|Schematic of MuCell injection molding process (injection molding of microcellular plastics with characteristic microstructures). Kramschuster et al. 2005. Click to enlarge.|
Ford researchers have been investigating the use of the MuCell microcellular foam injection molding process to reduce component weight; weight reduction is a key Ford strategy to help lower fuel consumption. (Earlier post.)
MuCell, developed and patented by MIT and now owned by Trexel Inc., involves the highly controlled use of a gas such as CO2 or nitrogen in its supercritical state (SCF) in the injection molding process to create millions of micron-sized voids or bubbles in uniform configurations. The voids are created as a result of homogeneous nucleation that occurs when a single-phase solution of polymer and gas passes through the injection gate into the mold.
The single-phase solution is created through the operation of a conventional injection molding machine which has been modified to allow the creation of a single-phase solution. Trexel says that the key modifications to the system involve the use of a precision SCF delivery system to deliver SCF to special injectors based on mass flow metering principles.
The SCF is then injected into the barrel where it is mixed with the polymer via a specially designed screw. A shut-off nozzle maintains the single phase solution while the injection molding screw maintains sufficient back pressure at all times to prevent premature foaming or the loss of pressure which would allow the single phase solution to return to the two phase solution.
In Ford research trials, the microscopic bubbles reduced component weight up to 10%, while retaining all necessary properties for a non-visible part and without sacrificing dimensional integrity or durability.
Using MuCell technology also eliminates pack and hold times in the molding/cooling process (to address warpage), reducing cycle times. With MuCell, the typical time to mold a part during trials ranged from 30 to 60 seconds, a 50% time improvement.
The MuCell process also requires less clamping force, allowing a reduction in injection press size by up to 50%; reduced cycle times and clamping forces translate to energy savings and reduced CO2 emissions associated with production.
Ford says that it is continuing to research potential uses of MuCell technology for production of interior components. Ford also has patents pending on improvements to the MuCell process.
Adam Kramschuster, Ryan Cavitt, Donald Ermer, Zhongbao Chen, Lih-Sheng Turng (2005) Quantitative Study of Shrinkage and Warpage Behavior for Microcellular and Conventional Injection Molding. Polymer Engineering and Science. doi: 10.1002/pen.20410
Jingyi Xu and David Pierick (Trexel) Microcellular Foam Processing in Reciprocating-Screw Injection Molding Machines