Force measuring instruments have come a long way. Today’s equipment opens up possibilities once in the exclusive domain of more complex systems. As illustrated in this article, force measurement instruments are able to address the testing needs of medical device manufacturers for a variety of applications.
The decision to purchase a piece of measuring equipment should never be taken lightly. Whether procuring a drop indicator or a multi-million dollar measuring system requiring multiple levels of approval, the result of a quality control measurement can only be as good as the equipment used to perform it. Reflexively seeking the lowest price or a familiar model number should be resisted until a thorough assessment of the quality testing challenge and available solutions has taken place. This couldn’t be truer when purchasing equipment to measure the amount of force required to break, tear, actuate, or otherwise achieve a quality control objective with regard to the mechanical properties of a material or a part.
Why Rethink Force Measurement?
Force measurement refers to the measurement of a compressive or tensile force. Closely related but not to be confused with weighing and materials testing, the medical device manufacturing industry is rife with applications for force measurement. Typical examples include determining packaging seal strength, tube termination pull-off testing, blade sharpness testing, spring testing, fastener torque testing, syringe plunger force testing, and many others.
The most familiar embodiment of force measurement – the ubiquitous force gauge – had its origins many decades ago. Having evolved from a simple spring weighing scale, the handheld force gauge of today can be as accurate and advanced as a closet-sized tensile testing machine from years ago.
Force gauges today are joined by test stands which have also grown in sophistication. Usually, a screw-driven crosshead moves a force gauge up and down to produce a force, and reverses when the test is complete. A test stand’s controller can regulate the rate of speed; stop the motor when a specified load, distance, or sample break has occurred; cycle the crosshead; and accept commands from a PC. Much like innovation in personal computing has progressed rapidly, so too has the ability of force measurement systems to provide useful and cost-effective answers to quality control questions. These answers have upended previously held notions about what a tension or compression testing apparatus must look like.
Determining whether force measurement equipment can help the Quality Control engineer or researcher satisfy an ISO, ASTM, or internally developed standard starts with a thorough inspection of the requirements of the standard. Consider some of the core parameters typical of a standard testing procedure:
- Accuracy and resolution of the measuring instrument
- Unit of measurement
- Proper sample preparation and dimensions
- Linear speed of the testing apparatus
- Nature of the sample gripper or fixture
- Requirement for force measurement only or force vs. displacement
- Interpretation and storage of results
When comparing these parameters to the capabilities of a force measuring instrument, it may become evident in many applications that a system consisting of a test stand, force gauge, suitable grips, and data collection software can satisfy the application.
A motorized test stand can be programmed to run at a specified rate of speed and can be fitted with an internal scale for displacement measurement. The force gauge can display peak and real time values with an internationally accepted level of accuracy, and numerous standard grips and fixtures are readily available. In many cases, custom fixtures are fabricated to reflect the great variety of sample shapes and sizes.
Data collection software captures peak readings or continuous force readings for force vs. time or force vs. displacement tabulation and plotting. Statistical calculations, exporting, and reporting tools are commonly available in basic software packages. In simpler applications, a printer can be used to print the results and basic statistics. Some test stands can be controlled by PC software to help reduce operator influence, increase testing throughput, and improve data collection.
In OEM type applications, force measurement can be a viable alternative to analog data acquisition systems. A force gauge can become an integral component by interfacing the gauge’s set point, digital, footswitch, and analog inputs/outputs with a programmable logic controller (PLC). One example involves a manufacturer of sutures requiring frequent testing of samples from the production line. The test requires a pull to failure. A force gauge is integrated into a semi-automated custom machine, and pass/fail limits determine whether particular production lots are cleared for onward shipment. Data is recorded for statistical analysis and reporting.
Test stand-based systems can also be integrated into complete systems, for example, as a station in a pacemaker production line. Serial communications between the test stand, force gauge, and PLC can automatically run the stand, collect data, and stop the production line if a non-conforming sample has been detected.
Calibration can be a simple process. Typically, the only part of a force measurement system requiring calibration is the force gauge itself. When it is due, it is removed from the test stand and transported to the laboratory, where deadweights or a master load cell is used to calibrate to full scale and several points in between. It is, therefore, not necessary for the calibration technician to travel to the test station to calibrate it.
When Does Force Measurement Take a Back Seat?
Throughout these examples, force measuring instruments were used in cases where, in the past, a more expensive alternative may have been the default choice. However, despite technological advancements, materials testing takes over where force measurement leaves off. Materials testing uncovers a wealth of scientific information about the mechanical properties of materials used in medical devices, including various metals, plastics, elastomers, ceramics, and others. Through a combination of sophisticated software and measurement of very small deflections, material testers have earned their place in quality control and research laboratories of raw materials suppliers, medical device manufacturers, and educational and research facilities.
Force measurement systems’ deflection measurement accuracy and resolution are typically too coarse for reliable measurements, which means that applications such as tensile testing of steel and compression testing of ceramics become challenging. In such cases, a materials tester fitted with an extensometer and analysis software is essential. Such software can capture a stress vs. strain curve to help characterize materials and automatically identify important features such as Young’s modulus and yield strength. For more specialized applications requiring the calculation of application-specific coefficients, materials testing software is often capable of providing such results.
With sophistication comes a learning curve, which is why materials testers have traditionally been used by trained professionals in a laboratory type environment. Force measurement equipment cannot replace a materials tester, but typically, have simpler interfaces, reflecting their more focused capabilities. For many companies, there is a need for force measurement equipment and materials testing equipment to coexist – often a materials tester to test incoming raw materials, and a force measurement system to measure a finished component or assembly.
The Bottom Line
In today’s competitive medical device manufacturing environment, it has become more important to carefully analyze test applications and the capabilities of available instrumentation. Educating oneself about the capabilities and limitations of force measurement equipment can better guide procurement decisions. Optimizing such decisions can translate into decreased equipment and maintenance costs, reduced operator training time, and faster throughput.
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