Volumes of production
Range from several thousand to hundreds of thousands per 24-hour period. The minimum production run to achieve an economical price is approximately 50,000 units. The weight of the glass, however, is one of the main determinants of speed, and rates of 170,000 units per day are not uncommon.
Unit price vs. capital investment
This is a process for high-volume mass-production. Tooling costs are high, and production runs for glass need to last for days, on a continuous 24-hour cycle, for the products to be cost-effective.
Speed
Depending on the bottle size, machines can be set up to hold several molds at the same time on a single machine. This can result in very high production rates, with some approaching 15,000 pieces an hour.
Surface
Excellent finish—look at any wine bottle.
Types/complexity of shape
Restricted to fairly simple forms. In large-scale glass production, the forms need to be carefully designed to allow for the easy opening of molds—for instance, they cannot have sharp corners, undercuts, or large, flat areas. The blow and blow method is, in fact, very inflexible, and you should consult a manufacturer for specific designs. Do not look at expensive perfume bottles for inspiration, because that is a different game altogether.
Scale
Because of the nature of the applications for blow-molded products (mainly for domestic glass vessels), most manufacturing is set up for a maximum of 12-inch-high containers.
Relevant materials
Almost any type of glass.
Typical products
Narrow-necked wine and spirit bottles, and oil, vinegar, and champagne bottles. Similar methods
While this method is suited to making narrow-necked glass containers, press and blow molding can make open-necked glass containers (p.124). For plastics, see injection blow molding (p.129) and extrusion blow molding (p.132).
Sustainability issues
Although this process has an incredibly high production rate that helps to make effective use of energy, the extreme heats required throughout the various stages of production make it incredibly energy intensive. On the positive side, glass is a natural and renewable material so has a low environmental impact while it can also be widely recycled.
Further information
www.vetreriebruni.com
www.saint-gobain-emballage.fr
www.packaging-gateway.com
www.glassassociation.org.uk
www.glasspac.com
www.beatsonclark.co.uk
Glass Press and Blow Molding
Product |
storage jar |
Materials |
soda lime glass with thermoplastic elastomer (TPE) seal |
Manufacturer |
Vetrerie Bruni |
Country |
Italy |
The open-necked shape of this jar is a typical example of a product for which you would have to consider press and blow molding in preference to blow and blow molding (see p.120).
A form of industrial glass blow molding, the technique known as “press and blow” is used to make wide-mouthed containers such as jam jars, rather than the narrow-necked items such as wine bottles, that are made with the blow and blow process (see p.120). The main difference between the techniques occurs during the molding process. Instead of being blown, to create wide-mouthed vessels the “gob” of glass is pressed onto a male former inside the mold cavity. This can speed up production cycles and allows greater control in the distribution of the glass, so that a thinner wall can be achieved. After the objects have been formed, the production line pushes them into an annealing furnace where, over the period of an hour, the objects are slowly cooled to room temperature, thus eliminating any tension in the glass.
Inside the factories, machines shoot out glowing, molten gobs of glass that look like shafts of light falling into the cavities of the empty molds. This process has none of the theater and craftsmanship of hand-blown glass: the automated, greasy, noisy, steaming machines can turn out hundreds of thousands of bottles per day with just a handful of men watching over this vast production.
Compared with the blow and blow process, which can produce more than 350,000 narrow-necked units per day, this process can churn out 400,000 units the size of, for example, jam jars. When it comes to small “press and blow” bottles, however, the machines can pump out up to 900,000 units of, say, small pharmaceutical bottles per day, running on a continuous 24-hour cycle. Uninterrupted production runs for some food packaging can last up to ten months, just producing the same objects over and over again.
– Very low unit price.
– Suited to making thin-walled, open-necked vessels.
– Exceptionally fast rates of production.
– Exceptionally fast cycle times.
– Very high tooling costs.
– Limited to fairly simple hollow forms.
– Adding color to glass can be expensive, as can running through colors at the end of production to clean out the machines.
– Demands high volumes in order to be economical.