Medical Plastic Data Service Magazine

 

A TECHNO-ECONOMIC NEWS MAGAZINE FOR MEDICAL PLASTICS, MEDICAL DEVICES, DIAGNOSTICS AND PHARMA INDUSTRY

Our 29th Year of Publication
Page  2 of 8
 

Materials

Drug Delivery Device Using Polymers With Improved Tribology

 

POLYMER COMBINATIONS

 

When identical unmodified polymers slide against each other, there is a tendency toward high friction. In general, it is best to avoid these pairings. However, in some cases these combinations are favorable because of other interactions and/or requirements, i.e. laser marking, suitable sterilization techniques or mechanical specifications.

 

For combinations of unmodified POM-on-unmodified POM the static and dynamic coefficient of friction shows a large difference resulting in a high risk of noise.

 

Changing to a combination of dissimilar materials may provide a solution. For example, a POM-on-POM combination may generate noise whilst a POM-on-Polyester pairing may run silently (Figure 3). This is due to the relatively small difference between the static and dynamic coefficients of friction in the POM-on-Polyester combination.

 

Figure 3: Sliding behavior of POM/POM vs. POM/Polyester, dotted lines: dynamic CoF, solid lines: static CoF

 

THE USE OF EXTERNAL LUBRICATION

 

On initial examination, the addition of external lubrication is attractive. Lubricants on the surface of the part create a film during operation and provide exceptionally low friction between sliding components, as illustrated in Figure 4. External lubrication is often adopted as a cost saving measure to reduce the overall bill of material in a device, or as a problem-solving step if a design exhibits unwanted noise in operation due to stick-slip. Lubrication can either be achieved through coating parts (postsiliconisation) or discrete lubrication. The downside of external lubrication is the management of the secondary process, potential migration of the lubricant to undesired locations in the device, and chemical resistance issues. It is difficult to tell whether or not lubrication will be consistent from device to device and whether surface lubricants will remain in the required place to do their job after protracted storage.

 

Figure 4: Effect of external lubrication (silicone oil), dotted lines: dynamic CoF, solid lines: static CoF

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