Correction of mould cavity geometry for warpage compensation

Abstract

Warpage is one of the most challenging defects occurring in plastic injection moulded parts. Various approaches to overcome this issue have been proposed in the literature, but they all provide only partial solutions to the problem. This paper proposes a new method for the compensation and minimisation of warpage. The method is based on Mould Cavity (MC) correction. In contrast to other similar methods, here the MC correction is accomplished through a direct comparison of the local deviations of the warped part's geometry to the desired geometry of the part. Modifying the MC shape accordingly yields parts with a lower shape discrepancy from the desired geometry compared to the nonadjusted shape. The key novelty of the paper is the development of software that iteratively adjusts the MC shape to minimise local deviations. In every iteration, the warped part is compared to the desired geometry and the MC geometry is adjusted accordingly. A curved thin-walled plate part case study demonstrates the method's capabilities. We show that the maximum warpage value of 0.005 mm (0.7% of the initial maximum warpage) was reached after three iterations of MC geometry correction and remained stable afterwards.

Comparison of Numerical Methods for Geometric Warpage Compensation

“…The normal-vector method, developed by Kastelic et al [25], involves an iterative adjustment of the cavity shape. This adjustment is carried out by computing the normal vector at each surface node of the warped geometry mesh and identifying its intersection point with the corresponding desired geometry surface.…”

“…This method is inspired by experimental cavity shape compensation, where discrete measurements of produced parts inform adjustments [18,19,24]. However, this so-called reverse geometry method [25] requires identical meshes with corresponding nodes for both the deformed and ideal shapes. Method (3): Kastelic et al [25] introduced an alternative method for updating the cavity shape.…”

“…However, this so-called reverse geometry method [25] requires identical meshes with corresponding nodes for both the deformed and ideal shapes. Method (3): Kastelic et al [25] introduced an alternative method for updating the cavity shape. The method determines the intersection point of the surface normal vector with the ideal geometry and adjusts the surface mesh nodes along the direction of the normal vector to compute the new cavity shape.…”

“…This method is advantageous when the deformed geometry and the ideal geometry do not share the same mesh. Both the reverse geometry method and the normal vector method by Kastelic et al [25] require very few iterations to compute the cavity shape. Method (4): Another approach is to formulate the problem as a shape optimization task [26].…”

Using Bayesian optimization for warpage compensation in injection molding

“…In another iterative method, the objective function is the difference to the desired geometry and it is computed locally [11]. With this local objective function, the geometry can be adjusted locally to reduce each objective function with each iteration.…”