micro parts to market... faster


05/14/2012: The preparation done to micro components at the design stage carries through all the way to automated assembly.  Because the tolerances are critical and diminishing to microns and sub-microns, the stack-up tolerances of the micro components and their design criteria are scrutinized and their method of manufacturing are also considered for long-term production.  The mechanics of micro assembly and the testing, inspection, application knowledge are required for fully functioning specks of dust working collaboratively and repeatedly.   Handling these components is a challenge requiring its own article alone but some methods to handling are tiny vacuum suction cups, static-free gloves, and sometimes with tiny tweezers (See Figure 1.6).  Below are some of the key joining methods for assembling micro components into micro assemblies.

  1. AVOIDANCE OF HANDLING: Because these parts are microscopic, the best way to handle them is to not handle them at all. Combining geometry into the least number of parts in a micro assembly is design work worth the effort because picking them up, registering them in nests, joining them to other parts of like or unlike materials can be much more costly than spending the time in the design stage.TWO-SHOT
  2. MICRO MOLDING/OVER-MOLDING: Two-shot molding (2 injection barrels of two different materials) injecting into a mold at two different locations or in the same location with a rotating mold enables one “set up” of combination geometry and materials. For example, if a pump component requires a seal (or silicone gasket), it is easier to 2-shot mold a gasket in place in an o-ring groove into the same mold as the pump piston than it is to bowl feed an o-ring onto s precision tracking mechanism, grip the o-ring with some scissor grippers and place it over the piston. Firstly, the o-ring is subjected to stretching and shape distortion with the scissors, bowl feeding, and riding on a track of an assembly system.
  3. LASER WELDING: If 3d geometry isn’t possible to combine, and material strength allows for it, laser welding is a good method for joining micro components. Careful control of laser energy and power densities can be used to selectively clean and strip materials such as wires quickly and non-destructively. Laser beam sizes in the sub-micron and below size range and therefore pinpoint accuracy of multiple laser welds is possible using a nano-positioning table or a multi-axis robot station mounted on the automation rotary or shuttle system. Laser welding can be done using existing wall thickness or an extra “tab” designed into the singular component being welded OR a small amount of similar material can be introduced using wires, etc as is done with soldering in electrical components.
  4. ULTRASONIC WELDING: There is a time and a place for all joining methods and ultrasonic welding is a good method to join thermoplastics as well as material compatible metals. Because of the size of the parts, spin welding is usually not a good option due to the amount of wall thickness in micro sized parts, however a very tiny (100-200 um) weld bead is sometimes possible and plausible. Specialized and low energy boosters are required for micro components as the amount of energy required for a strong weld is extremely low. Customized ultrasonic horns are also used to dissipate and distribute the correct “tuning fork” focused energy.
  5. SOLVENT BONDING: Solvent bonding is often used as a quick, low capital investment method for joining micro components in an assembly. Quick and dirty fixtures can easily test the feasibility of multiple and dissimilar materials in a micro assembly using micro and nano pipettes to distribute the tiny amounts of solvents. Appropriate solvents must be used that are compatible with the materials being solvent bonded, especially if the assembly is to be used as an implantable. Although quick and capital-friendly, ramping up a very high volume component/assembly using solvent bonding can be headache-ridden as the method is not easily automated and repeatable, is messy, and difficult to validate on a high volume scale as a result.
  6. STAKING: Micro staking is a very inexpensive method to join polymer and metal components. In battery cans, for example, crimping or staking is a very common practice to generate good seals for preventing very caustic fluids from escaping the battery vessel. Inexpensive progressive stamping dies enable a moderately fast method for staking both polymers and metals to one another using a pressurized “folding” of one material into another. Cons of this method are that material lot variations ramping up to high volume



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)