In the conventional structural foam injection molding, a chemical foaming agent is usually used, and since the foaming pressure generated is low, the produced article is limited in wall thickness and shape. Microcellular foam injection molding is limited by the use of supercritical inert gases. Microcellular foam injection molding uses supercritical inert gas (CO₂, N₂) as a physical foaming agent.
The process is divided into four steps:
(1) Gas dissolution: a supercritical liquid of an inert gas is injected into a polymer melt through a syringe mounted on a structure to form a homogeneous polymer/gas system;
(2) Nucleation: During the filling process, the gas is precipitated from the polymer due to the pressure drop to form a large number of uniform gas cores;
(3) Bubble growth: the gas grows under precise temperature and pressure control;
(4) Stereotype: When the bubble grows to a certain size, it is cooled and shaped.