With all of the blanks stacked on a pallet, they’re ready to be moved to a transfer line or to a tandem press line—both will include a destacker at the front of the line to separate a single blank from the stack and feed it into the press. To minimize the risk of blanks sticking to each other and overloading the press or damaging the dies, the line also should include a double-blank detection system. After destacking, the material may also pass through a washer/oiler or a centering station.

In a transfer line, typically three to eight die stations act simultaneously under a single press slide. Stamped parts move from one station to the next via a transfer system. Transfer presses typically are straightsided with at least two connecting rods; tonnage capacity ranges from 300 tons to more than 3000 tons. The bolster and slide must be long enough to accommodate all of the stations; some transfer presses exceed 275 in. in length. And these presses typically use an eccentric-gear or link-drive system. In some cases, an asymmetric drive—which positions a high-tonnage drive at the front of the line and a lower-tonnage drive near the end of the line—may be used to compensate for off-center loading. In an asymmetric 2000-ton press, for example, with two connecting rods, one connecting rod may apply 1200 tons and the other 800 tons. Lastly, a conveyor typically is placed at the end of the line. 

A tandem transfer line differs from a transfer press by featuring a separate press for each die station, typically four to six presses. The first press, called the head press or lead press, may include a hydraulic cushion and typically draws the sheet metal panel. The follower presses, also called rework presses, typically perform trimming/piercing, bending and restrike operations.

Many OEMs continue to seek ways to increase the stroke rate from tandem lines and reduce the number of presses. For example, Ford Otosan (a joint venture in Turkey) recently has reduced the number of presses in three of its tandem lines from five to four. By only investing in one lead press, it now operates four press lines, each consisting of four presses. And, Honda, which has been trying to reduce the number of presses in its tandem lines since the 1980s, converted, in 2015, a tandem operation for stamping floor panels into a one-press operation.

The largest press lines may have a bolster size of 4500 by 2500 mm or larger, and a lead press in excess of 2000 tons equipped with hydraulic cushions or double-action slides. Follower presses may have similar bolster dimensions and press force to 1000 tons. Smaller lines (bolster dimensions in the neighborhood of 3500 by 2500 mm) might have a 1600-ton lead press and 800-ton follower presses. Typical hit rate may be as low as 6 strokes/min. using standard mechanical presses operating in the intermittent mode, while a line using servomechanical presses and continuous operating mode can achieve production rates in excess of 18 strokes/min. 

Lastly, some stamping facilities run combined press lines, which may have a decoiler at the front of the line and an in-press transfer system. These lines, best-suited with servomechanical drives, can operate in three modes:

1. As a regular progressive press, at high speeds
2. As a transfer press (using a separate destacker, and at slower speeds)
3. As a combination: a progressive line for the first three or four stations, and then shearing the parts from the strip and transferring them to subsequent die stations using a tri-axis transfer system. 

Note: Most of the numbers provided here are considered typical for automotive applications. There always are exceptions to the rule. MF

Special thanks to Gerald Schulz, Fagor Arrasate Deutschland GmbH, for helping to prepare this article.

See also: Billur Metal Form

Technologies: Stamping Presses

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