Shrinkage and warpage in mold and tool making

Compensation of shrinkage and warpage is an omnipresent topic in the field of mold and tool making. With tools made strictly according to the drawings in most cases the parts produced are not true to size. This effect shows up particularly in complex plastic parts with high accuracy requirements. Conventional methods for compensation of shrinkage and warpage are often time and cost intensive. There are several steps in the compensation procedure needed to reach the desired quality. The company F. & G. Hachtel GmbH & Co. KG integrated a system into their process chain that reduces the required time for compensation of shrinkage and warpage in injection molding to a minimum.

F. & G. Hachtel GmbH & Co. KG is a medium sized company in the plastic processing sector, located in Aalen (Ostwürttemberg, Germany). The company focuses on testing and the application of all technologies concerning injection molding as well as mold and tool making. Also an inherent part of the company profile is the application of industrial computed tomography.

For fast and optimized compensation of shrinkage and warpage of plastic injection molding parts the company Hachtel uses a software product from WENZEL. The software PointMaster V5 allows the determination of exact correction parameters. The calculation is based on analyzed scanned data as a result of an actual-to-nominal comparison. The actual geometry data is captured with a precise computed tomography system. The significant advantage of CT scanned data lies in the non-contact and holistic acquisition of outer and inner structures with a high resolution of detail.

 

Actual Data Acquisition with Computed Tomography

The process chain for compensation starts with a CT-scan of the part. During scanning the part is completely radiographed. Based on the 2D radiography pictures a 3D reconstruction is carried out. The 3-dimensional point cloud is used to calculate a surface-model in STL-format (Figure 2). Subsequently the surface data is preprocessed; with STL-Modeling writing and ejection marks are removed. The data is smoothed and the number of triangles optimized afterwards. This means, triangles on flat surfaces are enlarged and therefore the number of triangles reduced. On highly curved surfaces the number of triangles remains unchanged.

Subsequent to data processing a comparison of the scanned data and a CAD-model is carried out. The deviations of this actual-to-nominal comparison are displayed in colors. Figure 4 shows the deviations of a box made of plastic. This view shows, the side wall of the box is shrunken to the inside. With PointMaster correction parameters according to these deviations are calculated. With this calculation method the deviations are not just simply mirrored, but the material properties are taken into account, too. The technological knowledge and the experience of the toolmaker can optimally be included into this process. In figure 5 the local differences between actual part (red) and nominal tool geometry (green) are shown. The blue intersection contour is the new contour to be manufactured. When the tool geometry is changed according to this correction and the part is injection-molded with similar process parameters a more accurate part can be generated.

In a further procedure the STL-data is reversed into CAD-surfaces (figure 6). Thus the compensated data can be imported to every conventional CAD/CAM system. This data is the basis for the tool design.

The following inspection of a part manufactured on the basis of the compensated tool design shows, that even after a single correction cycle parts true to size can be produced.

 

About F. & G. Hachtel GmbH & Co. KG:

F. & G. Hachtel GmbH & Co. KG, based in Aalen (Germany), is a medium sized injection molding company with mold and tool making. The company has 400 employees. The principal custumers are from the white goods, automotive, aerospace and machine building sectors. The focus is on multi-component injection molding technology and injection molding assembly technique. In addition to mold making and plastic processing, Hachtel offers qualification and testing of parts and component assemblies using 3D computed tomography as service.

 

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