Flexible Part Carriers

To transport the strip from one station to the next in a progressive die, some material must be left between the parts on the strip。 This carrier material may be solid across the width of the strip, or may be one or more narrow ribbons of material, see part carriers sidebar。

Many parts require the edge of the blank to flow inward during flanging, forming or drawing operations。 This may require the carrier to move sideways or flex vertically, or both, during the die operation。 A flexible loop must be provided in the carrier to allow flexing and movement of the blank without pulling the adjacent parts out of position, Fig。。

Another concern is the vertical "breathing" of parts in die stations during the closing and opening of the die in the press stroke。 For example, vertical breathing takes place between the draw stations of parts requiring more than one draw to complete the part, Fig。 。 Vertical breathing also occurs when a flange is formed "up" in a progressive die station that is adjacent to stations that use upper pressure pads to hold the adjacent parts down。

It is important to consider the flexing of the carrier during the upstroke of the press as well as during the downstroke because the action may be different。 This can be simulated in the design stage by making an outline of the cross-section of the part, the pressure pads and the stationary-mounted steels on separate sheets of paper and then placing these sheets on top of each other in layers over the die section views。 These sheets then are moved down in relation to each other to simulate how the upper die would close during the press downstroke。 This will show the relative position of the part as the die closes and during the reverse action as the die ram opens。

Upper Pressure Pads

Because of size or function, many progressive dies require two or more pressure pads in the upper die。 Each may require a different travel distance to perform the work in the inpidual die station, such as trimming or forming or drawing。

However, the upper pressure pads often are used to push the material lifters down by pressing against the strip, which pushes the lifters down。 In this situation, all of the pressure pads that push material lifters down should have the same travel distance。 If the upper pressure pads travel different distances, the strip will not be pushed down evenly。 This can pull adjacent parts out of the progression, making it difficult to locate the parts in their proper station position after the feed cycle。

If the part requires a flange to be formed up, the part carrier must have a flex loop to allow for vertical breathing of the part or provide a pressurized punch/pad with the same travel as the other pressure pads。 The force required by the pressurized punch/pad has to be adequate to form the flanges up during the downstroke while the punch/ pad is in the extended position。 This keeps the strip from breathing vertically as it is pushed down from the feed level to the normal work level。

When the strip reaches the work level, the pressurized punch/pad stops its downward motion while the upper die continues down for punching, trimming, down flanging and other operations。 Springs or nitrogen cylinders can be used for pressure in these pressurized punch/ pad stations, but they must have enough preload force to form the flanges up and to collapse the lower gripper pad before the upper punch/ pad recedes, Fig。 4。

Drawn Shells

Drawn shells are produced when strip material is changed from a flat plane to a cylindrical shape。 During the draw operation, the "diameter" of the blank is reduced to the "circumference" of the shell。 As the circumference is being reduced during the flow of material inward, the outer portion of the material goes into side or edge compression。

When this compression becomes too great for the material to stay flat, it begins to fold or wrinkle。 To prevent this, the material is allowed to flow in a controlled gap between a draw ring and a pressure pad。 The two main causes of failure in drawing a shell are to exceed the percentage that the blank (or shell) is reduced in diameter and an improper draw ring radius。