TeraMetrix Explores Off-line Terahertz (THz) Measurements of Bottle and Preform Layers

Manufacturers use both on-line process control measurements, and off-line quality control measurements.  The off-line measurements are typically those that take additional time, but often show detail that cannot be obtained on-line.  More in-depth study early in the process can actually decrease the setup time (possibly by days) for the manufacture of a new product and help characterize product specifications (e.g., wall thickness) and effectiveness of new materials (alternatives to plastics).  

Off-line measurements and optimization methods can be quite varied often providing more detailed measurements or analyses that can be used to improve the product or improve the process.  An example is the fast (< 40 sec) generation of whole bottle thickness maps for total and individual layer thicknesses in a multi-layer structure.  The current method requires time consuming destructive measurements at only a few places on the bottle. However, a T-Ray 5000 terahertz measurement system running at 1 kHz  deployed at-line or off-line could provide the full bottle detailed information necessary to significantly shorten the startup cycle for these complex multi-layer bottles and improve the layer thickness consistency of the product.

In the example case here, a multilayer bottle is placed on a Z - Θ rotation stage that can rotate around and translate along the length of the bottle.   The total wall thickness, as well as each individual layer can then be mapped.

Certain areas of the bottle do not provide a measurement, e.g. along seams and highly curved injection points.  In the image above, the cylindrical section of the bottle is mapped in this way.  The entire scan took approximately 40 seconds, but this is dependent on the resolution required.

For products with critical structures (e.g., barrier layers in pharmaceutical bottles and fuel tanks), confirmation of presence and proper manufacture of the structure is essential.  The absence, gaps or insufficient thickness of an internal barrier layer can be extremely problematic.  Spot checks are a common practice, but full product scanning can both be used for QA purposes and product optimization / cost savings purposes.  The ability to routinely and easily make such whole product scans quickly, without requiring any couplant, and measuring both clear and opaque materials further increases the value of the measurement.

Even very quick (1 – 2 seconds) simple scans can provide a quite complete picture of the product.  A simple linear scan of an object creates a B-Scan which is a cross-sectional image of the product along the scan line.  The image result can clearly reveal improper layer structure.  For example, a scan line can be the circumference around the round bottle.  The B-Scan image demonstrates variations in total and individual layer thickness values along the scan line.

The grey scale of the lines in the image correspond to the polarity and amplitude of the reflection peaks in the waveforms along the scan line.  The Y-axis of the B-Scan is time of the waveform which corresponds to depth into the bottle wall.

In addition to thickness variation, additional features such as potential gaps in the center barrier layer can be seen.

A similar offline application of THz is the measurement of preforms used in blow molding, especially the detection of internal layers.  One of the layers in a preform, with the addition of Carbon Black, can serve as a light blocking layer.  It is critical that the layer extend up under the cap.  THz is sensitive to Carbon Black due to the slight conductivity of the material which causes reflections of the THz pulse. 

An important measurement on the preform is the confirmation that the Black layer reaches sufficient far into the screw cap end for all 360 degrees around the sample.  For the sample pictured below, it can be seen that the Black layer is present at the two locations below the lower ridge (Green arrow), but not at both locations above the ridge (Red arrow).

A B-Scan (cross section view) along the circumference of the preform at the heights of the Green and Red arrows help demonstrate the presence or absence of the Black layer.

In the B-Scans, the bright white line near the top of the images is the outer surface of the preform.  The dark solid Black line near the bottom is the inner surface.  The lower intensity light / dark lines in between these are reflections for interior layers.

Note that for the scan at the Green arrow position, the interiors reflections can be seen as a clear layered structure between the White Top line and Black Bottom line.

However, for the full scan at the Red arrow, the consistent layer structure can only be seen for the beginning and end of the image.  The loss of the layer structure between these two areas means the Black layer does not extend up to the Red arrow position for all 360º of the sample.

This type of THz measurement, enabled by the T-Ray 5000 and accompanying motion hardware, is a powerful tool for the examination and evaluation of both bottles and preforms.  By measuring a bottle in this manner, the manufacturer can more quickly optimize a new production line, but also maintain a high quality product in ongoing production.