October 2, 2001
Originally Published MPMN October 2001
Long-Fiber Compound Reduces Nebulizer Costs
Molded compressor components provide repeatable, long-term creep resistance
Manufacturers of nebulizers, which are used by people suffering from asthma and other bronchial ailments, are seeking ways to reduce production costs for their devices. Increased competition coupled with changes in healthcare coverage are two of the factors driving this trend.
Thomas Industries' nebulizer relies on RTP's long-fiber compound for its motor-end bracket and housing.
One method Thomas Industries (Sheboygan, WI) used to reduce its production costs was to replace compounds used to mold two compressor components on its devices. The company's nebulizers are fitted with compressors in which injection-molded components are assembled by means of an efficient snap-fit method. After evaluating several other materials in terms of cost-effectiveness, Thomas turned to RTP Co. (Winona, MN) for a long-fiber compound for the two components—the motor-end bracket and the housing.
The compound the two companies developed provides the repeatable, long-term creep resistance that is critical to facilitating the assembly process, thus reducing costs.
"Initially, we tried long-fiber PET but did not obtain the required material or part properties," says Thomas Industries' senior key project leader Roy Rozek. "We then considered long-fiber nylon, but were very concerned that the dimensional stability could present a problem due to moisture absorption."
RTP's engineers provided a material that consistently processes to fixed dimensions. According to Rozek, changes of less than 0.001 in. occurred on critical dimensions during mold capability studies of a 4-in. part. This included comparing dry-as-molded parts with full moisture-saturated components.
The RTP 200–series nylon specialty compound is reinforced with long glass fiber to provide flexural strength. Unnotched Izod impact strength is 24 ft·lb/in. at 1/8 in., and flexural strength is 50,000 psi. The material is heat stabilized and carries a UL 94 HB flame-retardant rating.
Even after processing, the long-fiber compound from RTP Co. resists thermal and stress pressures.
The motor-end bracket is designed with extended fingers that retain a sintered bronze self-aligning sleeve bearing in a snap-fit pocket. The molded fingers were studied using finite element analysis to ensure that the design could maintain long-term spring retention and to prevent bearing rotation. At the same time, the material was subjected to lab testing to verify the long-term creep-resistance properties of the resin.
The housing features an insert-molded standard ball bearing and flexing cantilever supports. The supports, which were analyzed in the same way as the fingers, also withstand bending stress as the compressor head is snapped onto two upright parts.
Compressor parts, in their final assembled, or stressed, positions, survived thermal aging testing for 3 hours at 225°F. These tests simulate 8 to 10 years of aging at normal room temperature. According to RTP, the tests showed no noticeable flexural change in the cantilever beams and no movement or loosening of the bearings.
Copyright ©2001 Medical Product Manufacturing News
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