DESIGNER'S TOOLBOXDeveloping a successful product can depend on considering the effects of process conditions when designing plastics-based medical devices.
To effectively design medical devices with plastics in mind, it is necessary to know not only the nominal physical properties of the materials, but also the effects of processing on those properties. Plastics performance is very process dependent.
Polyethylene, a widely used material in medical applications, is strongly influenced by processing. For instance, depending on the cooling rate, the crystallinity of polyethylene can range from 20 to 80%. It is evident from Figure 1 that material stiffness over that range can vary more than tenfold from the minimum to the maximum crystallinity attainable in processing.
For example, when a flexible syringe bulb blow molded from low-density polyethylene was cloudy and tended to crack when pressed, it was necessary to decrease the material's stiffness for proper function. The inadequate control of the part's crystallinity was cured by both a design change and a process change. Reducing the wall thickness of the bulb, increasing the cooling rate, and cooling the bulb wall more rapidly during blow molding by injecting liquid carbon dioxide into the interior of the bulb solved the problem. The result was a clear, flexible, and crack-resistant part.
A rigid connector molded from polyethylene presented a different problem related to crystallinity level. The needle connector end of a syringe was adapted to accept a tapered needle connector. Because the designer desired a clear syringe barrel, the product required different cooling rates to control the plastic's crystallinity, which determines transparency and stiffness. The solution was to first make the syringe wall as thin as possible. Second, the wall in the connector section of the syringe was made thicker. To do these two things at one time, the process was changed by adjusting cooling flows to make the mold temperature hotter in the connector section and colder in the syringe wall.
A plastic's crystallinity level also affects other properties. Materials become hazy and disperse light at high crystallinity, but are transparent at low crystallinity. The permeability of a material increases at low crystallinity; so does the solubility of other substances in the resin. The material's surface wetting characteristics and surface electric charging patterns, both of which may affect performance in a medical application, change with crystallinity levels as well.
Several other conditions are important and process sensitive. Polymer orientation, the aligning of molecules that occurs in most processing, has significant effects on the properties of the material, with strength and stiffness increasing in the oriented direction. Highly oriented polymers have lower gas and liquid permeability and better solvent resistance than do less-oriented ones. They are tougher and resist impact forces.
Figure 1. Stiffness modulus of polyethylene as a function of crystallinity at 25°C.
Molded-in stresses--a related problem--are a result of processing conditions. These stresses are generated when a plastic melt is forced to flow after it is partially set, a condition that results from incorrect process settings on the molding machine. Extremely thin walls in the part can also cause molded-in stresses which, in turn, can cause deterioration of the physical properties of the material. They increase the danger of environmental stress-cracking and can lead to distortion of the part during heat sterilization.
Molded-in stresses, orientation, and crystallinity levels are dependent on part design and on mold and tool design. In the manufacturing process, the plasticized material flows through the filling paths, creating the part walls and other elements. Process conditions such as filling rate, filling pressure, melt temperature, and cycle time must be adjusted to make the part to specifications.
This brief article has addressed a few conditions to consider in the design of plastics-based medical products that use molded parts. Such processes as extrusion, sheet forming, and others have different limitations that design engineers must consider to avoid an unsuccessful product. Polymer physical chemistry conditions must also be considered in order to make successful low-risk products for high-risk applications.
Sidney Levy is the principal of Sidney Levy P.E. and Associates (La Verne, CA), an engineering and consulting firm. This article is based on material from Plastics Product Design Engineering Handbook, 2d ed, by Levy and Dubois.