Stretcher Jaw Scrap Allowance

Mechanical Gripping

The process of straightening an extrusion involves clamping the metal at both ends and applying a controlled tensile force. The stretcher jaw scrap allowance covers the area where the serrated teeth of the gripper bite into the aluminium to prevent it from slipping. This contact creates mechanical marks and often causes a slight narrowing of the profile at the grip point.

Because these marks are permanent and affect the dimensional accuracy of the part, this section cannot be included in the finished product. The amount of metal held by the jaws must be sufficient to withstand the stretching force without tearing. For very large or heavy profiles, the jaws must be larger, which increases the amount of scrap generated at each end.

Modern machines use specialized jaw inserts to minimize the depth of the marks, but some waste is unavoidable.

Yield Calculation

Designers and production planners must factor the loss of metal into their initial material requirements for every run. A stretcher jaw scrap allowance typically adds between thirty and sixty centimeters to the total length of the extruded strand. If the press produces a twenty-meter strand, and the allowance is thirty centimeters at each end, the usable length is reduced by three percent.

This percentage becomes more significant for shorter production runs or for machines that cannot handle long lengths. The cost of this waste is built into the per-kilogram price of the finished profile. To improve efficiency, manufacturers try to extrude the longest possible strands to spread the fixed scrap loss over a larger volume of good metal.

This strategic planning is a key part of maintaining a competitive manufacturing operation.

Manufacturing Cost

The cumulative effect of these small losses at the ends of every strand represents a large volume of metal over the course of a year. Managing the stretcher jaw scrap allowance is a priority for the plant manager who is responsible for the overall material recovery rate. The scrap generated is collected and sent to the remelt furnace, but the energy used to extrude that metal is lost forever.

Reducing the allowance through better machine design or more precise gripping techniques can lead to significant cost savings. However, if the allowance is too small, there is a risk that the grip marks will extend into the salable part of the profile, leading to customer rejections. Finding the optimal balance between yield and quality is an ongoing challenge for the engineering team.

The final allowance is often set by the physical constraints of the equipment and the specific requirements of the alloy being processed.