Surface Defect Analysis

Vehicle manufacturers aim to build their new car models by using state-of-the-art technologies, which reduce time intensive work processes and increase planning accuracy. By implementing such technologies, the period of time between the design freeze and SOP can be optimally reduced.

Within the product development process, an activity which takes a great amount of time is the optimization of surface defects, particularly surface lows during tryout. Surface lows are characterized by unwanted changes of curvatures, i.e. isolated deviations between actual and desired geometry.

In the tryout phase, the detection and measurement of surface lows is carried out by stoning or optical measurement. Experienced experts discuss and define the measures for tool adjustments, which usually includes the re-milling of tools. The extent to which these measures lead to an improvement of surface lows issue becomes apparent only after their implementation. In order to achieve a satisfactory result, several optimization loops are usually necessary. These loops are not only time consuming and costly but also difficult to plan. As a result, car manufacturers, particularly volume producers in the middle and upper class segments, must allow for time corridors for the appropriate measures.

In order to generate time and cost savings but especially to increase planning accuracy, these so called esthetic defect optimizations must be moved from the tryout phase up to early on in the virtual process validation phase. With this objective in mind, AutoForm has intensively examined the detection and measurement of these surface defects, with the focus on examining how and how well surface lows are detectable and measurable using AutoForm software. Mechanisms that play a major role in the appearance of surface lows were also examined. Based on these investigations, appropriate surface defect variables were developed and a workflow for virtual surface defect detection and optimization was defined.


Surface Defect Analysis According to AutoForm’s Workflow

Over recent years, the procedure for surface defect detection and optimization has been continuously improved and is presented in the following sections. The recommended workflow for the assessment of surface lows is structured as follows:

Simulation setup according to AutoForm’s recommendations

The analysis of surface lows requires high simulation accuracy and complete process definition. Therefore, tool functions have to be defined according to the real process, including insertion of the work piece and closing of tools. In addition, affected tools have to be scaled to avoid unwanted plastic deformations at tool closure.

Ensure part feasibility and part quality

The second step is to ensure part feasibility and part quality with respect to “Class A” surface requirements. In particular, the material movement across bending radii must be carefully checked.

Analyze desired geometry

Subsequently, an analysis of the reference geometry is carried out. In this way, process planners have a clear understanding of the part designer’s wishes regarding the part’s curvatures.

Compare simulation results with reference part

By comparing the desired geometry with the obtained simulation result, curvature deviations become apparent. For this task, AutoForm provides variables for direct curvature analysis, i.e. Three Point Gauging. More details about the extent of surface lows can be obtained by using more sophisticated referencing variables, i.e. Surface Lows “Stoning”. The figure below shows the results of the surface lows analysis carried out according to the recommended workflow.

Identify and implement measures for improvement

Based on the identified surface defects, the user decides how the forming process should be adapted in order to address these issues. It is then necessary to integrate these measures into the simulation setup.

Check success of measures taken

The final step is to run simulation in order to analyze and confirm whether the implemented measures were effective.

The process “Evaluation and Optimization of Surface Low” can be shifted from tryout to process engineering to a great extent. The associated benefits are time and cost savings in the tryout and a significant increase in planning predictability.