Affect of Assembly Tolerances on Medical Syringe Heating

Introduction:

Diagnostic X-ray examination using a contrast medium is a primary method of assessing circulatory system health in medical patients. Contrast medium is injected into the bloodstream of the patient and this allows diagnostic equipment to detect and visually display arterial blockage. To maintain the comfort and safety of the patient undergoing the diagnostic procedure, the contrast medium must be maintained at 37°C ± 5°C.

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Customer Challenge:

The customer employs a syringe fabricated from polyethylene terephthalate (PET) plastic resin that is externally heated via a polycarbonate (Lexan®) heater sleeve in order to control the injection temperature of the contrast medium. Because this is a medical application involving humans, concern for patient safety is paramount. As a result, the syringe heater design includes a primary active electronic temperature control and dual passive snap switch limit controls to provide redundant protection against thermal runaway. To reduce fabrication cost, all electrical components were encapsulated into a module molded onto the side of the Lexan® heater sleeve. The customer experienced a high scrap rate during production of this design due to failure of the primary control to consistently maintain medium temperature within specification and failure of the limit controls to consistently prevent an over-temperature condition. Failure analysis by the customer was unable to identify any individual component characteristic that could be attributed to acceptable or unacceptable assembly performance. Components tested good at the component level and the same components would work in one assembly but not another.

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Single Iteration Solution:

The symptoms of the customer's problem were immediately identified as a classic tolerance stack-up problem. To understand temperature performance at the assembly or system level requires a complete understanding of all mechanical and electrical interfaces, environmental conditions and measurement techniques present in the design. Through thermal modeling and analysis, Single Iteration was able to determine that the relatively poor thermal conductivity of encapsulation material resulted in the temperature sensors (control and snap switch) being thermally isolated from both the heater and the medium. This was further aggravated by the control module and sensors being located on opposite sides of the syringe. When coupled with the slow response time of the active control and the large thermal mass of the syringe and medium this was found to result in an unstable dynamic system response that allowed temperature swings significantly beyond the desired specification. System simulation was able to determine that relatively minor dimensional changes in the position of the sensor during assembly could result in significant changes in dynamic response.

To address this problem, Single Iteration identified a number of inexpensive layout changes in the assembly to better control stack-up and variation in thermal contact resistance. In addition, control set point changes were specified to account for previously uncompensated thermal losses in the assembly. This resulted in consistent assemblies with significantly tighter temperature control performance.

Through this development, Single Iteration enabled the customer to overcome a seemingly intractable manufacturing problem while avoiding a major redesign of a costly assembly.