Wednesday, April 2, 2014

Roll Forming: Post-Cut or Pre-Cut? Advantages and Disadvantages

When roll forming producers make serious decisions about new roll form products, it is critical to consider the best mill configuration to produce the desired product. Factors to consider include when to make the cut-off and choosing between roll forming pre-cut material or post-cutting the material. With pre-cut material, the strip is cut before entering the roll forming mill, whereas with post-cut material, the strip is cut after the product is shaped through the roll forming mill. Review these advantages and disadvantages to each choice to determine the best option for your roll forming application:

Advantages of a post-cut roll form line:
  • Increased production and less downtime, since material feeds continuously through the roll former
  • Typically, fewer forming stations are required, since there is no need for the material to self-thread
  • After a new coil is threaded, the leading edge of the profile does not get deformed
  • Tooling tends to wear longer between reconditioning, yielding more linear footage
  • End flare, twist, camber and bow defects are easier to control
  • Minor mill adjustments are possible with continuously fed material under load in the roll former
  • Part lengths are not limited by mill specs (i.e. horizontal centers); if floor space is an issue, the parts can be run directly to the outside of the building
Disadvantages of a post-cut roll form line:
  • Edge condition from the cut-off die can leave a slight burr and/or distortion with hemmed parts or non-supported areas of a cross-section
  • More cut-off die inserts are required, when there are many sizes or combinations for a given product
  • Sometimes it is better to form a pre-cut strip, when there is a severely notched feature in the product
  • When pre-notching is needed, the mill system requires additional capital to add the cut-off press to the system
Advantages of a pre-cut roll form line:
  • Generally, it is less expensive to use an existing electronic feed system already used in the pre-shear, as opposed to adding a second post-cut press at the end of the mill
  • Hand feeding strips is more economical for low-volume production
  • Some notch configurations make it easier to run pre-cut strips
  • Combination roll tooling that forms multiple strip widths does not require shear die changeover
  • Parts with severe notches or complex cross-sections may not cut well after roll forming
Disadvantages of a pre-cut roll form line:
  • Strips must self-thread without damage to the lead edges, normally requiring additional forming stations
  • Lead and trail ends of pre-cut strips are not supported against adjacent roll passes, resulting in straightening issues

Article also available on the Custom Roll Forming Institute website

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