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Using Dry Lubricants to Adjust for Stacked Tolerances
By overcoming stacked tolerance issues and improving medical device quality, dry PTFE lubricants are a powerful addition to an engineer’s bag of tricks.
April 17, 2015
10 Min Read
Complex shapes can be coated with nearly frictionless PTFE dry lubricants, enabling complex tools to move, slide, rotate, and pivot smoothly and reliably.
In a perfect world, every medical device component would be manufactured to exact dimensional specifications without variation. Sadly, because the world is far from perfect, engineers design tolerances into almost every component. These tolerances define the maximum degree of dimensional variation that any component can bear. Fundamentally, it’s all about costs: Since very tight tolerances generally drive up manufacturing costs, it makes economic sense to accommodate minor dimensional variations during the manufacturing process as long as these variations do not compromise the performance of the finished assembly.
But there’s a catch. How can manufacturing engineers accommodate stacked tolerances without impacting the functionality of many complex medical devices? The answer: by using dry lubricants. This article examines the use of dry lubricants for managing the cumulative effects of dimensional variability.
The Cost of Tight Tolerances
Stacked tolerances are an inevitable element of multipart mechanical assemblies with moving parts. Hypothetically, if a device features four or five nested components, random variations affecting all of the components can fall within the worst-case limits of their design tolerances. This possibility presents two challenges to manufacturers. First, during the assembly process, mated parts may fit too tightly, making assembly slow and difficult. Alternatively, friction from overly tight components can cause the device to function poorly because the accumulated tolerances restrict free movement.
Addressing tolerance issues has implications all the way down to the end-user, especially for medical devices that must perform mechanical functions precisely and smoothly. One option for dealing with stacked tolerances is to design components with tighter dimensional tolerances, ensuring higher performance levels. But in the interest of keeping costs low, ever-tighter tolerances usually are not the most effective choice.
Another option is to lubricate the components. To do so, design engineers often specify the use of plastic or nonferrous materials because they have inherent lubrication properties. Alternatively, they can specify the use of friction-reducing silicone- or hydrocarbon-based lubricants. However, these strategies have several drawbacks. End-use loading requirements can preclude the use of less-durable plastics and metals. And silicone and hydrocarbon lubricants can present cleanliness problems because they tend to transfer over to untreated surfaces, where they can attract and hold surface contamination.
Enter Dry Lubricants
In industrial applications, an often overlooked but cost-effective way to address the challenges of stacked tolerances is to use dry lubricants. Usually based on a form of dry polytetrafluoroethylene (PTFE), industrial dry lubricants function like powdered graphite, which consumers use to lubricate hinges and drawers.
How to Apply Dry Lubricants
While many engineers understand the benefits of using dry lubricants, they may not be aware of the myriad simple ways that the lubricants can be applied. In all cases, manufacturers should consult a coatings provider to determine which process is best for the particular application.
To apply dry lubricants, PTFE is microdispersed in a carrier fluid. The ratio of PTFE in the fluid is normally specified as a percentage by weight. After being applied to medical device parts using a spray or dip-coating method, the carrier fluid evaporates, leaving a smooth, dry PTFE coating on the surface of the part. Dry lubricant processes include:
Dipping. Most commonly used in high-volume production and suitable for coating small parts, coils of wire, and irregular shapes, dipping provides a consistent and uniform coating to virtually any surface geometry and internal surfaces. Fully finished devices can also be dipped into a PTFE carrier-fluid dispersion. The coating level is determined by the concentration of solids, the rate of withdrawal, and the number of applications. A single dip is adequate for most uses.
Wiping or Brushing. Useful for coating continuous surfaces such as rods, tubing, or sheets, wiping and brushing are appropriate for coating small, selected areas of a larger part. A variation of this method is flood coating followed by wiping.
Air Spraying. Spray equipment can be used to apply dilute dispersions. A carrier fluid engineered to decelerate evaporation can be specified to ensure a consistent coating.
Airless Spraying. Applied using a handheld spray gun or automatic spray heads, airless spraying can be either intermittent or continuous. Typically, this technique involves applying a succession of thin coats, allowing the surface to dry between applications. It typically has better results than the application of a single thick coat, which can take longer to dry and can cause ‘mud cracks,’ uneven coverage, and poor adhesion.
Aerosol Sprays. Suitable for use on injection-mold tooling, aerosol sprays offer convenient surface application and quick coverage. Some aerosol sprays have very tight spray patterns, enabling pinpoint lubrication and thus allowing them to accommodate hinges and pivot points. Many aerosol sprays contain additives that make them unsuitable for medical device applications.
Drying. All coatings dry best in dust-free areas. Drying times depend on the carrier fluid and the thickness of the application. Increasing the temperature of treated parts, especially the internal surfaces of complex assemblies, can reduce drying times.
Heat Curing. Some applications require a more durable coating. PTFE dry lubricant coatings can be made more durable by melting the coating to ‘fuse’ it to the substrate. Because heating turns many dry lubricants completely clear, it appears as if nothing is on the surface of the device. This is particularly helpful if the coating must be semipermanent or invisible.
By overcoming stacked tolerance issues and improving the quality and performance of finished devices, dry PTFE lubricants are a powerful addition to an engineer’s bag of tricks. They reduce the coefficient of friction on the surface of treated parts to 0.06, reducing actuation forces by 25 to 30% and smoothing device operation. In fact, many complex medical devices manufactured today would not be commercially viable without the use of a dry lubricant.
Clean and nonmigrating materials, dry PTFE lubricants can be applied in-house to surfaces in very thin to reasonably thick layers and can be incorporated into the assembly process. Compatible with most plastics and metals, they readily conform to virtually any surface geometry, including braided wire cables and complex meshing. They also penetrate easily into complex shapes and blind vias.
Because components coated with dry lubricants snap together easily, they speed the assembly of complex devices. Available with ISO 10993 certification, medical-grade formulas have nonpyrogenic properties and are fully compatible with a range of sterilization processes. PTFE materials exhibit few, if any, toxicity or handling issues, and nonflammable carriers can be specified to ensure maximum safety. The coating can also be used as is or after a brief heat-treating process has converted it into a hard, durable, and attractive finish.
PTFE Content Is Key
To ensure time and cost savings in the assembly room, manufacturers should consider a range of variables when choosing the correct dry lubricant for their application. While they can purchase PTFE in dry-powder form, they may mix the powder with an inappropriate quantity of carrier fluid. In addition, the resulting lubricant may exhibit particle-size variations, compromising its properties. In contrast, ready-made medical-grade dry lubricants are engineered and calibrated with high-purity carrier fluids to satisfy global regulations and meet user specifications, making process validation fast and simple.
A major challenge to maintaining a quality coating process is to manage the ‘hang-time’ of the PTFE particles in the carrier fluid. Physically heavy, the large PTFE particles found in many formulas have a brief hang time. In such cases, the PTFE particles sink to the bottom of the dip tank or storage vessel, separating from the carrier fluid and degrading the consistency of the coating application.
To extend the hang time, manufacturers can use a precalibrated fluid composed of microparticles. Microscopically small particles stay in suspension longer than heavier particles, resulting in a thin, even, smooth film over the entire treated surface.
Because PTFE does not dissolve in the carrier fluid but is suspended in it, heavier PTFE particles settle to the bottom (left) unless the liquid is agitated. Alternatively, a carrier fluid with a similar density to PTFE delays the settling process (right), resulting in smoother, more consistent coatings.
Naturally, there is no such thing as a free lunch. To ensure a smooth, streak-free coating over long production runs, the ratio of the PTFE solids to the carrier fluid must remain constant and correct. The first challenge to achieving this balance is to control the evaporation rate of the carrier fluid.
The carrier fluid by design evaporates very quickly because rapid drying enables high production rates and improves the consistency of the coating on the device. However, rapid evaporation can cause the carrier fluid to evaporate from the dip tank or storage vessel during the coating process, causing the PTFE-to-carrier ratio to fall out of spec and leading to inconsistent coating results. While many users replace the contents of their dispersion storage vessels after a defined period, this procedure, while effective, can be wasteful and expensive. Instead, manufacturers should develop an easy means to monitor the lubricant-to-carrier ratio.
Traditionally, the most precise method for measuring the microdispersion ratio has been to ‘boil down’ a measured sample and allow the carrier fluid to evaporate. The weight of the dried PTFE residue is then compared to the weight of the original sample. While, highly accurate, this method is slow, taking a minimum of several hours to achieve accurate results. To speed up the process, manufacturers sometimes introduce shortcuts that compromise coating consistency.
A more advanced option uses an automatic fluid test meter to measure the PTFE content with accuracy to two decimal places. In this method, a 50-mL sample is pulled from the PTFE dip tank and placed into a glass sampling vial. The vial is then inserted into the fluid test meter and analyzed instantly. A color touchscreen displays the PTFE content as a numerical percent readout. Taking the guesswork out of the testing regimen, this technology does not make contact with the lubricant, allowing it to be returned to the dip tank while eliminating the chance of operator error. If needed, a simple conversion chart makes it easy to determine the quantity of top-off fluid or concentrate required to maintain proper dispersion ratios.
The Bottom Line
For medical device design engineers and manufacturers, dry lubricant coatings can enhance the performance and consistency of finished devices. Although numerous factors can impact the final results, it is essential for engineers to understand the coating application process and maintain consistent PTFE dispersion ratios.
Developing a partnership with a vendor that understands the coating process will go a long way toward improving the quality and consistency of the finished product. The partner should be able to offer insights into regulatory compliance issues, the chemical parameters that affect the coating, and the methods for making the coating as consistent, efficient, and sustainable as possible. In addition, the supplier should be able to evaluate and optimize the client’s process, including the physical footprint of the system, the electrical requirements, maintenance requirements, and waste disposal and waste minimization measures. By emphasizing process control, manufacturers can ultimately maximize profitability.
Jay Tourigny is vice president of operations at New Britain, CT–based MicroCare Medical. Reach him at [email protected].
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