3D-Printed Foot Manikin For Determining Thermal Insulation Of Footwear

A thermal foot manikin was developed, 3D printed, and sensorized with the aim of determining the thermal insulation provided by footwear. Compared with existing thermal foot models, the manikin of the present work is cheaper, easier to set up, and simpler in its structure and components. It consists of a selective laser sintered (SLS) polyamide shell equipped with 16 digital temperature sensors and filled with water from the leg opening. The shell was designed in Rhino 3D using a Creative Commons licensed STL file selected from "thingiverse.com" website as the starting model, then printed at AIM Sweden AB. The sensors face the inner wall of the shell, thus in direct contact with the water inside. Data acquisition and processing are handled by a National Instruments interface device on the hardware side and a LabVIEW program on the software side. Instead of being actively heated and kept at a constant temperature during testing through power control (as most existing thermal foot models do), our manikin was initially filled with room temperature water, then exposed to cold while monitoring the cooling of the foot shell at 16 points of interest. The system was tested inside a freezer at (-40 ± 1) °C in four configurations: (a) bare foot, (b) wearing a liner (a thin sock), (c) wool sock over a liner, and (d) liner and slipper. Configurations (a), (b), and (c) were tested both with the manikin standing on the freezer floor and with the use of wooden spacers that avoided direct contact of the foot sole with the cold surface. The cooling curves of each sensor were plotted on a temperature-time chart for all tests performed. A qualitative interpretation of the general trends was provided, along with an objective analysis of the time it took each sensor to detect a temperature below 15 °C (pain threshold) and 7 °C (numbness threshold). Insulation values for each part of the foot were formulated using the original approach of relating the time it takes for a sensor to detect a temperature drop of 10 °C to the time that must elapse, according to EN ISO 20344 (for the test to be successful), before the insole temperature sensor detects a temperature change of 10 °C, or 1800 seconds. The results correlate well with the respective "steady-state cooling rates", or the slope of the cooling curves in their linear segment. Further studies are needed to refine the method, redesign the manikin shell to fit shoes and boots, and compare the results with those obtained from tests on human subjects.