In different extrusion processes, curvature (or bowing, sabre-running, katana effect, etc.) represents a major quality problem and is often difficult to get a grip on. The cause of the curvature is almost always an inhomogeneous thermal situation of the product during cooling and can often be greatly reduced or completely eliminated by adjusting the cooling conditions.
The cause for the deformation of extruded products is based in very many cases on an inhomogeneous thermal shrinkage of the material over the extrudate cross section. During the cooling process, different areas of an extruded product (e.g. edge area/core area of an extruded sheet different area of a profile with different wall thicknesses) are often cooled different and not homogeneous.
What are the effects of inhomogeneous cooling?
Uneven thermal conditions in a continuous extruded product lead to the formation of residual stresses. The already coold spots in a profile cross-section are already dimensionally stable and no longer deformable. Adjoining areas of the profile that still have elevated temperatures continue to cool and reduce their specific volume (density increases) while decreasing temperature. The following diagram shows the changing density of a PE-HD material during the cooling process. At temperatures above 200° C, the material has a comparatively low density of only 0.76g/cm3. During cooling, this value increases until it almost reaches the value of 0.98g/cm³ at 20° C.
The material thus shrinks by more than 20%, which is of course equivalent to a 20% change (reduction) in the specific volume. The material literally contracts during the cooling process. If this process is uniform for the entire profile, this large decrease in volume does not pose a major problem.
However, if parts of the profile have already cooled down and thus no more molecular chain rearrangement can take place there, the still progressive volume shrinkage of the adjacent areas lead to a build-up of stresses and if these stresses become too large also to a deformation of the material.
The curvature is formed in the direction of the warm extrudate areas
It is therefore obvious that deformation of the profile (if this is due to shrinkage) always occur in the direction in which the profile has the highest temperatures (over the time) within the cooling section, i.e. generally in the thickest areas. In the simulation result shown below (chillWARE 3D – profileSIM) it can be clearly seen that the ridge at the bottom right has already completely cooled down (T<60° C), while the internal ridge still has temperatures of more than 150° C. In this case, the shrinkage would proceed while the adjacent areas have already cooled down. The result in this application example is therefore not necessarily the formation of an arc but even a complete deformation of the profile (especially on the curved surface on the left profile wall and the right profile parts).
In further articles we will discuss special profile geometries and their deformation tendencies, and in our free download area we will present illustrative examples for avoiding sheet travel by means of special process parameterization or subsequent insertion of heating zones.