In micromolding, material flow follows its own set of rules
As components for medical devices and consumer electronics continue to decline, micromolding is playing a larger role in the manufacturing mix.
Generally speaking, according to MTD Micro Molding of Charlton, MA, parts qualify as “micro” if they have:
- Wall stocks between 0.002 and 0.004 in.;
- aspect ratios in the 250:1 range;
- a finished part weight so low that 520 parts can be made from a single plastic pellet.
However, notes MTD Micro Molding, “a part does not have to be microscopic to be considered a micro part.” That said, at this scale, material choices and process understanding begin at the molecular level, explained Patrick Haney, R&D engineer at MTD. The complexity of a part’s application can result in these considerations.
“Sometimes you need a material to perform a basic function, and other times the material is just as important as other criteria to function in an assembly,” Haney explained. “For example, for a mechanical part application – like staples, sutures, or screws – what you need to focus on are the mechanical characteristics of the part like strength, flexibility, etc. You can add the degradation rate if it is an implantable application.
“If you have a specific application, like a material that you overmold onto electronics to allow it to interact with a smart device – things like insulating properties, electrical conductivity, or in some cases ultrasonic damping – these characteristics are influenced by the molecular structures.
Fluid flow, or rheology, is a critical issue in the production of micro parts.
“In mic, it’s realistic that you hit an ultimate slimming threshold,” Haney said. “After this point, the viscosity of the polymer stops decreasing with increasing pressure. Once this happens, many of the tricks and tools of the trade that a processor uses to fill thin walls or troubleshoot don’t apply. After reaching this threshold, you are really working with a non-Newtonian fluid that acts Like a Newtonian fluid. There’s a whole new set of rules you have to follow when it comes to the flow of plastic fluids.”
Before the plastic cools and solidifies, he continues, “the flow front behaves differently. A material’s ability to fill high aspect ratios changes. Materials develop an “alternate personality” that must be relearned as flow characteristics have changed. Knowing the behavior of fluid flow at such high velocities and knowing how to manipulate plastic flow in the region beyond the thinning threshold is the key to success.
“When you mold in this region [the second Newtonian plateau], the microstructure of the material – especially if it is crystalline – is often completely different from what it would otherwise be. Depending on the material you are working with, this can affect anything from part stiffness and chemical resistance to things like shrinkage.
As the material cools, the polymer matrix undergoes phase changes. During this period, “crystal structures form and internal stresses are frozen in place,” Haney observed. “Because you exposed the material to such excessive pressures and shear rates, the microstructures that form are entirely different from what would form if exposed to ‘regular’ shear rates and pressures. Understanding the impact of changes is equally important when considering the part’s application and performance, and which material characteristics are important.
In MTD’s work on molding implantable microparts, validation of success criteria means “we monitor things like intrinsic viscosity and residual monomer content,” Haney said, “so molecular homogeneity of a resin is essential for MTD to develop robust process windows. When we partner with material suppliers, we challenge them to meet tighter molecular weight distributions from batch to batch, as well as maintain a higher standard for material consistency. »
The functionality of high-performance polymers must be rigid in micromolding because when the parts are so small, the mold steel surrounding the cavities acts as a quick heat sink, Haney explained.
“With this rapid heat transfer, the crystallization phase change cannot always be completed before the material reaches the ejection temperature. To optimize crystallization, which directly influences the stiffness of the part, without lengthening the cycle time, there are many injection molding parameters and their combinations to be manipulated, unless you understand the combination of effects that injection molding parameters can have on the crystallization mechanics of a material, you will not be able to optimize the properties of the material in this way.