Page 349 - Centrifugal Pumps 2E

314 Centrifugal Pumps: Design and Application

Table 15-2 continued
Comparison of Thermosets with Thermoplastics
Process Thermoset Thermoplastics
Obtain additional Not necessarily Yes
strength with ribs
Tooling Depends on Generally 15% to 20%
complexity and size higher than
compression molding
but offset by volume
of quantities
Process comment Compression Injection. Cannot use
compression molding
because not enough
heat to obtain proper
melt flow

Thermoplastics
Thermoplastics do not undergo a chemical change in their processing
and will become pliable upon reheating above their yield temperature.
Thermoplastic materials are available in a wide range of strengths and
application envelopes. They can be divided into fluoropolymers (PFA-
PTFE), engineering plastics (LCP-PPS), and general plastics (ABS
acrylics, polyethylene, PVC, and polypropylene). Thermoplastic pro-
cesses such as injection molding, vacuum forming, extrusion, and blow
molding offer the design engineer many selections for optimum cost con-
siderations. Selecting a suitable composite requires a complete under
standing of the end use application as well as a familiarity with the poly-
mer's physical, chemical, and processing properties. Although direct
replacement without design changes is feasible, more often the use of a
nonmetallic is optimized by a well-informed specialist.
Table 15-3 shows a general comparison of various resins applications.

Manufacturing Techniques
Two methods used in manufacturing the casing, casing cover, and im-
peller of nonmetallic pumps are compression molding and resin transfer.

Compression Molding
This process uses matched metal dies that have cored heat transfer pas-
sages to control the temperature of the process. The base resin is mixed
with appropriate amounts of chopped glass, fillers, and chemical cata-
lysts, inhibitors, and release agents to make a batch. This batch can be set
aside in plastic containers for a shelf life of approximately 30 days.