Quality assurance without destructive testing
Example: fit accuracy of a plastic screw-cap

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Assembly inspection in product development

Bottles made from polyethylene terephthalate, also known as PET bottles, are the most commonly used beverage packaging. About one-third of all drinks worldwide are bottled in PET containers. Glass is the second most-popular material for bottles, but only makes up approximately one-sixth of the packaging currently used. The shapes of PET bottles have changed many times since their introduction in the 1980s and become increasingly diverse. Advances in production technology have made it possible to significantly reduce the weight and the quantity of materials used. Customized shapes have helped beverage producers better position their products on the market.

Measure the important parts

The cap is a particularly critical area when developing a new bottle shape because it is vital for the function and safety of the PET bottle. The impermeability must be guaranteed in order to prevent both the beverage from flowing out of the bottle and germs from getting in. An additional security feature is the so-called tamper evident band which is designed to tear open or tear off when the cap is twisted. Thanks to the tamper evident band, consumers know that the bottle has not been opened after being filled.

The CT data display using ZEISS NEO insights software shows if the seal is properly positioned on the bottle.

The CT data display using ZEISS NEO insights software shows if the seal is properly positioned on the bottle.

Conventional inspection

If a CT system is not available, destructive testing is a standard process for inspecting the fit accuracy of the seal and the bottle. The area around the cap on a sealed bottle is embedded in resin. When cured, the resin gives the molded plastic stability, which is necessary for preventing deformations during sectioning. After being cut into many thin sections, the cross sections are checked optically to ensure defect-free contact between the cap and the bottle.

Different material densities are used for the color display of the bottle and cap.

Different material densities are used for the color display of the bottle and cap.

Disadvantages of destructive testing

The greatest drawback to the aforementioned process is the significant amount of time required. Moreover, destructive inspection is always incomplete: only those defects which appear on the cut surfaces are spotted. Defects in the areas in between the sections remain hidden. Although the 2D cut makes it easy to identify defects, information about the spatial dimensions of the defects is lacking. It is not possible to know where and how the molding tools need to be corrected without taking further steps. This process also poses the risk that the resin warms up when hardening, deforming the plastic. This distorts the inspection result because the introduction of heat can reconnect areas which originally had not been joined.

Inspections with the ZEISS METROTOM CT

With a ZEISS METROTOM CT system, it is possible to examine the quality and function of the cap in its original, screwed-on state without cutting apart the bottle. Using the CT data obtained, the cap can be cross sectioned at random on the computer using ZEISS NEO insights software so that the analysis is as complete as possible. A freely rotatable, semi-transparent 3D view facilitates orientation and interpretation. The different material density of the cap and the bottle can also be used for differentiated color rendition, further simplifying analysis. The complete 3D visualization makes it possible to locate all parts on the screw thread with missing material contact so that the origin of potential leakage can be determined. Moreover, the different display formats show where and how the tool shapes of the screw top or the bottle must be corrected.

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