Microcellular Foam Injection Molding
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.
Microcellular foaming is quite different from general physical foaming. First, a large amount of inert gas such as CO₂ and N₂ is dissolved in the polymer during the microcellular foaming process, so that the gas is saturated in the polymer, and it is impossible to achieve the polymer-gas homogeneous system by the general physical foaming method. Such a high gas concentration.
Secondly, the number of nucleation of microcellular foaming is much higher than that of the general physical foaming molding, which is a method of gradually changing the thermodynamic state, which easily leads to the occurrence of large cells in the product and uneven distribution of cell size. The thermodynamic state of the microporous plastic molding process changes rapidly, and the nucleation rate and the number of nucleation nucleus greatly exceed the general physical foam molding.
Microcellular foam molding has many advantages over conventional foam molding.
(1) The bubble formed by it has a small diameter, and can produce a thin-walled (1 mm) product which is difficult to produce due to the large micropores in the general foam;
(2) It is a pore structure of the microporous foam material, which is a closed-cell structure, and can be packaged with a barrier product;
(3) CO₂ or N₂ is used in the production process, so there is no environmental pollution problem.