Nondestructive product characterization of gas bubbles in gas-assisted injection-molded parts

In the past few years, gas-assisted injection molding has become one of the most important polymer processing operations in the plastic industry because of its ability to produce parts with both thick and thin sections while attaining structural rigidity without sacrificing good surface quality. In addition, this process reduces material usage, equipment cost, and cycle time while also overcoming many of the limitations of the conventional injection molding process. In gas-assisted injection molding, gas bubble penetration and associated geometry, namely size and shape, are mainly controlled by the part/gas-channel/mold design, processing conditions and material properties. While it is also closely linked with the quality of the final molded products, gas penetration is well known for its sensitivity to various surrounding conditions. Slight variations in process conditions or disturbances in the melt may cause the gas penetration to behave differently. Therefore, the study of the gas bubble characteristics is an important means to achieve a good understanding of relations between product quality and part/gas-channel/mold/process design for gas-assisted injection molding. A crucial step in studying the gas bubble characteristics is to view the gas bubbles in the gas-assisted injection molded parts. In this regard most of the previous reported work has involved using destructive methods or transparent molding materials, which yield either very rough measurements or ones not practical for industrial applications. In this study, three nondestructive testing techniques, namely Scanning X-ray Tomography, Scanning Acoustic Microscopy, and Ultrasonic Thickness Gauge measurement, have been used to examine the gas bubble characteristics in gas-assisted injection molded parts. Molded parts with different size, geometry and gas channel designs were examined using these techniques. Results show that nondestructive testing techniques can provide accurate information on all the important features of the gas bubble, such as length, width, residual wall thickness of gas channels, gas permeation into thin sections, and unbalanced gas penetration. Each nondestructive testing technique has its advantages and limitations. Comparison of these three methods is provided for various applications.