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Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

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314 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />

Table 15-2 continu<strong>ed</strong><br />

Comparison of Thermosets with Thermoplastics<br />

Process Thermoset Thermoplastics<br />

Obtain additional Not necessarily Yes<br />

strength with ribs<br />

Tooling Depends on Generally 15% to 20%<br />

complexity <strong>and</strong> size higher than<br />

compression molding<br />

but offset by volume<br />

of quantities<br />

Process comment Compression Injection. Cannot use<br />

compression molding<br />

because not enough<br />

heat to obtain proper<br />

melt flow<br />

Thermoplastics<br />

Thermoplastics do not undergo a chemical change in their processing<br />

<strong>and</strong> will become pliable upon reheating above their yield temperature.<br />

Thermoplastic materials are available in a wide range of strengths <strong>and</strong><br />

application envelopes. They can be divid<strong>ed</strong> into fluoropolymers (PFA-<br />

PTFE), engineering plastics (LCP-PPS), <strong>and</strong> general plastics (ABS<br />

acrylics, polyethylene, PVC, <strong>and</strong> polypropylene). Thermoplastic processes<br />

such as injection molding, vacuum forming, extrusion, <strong>and</strong> blow<br />

molding offer the design engineer many selections for optimum cost considerations.<br />

Selecting a suitable composite requires a complete under<br />

st<strong>and</strong>ing of the end use application as well as a familiarity with the polymer's<br />

physical, chemical, <strong>and</strong> processing properties. Although direct<br />

replacement without design changes is feasible, more often the use of a<br />

nonmetallic is optimiz<strong>ed</strong> by a well-inform<strong>ed</strong> specialist.<br />

Table 15-3 shows a general comparison of various resins applications.<br />

Manufacturing Techniques<br />

Two methods us<strong>ed</strong> in manufacturing the casing, casing cover, <strong>and</strong> impeller<br />

of nonmetallic pumps are compression molding <strong>and</strong> resin transfer.<br />

Compression Molding<br />

This process uses match<strong>ed</strong> metal dies that have cor<strong>ed</strong> heat transfer passages<br />

to control the temperature of the process. The base resin is mix<strong>ed</strong><br />

with appropriate amounts of chopp<strong>ed</strong> glass, fillers, <strong>and</strong> chemical catalysts,<br />

inhibitors, <strong>and</strong> release agents to make a batch. This batch can be set<br />

aside in plastic containers for a shelf life of approximately 30 days.

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