Extrusion Conversion Minimums and Start up Scrap Realities
Extrusion conversion minimums exist to dilute non-negotiable startup purge and calibration scrap across enough linear footage to stabilize part piece price.
Camber
Meaning
Lateral curvature along the length of a continuous extruded product indicates an imbalance in internal stresses or cooling rates. Technical specifications for plastic profiles often include a maximum allowable camber to ensure the part fits into its intended assembly. This bow usually results from one side of the extrudate shrinking more than the opposite side during the transition from melt to solid.
Material Distortion
Differential cooling in the calibration tank causes the polymer to pull toward the side that cools more slowly. When camber occurs in thin films, they create a baggy edge that prevents even tension across a winding mandrel. High crystallinity resins are particularly prone to this effect due to the substantial volume change that happens during the formation of molecular structures.
Processing Variable
Adjusting the temperature of individual sections of the die or the water flow in the cooling bath provides a method for correcting the straightness of the part. Proper alignment of the puller and the die exit prevents the introduction of mechanical strain that mimics thermal camber. Small changes in the take off speed relative to the extrusion rate can also influence the severity of the curve.
Downstream Alignment
Automated sawing or cutting operations require a straight substrate to maintain dimensional accuracy in the finished length. Excessive camber forces the material to bind in guides or creates uneven gaps in window frames and structural channels. Measuring the height of the arc over a fixed distance allows a quality technician to quantify the defect and adjust the line parameters accordingly.
Proper control of the cooling gradient ensures the finished profile meets the straightness requirements of the end user.