Design Guide Pt 1 Overview

Overview

Rotational molding, or rotomolding, is a plastics processing technique that is ideally suited to producing relatively large, hollow, seamless parts.

The rotomolding process has been in existence since the early 1930’s. The introduction of polyethylene powders in the late 1950’s provided the industry with a material which was ideal for the process. Since that time, rotomolding markets have grown globally, at a rate which often out-performs the general economic growth prevalent in the country where it is practised.

Designers of plastic parts turn to rotomolding, for a number of reasons.

The process is carried out at normal atmospheric pressure, which means that molds for rotomolding can be constructed with relatively thin walls. This means that the costs to produce rotational molds are generally lower than for alternative processes (eg injection and blow molding), where resistance to higher working pressures is required. The initial investment cost required to create a mold and to start to manufacture parts is therefore relatively low which, in many instances, makes a product viable to get to market.

There are many other advantages of the rotomolding process, but relatively low-cost molds is the single biggest driver for the dynamism and high growth of the process. In a related aspect, rotational molds can be manufactured relatively quickly, so products can be produced with short development times.

The low processing pressure involved has the added advantage of producing parts that are relatively free of residual stresses, compared to high pressure processes. This advantage is especially significant when considering large, load-bearing parts, in applications where residual stress may create the possibility of long-term part failure.

Rotomolding can be used to produce small parts, as can other processes. However, for larger parts, rotomolding may often be the only route to feasible mass production. Products as large as 30,000 gallon tanks have been made by rotational molding, as a one-piece part.

Rotationally molded parts tend to exhibit increased wall thickness on outside corners, which is a distinct advantage compared to blow molding and thermoforming; in these competitive processes there is a tendency for outside corner sections to thin. If these thin areas encounter high stress, they can act as a point of structural weakness. The advantage of thicker corners will offer a particular advantage in larger sized parts.

As an associated advantage, it is possible to manipulate process variables to ensure that some areas of a rotomolded part are thicker than others. For example, in a vertical storage vessel for liquids, the wall thickness can be increased in the lower sections of the part, where greater hydrostatic pressure would be expected.

Unsurprisingly, complex shaped molds are more expensive to make than simple shapes. However, once the mold is available, the additional costs of complexity are limited. In practice, there are almost no limits to the complexity of shape possible in a rotomolded part. This can present significant aesthetic attractions to the designer, but there are also practical implications. There are many examples where a series of separate components have been combined into a single rotomolded shape, saving assembly costs and increasing part quality and consistency.

Many parts are molded with little or no draft angle. With some common rotomolding materials, it is possible to produce parts with undercuts.

Aesthetic and performance features can be readily incorporated into rotationally molded products.

The base plastic material can be colored by various techniques, including pigment dry blending with natural powder and full compounding. In addition, specialist aesthetic effects may also be employed, including stone-effects, pearlescents and terracotta-look.

A wide variety of alternatives are available for the surface finish of parts, depending on mold construction. This can include high gloss, sandblast and shot peen textures. More complex textural effects, such as wood grain, are also available. The type of texture can be varied in different areas of the part.

Metal inserts can be strategically attached to the mold prior to molding. During the rotomolding process, the insert will be encapsulated by plastic and thus will become molded into the body of the part. Secure threaded bushings, for the attachment of ancillary items, can thus be provided.

Integrally molded-in threads are possible with rotationally molded parts, which can be designed to accommodate post-mold items like caps, pipe connectors and closures.

Spin welded fittings, such as pipe connectors and hole blanks, can be attached to parts after molding. This increases flexibility of design; for example, a standard rotomolded tank can easily be modified to have a variety of different connection points, in customized locations.

Reversal parts with closely spaced double walls is a frequently used design concept, which creates an internal cavity, fillable with foam, post-molding. This technique is commonly employed to provide rotomolded products with heat and sound insulation properties, or to increase the flotation characteristics of marine devices, such as marker buoys.

The plastic raw materials (aka polymers) that are used in rotomolding are described in a succeeding section of this Design Guide. At this point it should be noted that the range of plastics suitable for the process is relatively limited and that one polymer type, polyethylene, predominates.

It is possible, by a process modification, to rotomold parts having a multi-layer, or “sandwich” structure. This may be contemplated for a variety of reasons. A thick, relatively lightweight structure can be created using a sandwich structure of solid plastic outer and inner skins, with an intermediate layer of low-density plastic foam. This technique is often used in the manufacture of large canoes or boat hulls, as well as for water storage tanks. It is also possible to make a multi-layer part with two different plastics; an example would be gasoline containers, where the incorporation of a layer of a low-permeation plastic (eg nylon) complements the structural integrity of a polyethylene tank.

Production cycle time for rotomolded parts tend to be lengthy, in comparison to many other plastic processes. This results in rotomolding being preferred as a manufacturing method for small / medium sized run rates. If larger production quantities are required, this can be achieved using multiple molds. As a guide, a typical medium sized mold will produce approximately 5-10,000 parts per year, if run on a 24/7 basis. There have been cases where products have started their production life as rotomolded parts and quantities required have grown to the point where they have been converted to blow molding, which is a significantly more capital-intensive process. However, in reality, such cases are relatively rare; if the designer has fully utilized the advantages of rotomolding in the original product development process, it may be difficult to transfer to more restrictive alternative processes.