How to measure stress in transparent plastics.

You probably know that residual stress can affect your products' performance. But chances are that you don't have a fast, easy, and precise way to measure stress. Don't rely on qualitative judgment - here are several quantitative methods for more reliable quality control.

If you mold, form, or extrude clear plastics, you may have heard the Four Myths about evaluating residual stresses in products:

* Simple cross-polarizers Will tell you how much stress is in a part.

* Closely spaced fringes indicate areas of high stress.

* Bright colors in those fringes indicate high levels of stress.

* You can tell the stress level by the color of the fringe: e.g., red>green>yellow.

These common misconceptions about stress measurement reflect a widespread lack of understanding of a very important subject. Evaluation of residual stresses is a major concern for producers of transparent plastics used in medical devices and packaging, automotive lenses, optical components, compact disks and other optical data-storage media, and many other products. When residual stresses are controlled, parts are less likely to warp or be susceptible to chemical crazing or mechanical failure.

Stresses also can interfere with proper light transmission, which is critical in products like CDs and optical lenses. Some products, like oriented PET and PS films, include certain minimum stress levels as part of their design specifications. Such products must maintain consistent, controlled stress in order to provide specified levels of barrier properties, strength, and heat resistance.

While the effects of residual stresses are widely recognized and often discussed, quantitative methods of stress evaluation have not been well known or understood in the plastics industry at large. Some destructive methods are too time-consuming and imprecise to be valuable for production-floor quality control. Most non-destructive methods commonly in use are qualitative in nature, and thus prone to subjectivity and misinterpretation. Fortunately, a number of precise, quantitative methods have been developed in recent years and have proven useful in plastics-processing environments.

Making stress visible

For transparent plastics, the pre-eminent non-destructive method is birefringence measurement, which applies the principle of photoelastic response. Brewster's photoelasticity law states that the index of refraction in a strained material becomes directional. That is, where there is a difference in stress in the x and y directions, the index of refraction in these directions is different and the difference (or birefringence) is proportional to the stress level.

When polarized light passes through a stressed plastic, the components of the light wave that are parallel and perpendicular to the direction of the stress propagate through the plastic at different speeds. This "retardation" effect is proportional to the level of birefringence, thickness of the plastic, and a material constant known as the Brewster Constant.

When light that has experienced such retardation is viewed by a polarizer oriented at 90 [degrees] to the original plane of light polarization, the two components of the original light beam interfere with one another. This results in a change in color and intensity of the observed light. Observed colors correspond to different levels of retardation at that point, which in turn correspond to stress levels. (The standardized test method ASTM D 4093 describes this phenomenon in detail.)

Qualitative is not enough

The most basic way to look at stresses in clear plastics is through cross-polarizers - placing a sample between two polarized sheets and shining white light through all three. When these elements are properly aligned, the stress pattern becomes visible. In some cases, the color fringes can be used to evaluate residual stress, since each color band correlates to the degree of stress in the inspected sample. A retardation-versus-color chart is used to interpret the observed pattern (see the accompanying table on p. 69).

In a great majority of cases, the observation of these colors is difficult to interpret and can easily be misunderstood. For example, a red or green fringe can often be observed at multiple stress levels in the same sample. Still, this method can be very useful for simple identification of maximum-stress areas. However, today's quality-control standards generally require precise, reproducible, quantitative measurements that cannot be achieved using this approach.

Get quantitative results

Birefringence is easily measured quantitatively, and both off-line and on-line instrumentation is available. The most appropriate type of instrumentation depends on the application, level of accuracy or reproducibility required, and operator skill level.

Measuring can be accomplished using a polarimeter equipped with a "compensator" - a calibrated wedge that is placed between the analyzer and the illuminated item. The operator adjusts the wedge position until a black fringe shows at the point of interest on the sample, as shown on p. 68. A black fringe shows when the degree of retardation in the compensator is equal and opposite in sign (plus or minus) to that of the retardation at the point of interest. A scale on the compensator supplies a quantitative reading of retardation.

The procedure for measuring birefringence using a compensator is a standard test method described in ASTM D 4093. This is a particularly effective QC laboratory tool for clear plastics.

Another approach is known as the "Senarmont" or "Analyzer Rotation Method." Here the analyzer is rotated from its crossed position relative to the polarizer until the light traversing the sample is extinguished. The fraction of 180 [degrees] through which the analyzer is rotated correlates to the fraction of a complete wavelength of retardation of the light signal.

Page 68 shows a molded plastic part for medical packaging as viewed using a polarimeter. Note the many color bands, each of which indicates an increasing stress level. By learning how to observe these bands, an operator can qualitatively identify the region where the residual stresses are highest. At that location, a compensator is introduced to provide a quantitative reading.

The photo also shows a similar part that has undergone a post-molding annealing cycle. Here, the level of stress has decreased substantially. Also shown is a part that has been annealed to the point where almost no stresses remain. The black regions of the part show where stress is near zero.

This type of evaluation is quick, inexpensive, quantitative, and reveals a wealth of information. This method requires no harmful chemicals, has no deleterious effect on the measured sample, and can be performed using inexpensive equipment.

The cost of instruments for these test methods runs $10005000, depending on the size of the polarimeter and accessories such as microscopes.

More advanced approaches

Sophisticated computer-based instrumentation is commercially available to plastics processors for applications that require extreme speed or accuracy, or where automated on-line inspection is needed. These systems replace the human observer with a computer sensor. Operators need only enter information on variables such as product thickness, material constants, and product code (if needed). These systems then automatically provide information on retardation, birefringence, and stress in transparent films or discrete parts.

One type of PC-based instrument passes the transmitted light signal to a spectrophotometer. This patented method, known as Spectral Contents Analysis, or SCA, offers a high level of accuracy by employing spectrophotometric analysis of the transmitted light. It provides a graph of retardation, birefringence, or stress for any point or across any scanned line. The SCA method has gained increased attention for PC-based evaluation of biaxially oriented films.

Another method, Digital Image Analysis, uses a digital camera to replace the human observer. The captured image is analyzed by the computer, which measures transmitted light intensity to calculate stress levels over the full field of view. The computer can display either a graph of stress versus position on the product or a full-field color stress map. Digital Image Analysis is suitable for CD inspection or other applications in which very low levels of birefringence will hamper product performance.

Both of these methods provide quick, accurate, highly repeatable information about residual stresses in transparent parts and are free from operator-to-operator variations. Both types of systems can be supplied for off-line or on-line application. Off-line systems sell in the $25,000-50,000 range, depending on the complexity of the equipment, accessories, sample fixtures, and degree of software customization. On-line systems, which can include customized rails and linear motion systems, cost up to $80,000.

Take action

These quantitative methods open up new possibilities for processors to use residual-stress measurements for quality control. It becomes readily feasible to develop standards for the optimum level of stress and minimum/maximum limits. From there, a program of on-line or off-line QC stress measurement will detect when changes in part design, material, temperature, cooling rate, air flow, gate placement, part thickness, or numerous other factors cause variations in residual stresses or their distribution throughout the part.

A commitment to evaluation of residual stresses will greatly contribute toward product quality and consistency. Whatever the instrumentation or method used, the inclusion of stress measurement in QC procedures should be fundamental in monitoring, identifying, and fixing stresses before parts fail.

Alex S. Redner is president and Barbara R. Hoffman is marketing manager of Strainoptic Technologies, Inc. in North Wales, Pa. This firm supplies stress-measurement instrumentation and develops new methods of measuring residual stress.