Rotational molding

Rotational molding or 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.

Process

The rotational molding 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 molding, 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 molding process consists of four distinct phases:

# Loading a measured quantity of polymer (usually in powder form) into the mold.
# Heating the mold 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 mold, 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.
# Removal of the part.

Recent improvements

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 mold pressurisation. It is well known that applying a small amount of pressure internally to the mold at the correct point in the heating phase of 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.

Mold Release

A good mold release will allow the material to be removed quickly and effectively. Mold releases can reduce cycle times, defects, and browning of finished product. There are many mold 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: It's only been possible relatively recently, but natural materials can also be used. Through the use of real sands and stone chip, sandstone composite can be created which is 80% natural non-processed material.

Rotational moulding of plaster is used to produce hollow statuettes.

Product Design

Rotationally molded 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 molding 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.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.

Tooling

Molds (or tooling) are either fabricated from welded sheet steel or cast. The fabrication method is often driven by part size and complexity; Most intricate parts are likely made out of cast tooling. Molds are typically manufactured from stainless steel or aluminium. Aluminum molds are usually much thicker than an equivalent steel mold, as it is a softer metal. This thickness doesn't affect cycle times significantly since aluminum's thermal conductivity is many times greater than steel. Aluminum molds tend to be costlier than similar steel molds.

Bibliography

* Crawford, R, Throne, James L., "Rotational Moulding of Plastics", William Andrew Inc. (2002). ISBN 1884207855
* Crawford, R, Kearns, M, "Practical Guide to Rotational Moulding", Rapra Technology Ltd. (2003). ISBN 1859573878

External links

* [http://www.armo2008.com http://www.armo2008.com] Worlds Largest Rotational Moulding Conference
* [http://www.bpf.co.uk/bpfindustry/process_plastics_rotational_moulding.cfm British Plastics Federation - Rotational Moulding Group Overview]
* [http://www.tecni-form.com/moulding-animation.php Animation of the rotational molding process]

* [http://www.AllRoto.com http://www.AllRoto.com] All Rotational Molding Resources with a Comprehensive Discussion Forum on Rotational Molding