Medical Device Link.

Originally Published PMPN July 2004

Inspection

Film Inspection Using Cross-Polarized Light

The old method can give new life to film inspection procedures.

Earl T. Hackett Jr.
Senior Associate, Montesino Associates LLC

Observing defects in films by using polarized light is an old, effective, and inexpensive technique that is seldom used in the medical device industry. However, it is a proven inspection method that can be used to advantage in medical device packaging. Not only is it an inexpensive improvement to the visual inspection methods currently in use, it greatly improves the probability of detecting a defect and provides some information about its cause. 

Figure 1. An illustration showing the mechanism by which crossed polarizing filters can observe strain in transparent plastics (click to enlarge).

In such procedures, the object to be inspected is placed between two cross-polarized filters. As illustrated in Figure 1, the rear filter passes vertically oriented light. The front filter is horizontally oriented to block all light coming through the rear filter. When a piece of transparent plastic is placed between the two filters, it rotates the light. The amount of rotation varies with the type of plastic and the amount of internal strain within the plastic. With the light rotated out of the vertical plane, some light passes through the front filter. If the light is rotated 90º, most of the light will pass through the front filter. If the light is rotated in excess of 90º, the amount of transmitted light will begin to decrease. With some plastics that have been stressed excessively, alternating light and dark bands can be observed. 

Figure 2. Prototype of a cross-polarized light inspection unit (click to enlarge).

Any process that can affect the internal strain of a film will have an impact on its appearance when viewed in cross-polarized light. Internal stressing increases when a plastic is distorted. Some of this stress remains after the distorting force is removed. Exposure to heat will tend to relieve internal stress, particularly if the plastic is free to move, following the very principle used in heat-shrinkable tubing. Residual stress can be observed after vacuum forming rigid trays, as can defects in films and packages. 

Applications that can benefit from cross-polarized light include:

Figure 3. Polyester/polyethylene film with a puncture viewed under normal transmitted light (click to enlarge).

• Inspection of the film components of a package after transportation testing.
• Nondestructive inspection of packages made from transparent films.
• Nondestructive inspection of the film portion of vented pouches.
• Incoming inspection of films.

Figure 4. The same defect as shown in Figure 3 under cross-polarized light. Distracting dust particles are far less noticeable and three small impact points to the right of the primary defect are plainly visible (click to enlarge).

The technique will not work on packages in which light must pass through a fibrous or pigmented film layer. The multiple reflections of the light as it passes through the fibrous or pigmented material will destroy the polarization.

All photographs in this document were made using a handheld digital camera and the inspection equipment shown in Figure 2. 

Punctures and tears

Figure 5. A cut and tear in a nylon/polyethylene film under normal light (click to enlarge).

When a film is punctured or cut, it is stressed in an area that extends beyond the actual defect. The pattern of the stress is indicative of the mechanism that caused the failure. Figure 3 is a photograph of a puncture in a polyester/polyethylene laminated film under normal lighting. Two scratches to either side of the puncture are readily seen as well. Figure 4 is the same sample viewed in cross-polarized light. The difference in light transmission caused by differences in internal stress in the film generated by the puncture force is readily apparent. Note that the entire surface of the tab that has been torn from the film is heavily stressed. This is caused by the distortion of the film prior to failure. Areas where there is a direct line of sight through a hole in the film appear black because the light was not rotated and cannot pass through the second filter. The three small impact points to the right of the puncture are readily visible in cross- polarized light, but are barely noticeable under normal lighting. Unlike techniques that detect holes, inspection with cross-polarized light will detect impact points that may not result in an actual hole. 

Figure 6. The same tear in Figure 5 under cross-polarized light (click to enlarge).

Figure 5 is a photograph of an impact with a sharp object and subsequent tear in a nylon/polyethylene laminate. Nylon has different optical properties than polyester under cross- polarized light, but strain in the film is still very evident. This was initially identified as a puncture by the device during a shipping test. Under cross-polarized light, this was clearly not the case. The film in the area of the tab does not show the large internal stresses associated with puncture damage. Under cross-polarized light, stress is shown to be present only along the edge of the tear. Additionally, a small mark, parallel to the machine direction and pointing to the apex of the tear, is readily apparent. This mark is a scratch probably caused by the same object that initiated the tear. As received by the pouch manufacturer, this scratch is on the polyethylene surface facing the inside of the roll. There is no opportunity for this damage to occur once the film is in its final shipping configuration. Therefore, the damage most likely occurred during film manufacturing.

Figure 7. Point of impact of a hypodermic needle viewed under normal light (click to enlarge).

Pinholes

Today’s film design and production techniques provide packaging films that are essentially free of pinholes and other functional defects. Holes in films occur, but the vast majority of these are created by impacts during transportation. 

Figure 8. The impact point in Figure 7 as viewed in cross-polarized light (click to enlarge).

There have been many innovative test methods developed to detect pinholes in sterile packaging. Many are nondestructive test methods, but they are costly and can be time consuming. As a result, the most common means of detecting a defect continues to be visual inspection. Holes in clear films have little visual contrast, making them particularly difficult to see. A round-robin test conducted by the ASTM F02.06 subcommittee showed that the smallest hole that could be reliably detected, at an 80% detection rate, was between 75 and 125 µm (0.003 and 0.005 in.). Experiments have shown that pinholes of this size will have a negative impact on the ability of the packaging to maintain the sterility of the device. 

Figure 9. Laboratory seal with channels made with 25-µm (left) and 50-µm (right) wires, with contamination from fingerprints (click to enlarge).

Probably the most difficult detection challenge for visual inspection is an impact from a very sharp object. This may create a small hole or slit that could compromise sterility and would generate little or no visual deformation in the film. Figures 7 and 8 are of an impact point generated by dropping a hypodermic needle from a height of about 6 in. onto the polyester/polyethylene film of a pouch. The resulting damage is an almost invisible imprint a few thousandths of an inch long. The impact point is not visible with normal transmitted light (Figure 7), but the stress around the impact is clearly visible in polarized light (Figure 8). The light transmitted through the cross-polarized filters can be seen with the unaided eye from a distance of 3–4 feet. 

Seal inspection

Figure 10. Seal variability caused by variations in the rubber backing sheet (click to enlarge).

Sealing is a critical step in package production. Polarized light can reveal a surprising amount of information about the sealing process when two clear films are sealed together. In all cases, except for Figure 9, nothing unusual about the seals could be seen when viewed in normal light.

Figure 11. Variable seal consistency as seen in polarized light (click to enlarge).

Figure 9 is a photograph of a 15-mm wide laboratory seal. Two wires, 25 and 50 µm in diameter, were imbedded in the seal and removed. These are easily seen, even in normal light. What was not expected was the ability to detect contamination at the seal interface, in this case deposited by a fingerprint. This fingerprint is undetectable in normal light.

Variability in the seal can also be observed. Figure 10 shows the effect of a worn silicone rubber backup sheet. When rubber is exposed to heat and pressure over time, its hardness changes, and it may take on a permanent compression set. As a result, when used with another tool, there are regions of high and low pressure along the seal. These regions become visible under polarized light. The only evidence of these regions are small wrinkles in the film at repetitive positions along the seal. In this case, the variable pressure was confirmed by identical patterns produced on pressure-sensitive film.

Figure 12. A normal peelable seal in polarized light (click to enlarge).

Another instance of variability in a seal is shown in Figure 11. The mottled appearance of the seal can be seen in the seal area after the package has been opened. The root cause of this problem was not revealed to the author.

Figure 13. A normal weld seal in polarized light (click to enlarge).

Finally, Figures 12 and 13 show the appearance of a normal peelable and a welded seal, respectively, under polarized light.

Package formation

Manufacturers have used polarized light to observe the stress present in rigid trays for many years. To the author’s knowledge, polarized light is seldom, if ever, used to inspect flexible packages made on a form-fill-seal (FFS) line. The usual visual criterion used to evaluate the performance of a flexible film is its ability to conform to the surface of the cavity. The stress patterns in a flexible FFS package are shown in Figure 14. The center area of the blister shows little residual stress as compared with the sidewalls. The surface area of this unstressed region may prove to be a good indicator of the ability of the process to distribute material into the walls and corners of the package.

Figure 14. Strain patterns in a flexible film formed on an FFS machine (click to enlarge).

Conclusion

Using polarized light with visual inspection has several benefits. It is inexpensive and allows for the rapid location of the smallest defect in a film structure. Dust specks and small manufacturing irregularities that are apparent in reflected light have far less contrast and do not distract the inspector’s attention. Even large defects, which should be found during visual inspection, have far greater contrast, improving the probability that the inspector will see them. 

Earl T. Hackett Jr. is a senior associate with Montesino Associates (www.montesino.com).   

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