Stamping Die Essentials: Carrier strip design for progressive dies in metalforming

What Is a Carrier Strip?
In a progressive die, a strip of material is fed continuously and sequenced through various cutting and forming operations until one or more completed parts are made.

For the part to move from station to station, it is tied with a portion of the base material used to make it. This extra material outside of the part, called the carrier web, is cut up and discarded as scrap after the part is complete.

Carrier Webs
Carrier webs vary depending on the operation being performed in the progressive die.

A solid carrier (see Figure 1) usually is used for basic cutting and bending operations. It allows no vertical movement of the parts; the entire strip must remain flat and straight throughout the entire press stroke. Having stations at different heights within the tool will result in an inconsistent pitch length or distance between each part. Solid carriers can be used only when there is very little or no metal flowing or moving within the part geometry.

A stretch web carrier (see Figure 2) is more suitable for applications involving a lot of deep forming, drawing, or embossing, as well as multiple reductions. It allows metal to flow freely without upsetting the center line distance between each of the parts, and it works with progressive die stations of varying heights.

Keep in mind that even if a great deal of metal has been removed from around the blank, the carrier is not necessarily a stretch carrier. For instance, the carrier design shown in Figure 3 often is described as a stretch carrier, but it does not allow for metal flow inward or for each part to move up and down independently.

Carrier Strength
While the carrier must be designed to flex with the metal’s movement, it also must remain strong enough to resist bending during the feeding process. It takes about 10% of the part’s weight to move it horizontally. For instance, if the finished part weighs 1 lb., 1/10 lb. of force is needed to move it. If there are eight progressions, the total strength of each carrier or flex attachment must resist 8 lbs. of force without bending.

No formulas exist for determining carrier strength, so use your judgment based on material type and thickness. If a carrier is designed and built too weak, you can add stiffening ribs before or after the cutting process to increase its strength.

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