Dimakopoulos Y., Tsamopoulos J.
We examine the displacement by pressurized air of a liquid, which only partially occupies straight or complex tubes, according to the Gas-Assisted Injection Molding (GAIM) process. The process involves the formation and continuous elongation of a gaseous finger, which sets in motion the liquid, which, in turn, forms a second moving interface with gas downstream, the advancing front, and is simultaneously deposited on the tube walls. The motion of the advancing front is simulated using a Naviertype slip condition. A complete parametric analysis is performed in order to examine the effects of the initial amount of liquid, inertia, liquid compressibility, and the slip coefficient. Simulations under creeping flow conditions and relatively large initial amounts of liquid show that the thickness of the deposited film on the inner tube wall is uniform for the most part, except near the tube entrance and where the still moving portion of the liquid is nearly depleted. Increasing inertia causes flattening of the liquid front, non-uniform film distribution along the wall and eventually a tip-splitting instability. Liquid compressibility influences the phenomenon only slightly. The difference between the two interfacial velocities increases as the no-slip condition is approached and eventually leads to their collision. Finally, coating of an expanding tube of finite length and either closed or open downstream is examined for various amounts of liquid initially placed in it. © 2005 Society of Plastics Engineers.