Volumes of production

From batch production to high-volume mass-production.

Unit price vs. capital investment

Less expensive to set up and operate than injection molding (see p.196). Because there is no pressure involved, molds are simpler and cheaper. Unit costs are still very low.

Speed

This is affected by the size of the component and the wall thickness, both of which affect the cooling-cycle time. Some components, such as plastic drums for storing liquids, may require entry and exit holes for the liquid to be cut by hand.

Surface

The internal surface may reveal the swirls of the plastic as it was being formed, similar to the swirls of chocolate you can see on the inside of an Easter egg. The surface that is in contact with the mold is of much higher quality. While it may not be possible to achieve a super-glossy finish, matte finishes can be built into the mold to hide small defects. Inserts with graphics on can also be molded into parts.

Types/complexity of shape

Adaptable to a range of shapes. Even undercuts are possible. Wall thickness should be kept uniform, between approximately 1/12 and 5/8 inch. Unlike with other processes, there can be a buildup of material in corners which makes them the strongest part of the component.

Scale

Starting with chocolate eggs, it is possible to manufacture hollow products up to 23 feet long by 13 feet wide, such as panels for construction workers’ temporary huts.

Tolerances

Compared with other plastic molding methods, tolerances are low due to shrinkage, cooling rates, and the wall thickness, which varies slightly across the molding.

Relevant materials

Polyethylene, which has that Edam-cheesy feel, is a common material for rotational molding. Other resins can also be used, including acrylonitrile butadiene styrene (ABS), polycarbonate, nylon, polypropylene, and polystyrene. Reinforcement fibers can also be introduced to increase strength in the final component.

Typical products

Chocolate eggs, plastic road-traffic cones, portable toilets, tool cases, large toys that take up half your living room, as well as many other hollow products.

Similar methods

Centrifugal casting (p.161) is a similar process for plastics, but it is not widely available and can only produce small parts. Also blow molding in all its forms (pp.120–33), and dip molding (p.134).

Sustainability issues

As with most plastic processing, high temperatures are needed to melt the plastic, which makes the process quite energy intensive. However, this is a pressureless process. It is difficult to control the precise wall thickness and therefore the amount of material that is needed. Any faulty moldings produced can be melted down and reused in the process too.

Further information

www.bpf.co.uk

www.rotomolding.org

Slip Casting

Product	Wedgwood® teapot, before finishing
Materials	bone china
Manufacturer	Wedgwood
Country	UK

It is often the unfinished article that best reveals the production process, rather than the finished product. This image was taken while the clay was still wet, before the excess material at the top was trimmed off. The parting lines, where the two halves of the mold met, are still visible on the sides of the teapot.

This is a manufacturing process that is just as likely to be used in a college art and design program as in the industrial workshops of Wedgwood^® or Royal Doulton. In slip casting, ceramic particles are first suspended in water to form “slip,” which is something like the color and consistency of melted chocolate. This slip is tipped into a plaster mold. Because the dry plaster mold is porous, the liquid is absorbed from the outer layers of the slip, leaving a coating of leathery and hard ceramic on the inner surface of the mold. When a sufficient thickness has built up, the mold is turned upside down and the remaining muddy liquid is poured out. The excess ceramic around the opening of the mold is trimmed to produce a clean edge before the mold is opened and the molding, now in its “green” state, is removed, ready for firing.

Pressure-assisted slip casting (see p.234) is a process that is employed for larger components.