MICRO COMPUTATIONAL ANALYSIS
4/16/2012: A first look at device and assemblies of this critical nature, we must first take a long look at the solid models and perform computational analysis on their design. Because micro molds and initial parts can be costly in their development cycle, micro mold flow analysis provides an insurance policy that a particular design will fill with a particular material. Micro molded components can be as small as dust specks and/or have features that are this small. The state of art micro molding features replicated from mold to molding are ~ 3 micron diameter dimples, 3 microns in depth- 80+Million of them across a microscope slide! (see Figure 1.1)
Figure 1.1: Microscopic Molded Features in Silicone, photo provided by MES
An all too often assumption when comparing parts made by conventional molding methods to micro molding methods that parts can be filled with the same software and same modeling methods. Flow analysis and modeling parts flowing through a .020” (500 microns) gate is VERY different as parts flowing through a 0.003” (75 micron) equivalent gate.
The difference between a part going through a .002-.003” (75 micron) equivalent gate vs. a .020” (500 micron) equivalent gate is that it sees much more thermal energy from the shear heat (see Figure 1.2) pushing through that small orifice. As a result, the solid model mesh resolution (see Figure 1.3) is required to be extremely high resolution to determine what is happening in the gate and thin walled areas. Similar to this is using an inspection gage requiring one decimal point higher resolution than the specification requires, the mesh of a solid model used in a mold flow simulation requires single digit microns (0.0001-0.0002”) mesh when tens of microns (0.0005-0.001”) mesh are required in the part.
Another important factor in a mold flow or thermal simulation is processing knowledge and direct application knowledge with injection molding and mold making. It is important to know the practical experiences in these processing techniques along with the mold and die design to accurately depict proper mold flow. Knowledge in plastics engineering, micro mold design, gate location and size, and runner and sprue geometry is critical to properly analysis of the results of a micro mold flow simulation.
Figure 1.2 Shear Effects Modeled in 50 micron Gate orifice.
Figure 1.3 Micro Molding Mesh Rendering used in Mold Filling Software
Process-specific and material-specific knowledge helps to draw from experience. Critical to many micro molded components the gates will have to be properly sized so as not to place undue heat stress on the material entering the cavity. For heat sensitive materials such as bioresorbable and biopharmaceutical polymers, the residence time in the injection barrel, nozzle, and hot runner is important to understand and to minimize such that additional heat is not placed on the material during processing.