Most thermoplastic pipes and fittings
are made from materials containing no reinforcements, although
fillers are occasionally used. Pipe is manufactured by the extrusion
process, whereby molten material is continuously forced through a die
that shapes the product.
After being formed by the die, the soft
pipe is simultaneously sized and hardened by cooling it with water.
Fittings and valves are usually produced by the injection-molding
process, in which molten plastic is forced under pressure into a
closed metal mold.
After cooling, the mold is opened and
the finished part is removed. Some items, especially larger-sized
fittings for which there is insufficient demand to justify
construction of injection-molding tooling, are fabricated from pipe
sections, or sheets, by utilizing thermal or solvent cementing fusion
techniques.
To compensate for the lower strength,
the fitting may either be made from a heavier wall stock or
reinforced with a fiberglass-resin overwrap. The engineer designing a
pressure-rated system should make sure that the pressure ratings of
the selected fittings are adequate.
There is some thermoplastic pipe made
of a cellular-core construction (for example, ASTM* F 628) in which
the pipe wall consists of thin inner and outer solid skins
sandwiching a high-density foam. The primary benefit of such
construction is improved ring and longitudinal (beam) stiffness in
relation to the material used.
Because the foam-wall structure results
in some loss of strength, applications for cellular-core pipe are in
nonpressure uses, such as for above- and below-ground drainage
piping, which can take advantage of the more material-efficient ring
and beam stiffness.
For buried nonpressure applications, a
composite pipe (ASTM D 2680) is produced that consists of two
concentric tubes that are integrally braced with a truss webbing. The
resultant openings between the concentric tubes are filled with a
lightweight concrete. This construction increases both the ring and
the beam stiffness. Composite pipe is used only for nonpressure
buried applications such as sewerage and drainage.
Several other processes for improving
the radial (i.e., ring) stiffness of thermoplastic pipe for buried
applications have in common the formation of some type of rib
reinforcement.A well-established technique is forming corrugations in
the pipe wall.
Corrugated polyethylene pipe (ASTM F
405) in sizes from 2 to 12 in (5 to 30 cm) is widely used for
building foundations, land, highway, and agricultural drainage, and
communications ducts. Ribbed pipe also is commercially made by the
continuous spiral winding of the plastic over a mandrel of a
specially shaped profile.
Adjacent layers of this profile are
fused to each other to form a cylinder that is smooth on the inside
and has ribbed reinforcements on the outside. The smooth inside
diameter is preferable for many applications, such as sewerage,
because it creates no flow disturbances. Pipes with ribbed
construction are available in PVC and polyethylene (PE).
PE pipes, which are made with hollow
ribs to minimize material usage, are available in sizes from 18 to
120 in (45 cm to 3 m) in diameter.
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