High Aspect Ratio, Thin-Wall Micro Molded Devices
The smallest components in a medical device or drug delivery device are often times the enabling components to the device. Examples of these devices are: Polymeric needles, catheter sheaths, microfluidic channels, endoscopic links, and PCR wells. As the components of these devices get smaller, the tolerances and wall thicknesses follow suit. Tolerances of +/- 5 microns (~+/- 0.0002”) and wall thicknesses of 25-250 microns (0.001”-0.010”) are common. Although challenges exist in creating these tolerances in both the steel mold and the micro molded components, the determining factor for wall thicknesses this thin are dependent on the aspect ratio, where the wall thickness is with respect to gate location and end of fill.
The following are general guidelines to consider when designing thin-walled micro molded components:
Mold halves form parting line witness marks on the order of 5-10 µm (0.0002-0.0004”). It’s all relative of course as these parting lines would be considered excellent in the macro world, but not so when the parts or features assembled are pepper flake size. These tight tolerance parting lines still can prevent proper assembly fit if they are not “guided” or moved through the assembly process properly. These 5-10 microns can affect assembly to other micro molded components and purposefully clocked in the assembly to avoid the parting line witness shifting a tolerance stack-up.
More is better but can be as small as 0.1° of taper. But this taper (inside or outside) of a molded part can be cumbersome to deal with. Having your micro part “ride” on a taper will provide an arbitrary or irregular surface with which to improperly position it for assembly to other parts. Work-arounds for this design attribute can be; eliminating draft on a small portion of the part being positioned, drafting the assembly station/fixture with the matching draft angle, or adding a feature to the part or runner that can be used and removed later on.
The runner and/or sprue (if one exists), can be our friend or foe in micro assembly. We could use it as part of an assembly aid to hold onto a part in the automated assembly or add special locating “jogs” in the runner aid in the positioning in an assembly nest.
It is critical to choose a gate location that will actually create uniform flow in a micro molded part. Without a uniform flow, the part may not fill the thin-walled cavities and risk damage to the delicate pins and cavities. Thinking ahead as to how to remove this gate later on in the assembly process is important because if it has already been de-gated, the gate trim job may have left a divot or a proud protrusion that has to be rotated away from nesting, guide rails, or other parts.
Most micro molded parts are tunnel gated (parts are free from the runners upon ejection in the mold). Some are kept on an edge gate. If designed correctly, tunnel gates are best for scalability and cost. In the event the part is edge-gated, de-gating avoids issues with small “picks” of material causing damage to an artery, or causing issues with automation and assembly. These small picks can be addressed in the mold design by placing a recessed “dimple” in the wall (if they are thick enough) so the vestige will “sit” below the surface of a guide or a mating component in the assembly.
Often overlooked, the surface finish of a molded part is important in “riding” or “guiding” features into other features. Some surface finishes in assembly are best served with roughening the surface to provide improved surface area for bonding, for example. Depending on material selection, smoother surfaces are generally best for ejection from the mold, however.