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Progressive-Die Carriers

By: Peter Ulintz

Friday, July 1, 2016
 

With all material running through progressive stamping dies we find the carrier—the material that connects, holds and transports the stampings from station to station as they move sequentially through the die. Also known as webs, strips, ties and attachments, the types of carriers and their shapes vary based on product geometry, material thickness and strength, material flow during deformation, part stability in the die-tip position, lifting distance, and press speed. The carrier-strip profile may be straight, zig-zagged or looped, depending on the optimum location of the attachment points on the part, and the clearance required in the die.

Tool and die design engineers select among several types of carriers:

  • Center carriers, created by cutting a periphery around the part that leaves a strip of material near the center of the part. A narrow strip allows the die to perform forming and other work all around the part, while a wide strip allows work to be performed along the outside of the part.
  • Inboard carriers, attached to the part somewhere between the center and the outside of the part. This type of carrier stays within the stock width, avoiding the need for extra material to create the carrier.
  • Lance carriers, created by lancing the coil stock rather than trimming material away. Lancing eliminates scrap material between parts but tends to leave shavings in the die—this problem can outweigh the benefit of reduced scrap.
  • Outside carriers, generally attached to the sides of a stamped part to allow work to be performed on or around the center of the strip. Outside carriers make it easier to carry the strip on outside lifter rails, and also provide good balance to push-feed the strip through the die. Outside carries are created by trimming, lancing, or by punching dog-bone shapes for drawing.
  • One-sided carriers, which attach to the stamped part on one side only. This allows work to be performed on three sides of the part. However, one-sided carriers may allow the strip to veer off to one side of the die, creating problems with part location in the die stations.
  • Solid carriers, which find use when there is no material movement expected during the stamping process. The coil stock remains solid until the cutoff or blanking station generates the final periphery.

Design Considerations

Try to design carriers within the stock width and pitch required for the blank. Otherwise, additional material must be allocated for the coil-stock width or the progression to accommodate the carrier. Strive for a minimum carrier width not less that two times the material thickness to help ensure good cutting. Typical carrier width may be 1⁄8 to 1⁄4 in. for medium-sized dies requiring carriers that must flex, while larger dies may require wider carriers in order to push the coil stock through the die.

When using two carriers, keep their lengths as consistent as possible to balance strip movement. Consistent carrier length will help prevent carrier-strip distortion and twisting of the stamped parts. Use a “natural” attachment point on the parts that allows for easy cutoff at the end of the die. But remember: The direction of the cutoff burr can be a significant factor for the part edge.

If the action of the die requires the carriers to stretch or flex, be sure to design the carrier with loops long enough to stretch or flex without breaking. The resulting carrier must be strong enough to feed all of the parts through their full progression. In some cases, three flex carriers may be required.

Make the radii in flex loops as large as practical. Avoid mismatch steps or nicks in the cut edge of the carrier. Small radii and other carrier-edge defects concentrate stresses in these areas, causing the carrier to break when flexed.

When upper punch blocks must extend below the coil stock, or when lower die blocks extend above the coil stock when the die closes, clearance will be required relative to the parts and their carriers. Here, a loop carrier can be stretch-formed vertically (up in the die) to provide the necessary clearance.

Large parts produced from thin materials often require carriers with stiffening beads to add strength and stability. Another stiffening option is to lance-form the edge of the coil stock instead of trimming it entirely away when creating a pitch notch. Here, the formed edge provides rigidity while the lance-cut edge serves as a pitch notch.

Heavy parts in large progressive dies require greater force to move the strip through the die. Fortunately, the weight of the part usually is the result of using a thick material, much stiffer than thin a material. A word of caution: Tool and die designers often design carriers in thick materials that are too stiff to easily flex when a flex carrier is desired.

Stock Guiding and Lifting

Designers must ensure the use of sufficient stock lifters, so that the coil stock will move freely when progressed forward. If the coil material sags between stock lifters, it will encounter resistance when progressing to the next station. This resistance can cause the carriers to buckle, resulting in a short feed.

When round spool lifters are spaced too far apart, especially when running thin materials, it can be difficult for the carriers to remain flat. This condition can cause the parts to be pulled too far out of progression for the pilots to properly locate the strip in the next station. Here, a bar lifter may provide better stock support to prevent sagging.

It is generally more difficult to start a coil of material through outside spool lifters than it is to route it through outside rail lifters. The addition of an internal bar lifter makes it easier to feed the material through outside spool lifters, since a lifter bar can span the space between two or more parts.

Regardless of how well a progressive-die carrier is designed or how well the stock guiding and lifter systems function, there always remains the possibility of a short feed or misfeed occurring in a progressive die. Therefore, take care to design and install all of the equipment necessary to reliably detect short-feed and material-buckle conditions. MF

 

Related Enterprise Zones: Tool & Die


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