About Rotomolding

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by Roy J. Crawford, University of Waikato, New Zealand, and Susan Gibson, JSJ Productions, Inc.

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

Table 1: compare the characteristics of the different process that can be used to make hollow plastic products.

Table 1

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.

Table 2

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.

Molds

The molds used in rotational molding are shell-like constructions. They are normally made in two halves, although complex parts may require molds to separate into three or more pieces. The molds are held closed at the parting line by clamps. The mold almost always has a vent tube (“breather”) to ensure equalization of pressure between the inside of the molded part and the external environment. The positioning of the vent tube depends on the nature of the plastic part – for example, the fill port of a tank is a convenient place to locate the vent.

The most common mold materials are cast aluminum or fabricated sheet steel. The latter is favored for large articles such as tanks, whereas casting is used for smaller parts that contain complex details, or where several identical molds are required. Electroformed or vapor-formed nickel plate molds are also used, particularly for PVC parts. In recent years, the use of CNC machined molds is becoming common, and this is resulting in exciting improvements in mold quality, particularly at the parting line. Molds undergo high thermal stresses as they are regularly cycled from room temperature to over 300˚C (over 600˚F), and finite element analysis of CNC machined molds ensure that high performance can be maintained over long periods of time. The desirability of having a small positive pressure inside the mold can also be realized more easily in computer-designed molds. Automation of mold opening and mold filling is also helping to reduce cycle times and improve consistency in the molded part.

In the latest types of ‘Leonardo’ machines developed in Italy, the mold and the machine are a single unit. This enables very precise control over mold rotation, as well as the temperature and pressure inside the mold.

Design Guidelines

The nature of rotational molding creates some special effects not seen in other manufacturing processes for plastic parts. Outside corners of rotomolded parts are usually thicker than the general wall thickness because the plastic powder gathers in the corners of the mold. Conversely, inside corners are usually thinner because the plastic powder falls away from the mold in these areas. The generally recommended radii for inside the outside corners are illustrated in figure 4. The general rule of thumb for inside and outside corners it that larger radii will give more uniform wall thickness.

Rotational molding, like blow molding and thermoforming, is a free surface molding method. Wall thickness variations can occur in rotational molding, but the molder can exercise close control over wall thickness by altering the speeds and speed ratios about the major and minor axes. In addition, some areas of the mold can be shielded to reduce material build up, or extra heat can be directed to areas where more thickness is required. The shrinkage in polyethylene is large, typically 3-4%, but this can be allowed for and tolerances of 1-2% are quite normal. A flatness tolerance of 2-5% is usually the best that can be achieved due to the one-sided cooling in rotational molding. Wherever possible, large flat areas should be avoided in a molded part, and the use of curved surfaces is highly recommended to conceal warpage effects. Internal cooling of molds is becoming much more common in the rotomolding industry to overcome warpage effects – as well as reducing manufacturing times.

Threads, both internal and external, can be molded, although coarser thread profiles are preferred. Commercial “flow enhancers” can be sprayed onto the mold in areas such as thread profiles and these improve the reproduction of the mold details considerably.

Metal inserts are also very common in rotationally molded parts. The relatively large shrinkage of the polyethylene ensures that inserts are tightly gripped during molding, but it must be recognized that the resulting restriction of the shrinkage will introduce residual stress. As is often the case with polyethylene, care must be taken with the use of inserts if there is the possibility that the molded part is exposed to a stress-cracking environment.

Conventional ribs are difficult to create by rotational molding because the plastic powder does not flow easily into the deep recess needed to create the rib. Instead, the same type of stiffening effect can be created using corrugations as shown in figure 5. The recommended depth of the corrugations is about four times the material thickness and the width should be about five times the material thickness. This is to ensure a good balance of axial and transverse stiffness. Special stiffening features called “kiss-offs” are very effective in rotational molding (see figure 6). These are created in double wall parts by conical features in the mold that cause the two walls of the part to be “linked” together. The resulting molding is very stiff and in some cases, such as in pallets, foaming is added to provide excellent stiffness to weight ratio.

Improvements in mold design, and powder quality, are enabling some types of stiffening ribs to be created in rotomolded tanks as illustrated in Figure 2 (i). These solid ribs are about 70mm deep and 10mm thick.

Draft angles are usually not necessary in female parts of the mold because the plastic will shrink away from the mold. However, in male parts of a mold where there is a tendency for the plastic to shrink onto the mold, draft angles of 1-2o are usually sufficient. These are illustrated in figure 7. If the mold is textured, an extra 1o taper should be allowed. These values of draft angles are for polyethylene. An extra degree should be allowed for stiffer materials such as polypropylene and nylon. Amorphous materials such as polycarbonate will require a further 2o in all cases.

Some undercuts are permissible in rotational molding where the material shrinkage or flexibility will allow the material to pull away from the mold. The designer will need to determine what is possible based on knowledge of the shrinkage. Generous draft angles on external undercuts are not permissible since the shrinkage of the material will prevent ejection of the part. These effects are shown in Figure 8. Note that the undercuts drawn here are for illustration purposes only.

Holes cannot be molded-in using rotational molding; they must be machined afterwards using normal types of cutting tools. During molding, it is common to shield the areas to be cut out so as to avoid material wastage. Although polyethylene is difficult to paint, sophisticated decorating methods have been developed for rotationally molded parts. Several techniques are available. In one case, special transfers are picked up by the polyethylene during the normal molding operation. In other cases, the transfer can be applied after molding. Both methods are extremely effective at providing excellent graphics on rotomolded parts as shown in figures 2(a) and (e).

Concluding Remarks

Rotational molding has always been known as a versatile manufacturing method for plastic parts. Over the past decade it has come of age in terms of responding to the need to provide high performance parts in demanding market sectors. In doing this, the process has retained its advantages of producing stress-free parts, available with short lead times and attractive economics. The rotational molding industry is a dynamic sector with molders and suppliers to the industry always ready to respond to challenges.

Bibliography

Those wishing to obtain more detailed information on the rotational molding of plastics are referred to the following sources of information [1-8]:

1. Crawford, R.J and Kearns, M. P., Practical Guide to Rotational Moulding, 2nd edition, RAPRA Technology, Shawbury, Shrewsbury, UK (2012)
2. Nugent, P., Rotational Molding: A Practical Guide., www.paulnugent.com (2001)
3. Carvani, M., Mondini, F and Romboli, E., Rotational Moulding: Theory & Practice., Association of Rotational Moulders Australasia (2006)
4. Beall, G.L., Rotational Molding – Design, Materials, Tooling and Processing, 1998, Munich: Hanser. 245.
5. Beall, G., A Designer’s Guide to Rotationally Molding, SPE RETEC, 1999. Cleveland, Ohio, USA.
6. Dodge, P.T., Rotational Molding – The Basic Process, 1995, The Association of Rotational Molders: Chicago, Illinois. p. 14.
7. Crawford, R.J. and Throne, J.L., Rotational Molding Technology, William Andrew Publishing (now Elsevier) 2002
8. Crawford, R.J Rotational Moulding of Plastics, 2nd edition, Research Studies Press, UK (1996)