MICRO MOLDING NEEDLES AND SHARPS: THE SCIENCE OF COMPLIANCE
June 18, 2018
MICRO COMPUTATIONAL ANALYSIS
June 18, 2018

MICRO TOOLING

05/01/2012: When the design and material is nailed down to the point of spending capital to test the components, put them in the hands of investors, customers, and surgeons, it’s time for a micro mold to be made.  With any micro molding technology (thermoplastic, silicone, or metal), the tooling is THE most critical component to success.  Because the parts and molds are so small (see Figure 1.4) , the tolerances get smaller and the tooling must still be made to 25% of part tolerance to provide a good processing window.  With tolerances of +/-0.0005” (+/- 0.01mm), the steel tolerances must be +/- 0.0001” (+/-0.003mm) to achieve a good window.  There are not many tooling suppliers in the world that will sign up for these tolerances for 2 main reasons:

  1. They can’t measure +/- 0.0001” (+/-0.003mm) and therefore cannot validate
  2. They do not have the equipment/skill set to achieve these tolerances

Figure 1.4 Mold Insert and Molded Part on left and and Micro Transdermal Patch with <0.050” (1.3mm) tall needles on right.

Because these parts can be dust speck in size (and less in some feature sizes), the micro mold is the true enabler to their success.  There are some nuances to consider with micro molded parts whether they are thermoplastic, silicone, or metal injection molded.

  1. RUNNER/SPRUE- 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.
  2. PARTING LINES- Molded halves come together and form parting lines on molded parts on the order of ~0.0002-0.0003” ( ~ 10 microns).  These parting lines need to be considered when they will be assembled to other parts, they can prevent proper fit if they are not “guided” or moved through the assembly process properly.  These 10 microns can easily make or break your assembly and may need to be positioned in the assembly to avoid these features being stacked up against each other.
  3. DRAFT- More is better but can be as small as 0.2 degree 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.   Ways around this are to eliminate draft on a small portion of the part being positioned, draft the assembly station/fixture with the matching draft angle, or add a feature to the part or runner that can be used and removed later on.
  4. GATE LOCATION- 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 mold and do 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.
  5. GATE VESTIGE- Most micro molded parts are kept on an edge gate.  If so, they need to be de-gated properly to avoid 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 dimple in the wall thickness (if the walls are thick enough that is) so the vestige will “sit” below the surface of a guide or a mating component in the assembly.
  6. SURFACE FINISH- 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 vapor honing or roughening the surface to provide improved surface area for bonding, for example.  Smoother surfaces in assembly can cause problems in ejection from the mold and a trade-off surface may be required in order to “steal from Peter to pay Paul”.  Which one will be the lesser of the two evils is dependent on material selection (see below)