Introduction
Rotational molding, also known as
rotoforming or rotomolding. is a process in which finely ground powders usually
thermoplastics, are heated in a rotating mold unti melting or fusion occurs.
(The process was originally don with liquid PVC plastisols and was called slush
molding. a method for molding hollow plastic objects by placing finely
divided particles in a hollow mold that is rotated about two axes, exposing it
to heat and then to cold.
History of Rotational molding
In 1855 a patent taken out by R. Peters in
Britain documented the first use of a rotating mechanism producing “two centrifugal
motions at right angles to each other” by means of beveled gearing and heat.
This rotational molding process was used to create artillery shells and other
hollow vessels the main purpose of which was to create consistency in wall
thickness and density.
In a
U.S. patent in 1905, F.A. Voelke described a method including a polymer for the
production of articles using paraffin wax. Development led to G.S. Baker's and
G.W. Perks' process of producing hollow chocolate Easter eggs in 1910.
Rotational molding had developed further when R.J. Powell made mention of the
commonly used ratio of 4:1 between major and minor axes of rotation at slow
rotation speeds. His patent covered this process for molding hollow objects
from plaster of Paris in the 1920s
Material use in Rotayional molding
Most rotational molded products are made
from some sort of polyethylene (PE) a commodity based thermoplastic. PE is offered in a number of grades and
classifications. Other materials can also be used but there are limitations on
the material selection. This is due to the need for the materials to be
pulverized. You can see a full list of PEs and other rotomolding materials
below
Rotomolding Materials
Ø Linear Low Density Polyethylene LLDP
Ø Medium Density Polyethylene MDPE
Ø High Density Polyethylene HDPE
Ø Low Density Polyethylene LDPE
Ø Cross Link Polyethylene XLPE
Ø EVA Co-polymer EVA
Ø Polyvinylchloride PVC
Ø Nylon
Rorational molding process
There are
Four Basic Steps in rotational molding describe
in below
Rotational molding is a simple process. It
utilizes high temperatures, thin-walled metal or composite molds biaxial
rotation in two perpendicular axes finely divided powder or liquid polymers and
cooling using air and/or water to produce hollow seamless low-stress parts.
Rotational molding has four basic steps
1. Loading. A preweighed amount of powdered
or liquid plastic is placed in one half of a thin-walled hollow metal mold that
is mounted on the arm of a molding machine. The mold is then closed using
clamps or bolts.
2. Heating. The mold then begins to rotate
biaxially about two perpendicular axes while being moved into an oven where
heat is applied. The metal or composite mold becomes hot and the powder–liquid
tumbling inside rises in temperature. Hot powder material sticks to the mold in
successive layers to form the part, while liquid materials typically react as
they form the part shape.
3. Cooling. When the material has melted and
has been consolidated, the mold is moved to a cooling station, where forced
air, water mist, or a combination is used to bring the part temperature down to
a point below the crystallization or solidification point of the material.
Uniaxial or biaxial rotation continues to prevent the molten material from
sagging.
4. Unloading. Once
the part is cool, the mold is moved to the unloading station where the part is
removed. The mold is then ready to begin the process again. Stages 1 and 4 are
often combined into a single operating station (mold servicing) in machine
design so that the most basic of machine configurations typically consist of three
workstations: heating, cooling, and servicing
Advantages of Rotational Molding
1. The tooling required is usually very
simple and relatively inexpensive
2. The process is well suited for making very
large and/or very complex parts with single or double walls
3. Parts produced are strain free
4. Parts with square corners are thicker at
the corners rather than thinner as is the case with thermoformed parts
5. Parts are usually made with very uniform
wall thicknesses except as indicated above
6. Parts are usually made with no weld lines,
sprue marks ejection marks etc
7. No scrap, or very little, is produced
8. Hollow parts are easily produced with
simple tooling
9. Usually no secondary operations are
required
10. Wall-thickness changes can be easily made
with no need for new tooling or modification of the starting raw material
Disadvantages of Rotational Molding
1.
Material costs
are relatively high since most materials are produced as pellets and then
reduced to a fine powder
2. The process is not suited for production of parts with wall thicknesses of less than 0.030 inch
3.It is not suited, generally, for large production runs of small parts Rotational molding is well suited for prototype production of large parts such as boats
Application of rotational molding
• Agriculture: storage tanks, spraying
equipment tanks
• Automotive: interior panels, fuel tanks,
ductwork, air-intake systems
• Building–construction: water tanks, septic
tanks, highway barriers
• Electrical–electronic: aboveground
pedestals, belowground chambers
• Floor care: vacuum cleaner parts, floor
cleaner tanks
• Industrial: chemical tanks, cases, shrouds
and housings, corrosion and pollution control equipment parts
• Lawn and garden: mower shrouds, fuel tanks,
ductwork
• Marine products: dock floats, hulls, fuel
tanks, seating, fenders, live wells
• Material handling: tanks, drums, barrels,
hoppers, IBCs, pallets
• Medical equipment: spine boards, anatomical
figures, inflatable masks, implants
• Playground equipment: slides, climbing
frames
• Signs and displays: point-of-purchase
displays
• Sports and recreation: toys, play balls,
kayaks, canoes, helmet linings, pet products
• Transportation: road traffic barriers, cones, signage, aircraft ductwork
