The HOW, WHAT, and WHY of liquid impact forming
Hydroforming in a conventional stamping press
Liquid impact forming, a patented process developed by Greenville Tool & Die Co., Greenville, Mich., uses a conventional stamping press with a liquid medium to manufacture parts that otherwise would require hydroforming.
Hydroforming has revolutionized sheet metal stamping and tube forming. It can expand, bend, bulge, and change a simple pipe into geometric shapes without a hint of wrinkling.
This process has allowed automotive exhaust manufacturers to produce a variety of shapes to accommodate almost any product design. Automotive manufacturers have been—and always will be—investigating processes that reduce product weight, save material cost, and increase production speed while using existing production stamping presses.
As an innovative process, hydroforming doesn't come without cost. One disadvantage of conventional hydroforming is an initial investment in special presses and tooling. Another drawback is the additional cycle time needed to produce a single part—up to three times longer than conventional stamping. This can result in decreased output and smaller profits. However, the cost of tooling required is comparable to the cost of a conventional single-station, offline forming die.
Liquid Impact Forming
Liquid impact forming, a patented process developed by Greenville Tool & Die Co., Greenville, Mich., uses a conventional stamping press with a liquid medium to manufacture parts that otherwise would require hydroforming. This process doesn't replace either conventional stamping or hydroforming, but rather fills the gap between them. It allows certain products to be manufactured at a fraction of the cost compared to conventional hydroforming.
With this process, single parts—replacing entire assemblies—can be manufactured. This can reduce the need for welding and assembly operations. In addition, because liquid impact forming uses a conventional stamping press, cycle times can be up to 212 times faster than conventional hydroforming. Piercing and forming can be done in a single-station liquid impact die. Liquid impact tooling also can be placed inline with conventional stamping dies.
During liquid impact forming, a tube is submerged in water, the ends are capped off, and the tube is squashed in a specific area. The displaced water reshapes, bulges, or supports the tube in a specific area.
How the Process Works
In liquid impact forming, a tube is submerged in water, the ends are capped off, and the tube is squashed in a specific area. The displaced water reshapes, bulges, or supports the tube in a specific area. At the bottom or any point during the press stroke, pressure is released by a mechanical pressure release valve and the finished part is removed (see Figure 1).
This process can turn a simple tube into a variety of straight, twisted, or curved shapes (see Figure 2). Depending on the final product shape, it may be necessary to prebend the tube to fit into the liquid impact die. Because the tube is filled with and supported by liquid, it can be pierced at the bottom of the press stroke, reducing the tendency for the metal to cave in around the pierced hole.
Benefits of Liquid Impact Forming
Water has many advantages in the metal forming industry:
- Water does not compress—it fully supports the areas of a tube that otherwise might wrinkle during compression.
- Because water doesn't have a specific shape, it can conform to any geometric configuration.
- Forming with liquid can promote good stretch distribution. This maximizes the amount of stretch that can occur within a given boundary. Even stretch increases the total strength of the formed product.
- Water is readily available and inexpensive.
- Water can be combined with biodegradable rust preventive.
Limitations of Liquid Impact Forming
Unlike conventional hydroforming, liquid impact forming doesn’t inject water into the tube or compress its original length. The amount of bulging that can be achieved is limited to the stretchability, formability, and tube material thickness. Products requiring severe bulging and extensive metal flow don-t lend themselves to liquid impact forming and are more suited for conventional hydroforming.
Also, the amount of deformation that takes place is controlled by the amount of liquid displaced when the tube is squashed.
Not Replacing Conventional Hydroforming
Keep in mind that liquid impact forming is not intended to replace conventional hydroforming, but to offer a viable alternative. Straight, simple tubes whose shape circumferences must be changed from a simple circle to a square, octagonal, or polygonal shape are prime candidates for liquid impact forming.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.