Non-destructive testing of polymer foam models with THz pulses

Detecting density fluctuations

Foamed polymers are widely used in industry as packaging and insulating material as well as modeling material for lost-foam casting. Especially in the latter process, high demands are made for the polymer foam quality.

A homogenous density distribution of the foam model is a deciding factor for the quality of the resulting casting. As the models may display process-induced density fluctuations, these must be identified and the foaming process must be adjusted accordingly.

Due to the lack of suitable metrology, the densities have been determined with the invasive methods which are slow and are performed with low local resolution. Therefore, it was not possible to establish a direct comparison between the model and casting quality.

The Fraunhofer IOF in cooperation with Fraunhofer IFAM developed a new measurement method for the non-destructive determination of the density distribution and fluctuation based on the THz imaging technique.

 

THz metrology

Ultrashort terahertz (THz) pulses (ʋ = 0.1 – 10 THz, ʎ = 30 μm – 3 mm) are electromagnetic pulses with just one oscillation cycle of the electrical field strength.

The low-energy radiation penetrates paper, plastics, dry wood and the majority of ceramics, and exhibits a high sensitivity to water. It is possible to determine material densities and thicknesses using time of flight measurements of THz pulses through polymer foam and plastic samples.

The images obtained after the processing of the measured data reveal both density fluctuations and inclusions, as well as form deviations of a sample, thus, the detection is performed non-destructively.

 

Density distribution of a polystyrene sheet

Figure 1 shows a typical measurement result of the density distribution performed on the polystyrene foam sheet in the area of an injector nozzle, through which prepared foaming mold is filled with pre-foamed polystyrene granules during the manufacturing process of the models.

During the verification the foam sheet was systematically divided into sections; the density of each segment was measured by a destructive method and compared with data achieved from the THz measurement.

The densities in the individual quadrants are given in g/l. The measurement results reveal a process-induced density increase in the area of the injector.

 

Detecting thermosetting adhesive

Individual parts are frequently joined by thermosetting adhesives into a complex model. By means of the THz imaging  the position of the adhesive seams can be identified with great precision and its quality can be monitored due to the higher density of the adhesive.

 

CAD data comparison

Figure 4 demonstrates a polymer foam model used for the lost foam procedure from which an aluminum, iron or steel casting can be produced. The CAD data is used to calculate the optical path through the sample and create a theoretical model which is displayed in Figure 5.

Further, the theoretically calculated values are compared with the optical paths measured with the THz pulses through the sample. Thus, the inhomogeneities can be identified even in the complex samples.

Measurement with ultrashort THz pulses (Fig. 6) matches perfectly the CAD data (Fig. 5) and highlights local defects and inhomogeneities.

 

We Offer

  • Customized design of THz systems
  • THz measurements (imaging, spectroscopy, tomography)
  • Studies on the applicability of THz radiation
  • Comparison of CAD and measurement data

 

Expertise at the Fraunhofer IFAM

  • Production of polymer foam models
  • Non-destructive density control
  • Destructive density determination with buoyancy-flotation method
  • Optimization of foam and lost foam casting production of prototypes (AI, Fe, Mg, Zn)
  • Expert advice/Consultation

In cooperation with Fraunhofer IFAM

Contact:

Daniela Schmidt

Phone: +49 (0) 421 2246-108