We investigated the foaming mechanisms associated with the gas counter pressure (GCP) and mold opening processes during foam injection molding for products with high quality surface. The molding process involved injecting a polymer/nitrogen solution into a nitrogen-pressurized mold, cavity. The pre-loaded cavity pressure (that is, the GCP) prevented cell nucleation and growth within the polymer melt. This was the case until the GCP was released (without opening the mold) or until the melt filled the mold cavity completely. Then the cavity thickness was suddenly increased by opening the mold. In an attempt to understand the mechanism of cell nucleation and growth, we monitored the pressure profile at several locations in the mold cavity during the molding process. We found that the pressure history in the mold cavity, both as a function of space and time, was the dominant factor that determined cell morphology. By applying the GCP and the mold-opening process, the cell size uniformity was improved when compared with conventional foam injection molding. These methods did not compromise the achievable void fraction. Using GCP and the mold-opening process also significantly improved the surface qualities of the foamed parts. This was because the premature cell growth at the flow front during the mold filling stage had been eliminated. (C) 2017 Elsevier Ltd. All rights reserved.