ROTATIONAL MOLDING

Rotational molding

Rotational molding (also popularly known as
rotomolding) is best suited for large, hollow
products requiring stress-free strength, complicated
curves, a good finish, a variety of colors,
a comparatively short (or very long) production
run, and uniform wall thickness. It has been
used for products such as fuel tanks, furniture,
tilt trucks, industrial containers, storage tanks,
portable outhouses, modular bathrooms, telephone
booths, boat hulls, garbage cans, light
andRotational molding offers a number of advantages

1.Virtually unlimited design possibilities(parts as

small as a golf ball to a 22,500-gallon agricultural tank).

2.Relatively low machinery cost.

3. Low tooling costs.
4. Economical prototyping

5 . Strong outside comers in virtually stress-
6. Part finish from matte to high gloss.
7, Simultaneous processing of multiple
8. Simultaneous processing of different
9. Quick mold changes.
10. Possibility of molding in metal inserts.
11. Molded-in multicolor graphics.
12. Multilayer molding for chemical resis-
13. Double-walled parts molding for addi-
14. Possibility of minor undercuts.

DIFFERENT TYPES OF MACHINES

Batch-Type Machines
This type is the least expensive of the rotational
molding machines because it is the least sophisticated
and requires the most manual labor.
In a typical batch operation, the charged mold
is rolled into the oven for rotation and heating.
At the completion of the cycle, the mold is removed
and a newly charged one inserted in its
place. The completed mold then is transferred
manually on rollers to a cooling station for cure
before removal of the parts.
Carousel-Type Machines
The most common rotational molding machine
in use is the carousel unit, which is essentially
a three-station rotary indexing type with a central
turret and three cantilevered mold arms .In
operation, individual arms are involved in
different phases simultaneously, so that no arms
are idle at any time. The arms or mold spindles
extend from a rotating hub in the center of the
unit. Thus, while one arm is in the loading/
unloading station, another is rotating within the
oven, and the third is rotating within the cooling
station. All operations are automated, and
at the end of each cycle the turret is rotated
120", thereby moving each mold arm to its next
station.
Newer carousel machines being offered today
use four arms. The additional arm can be used
in a second oven, cooler,or load station,
depending on which is the most
time-consuming part of the overall cycle .
The four-arm carousel machines increase
production by allowing the index from
station to station to occur more frequently that
could be managed on a three-arm machine
Independent-Arm-Type Machines
The independent-arm-type machine offers even
more flexibility. Featuring five stations-one
oven, one intermediate station, one cooler, and
two loadhnload stations
Straight-Line Machines
Used primarily for molding large parts, this
type is a shuttle carriage machine that is generally
a straight-line operation with the oven on
one side, the loadinghnloading station in the
middle, and the cooling station on the opposite
end. The carriage is guided on parallel tracks
that ensure positive placement in the stations.
Variations of this particular design, involving
the placement and relationship of the oven,
cooling stations, and loading/unloading stations,
also are available for handling particular
jobs.
Jacketed-Mold Machines
These units offer precise temperature control up
to a 300°C heat-transfer medium temperature.
This type of machine finds its biggest market
in molding heat-sensitive polymers because of
its accurate temperature control.
Key to the machines are double-walled jacketed
molds that are charged with a hot liquid,
usually oil, to attain temperature control. When
the heating period is over, the oil is drained,
and a coolant is introduced.
The molds rotate biaxially through all production
operations. Significant savings are effected
because the heating medium retains its
heat, and the heat transfer is more rapid, resulting
in faster thermal cycling at lower temperatures

MOLD MATERIAL

Cast aluminum molds are probably the most
widely used, and are the most practical for
small to medium parts requiring a number of
cavities.The initial cavity cost
may be relatively high because a model and/or
a pattern is required. However, subsequent
cavities are moderate in price, and a fair reproduction
of the mold surface on the finished part
may be expected. The process used for casting
aluminum molds is somewhat specialized; an
experienced rotational casting moldmaker is required
Electroformed nickel molds are best used
where precise detail is required on the finished
part, or where no parting line can be tolerated.
This type of mold is not so durable as cast aluminum
or sheet metal; however, it is widely
used by molders of such objects as automotive
headrests, armrests, and so forth, and by plastisol
molders.
Sheet metal is appropriate for extremely large
parts or single cavities requiring inexpensive
tools, where a simple sheet metal mold is generally
adequate.