod-seal gas spring, in many cases, will not perform as well as a manifold system in this type of environment.”

Those storing dies outdoors also should take precautions with dies featuring rod-seal springs. To protect against piston-rod surface damage due to weather or environment, store dies in a closed position with nitrogen drained from the cylinders.

Consider Gas Volume

The difference in available gas volume between manifold systems and individual gas springs also should be considered.

“A manifold system contains significantly more gas volume than typical gas springs,” Sanchez says. “In the event that over the course of the system’s lifecycle a leak develops, the initial gas volume is so large that the system will take quite some time to bleed down. However, a quick hiss or two coming from a gas spring could be a significant amount of escaping nitrogen—a large percentage of gas volume, depending on the cylinder diameter.”

So if you are looking for longevity, manifold systems again get the call.

Individual Gas Springs Best for Small Stripping Force

So when do hosed gas springs get the call? When applications require minimal stripping force and manifold systems simply would be overkill.

“An example is the use of pre-lubricated material where the compressed oil creates a small vacuum,” Sanchez explains. “Another: where metalformers want to avoid any imperfections on parts, mini gas springs provide minimal force without causing damage.”

Do Homework Up Front

Whatever force method is applied, it’s best to make the correct selection during die design. Changes after die construction bring higher costs and in some cases long delays.

“Be careful when designing complex geometries or deep-drawn flanged channels without a pad delay, which can lead to part inversion—turning the part inside-out,” says Sanchez. “If a die is already built without a neded delay, changes can be expensive and time-consuming. Thorough knowledge of part geometry and the means to produce that part prior to die build is the better alternative. Consult an expert beforehand and conduct simulations.”

Over-utilizing parallels provides a case in point for planning that can save time and money.

“A designer may know that the die will contain a certain amount of parallels due to the shut height of a particular die,” Sanchez says. “He can design clearance holes through the die shoe and place a nitrogen system, whether pocketed cylinders or a manifold system, right into a parallel. This proves beneficial, especially to a shop that standardizes parallel heights, because the nitrogen systems can be used on various dies.” MF