Product Code Database
The creation of microcellular foams as we know today was inspired by the production of traditional foams. In 1979, MIT masters students J.E. Martini and F.A Waldman, under the direction of Professor Nam P Suh, are both accredited with the invention of microcellular plastics, or microcellular foams. By doing pressurized extrusion and injection molding, their experimentation led to a method that used significantly less material and a product with 5-30% less voids that were less than 8 microns in size. In terms of mechanical properties, the fracture toughness of the material improved by 400% and the resistance to crack propagation increased by 200%. First, plastic is uniformly saturated with gas at a high pressure. Then, the temperature is increased, causing thermal instability in the plastic. In order to reach a stable state, cell nucleation takes place. During this step, the cells created would be much smaller than that of traditional foams. After this, cell growth, or matrix relaxation would initiate. The novelty of this method was the ability to control the mechanical properties of the product by varying the temperature and pressure inputs. For example, by modifying the pressure, a very thin outside layer could be formed, making the product even stronger. Experimental results found to be the gas that produced the densest foams. Other gases, such as Argon and Nitrogen produced foams with mechanical properties that were slightly less desirable.
The production of microcellular plastics is dependent on temperature and pressure. Dissolving gas under high temperature and pressure creates a driving force that activates nucleation sites when the pressure drops, which increases exponentially with amount of dissolved gas.
Homogeneous nucleation is the primary mechanism for producing the bubbles in the cellular matrix. The dissolved gas molecules have a preference to diffuse to activation sites that have nucleated first. This is prevented since these sites are activated nearly simultaneously, forcing the dissolved gas molecules to be shared equally and uniform throughout the plastic.
Removing the plastic from the high pressure environment creates a thermodynamic instability. Heating the polymer above the effective glass transition temperature (of the polymer/gas mixture) then causes the plastic to foam, creating a very uniform structure of small bubbles.
The foam injection process itself introduces surface defects such as swirl marks, streaking, and blistering, which also influence how the part reacts to external forces.
With the porous nature of this material, the overall density is much lower than that of any solid plastic, considerably dropping the weight per unit volume of the part. This also entails less consumption of raw plastic with the addition of the tiny gas-filled pockets, allowing for further cost reduction, up to 35%.
When observing the mechanical properties of these foams, a loss of tensile strength is correlated with the decrease in density, in a nearly linear fashion.
Injection molding and blow molding differ in regards to the type of product in need of being manufactured. Injection molding, much like casting, is centered around creating a mold for a solid object, which is to later be filled in with the molten plastic. Blow molding on the other hand, is more specialized for hollow objects, although it is less accurate regarding wall thickness with this dimension being an undefined feature (unlike in an injection mold where all dimensions are predetermined). In respect to MuCell ® and microcellular plastics, these processes vary from that of traditional plastics due to the additional steps of gas dissolving and cell nucleation before the molding process can begin. This process removed the "pack and hold phase" that allowed for imperfections within a mold, creating a finished product with greater dimensional accuracy and sound structure. By removing an entire step of the molding process, time is saved, making MuCell ® a more economical option since more parts can be manufactured in the same time compared to standard resins. A few examples of applications include automobile instrument panels, heart pumps, storage bins, and the housing on multiple household power tools.
|
|
