Residual Stress Testing for Transparent Polymers

All medical plastics manufacturing processes— including injection molding, extrusion, vacuum forming, and machining—inherently introduce residual stresses. These stresses sometimes have an intentional and highly desirable purpose, as in the case of biaxially oriented films, whose carefully designed orientation enhances mechanical properties. In other products, residual (or "frozen-in") stresses can be a problem, reducing end-use performance and resulting in increased scrap and rejects. When high levels of stress are present in a part, impact strength is lowered, high-temperature performance is diminished, and environmental stress cracking becomes more prevalent.

EFFECTS OF RESIDUAL STRESS

Distortion from Stress Relaxation. Stress relaxation results in deformation and improper fit during product assembly and is a direct cause of a deterioration in product appearance and of ultimate product failure. Frozen-in stresses become real loads applied to the material upon exposure of the part to slightly elevated temperatures. Medical manufacturers often do not realize that this problem exists until parts are subjected to sterilization or heat-sealing processes.

Cracks. Cracks in a material are the most visible result of excessive residual stress. Cracks are accelerated by the presence of solvents, but can also appear when a molded product is restrained and cannot relax to a shorter, stress-free dimension. Crazing is the appearance of many fine microcracks across a material. This condition may not be evident during production but can be triggered by subsequent exposure to chemicals. For example, styrene parts dipped in kerosene will craze quickly in stressed areas. Proper annealing can minimize these stresses and prevent crazing.

Deterioration of Optical Performance. Clear plastics are used extensively in the production of lenses and optical transparencies. Residual stresses introduce birefringence—an asymmetry of the index of refraction—that can make a plastic lens unable to focus properly. Even a small level of birefringence hampers optical properties and product performance.

Changes in Mechanical Properties. Oriented polymers have different mechanical properties than their annealed counterparts. Drawing, forming, and cooling procedures can and do introduce orientation stresses in the part that can lead to property changes. An extruded polymeric tube or sheet has a certain inherent tear strength; maximum elongation inherently creates anisotropy in the material, resulting in a decrease of performance in the unstretched direction. If processed at too low a temperature, the part can shorten in the machine direction and thicken in the cross-machine direction.

HOW TO EVALUATE RESIDUAL STRESS

Evaluation of orientation and birefringence can help processors identify potential field failures. For example, examining a transparent molded product under polarized light will reveal where stress or orientation has been created in the product due to flow patterns and shrinkage in the mold. This test is often employed to check areas of potential weakness in a complex part. During production, it is important for processors to check residual stresses periodically in the run to verify that processing conditions have not drifted.

Photoelastic evaluation is used to make stresses visible in transparent parts. Stressed areas of polymers have refractive characteristics: when subject to stress, transparent materials become "birefringent." Viewed with polarized light, stresses appear as a series of multicolored bands or fringes. This fringe pattern is rich with information.

Figure 1. Medical packaging with excessive residual stresses (top) and the same type of package after annealing has substantially reduced stresses (bottom).

A simple polariscope will allow you to view stresses in transparent parts. Polariscopes essentially consist of a white light source and suitable polarizing elements. The packages illustrated in Figure 1 were evaluated using the polariscope shown in Figure 2. This type of evaluation can be employed to survey stress distribution in a part or to compare stresses in two identical parts.

Figure 2. A polariscope will reveal stresses in transparent parts. (Model SV-2000, Strainoptic Technologies Inc.)

MEASURING BIREFRINGENCE FOR QUANTITATIVE RESULTS

Stress-measurement instrumentation can deliver accurate, quantitative results that cannot be derived using crossed polarizers alone. Quantitative evaluation is preferred over qualitative methods and is far more reliable for quality control. With the right instruments, stress measurement can be easily conducted for transparent parts.

Both on- and off-line instrumentation is available for quantitative measurement of birefringence. Determining the most appropriate tool depends on the application, the level of accuracy or reproducibility required, and the level of operator skill.

A simple test can be accomplished using a polariscope or polarimeter equipped with a compensator—a type of calibrated wedge. The operator adjusts the wedge position until a black fringe appears at the measurement point, as shown in Figure 3. A scale on the compensator supplies a quantitative reading of optical retardation.

Figure 3. Stress birefringence can be quantified using a compensator.

The procedure for measuring retardation/birefringence using a compensator is a standard test method described in ASTM D 4093 and is a particularly effective quality control test for clear plastics. The procedure is nondestructive, requiring no chemicals or layer removal. In addition, results are fast, enabling processors to make on-line adjustments as needed.

ADVANCED TESTING APPROACHES

Sophisticated computer-based instrumentation is commercially available for applications that require very fast, accurate results, or for those situations in which automated inspection is preferred. These systems replace the human observer with computerized vision systems. PC-based instruments can provide information about retardation, birefringence, and residual stress in transparent films or discrete parts.

One type of PC-based instrument offers a high level of accuracy through spectrophotometric analysis. Using a method known as spectral contents analysis or SCA, such a system can quickly and automatically report quantitative retardation, birefringence, or stress for any selected point as well as generating a graph of stress versus position for any scanned line.

Figure 4. A PC-based stress scanner for biaxially oriented materials can measure retardation in both normal and oblique paths. (Model SCA-1500, Strainoptic Technologies Inc.).

While not limited to film applications, this advanced method is particularly effective for evaluating both uniaxially and biaxially oriented films. Figure 4 shows a system used for laboratory evaluation of biaxially oriented film. A 50-mm-wide (2-in.-wide) strip of film of any length is placed on the unit's specimen holder. Motor force feeds the ribbon while the measurements are obtained at a preset number of menu-selected points, typically in 20-mm increments. Upon completion of the scan, the results are printed in the form of a graph, and a data file is saved. This compact, self-contained instrument can be used in the laboratory or on the factory floor.

Similar instrumentation is available for in-line inspection, with the optical scanning heads bolted to a carriage to scan the film during the production process. The SCA method can be easily adapted to perform process monitoring for real-time process control.

Another method, digital image analysis, employs a digital camera to replace the human observer. This technique is ideal for inspection of optical elements or other annealed parts that exhibit very low birefringence. In addition to supplying measurements for any point or along any scanned line, a digital image analysis system can display a full-field stress map, as shown in Figure 5. Using this feature, inspectors can readily identify maximum stress regions in samples, specify retardation/birefringence thresholds, and automatically select or reject parts for quality control.

Figure 5. Comparing residual stress in two copolymers using digital image analysis.

Both the SCA and digital image analysis methods are free from operator-to-operator variations and provide quick, accurate, highly repeatable information about residual stresses in transparent parts.

CALIBRATION AND CERTIFICATION

Calibration standards have been in existence for years for most measurement functions, but until recently these tools have been rare and expensive for stress-measurement applications. Such standards are useful to verify both human and PC-based measurements and to ensure proper alignment of polarimeters and other stress-measurement instruments.

The most practical tool for calibrating both visual and PC-based stress measurements is a calibrated retarder, which can exhibit uniform retardation (e.g., 100 nm). The retarder is traceable to the National Institute of Standards and Technology and can be supplied with certification documents to satisfy ISO requirements.

CONCLUSION

A commitment on the part of medical manufacturers to evaluating residual stresses in their parts can contribute greatly to improved product quality and consistency. Stress-free parts are more likely to maintain their strength, optical clarity, stability, and resistance to environmental stress factors. For transparent parts, the test methods described here can be performed using inexpensive, nondestructive procedures that can be used either on- or off-line without slowing down production. The benefits can be significant, since these simple tests can help processors monitor and identify problems before parts fail.

BIBLIOGRAPHY

ASTM D 4093, Standard Test Method for Photoelastic Measurements of Birefringence and Residual Strains in Transparent or Translucent Plastic Materials. Conshohocken, PA: ASTM.

Hoffman, BR and Redner, AS. "How to Measure Stress in Transparent Plastics." Plastics Technology (November 1998): 68–72.

Hoffman, BR and Redner, AS. "Measuring Residual Stresses in Transparent Plastics." Medical Plastics and Biomaterials 4, no. 1 (1997): 16–27.

Redner, AS. "Photoelastic Measurements by Means of Computer-Assisted Spectral Contents Analysis." Experimental Mechanics 25, no. 2 (1985): 148–153.

Redner, AS. "On-Line Birefringence Measurements in Production of Biaxially Oriented Polymers," in Proceedings of the Society of Plastics Engineers Inc., Annual Technical Conference (ANTEC 98). Brookfield, CT: SPE, 1998, 1598–1601.

Tallmadge, B. "Finding and Fixing Molded-in Stresses before Parts Fail." Injection Molding (November 1993): 53–55.

Alex S. Redner is president of Strainoptic Technologies Inc. (North Wales, PA), a company that supplies stress-measurement instrumentation as well as consulting and testing services. Barbara Hoffman is marketing manager at Strainoptic, responsible for sales and market development.

Copyright ©1999 Medical Device & Diagnostic Industry