Volumes of production

From batch production to high-volume mass-production.

Unit price vs. capital investment

One of the least expensive ways of mass-producing plastic components, with reasonably cheap tooling and easy-to-produce samples, while still allowing for cost-effective unit parts.

Speed

This process involves many steps, including preheating of the former, dipping, curing, and finally peeling the finished molding from the former, which makes for a slow process if performed manually. Complex moldings may take up to 45 minutes to complete, while the production of very simple shapes, such as end caps (for example, simple bicycle-handlebar grips) can be fully automated and may take only 30 seconds.

Surface

The exterior of the component is determined by the natural state of the material, and may have a small nipple as evidence of the polymer dripping from the mold.

Types/complexity of shape

Soft, rubbery, flexible, though simple, forms. Products must be shaped in such a way that they can be unpeeled from the mold.

Scale

The scale of dip moldings is theoretically only limited by the size of the bath containing the polymer, but generally moldings range from 1/25-inch-diameter end caps to 24-inch industrial pipe covers.

Tolerances

Dip molding does not achieve a high level of accuracy, apart from on the internal dimensions.

Relevant materials

Because of the nature of the process, which involves the former being “undressed” as the part is removed, it is limited to soft materials and parts that can be stretched over the molds, including PVC, latex, polyurethanes, elastomers, and silicones.

Typical products

A whole range of flexible and semirigid products, from kitchen and surgical gloves to balloons and those soft, waxy plastic handlebar grips for children’s bikes.

Similar methods

An economical alternative to plastic blow molding (p.127) and rotational molding (p.137).

Sustainability issues

Heat is required to keep the polymer bath in its molten state throughout processing so dip molding is energy intensive. Furthermore, some plastics such as latex and silicone are often not widely recyclable. On a brighter note, latex products such as balloons can, in fact, be composted, which prevents the material entering the waste stream.

Further information

www.wjc.co.uk

www.uptechnology.com

www.wade.co.uk

www.qualatex.com

Rotational Molding

AKA Roto Molding and Rotational Casting

Product	Rotationalmoldedshoe
Designer	Marloes ten Bhömer
Materials	Polyurethane rubber and stainless steel
Manufacturer	Marloes ten Bhömer
Country	UK
Date	2009

The shoes demonstrate a production process being transferred into a completely new type of product. The image above shows how the two parts are separated (not actually with a knife) and rejoined to make the shoe.

Rotational molding is all about making things that are hollow. If you have ever wanted to know how chocolate Easter eggs are made, then the answer lies in this method of production. One of the interesting things about rotational molding is that the soft and rounded products that are typical of this method very much take their aesthetic from the limitations of the process. This is quite unlike injection molding (see p.196), which uses pressure to inject material into the mold, producing sharp edges and fine detail. Roto molding, as it is sometimes known, uses only heat and the rotation of a mold to form parts and thus lacks the fineness of pressure-formed parts.

In a sense, rotational molding is based on a similar idea to ceramic slip casting (see p.140). In both methods, a liquid material is built up on the internal cavity of a mold, allowing the manufacture of hollow parts. It is a simple, four-stage process, which begins with adding powdered polymer to a cold die. The amount of powder in relation to the size of the die determines the wall thickness of the final component. The second stage involves the die being uniformly heated inside an oven, while simultaneously being slowly rotated around two axes. This allows the polymer (which is now liquid) to tumble around the inside of the die, where it builds up on the walls and creates a hollow form. Finally, while the die is still rotating, it is cooled using air or water before the component is removed.

These images show (top) close-up of half of the rotational molding tool; (middle and above) tabletop rotational molding.

–  Ideal for hollow shapes.

–  Suitable for low-volume production.

–  Simple process.

–  Allows for cost-effective production of large components.

–  Not suitable for making small, precise components.