Consistency in the coating of a dry lubricant to reduce friction from stacked tolerances is very important. Above, the surfaces of three eyelets are compared. The shiny surface on the left is uncoated, while the center eyelet demonstrates an irregular coating which can come with powder coatings with large powdered PTFE particles that settle out due to low “hang time.” On the right is the dry lubricant from MicroCare Medical, which uses microdispersion technology to create a better hang time for the PTFE particles in the carrier fluid.

One of the most familiar challenges associated with the designing and engineering of medical devices is achieving stacked tolerances.

Tolerances refer to the permissible limit or limits of variation in a physical dimension and are specified by the design engineer to allow reasonable leeway for manufacturing imperfections and variability without compromising performance.

Stacked tolerances are a common challenge when it comes to complex, single-use mechanical assemblies such as staplers and arthroscopic devices. This challenge becomes critical for design and manufacturing engineers because it can lead to problems in the performance of a finished device. For example, when a mechanical assembly such as a medical stapler is put together, the tolerances of each metal stamping, spring or plastic part may begin to combine in such a way that the assembled device requires more force to actuate, or execute. This is most common in high-volume production, when tooling used to manufacture metal stampings, springs, and plastic parts begins to wear. In this case, manufacturers want to maintain the quality and functional consistency of the finished device while controlling costs.

The way that these stacked tolerances are addressed has implications all the way into the operating room, where the doctor needs the device to function precisely, predictably, and smoothly in order to complete a procedure. One way engineers can address these tolerances is by choosing to design everything with tighter tolerances to gain higher precision. But “precision” usually means more frequent inspection and maintenance of tooling and fixtures used throughout the manufacturing process, driving up the unit price of a finished device. A more common way of dealing with stacked tolerances is to apply a lubricant coating such as PTFE or silicone on the finished assembly to reduce friction.

The dry lubricant advantage(s)

Dry lubricants using PTFE particles are used on many devices or mechanical assemblies found in operating rooms: catheters, cutting tools, staplers, hypotubes, and other surface-to-surface complex assemblies. These easy-to-apply lubricants are an effective means for the design engineer and manufacturer to reduce the impact of tolerance stacking on device functionality.

In fact, many single-use medical devices used today would not be commercially viable without a lubricant coating. When used properly, dry lubricants typically reduce the force needed to actuate or execute a device by 25% to 30%, and provide an almost effortless actuation for the medical professional performing the procedure.

Photos show importance of utilizing a dry lubricant formula with a long PTFE “hang time.” On the assembly line, in dipping bins, some dry lubricant solutions will allow PTFE solids to rapidly settle so the coating is inconsistent from one production lot to the next; thus the functionality of treated assemblies may vary significantly between different production lots. Left photo was taken moments after the solutions were poured. Middle photo was taken after seven minutes. Right photo was taken after 20 minutes. On the left, you can see the PTFE particles have less hang time, and the particles are no longer uniformly suspended in the carrier fluid; frequent re-agitation would be needed in order to return this to a mixed state. On the right is MicroCare’s microdispersion lubricant, which after 20 minutes remains by comparison, uniform and consistent throughtout the fluid.

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