Volumes of production

Range from several thousand to hundreds of thousands per day. This level of high-volume production is usually determined by time, rather than by numbers of units produced per hour. It may take up to eight hours for production to be in full swing, so a minimum production cycle is likely to be around three days, with machines running without interruption.

Unit price vs. capital investment

As with the similar process of blow and blow molding (see p.120), this is a process only for high volume mass-production. Tooling is prohibitively expensive unless you have production runs of several tens of thousands of units.

Speed

The press and blow method is generally slightly faster than blow and blow glass production, though they have in common the fact that the weight of the glass is a determining factor for speed. Rates of 250,000 units per day for a typical large cooking-sauce jar are fairly standard.

Surface

Just look at a jam jar and you can see the excellent finish. However, just as with blow and blow bottles, the witness lines will need to be taken into account if labels are to be added.

Types/complexity of shape

Restricted to fairly simple forms with wide, open necks. In large-scale glass production these forms cannot have sharp corners, undercuts, or large, flat areas, all of which would make releasing them from the mold difficult. Compared with blow and blow molding, press and blow allows a greater degree of control over the thickness of the glass.

Scale

As with blow and blow, manufacturing is set up for a maximum of 12-inch-high containers.

Relevant materials

Almost any type of glass.

Typical products

Open-necked jam jars and spirit bottles, open-necked pharmaceutical and other containers, and food packaging.

Similar methods

For glass, blow and blow molding (p.120), lampworking (p.118), and glass blowing by hand (p.116). For plastics, plastic blow molding (p.127) and extrusion blow molding (p.132).

Sustainability issues

Similar to blow and blow molding, the extreme heats used throughout various stages of production amount to an exceedingly high energy consumption. Yet, the exceptionally high production rate and fast cycle times are optimized to make economical use of this energy, while the recycling of glass back into the process helps to reduce the use of raw materials.

Further information

www.vetreriebruni.com

www.britglass.org.uk

www.saint-gobain-conditionnement.com

www.beatsonclark.co.uk

Plastic Blow Molding

Blow molding is an umbrella term that describes one of the major industrial mass-production methods for producing a whole host of hollow products. In one sense it is unusual, because it is a process that can be used for molding plastic containers as well as glass bottles (see glass blow and blow [p.120] and glass press and blow [p.124] molding).

There are several forms of blow molding suitable for plastics, including injection blow molding and injection stretch molding (see p.129), and extrusion and co-extrusion blow molding (see p.132). All have differing potential to create shapes, but, in simple terms, all of them involve a process that is like blowing a balloon into a mold to form a shape. The process starts with a pre-form being fed into a two-part mold. The closing of the mold snips the material to an appropriate length, forming a seal at one end of the plastic. This pipelike form is fed into a second mold where air is blown into it, forcing the plastic to expand against the mold cavity to form the final shape, after which the mold opens and the part is released.

–  Very low unit price.

–  Exceptionally fast rates of production.

–  Details, such as threads, can be molded in.

–  High tooling costs.

–  Demands high volumes in order to be cost-effective.

–  Limited to fairly simple hollow forms.