By Sean Fenske, Editor-in-Chief
 
Molding is a fairly well understood part fabrication process. Similarly, many materials used for this technique for medical device components are also predictable and don’t present many surprises when used. Molding at the micro level, however, is not as clear cut. Some designers may not be too familiar with the actual method while others are unsure how a material will behave at the micro scale.
 
With micromolding gaining interest in medical device manufacturing as products and technologies are fulfilling the needs of healthcare professionals by shrinking in size, there are a number of questions that need to be answered. Ensuring repeatability, reliability, and patient safety is critical when developing and manufacturing these devices at the micro scale, which can be unfamiliar territory for some OEMs.
 
Fortunately, Patrick Haney, R&D engineer at MTD Micro Molding, has offered responses to a number of questions around micromolding and the effect this process can have on materials. He addresses the driving forces behind the interest in micromolding, what happens when processing the materials at this scale, and considerations to keep top of mind when designing micro injection molded parts.
 
Sean Fenske: When molding at a micro scale, what materials are most often used? Are there materials that can’t be used at a certain (small) size?
 
Patrick Haney: In general, any thermoplastic can be used in micro injection molding. Micro molding is not restricted by materials—it’s more that the material is restricted by specific scenarios and expectations. Materials can be limited by part geometry more than the material characteristics itself and some materials are better at filling certain geometries. Plastics behave differently in micromolding most of the time and semi-crystalline ones are typically better at filling thinner walls compared to amorphous ones. This is true for both micro and macro; at MTD, we take these materials and push them beyond their published limits.
 
Fenske: Why do parts need to be made so small? What applications are most commonly requiring such components?
 
Haney: Medical implants are increasingly becoming smaller and more powerful, requiring the best of the best in micro manufacturing to accomplish. Parts are miniaturized for patient comfort and minimally invasive devices continue to shrink to smaller sizes with the goal to be inserted, installed, or moved through smaller needle and tubing diameters for certain medical applications. The focus is on improved comfort and better outcomes, while decreasing procedure time, risk, and recovery time for the patient. Smaller devices mean gaining more and better access to certain parts of the body for targeted treatment.
 
In some cases, devices need to be ultra-small to work in a certain application, like optometry for example. But in the medical device world, there is a need to shrink components and devices across the board.
 
With devices and implants being miniaturized comes a need to be able to visualize and track these tiny parts once implanted in the body. This can be accomplished by incorporating radio opacity into the material.
 
Fenske: What material properties are most important when molding at the micro level?
 
Haney: With micromolding, engineers are focused on viscoelasticity and shear sensitivity. Some materials hit a maximum shear thinning threshold sooner than other ones do. Generally, materials that reach that limit faster are more difficult to coerce into micro design features. Looking at how a material is going to behave under extreme shear rate exposure is most important for optimal processability, in my opinion.
 
Fenske: Are there tradeoffs made at that size such as strength or other functional characteristics?
 
Haney: Nothing will get stronger the smaller it gets. A feature that is only 0.005 inches thick compared to one that is 1” will be different regarding strength and functionality. OEMs need to consider how the size of a device will affect the mechanical properties.
 
A material’s thermal and chemical properties will act relatively the same at the micro scale as they would at the macro scale. However, in some cases, the microstructure of the polymer may be different. For example, certain crystalline structures will form differently when exposed to extreme shear rates and rapid cooling environments.
 
The impressive part of successful micromolding is preserving molecular weight and properties in a molded part after introducing a material to the violent process, high temperatures, and pressures that typically come with micro injection molding.
 
Fenske: When designing in micromolded parts, what considerations are there versus more traditional molded components? What do designers need to be aware of?
 
Haney: A material that worked well for an original medical device design may not be as well-suited for the miniaturized version with smaller, nominal wall stock and thin-wall features. Part designers need to be aware of the pressures and temperatures involved with coercing a viscous material into a high aspect ratio geometry and, in some challenging cases, will need to make concessions that wouldn’t be needed at a larger scale. This is to prevent a scenario where material is seeing extreme temperatures and pressures that can cause warpage, discoloration, and even destroy the material integrity. The goal is to produce successful initial samples as well as create a robust molding window to sustain high volume production, and the optimized relationship between part design and material can be key to that.
 
Fenske: What challenges exist in getting even smaller with micromolded components? Have we reached the “bottom” yet in terms of size?
 
Haney: The “basement” is different for every part design scenario and the relationship between the design and the material will determine how small the overall size of the part can be.
 
MTD produced an EVA ophthalmic part that weighs in at 0.00000313 grams. This required MTD’s Sarix technology to make a precise gate needed for such a tiny part. The gate measured 0.0018 by 0.0008 inches.
 
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