Volumes of production
One of the best things about this type of semimanual process is that there is no limit to the number of units that can be produced—it can be used for anything from one-offs to runs of several thousand. If you want to produce more than 1,000 units, then it might be worth considering using a semiautomated setup.
Unit price vs. capital investment
Unit price is relatively low for a product that can be tailored and easily adapted. Capital investment is nonexistent because there are no tools.
Speed
Varies, depending on the complexity of the shape.
Surface
Excellent.
Types/complexity of shape
Limits to the shape are based on symmetry because of the fact that the glass tube rotates around a single axis. However, post working of the glass once it is taken off the lathe can allow for design details to be added. Laboratory glassware is made using this method, which may give you an idea of its complexity. Wall sections are generally thin.
Scale
The scale of products is limited by the type of lathe and the skill of the craftsman.
Tolerances
Because this is a handworked process, tolerances are not very high.
Relevant materials
Mainly restricted to borosilicate glass.
Typical products
Anything from special laboratory apparatus and packaging, to oil and vinegar containers (the kind you find in gourmet food stores, where the vinegar bottle is trapped inside the oil bottle), thermometers, and lighting.
Similar methods
Glass blowing by hand (p.116).
Sustainability issues
Although glass is a natural and renewable material, its production and manufacture is less eco-friendly because of the extreme heat required. However, during lampworking the glass is shaped by and which, although time consuming, requires no machinery and therefore helps to balance this high use of energy. Additionally any glass wasted through breakage or error can be recycled back into the process to reduce material consumption and save raw materials.
Further information
www.asgs-glass.org
www.bssg.co.uk
Glass Blow and Blow Molding
Product |
Kikkoman bottle |
Designer |
Kenji Ekuan |
Materials |
soda-lime glass |
Manufacturer |
Kikkoman Corporation |
Country |
Japan |
Date |
1961 |
The proportions and narrow neck of this classic soy sauce bottle are typical of the blow and blow process for glass forming. The parting lines, which are just visible, show the point where the two halves of the mold have separated. The red plastic cap is injection molded.
There are a number of different ways in which blowing air into, or out of, a material can be used to manufacture products, many of which are described in this book. Although varieties of blow molding can be used for plastic (see, for example, injection blow molding, p.129) and even—on a limited scale—metal (see inflating metal, p.76, and superforming aluminum, p.70), it remains one of the major industrial mass-production methods for making blown glass objects. The industrial blow molding of glass today consists of two main methods: blow and blow, and press and blow (see p.124). The blow and blow method discussed here is used to make bottles with narrow necks, such as wine bottles. The term “blown glass” can, of course, also be applied to one-off handmade pieces (see glass blowing by hand, p.116), but we are talking here about the sort of large-scale process that is capable of producing hundreds of thousands of units per day.
To form a product using blow and blow molding, a mixture of sand, sodium carbonate, and calcium carbonate is carried to the top level of the factory, where it is heated to 2,820°F in a furnace that can be as large as a small living room. The molten glass is released in a series of fat sausage shapes, known as “gobs,” which are drawn down by gravity into the forming machines. At this stage, air is injected into the gob to partially form the bottle, including the neck. This semiformed glass is then removed, rotated 180 degrees, and clamped into a further mold. At this stage, air is injected into the mold to form the final shape. The various parts of the mold then open and the bottle is lifted onto a conveyor belt, which carries it to an annealing oven to eliminate any tension in the glass.
1 Gobs of heated glass are dropped from an elevated furnace.
2 The glass gobs are cut to length before being dropped into the mold. – Very low unit price.
3 Hot bottles leaving the mold.
4 A series of eight molding machines feed bottles onto the production line, ready for annealing.
– Able to make narrow-necked containers.
– Exceptionally fast rates of production.
– Versatility is very low in this high-volume method of production.
– Very high tooling costs.
– Demands very high volumes.
– Limited to fairly simple hollow forms.
– Adding color to glass can be expensive as it involves “running through” colors at the end of production to ensure that there is no bleeding between colors.