Understanding the fundamental mechanisms that govern the foaming process is the most essential universal prerequisite for developing effective processing strategies for fabricating high-quality foamed plastic products using any type of foaming technology. Despite chemically blown foaming of thermoplastics under atmospheric pressure has been successfully implemented in rotational foam molding over the last decade, the related open literature does not provide substantial research addressing the fundamentals of this unique foaming mechanism. The present study focuses on clarifying the fundamental foaming mechanisms that govern the successful manufacture of thermoplastic foams using a chemical blowing agent under low-pressure (atmospheric) conditions. The presented research is mainly based on observing a series of visualization experiments conducted using a custom-made visualization system including an optical microscope and a computerized CCD camera imaging system, which was utilized for investigating the behavior of foamable polyethylene samples prepared by using the compression-molding method. (C) 2008 Wiley Periodicals, Inc.