About Rotomolding
Rotational molding is a method for manufacturing hollow plastic products. It is best known for the manufacture of tanks, but many designers all over the world are using the technology to make many different types of plastic parts. Some of the market sectors that it services include medical products, consumer’s products, agricultural and garden equipment, automotive and transportation components, toys, leisure craft, sporting equipment, furniture, materials handling articles, and highly aesthetic point of sale products.
The rotational molding industry offers exciting opportunities to designers and end-users. Over recent decades a number of significant technical advances have been made, and new types of machines, molds, and materials are becoming available. Important new market sectors are emerging, as rotational molders are able to deliver high-quality, high-performance parts at competitive prices. In the portfolio of manufacturing methods available to designers, rotational molding can now take its rightful place alongside other major processes, such as structural blow molding, twin-sheet thermoforming, and injection molding.
This article provides an overview of the key features of rotational molding. It describes the basic nature of the process and gives the reader a primer on some of the characteristics that must be taken into account when designing plastics parts that are to be rotomolded. Detailed design guides for rotational molding are cited at the end of the article.
The Process
The principle of rotational molding of plastics is relatively straightforward. Indeed, the simplicity of the process is a key to its success because it allows the molder to exercise close control over part dimensions and properties. Basically, rotational molding consists of introducing a known amount of plastic in powder, granular, or liquid form into a hollow, shell-like mold. The mold is heated and simultaneously rotated about two principal axes so that the plastic enclosed in the mold adheres to and forms a layer against the inner mold surface. The mold rotation continues during the cooling phase so that the plastic retains the desired shape as it solidifies. When the plastic is sufficiently rigid, the mold rotation is stopped to allow the removal of the plastic product from the mold. The process is distinguished from spin-casting or centrifugal casting by its relatively low rotational speeds, typically 4 – 20 revs/min.
The basic steps (a) mold charging, (b) mold heating, (c) mold cooling, and (d) part ejection are shown in figure 1. This diagram is for illustration purposes only. In reality, there are many types of commercial and custom-made machines for manufacturing plastic parts using the rotational molding principle. Most large commercial machines are of a “carousel” design. In these machines, the mold or molds are mounted on an arm that imparts the biaxial rotation to the molds and carries them sequentially into the heating zone, the cooling zone, and finally into the demolding/charging area. Three arms are often used so that heating, cooling, and servicing can be carried out simultaneously on three different sets of molds. In some cases the arms are fixed together at 120˚ spacing. In more recent designs, the arms can be moved independently of each other so that, for example, the molds being heated can be moved out of the oven if the homogeneous melt coating has been formed on the mold wall before the cooling has been completed on the preceding arm.
In the other main type of machine design, the molds go through a “Rock and Roll” motion” – that is, full 360˚ rotation about one axis and a rocking motion about a perpendicular axis.
In both types of machine there are many permutations of the sequencing of heating, cooling, and mold servicing. Conductive, inductive, and dielectric mold heating methods are also used.
Characteristic Features Of Rotational Molding
Rotational molding is an atmospheric pressure process that produces nearly stress-free parts. The fact that there are no stresses on the melt as it is shaped is a major advantage that rotational molding has over all other manufacturing methods for plastics parts. Also, as there are no forces on the plastic melt during forming, rotational molds can have thin walls and are relatively inexpensive to fabricate. For simple parts, mold delivery times can be a few days or weeks. Modern, multi-armed machines allow multiple molds of different sizes and shapes to be run at the same time. With proper mold design, complex parts, such as double-walled containers, that are difficult or impossible to mold by any other method, can be rotationally molded. With correct process control, the wall thickness of rotationally molded parts is quite uniform, unlike structural blow molding or twin-sheet thermoforming. And unlike these competitive processes, rotational molding has no pinch-off seams or weld lines that must be post-mold trimmed or otherwise finished.
The main attractions of rotational molding are:
- A hollow part can be made in one piece with no weld lines or joints
- The molded part is essentially stress-free
- The molds are relatively inexpensive
- The lead time for the manufacturer of a mold is relatively short
- Wall thickness can be quite uniform (compared with other free surface molding methods such as blow molding)
- Wall thickness distribution can be altered without modifying the mold
- Short production runs can be economically viable
- There is no material waste in that the full charge of material is normally consumed making the part
- It is possible to make multi-layer moldings, including foamed parts
- Different types of products can be molded together on the one machine
- Inserts are relatively easy to mold in
- High quality graphics can be molded in
The main limitations of rotational molding are:
- The manufacturing times are long
- The choice of molding materials is limited at present
- The material costs are relatively high due to the need for special additive packages and the fact that the material must be ground to a fine powder
- Some geometrical features (such as ribs) are difficult to mold
Application Areas
Nowadays rotationally molded parts are used in practically every market sector where plastics parts are found. This includes high technology sectors such as the aircraft industry. The process is best sited for the manufacture of one-piece hollow parts or double-wall open containers. Secondary operations can be used to split moldings or cut out panel so that all types of single-wall open containers and products can be created. Areas that are to be cut out of a part can be shielded from the heat during molding so that there is very little material waste as a result of the cutting/trimming operation. Table 2 gives examples of typical types of rotationally molded parts. It may be seen that the variety of products is impressive and although polyethylene is the primary material in most cases, high performance, structural parts are possible through the strategic utilization of unique features such as internal “kiss-off” points between the double walls of the hollow part.
Some examples of rotationally molded parts are shown in figures (a-i). In most cases a high quality finish and close tolerances are achieved in these parts. A key point is that they are all complex 3-dimensional shapes, and they are all made in one piece. Foaming is also very common in rotationally molded parts to provide thermal insulation or high stiffness at minimum weight.
Materials
Nearly all commercial products manufactured by rotational molding are made from thermoplastics, although thermosetting materials can also be used. The polyolefins (mainly polyethylenes) dominate the market for rotationally molded parts. There are several reasons why this situation has arisen. One is that this material is readily converted from granules to the powder form needed for rotational molding. Another is that polyethylene remains more stable than most plastics during the relatively long heating period. Currently, polyethylene, in its many forms, represents about 85% to 95% of all polymers that are rotationally molded. PVC plastisols are quite widely used and polycarbonate, nylon, polypropylene, unsaturated polyesters, ABS, acrylics, cellulosics, epoxies, fluorocarbons, phenolics, polybutylenes, polystyrenes, polyurethanes and silicones make up the rest.
The relative proportions of the usage of these various materials are shown in figure 3. High–performance materials such as fiber-reinforced nylon and PEEK show potential to be used in this technology but represent a very small fraction of the industry output.
