This article is a compilation of those previously written by John R. Nebbia and Jacob Nemec.
Overview
Cross-sectioning is a crucial resource in an engineer’s toolbox. An engineer cannot evaluate what they cannot see, which is where cross-sectioning becomes a valuable tool. It allows for examination of features or defects within the thickness of a part. A typical cross-section involves selection of a sectioning plane through the area of interest in the part. The part is then cut through the plan and the sample is mounted/potted in an epoxy compound and polished to a mirror finish. The polished cross-section can then be examined visually and microscopically at high magnification. This article will examine some common features that can easily be exposed and studied via cross-sectioning.
Some of important areas of analysis where cross-sectioning can be of benefit include but are not limited to the following:
- Thickness measurements
- Interfacial effects
- Cracking
- Assemblies and weld joints
- Knitlines
- Defects
- Contamination
Thickness Measurements
Cross-sections can be taken of molded parts, and their wall thicknesses can be measured microscopically or mechanically, in order to verify a wall thickness distribution has been met. This technique is valuable with injection-molded parts. It has also been proven to be a valuable method for various manufacturing techniques, such as blow molding, where the thickness distribution is not as easily controlled. Thickness measurements are also important when evaluating the quality secondary processes including, plating and painting.
Interfacial Effects
In injection molding, there is always a surface interaction between the polymer melt and the material it is flowing across, such as the mold steel, the first shot in a two shot part, an insert or in mold label. When a sample is cross-sectioned, this interaction can be studied, which often answers simple questions such as: Was the mold temperature too cold? Was too much shear heat produced? Having too cold of a mold will often lead to a visible skin-core effect. Having too much shear heat in a multi-shot molding process can result in melting and washing of the first shot, Figure 1. There are many processing parameters that can impact the bond of plastic substrate to an in mold label. If these parameters are not optimized poor bonding between the label and substrate can result, Figure 2.
Cracking
Cross-sectioning can be a useful tool to identify early-stage cracking in a component or to view a chemical agent that has diffused into a sample, Figure 3. Cross-sectioning can also be used to help determine where cracking is initiation within a component, Figure 4.
Component Assemblies and Welded Joints
One of the most common uses of cross sectioning is to examine an assembly. How do two sub-components fit, function or interact together, Figure 5. Further, cross-sectioning can also be used to examine the integrity of welded joints in an assembled part, Figure 6.
Knitlines
In a failure analysis investigation, failures had occurred adjacent to a knitline in parts produced using the color concentrate. There were no failures reported in the same parts manufactured using the pre-compounded resin. Cross-sections were prepared through the knitline region of parts produced using a pre-compounded resin and a color concentrate. It was observed that the dispersion of the colorant in the pre-compounded part was excellent, while the colorant dispersion in the color concentrate part was poor, Figure 7. The parts were produced on the same press under similar conditions, however, the different coloring methods led to a well-fused knitline on the pre-compounded parts and a large internal void at the knitline of the color concentrate parts.
Defects
Cross-sectional examination can expose internal defects in molded parts. These defects could include voiding within part walls or defects associated with secondary operations. The cross section of the plating defect shown in Figure 8 revealed an irregularity at the surface of the molded part.
Contamination
Often times particulates can make it into an injection-molded part. These materials can act as a contaminant, and can weaken the part on both a mechanical and/or molecular level. Cross-sectioning can act as a means to view these encapsulated particles.
Conclusions
Cross-sectioning is a cost effective and timely way to reveal of obscured features of a part or assembly. A drawback of cross-sectioning is that it is limited to only the plane(s) selected. Other methods, such as CT scanning provides a more broad and detailed view of the part or assembly as a whole. Additionally, cross-sectioning is a destructive technique, where CT scanning is non-destructive. In many cases, a simple cross-section can successfully identify whether and where a problem exists, and can offer valuable insight into how to fix the problem.
