As with so many test methods, the right equipment (calibrated), a good workspace and a well-defined technique to apply the method can all help achieve higher levels of data agreement. The Technical Group of the Flexible Packaging Association's Sterilization Packaging Manufacturers Council (SPMC) develops defined test methods with equipment and technique issues in mind. SPMC is composed of FPA member converting companies servicing the medical device and pharmaceutical markets. The group meets regularly to discuss and develop test methods and guidance documents designed to improve understanding and communication with customers and suppliers at any stage of the packaging supply chain. Taking linear dimensional measurements with a calibrated steel rule is one recent example of SPMC's efforts.

Developing A Test Method
On the surface of a dimensional measurement test method there are some usage techniques that may seem obvious to some; use good equipment, prep your material, create an effective workspace and read your ruler from the proper angle. Unfortunately, any one of these can be the cause of the very small difference between meeting specifications or not. SPMC recommends the following in order to obtain accurate results.
· Equipment: Use a steel rule that is calibrated, maintains its shape and size over time, and withstands the effects of the environment and handling. The ruler increments must be fine enough to reach the accuracy required in the standard or metric system specified.
· Material: Any creases or wrinkles not designed into a material can easily affect the results as will stretching a sample that is extensible by pulling it too taut. Materials that are sensitive to temperature and humidity will shrink or grow with their environment and may require conditioning prior to measurement.
· Workspace: Ensure that the area used to measure your sample has sufficient lighting and a clear flat space that will not affect the outcome of your work.
· Reading: Parallax error is the error caused by reading a gage or device at an improper sight angle. Looking at a ruler from an angle can change a good measurement into a longer or shorter result.

Reproducible Results
Once the test method was developed, SPMC implemented a repeatability and reproducibility study to determine the precision of the method. If all the instructions written in the test method were used by one person measuring the same set of dimensions at different times, what would the results indicate? If this method was tested in six independent labs, how would they compare? If the potential for error suggested a problem in the tolerances written in the specifications, how would SPMC respond to the vendor with the rejected shipment, the press operator and inspector on the line, or the customer with the quality complaint? SPMC's results showed that when using a +/- 1mm tolerance, the repeatability error (same operator measuring the same sample) might take up 48 percent of that tolerance range and the reproducibility error (different laboratories using different equipment) as far as 74 percent!

This in effect means that process variability demands are higher and machines must now perform within a much tighter range. When measurements are extremely important for function and tolerances are tight, more accurate methods for measurement may be required; comparators, in-line electronic sensors, scales with eyepieces for visual enlargement and control of parallax. Whatever the requirements at manufacturing, the tolerance applied to any dimension should be well thought out during the design phase to ensure that the needs are not overstated by tight tolerances in non-critical areas and are instead more stringent in areas having the highest impact on the functionality of the package.

The efforts of this technical group are focused on developing effective tests and techniques to measure products whether they are in the research and development evaluation stage or the process control quality assurance stage. SPMC's current focus is on test methods that are common in the industry but are not published standards. Further examples of the SPMC focus can be found in documents currently under development for thickness measurement with calipers, alcohol resistance testing of printed inks, coat weight determination and ink and coating adhesion to substrate.

The Flexible Packaging Association's Sterilization Packaging Manufacturers Council and its technical group member companies include Beacon Converters, Perfecseal, Rexam, Rollprint Packaging and Tolas Health Care Packaging.