Rotational molding F
Rotational molding or rotational moulding is a versatile process for creating
many kinds of mostly hollow plastic parts. The phrase is often shortened
to rotomolding or rotomoulding. The process was developed in the 1940s
but in early years was used little because it was a slow process restricted
to a small number of plastics. Over the past two decades, improvements
in process control and developments with plastic powders have resulted
in a significant increase in usage.
The rotational moulding process is a high-temperature, low-pressure plastic-forming
process that uses heat and biaxial rotation (i.e. rotation on two axes)
to produce hollow, one-piece parts. Critics of the process point to its
long cycle times—only one or two cycles an hour can typically occur, as
opposed to other processes such as injection moulding, where parts can
be made in a few seconds. The process does have distinct advantages. Manufacturing
large, hollow parts such as oil tanks is much easier by rotational molding
than any other method.
Rotational molds are significantly cheaper than other types of mold. Very
little material is wasted using this process, and excess material can
often be re-used, making it a very economically and environmentally viable
manufacturing process. The rotational moulding process consists of four
distinct phases: Loading a measured quantity of polymer (usually in powder
form) into the mould. Heating the mould in an oven whilst it rotates,
until all the polymer has melted and adhered to the mold wall. The hollow
part should be rotated through two or more axes, rotating at different
speeds, in order to avoid the accumulation of polymer powder. The length
of time the mold spends in the oven is critical. Too long and the polymer
will degrade, reducing impact strength. If the mold spends too little
time in the oven, the polymer melt may be incomplete. The polymer grains
will not have time to fully melt and coalesce on the mold wall, resulting
in large bubbles in the polymer. This has an adverse effect on the mechanical
properties of the finished product. Cooling the mould, usually by fan.
This stage of the cycle can be quite lengthy. The polymer must be cooled
so that it solidifies and can be handled safely by the operator. This
typically takes tens of minutes. The part will shrink on cooling, coming
away from the mold, and facilitating easy removal of the part. The cooling
rate must be kept within a certain range. Very rapid cooling (for example,
water spray) would result in cooling and shrinking at an uncontrolled
rate, producing a warped part.
Until recently, the process was largely empirical, relying on both trial
and error and the experience of the operator to judge when the part should
be removed from the oven, and when it was cool enough to be removed from
the mold. Technology has improved in recent years, allowing the air temperature
in the mold to be monitored, removing much of the guesswork from the process.
Much of the current research is into reducing the cycle time, as well
as improving part quality.
The most promising area is in mould pressurisation. It is well known that
applying a small amount of pressure internally to the mold at the correct
point in the heating phase accelerates coalescence of the polymer particles
during the melting, producing a part with fewer bubbles in less time than
at atmospheric pressure. This pressure delays the separation of the part
from the mold wall due to shrinkage during the cooling phase, aiding cooling
of the part. The main drawback to this is the danger to the operator of
explosion of a pressurised part. This has prevented adoption of mold pressurisation
on a large scale by rotomolding manufacturers.
Mould Release Compound
A good mould release compound will allow the material to be removed quickly
and effectively. Mould releases can reduce cycle times, defects, and browning
of finished product. There are many mould releases on the market. Some
perform at higher temperatures, some are biodegradable. Others can be
hazardous to the environment and the worker. Fortunately, more and more
companies are developing user and environmentally safe alternatives.
Materials
More than 80% of all the material used is from the polyethylene family.
Cross-linked polyethylene (PE); linear low density polyethylene (LLDPE);
high density polyethylene (HDPE). Other compounds are PVC plastisols,
Nylons, and polypropylene. Natural Materials Recently it has become possible
to use natural materials in the moulding process. Through the use of real
sands and stone chip, sandstone composite can be created which is 80%
natural non-processed material.
Rotationally moulded parts have to follow some restrictions that are different
from other plastic processes. Being a low pressure process, sometimes
designers face hard to reach areas in the mold. Good quality powder may
help overcome some situations, but usually the designers have to keep
in mind that it's not possible to make some sharp threads used in injection
molded goods. Some products based on polyethylene can be put in the mold
before filling it with the main material. This can help to avoid holes
that otherwise would appear in some areas. This could be also achieved
using molds with movable sections. Rotational moulding offers design advantages
over other molding processes. With proper design, parts assembled from
several pieces can be molded as one part, eliminating high fabrication
costs. The process also has inherent design strengths, such as consistent
wall thickness and strong outside corners that are virtually stress free.
For additional strength, reinforcing ribs can be designed into the part.
Designers can select the best material for their application, including
materials that meet FDA requirements. Additives for weather resistance,
flame retardation, or static elimination can be incorporated. Inserts,
threads, handles, minor undercuts, flat surfaces without draft angles,
or fine surface detail can be part of the design. Designs can also be
multi-wall, either hollow or foam filled. Roto-moulded traffic cones with
reflective sleeve for night visibility.Products that can be manufactured
using rotomolding include storage tanks, bins and refuse containers, doll
parts, road cones, footballs, helmets, and kayak hulls. Playground slides
and roofs are also generally rotomolded.
Courtesy Wikipedia
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