Rotational moulding, also sometimes known as rotomoulding, is a thermoplastic fabrication process by which plastic goods are formed and manufactured using a revolving mold and high heat. Rotomolded products are hollow and have single piece construction, with uniform wall thickness and high tensile strength. They are often inexpensive to manufacture and require little further fabrication work.
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The process begins with a rotomoulding machine with arms called spindles. These spindles support the molds in which the plastic material is contained. The molding machine transports the molds from the loading area to a heating chamber and then to a cooling chamber. From start to finish, there are four different stages that are completed in anywhere from 15 to 90 minutes. In the first stage, which is referred to as the charging phase, the metal mold is loaded with raw plastic material and is sealed shut. From there, the mold is heated to high temperatures while slowly revolving on the vertical and horizontal axes of the spindles. This heats the plastic powder until molten, so it evenly coats the interior walls of the mold. Because roto moulders use low pressure, the plastic resin is in the form of fine powder to help it reach all surfaces of the mold cavity evenly. The plastic material can feature additives that contribute to corrosion and static resistance, and the finished molded product assumes these properties when it emerges from the molding process. Phase three is a cooling stage that cures the plastic within the mold, which is accomplished by using cold air and water for about 20 minutes. After the plastic has fully dried, the mold is reopened and the solid plastic part is manually removed. Because shrinking often occurs, removal can prove to be difficult. For applications that require additional strength, reinforcing ribs can be fabricated into the part.
Rotational molding services have several advantages over blow molding, thermoforming and injection molding. Molders are able to change colored plastics without extensive line cleanings, which saves on time and additional costs. Rotational molding does not require pressure, so the molds do not endure the same pressure as in injection molding or extrusion, which helps to limit equipment costs as well as energy-related costs. Rotational molding is often the method of choice in the fabrication of large plastic vessels and tanks, which are otherwise welded together with a laser, hot pressurized gas, ultrasonic vibration or a hot plate and pressure. While welding provides an air and water tight seal, bursting and cracking under high pressure is a concern. Rotomolded plastic tanks, however, are seamless and able to withstand higher amounts of pressure. Examples of other rotomolded products include outdoor furniture, kayaks and canoes, playground slides, safety cones, footballs, sports helmets, plastic rotomolded tanks, large trash containers and beverage bottles.
There are many other advantages to the rotational molding process. While being heated and rotated, the plastic resin adheres to the hot mold, coating it very evenly and seamlessly, producing strong outside corners with virtually no stresses. Rotomolding can produce one-piece constructions, and complex parts are formed without any part assembly required. The plastics used for rotomolds are often lightweight and may be high or low in density. For applications in the food and beverage and pharmaceutical industries, the thermoplastics used can be easily fabricated to be FDA approved, meaning they must not contain any recycled plastic or dies and cannot contaminate substances under any conditions, even high temperatures. The low cost of the materials and tooling makes rotational molding just as cost effective for producing prototypes as it is for large production runs. A variety of finishes, colors and textures are available. Companies are able to mold their logo, graphic or labels right into the parts. These imprints will not come off and are parts of the mold construction. Rotomolding is a versatile molding technique that provides durability to products, eliminates costly fabrication and assembly processes and reduces the need for replacement parts.
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- A common process used to form hollow, thermoplastic shapes, such
as bottles and containers. Two general types of blow molding are extrusion
and injection blow molding.
- The
process in which individual polymer particles are joined together by a
molecular bond.
- The ability
of a material to withstand various chemical agents. Plastic is capable
of enduring many of these chemicals, making it very useful for holding
tanks and other containers.
- The oxidation and
deformation of a material, such as the rusting of iron, due to a chemical
reaction.
- The time it takes
from the first stage of rotational molding, in which the plastic is loaded,
until the plastic completes its final stage and is removed from the mold.
- The removal of
unwanted material from a molded product, especially from the areas where
parting lines of the mold may have caused excess material to be formed.
- A significant parameter
for polyethylene (PE) that affects the properties, such as stiffness and
impact strength, of the end product.
- The ability of
a material to return to its original form after stress has been applied
and quickly removed.
- The temperature
at which the plastic resin melts and begins to adhere to the surface of
the mold.
- The ability
of a plastic to be ground into a fine powder. Most plastics are ground
prior to being loaded into the mold.
- The measure
of the ability of a plastic to withstand fracture by shock.
- A generic name for
semi-synthetic or synthetic materials that can be extruded or molded into
films or objects. Plastic
materials are also used for coatings and adhesives.
- A chain of molecules
that is formed by the bonding of individual repeating units (monomers).
- The measure of
the speed of a plastic powder when poured through a funnel. For a plastic
powder to be acceptable for rotomolding, it must be capable of flowing
at least 185 grams/minute.
- Any of various semisolid
or solid natural organic materials that are usually translucent or transparent.
Resins are mostly utilized in plastics, adhesives and coatings.
- The "arms"
of the rotational molding machine that are responsible for moving the
molds between different stages. Spindles also spin on the horizontal and
vertical axes to create a uniform filling within the mold.
- The ability
of a resin to withstand specified temperatures. Less thermally stable
materials can be rotomolded only under controlled conditions with extreme
care.
- A material that
will soften when repeatedly heated and cooled. Examples of thermoplastics
include polyethylenes, polypropylene (PP), vinyl and nylons.
