The microcellular process consists of four main, sequential steps, that end with solidification.
Figure 1: Microcellular foaming process, prior to solidification. (A) Single phase gas/polymer mixture, (B) bubble nucleation, (C) cell growth
The four main steps are:
Gas dissolution - In the plastication period of the microcellular injection molding process, a supercritical fluid (SCF) or blowing agent, nitrogen (N2) or carbon dioxide (CO2), is injected in to the polymer to form a single-phase solution. The gas is dissolved in the polymer melt due to applied high pressure.
Nucleation - this is the 'seeding' of bubbles and occurs as a result of a pressure drop. There are two types of processes, either short shot or full shot:
Bubble growth - this is the increase in size of the bubble seeds as the gas diffuses out of the melt and into the bubbles.
Solidification - During the foaming process the mold is continuously cooled down, creating the internal cellular structure. The result is a foamed material with cell size of around 5-100 microns, the actual size depending on processing conditions. The lack of a formal packing phase reduces the residual stress in the material and results in extremely low warpage.