MuCell

Microcellular plastic foams can be made by mechanically or chemically dispersing a gas, usually carbon dioxide or nitrogen, into the polymer melt.  A mechanical process employing supercritical fluids for this purpose, developed and licensed by Trexel Inc., is called MuCell (µCell).  Further physical foaming methods are Optiform (Sulzer Chemtech) and Ergozell (Demag Ergotech).

The critical point is the highest temperature and pressure at which a substance can exist as a vapor and liquid in equilibrium.  A gas above its critical temperature becomes a supercritical fluid. Carbon dioxide at about 1100 psi or nitrogen at about 750psi become supercritical and dissolve into the polymer melt.  As molding pressure decreases, the gas undissolves from the polymer to form a uniform cellular structure.  Such foams have cell sizes ranging from 5 - 100 microns and exhibit excellent mechanical properties and can provide thermal and acoustic insulation.  

The benefits of microcellular foaming include:

• Reduced weight
• Improved flowability
• Lower apparent viscosity
• Less warpage, shrinkage and molded-in stress
• Improved cycle time
• Reduced clamping force

General observations regarding the MuCell process include:

• Nitrogen yields smaller cells while carbon dioxide provides better flowability
• Thin walled parts can be foamed
• Cycle time can be reduced by 40%
• Weight reduction up to 30% is possible, for technical demanding parts the weight saving lies between 8 - 15%
• The advantages of physical foaming can be good combined with film- or textile injection molding.
• To make best use of the physical foaming method, the tool needs to be optimized (temperature, gating,first cut, ventilation) and potentially adapted to the process.