Page 201 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.71

3.2.1.6 Rotational Molding (Rotomolding). Liquid or powdered resin is placed in a
closed, heated mold and rotated on two axes so that the resin coats the walls of the mold.
This produces hollow products, ranging in size from golf balls up to 22,500-gal tanks.
Capital investment is low, but operating cost is high, so it is best for short production runs.
The most common equipment is a three-arm carousel, with separate stations for load-
ing/unloading, heating, and cooling. The mold is usually cast aluminum, or electroformed
nickel for fine detail.
It rotates on two perpendicular axes at speeds up to 40 RPM. It is heated in an air oven
or by oil in a jacketed mold. The resin is distributed by the force of gravity—the speed is
not high enough for centrifugal force. A complete cycle may be 5 to 30 min. Yields are
high, because there is no waste of material.
The process is applied mainly to polyethylene but also to other thermoplastics and to a
number of thermosetting resins: epoxy, phenolic, polyurethane, and silicone. It is used to
produce tanks, trucks, bathrooms, boat hulls, appliance housings, and toys. It is possible to
produce multiwall construction by successive loading with different resins.

3.2.2 Reinforced Plastics Processes
Addition of reinforcing fibers to plastics increases their modulus, strength (especially im-
pact strength), dimensional stability, and heat deflection temperature. Short, chopped fi-
bers can be dispersed in all thermoplastics and thermosets; melt processing remains fairly
conventional, and property improvements are significant. Maximum improvement of
properties, however, requires continuous fibers, and especially fabrics. These defy conven-
tional melt processing. They require the fluidity of low-molecular-weight thermosetting
oligomers and the development of new processing techniques, which have come to charac-
terize reinforced plastics processing. They are often called laminated plastics, and the pro-
cesses are called lamination.
3.2.2.1 Hand Layup (Contact Molding or Open Mold Process). The oldest and sim-
plest technique is primarily manual. An open mold is waxed to lubricate it. The surface of
the proposed product is applied to the mold, usually as an unreinforced gel coat or some-
times as a thermoformed plastic sheet. A layer of fabric is hand cut and hand laid into the
mold. Catalyzed resin is poured over the fabric and worked into it with brush, roller, and
squeegee to eliminate air bubbles. Another layer of fabric is laid over this, often oriented
in a specific direction to optimize properties. Again, catalyzed resin is poured over it and
worked into it. The process is repeated as many times as required to build up the desired
thickness and optimum orientation of the layers. If the cure reaction is inhibited by air, a
wax may be dissolved in the resin; it exudes to the surface and forms a barrier to exclude
air and permit complete cure. The resin may be allowed to cure at room temperature, or
the assembly may be heated to complete or hasten the cure reaction.
Polyester and epoxy resins are the most common. Capital investment is very low; labor
cost is high, and skill is required. The process is ideal for small production runs of large
products such as boats, tanks, flat panels, tools, and prototypes.
3.2.2.2 Sprayup. Sprayup is a mechanized version of hand layup. The spray gun is fed
with (1) glass roving, (2) liquid resin, and (3) catalyst. The gun chops the glass roving into
short lengths, mixes them with the catalyzed resin, and sprays them into the mold. If the
gun is handled manually, skill is required, and quality will vary. If the gun is controlled au-
tomatically, less manpower is required, and quality is more uniform. As in hand layup, the
sprayed glass-resin mixture must be pressed down with a roller to squeeze out air bubbles.
Since hand layup can use woven fabric, it gives greater strength; sprayup delivers short,

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