Fluid flow behaves differently in micromolding than it does in conventional injection molding, and many of the typical flow-management tools used by plastics processors cease to work, writes GEOFF GIORDANO.
As components for medical devices and consumer electronics continue to shrink, micromolding is playing a bigger 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 pellet of plastic.
However, MTD Micro Molding notes, “a part does not need to be microscopic to be considered a micro part.” That said, at this scale, material choices and process understanding start at the molecular level, explained R&D Engineer at MTD, Patrick Haney. The complexity of a part’s application can drive those 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 instance, for a part application that is mechanical — like staples, sutures, or screws — what you have to focus on is the part’s mechanical characteristics like strength, flexibility, etc. You can add in degradation rate if it is an implantable application.
“If you have a specific application, such as a material that you overmold on electronics to allow it to interact with a smart device — things like insulative properties, electrical conductivity, or, in some cases, ultrasonic dampening — these characteristics are influenced by the molecular structures.”
Fluid flow, or rheology, is a critical issue in producing micro parts.
“In micro, it is realistic that you reach an ultimate sheer thinning threshold,” Haney said. “After that point, the polymer viscosity stops decreasing with increasing pressure. Once that happens, many tricks and tools of the trade a processor uses to fill thin walls or troubleshoot don’t apply anymore. After you reach that threshold, you are really working with a non-Newtonian fluid that acts like a Newtonian fluid. There is a whole new set of rules you need to play by when it comes to plastic fluid flow.”
Before plastic cools and solidifies, he continued, “the flow front behaves differently. A material’s ability to fill high aspect ratios changes. Materials develop an ‘alternate personality’ that needs to be relearned as the flow characteristics have changed. Being familiar with how the fluid flow behaves at sheer rates that high and knowing how to manipulate plastic flow in the region past the sheer thinning threshold is key to success.
“When you mold in this region [the second Newtonian plateau], the microstructure of the material — especially if it’s crystalline — is often completely different than it would be otherwise. Depending on the material you are working with, it could affect anything from part rigidity and chemical resistance to things like shrinkage.”
As material cools, the polymer matrix undergoes phase changes. In that period, “crystalline structures are formed, and internal stresses are frozen in place,” Haney observed. “Because you have exposed the material to such excessive pressures and sheer rates, the microstructures that form are entirely different than what would form if exposed to ‘regular’ sheer rates and pressures. Understanding the repercussions of the changes is just as important when you consider the part application and how it needs to perform, as well as what material characteristics are important.”
In MTD’s work molding implantable micro parts, validating criteria for success means “we monitor things like intrinsic viscosity and residual monomer content,” Haney said, “so a resin’s molecular homogeneity is critical for MTD to develop robust process windows. When we partner with material suppliers, we challenge them to meet tighter molecular weight distributions from lot to lot as well as to hold a higher standard for material homogeneity.”
The functionality of high-performance polymers must be rigid in micromolding because when parts are so small, the mold steel surrounding the cavities acts as a rapid heat sink, Haney explained.
“With that rapid heat transfer, the crystallization phase change cannot always complete before the material reaches ejection temperature. To optimize crystallization, which directly influences part rigidity, while not extending cycle time requires manipulating many injection molding parameters and their combinations. Unless you understand the combination of effects that injection molding parameters can have on the crystallization mechanics of a material, you won’t be able to optimize material properties in that way.”