Biaxial Extensional Deformation Dynamics of Semi Crystalline Polymer Sheets
Biaxial extensional strain hardening controls sheet thinning, requiring precise melt strength, tool friction, and temperature tuning to set uniform container wall thickness.
Ultrasonic Thickness Gaging
Meaning
Non-destructive measurements utilize high-frequency sound waves to determine the wall thickness of plastic parts. In the plastics industry, ultrasonic thickness gaging provides a reliable method for inspecting hollow or enclosed structures such as pipes, tanks, and blow-molded bottles. The technique requires access to only one side of the part, which eliminates the need to cut or destroy the specimen.
It is not suitable for highly porous materials or plastics with high levels of glass fiber reinforcement, which scatter the sound waves.
Measurement Physics
The instrument works by transmitting a high-frequency sound pulse from a transducer into the polymer material. This pulse travels through the material until it encounters a boundary, such as the back wall of the part, where it is reflected back to the transducer. By measuring the round-trip transit time and knowing the velocity of sound in the specific polymer, the gauge calculates the material thickness.
This calculation is highly dependent on the temperature of the material, as sound velocity decreases as temperature rises. Therefore, calibration must be performed at the same temperature as the production run to ensure accurate measurements.
Instrument Calibration
Precise measurements require calibrating the gauge using a sample of the same polymer with a known thickness. This calibration accounts for the differences in sound velocity between various resins, such as high-density polyethylene and polystyrene. During production, a couplant liquid is applied between the transducer and the plastic surface to ensure efficient sound transmission.
If the surface is too rough or the couplant is missing, the sound wave will not enter the material, which prevents a reading.
Quality Control
In blow molding and pipe extrusion lines, this measurement technique is used to verify that the wall thickness meets the specification along the entire length of the part. This feedback helps operators adjust the parison or extrusion die to prevent thin spots that could cause part failure under pressure. Automated systems can integrate multiple transducers to provide continuous, multi-point thickness measurements across the production run.
This real-time data allows the line to run closer to the lower tolerance limit, which reduces material usage and lowers overall production costs.