Volumes of production
Glass pressing is a term that can be applied to a hand process, a semiautomated, or an automated machine process. Semiautomated production can be used for a minimum of 500 units, and is often employed for sampling large production runs for fully automated production.
Unit price vs. capital investment
In fully automated production, the unit prices can be extremely low, but like most high-volume production, it requires expensive tooling.
Speed
In an automated setup, and depending on the component size, a single machine can be set up to hold several molds at the same time. This can result in huge production rates—some approaching 5,000 pieces per hour.
Surface
Dimples, serrations, and diamond patterns are all achievable in pressed glass, although the definition is less pronounced than such patterns in cut glass.
Types/complexity of shape
While blown glass (see p.116) lends itself well to rounded shapes, pressed glass is a lot more versatile because it allows for more complex detailing and decoration. One of the key design features to bear in mind is that it is not possible to have a closed shape and, as in thermoforming (see p.64), the component must have a draft angle to allow for the mold to open at the end of production. Pressed glass is also more suited to thick-walled hollow ware.
Scale
Some semiautomated production allows for a maximum of about 24 inches in diameter. Larger pieces can be made, depending on both the volume of production and the manufacturer.
Tolerances
Due to contraction and expansion of the material, glass pressing is able to match the high tolerance of engineered components. However, a typical tolerance is ±1/25 inch.
Relevant materials
Almost any type of glass.
Typical products
Lemon juicers, railroad signal lamps, lenses, street and display lighting, laboratory glass, glass ashtrays, sidewalk lenses, wall blocks, marine and ship lighting, aircraft and airport runway lenses, and road and traffic signals.
Similar methods
Cut glass can be used for fine-detailed patterns, but this is really your best bet for producing open glass shapes with the potential for decoration on both sides. For plastic components, you might want to consider compression molding (p.174).
Sustainability issues
Glass is a renewable material that is widely recyclable. Recycling it can reduce waste and virgin material consumption, and it retains its excellent clarity and appearance after reprocessing. However, its production and manufacture is less environmentally friendly as pressing glass requires various stages of intense heating, which consumes large quantities of energy. Some harmful air-polluting compounds and particles are released during processing.
Further information
www.nazeing-glass.com
www.glasspac.com
www.britglass.org.uk
Pressure-Assisted Slip Casting
with pressure-assisted drain casting
Product |
bath from the Loo range |
Designer |
Marc Newson |
Materials |
ceramic |
Manufacturer |
Ideal Standard |
Country |
UK |
Date |
2003 |
This bath is a typical example of the scale of casting that can be produced in ceramic.
Pressure-assisted slip casting is a development of conventional ceramic slip casting (see p.140). Compared with the traditional form, it offers several manufacturing advantages that affect the speed and complexity of the final component. Conventional slip casting involves the use of plaster molds into which the ceramic “slip” is poured. The “de-watering” of this slip is based on a capillary action that draws water from the slip into the plaster, leaving the clay to form a dry layer against the internal wall of the mold. This can be quite slow and the plaster molds have a limited life.
In pressure-assisted slip casting, a more resilient material, with larger holes, is used for the mold. The size of the holes means that the capillary action is reduced and replaced by the use of pressure (typically between 10 and 30 bar, depending on the size of the product). This involves pumping the slip into the porous plastic mold. Under this pressure, the water seeps out through naturally occurring capillary tubes in the mold. Once dried, the form is taken out of the mold and any imperfections are cleaned off. The product is then dried in fast dryers and sprayed with a glaze before firing.
A project called Flexiform, led by Ceram Research in the UK, has enhanced pressure-assisted slip casting, coming up with a process it calls “pressure-assisted drain casting.” In this development, the conventional synthetic mold is replaced by a machinable plastic, which can be machined directly from the product designer’s original CAD drawing. This offers a number of further advantages, including cheaper tooling and the possibility of the mold being re-cut, which is not possible with the molds used for pressure-assisted slip casting.
– The plastic mold allows higher pressure to be used in the production of large pieces.
– Plastic molds have a longer life (approximately 10,000 casts) before they are thrown away.
– Fewer molds are needed and less storage is required.
– The molds add to the setup costs (however, for drain casting, Flexiform molds greatly reduce the tooling costs).