Production of Hollow Ware Glass

Current machines for hollow ware glass are drop fed and are divided in two categories depending on the methods with which the 'draft' is formed:
1) by blowing;
2) by casting.
In both cases, the various phases of the mechanical processes of the hollow ware glass are the same as manual production: after the withdrawal of the 'gob' of glass (also called removal) through the blow pipe, the blower does not immediately introduce this gob in the mould for the final blowing. In fact the glass gob must be appropriately cooled, shaped and pre-blown before the final blowing, so as to obtain a good quality finished product.

Maximum development came with the invention of the blow pipe and is based on the energy that human lungs possess: the air introduced through the blow pipe in a mass of incandescent glass, removed from the crucible in quantities in proportion to the object to produce, by heating itself up, acts by the pressure of the internal walls, it generates the stretching of the mass as it assumes the desired form from the ability of the maestro blower.
The blow pipe is a simple steel tube: at one end it is narrower and forms the mouth-piece from which to blow; while at the other end, it widens out and is used to dispense and hold the glass.
When the glass is dispensed, a gob forms at the end of the blow pipe and then, once again dispensed, there is the full removal, which is called the paraison, using the French word which has become part of the glassmaker's jargon; then follows marvering which is obtained by rolling the gob (the rotation axis is the blow pipe) on a metal sheet, to give it homogeneity of its initial form and consistency, which is an operation that is generally repeated several times and alternated with light blows in the blow pipe and with heating of the glass in the furnace to maintain its plasticity.
In doing so, the gob is shaped into the most appropriate form depending on the object required (cylindrical, spherical, conical etc.) and finally it is pre-blown; it is at this point that, the gob now having been transformed into a draft, it is blown into a mould.
It is evident that while the external form is determined by the mould, the distribution of the thickness of the finished object is tied to the form and distribution of the temperatures of the draft.
The distribution of the temperatures must be as uniform as possible because the parts of glass at higher temperatures and thus lower viscosity, become more stretched in the final blowing.
In short, the role of the finisher mould must be seen in its right perspective: its sole functions are to confer to the object the external form and to lower the temperature of the glass to a level so as to avoid successive deformations. The quality of the finished object is, rather, determined largely by the previous draft preparation.
There are thus two main production phases in manual production: the first for the preparation of the draft and the second for the final blowing, which gives the object its definitive form.

In mechanised production, whatever the feeding system may be, these two phases are still respected. The formation of the draft is generally obtained in a first mould, called a drafter, from which, with systems that differ according to the type of machine, the draft is transferred to the second mould, the finisher, where the final blowing takes place.
The draft, as already mentioned, can be obtained by blowing or pressing, while the final form is obtained, as always, by blowing.
There are thus two processes:
1) blowing  blowing;
2) pressing  blowing.

The temperature of the draft must be as uniform as possible to obtain a good quality finished object. For this reason, the mould must be fed with a gob of glass of a uniform temperature and which corresponds to the viscosity of the beginning of the production. Considering that the temperature of the bath of glass in the working tank is higher than 1300°C, while the feeding temperature can vary from 1050 to 1250°C, it is clear that it is necessary to 'condition the temperature of the glass'.
This is the purpose of the feeder, a canal located at the end of the furnace. It consists of several areas: the first one for cooling, the second for conditioning and finally a holed tank which allows the exit of the gob.
The first area allows the glass to cool by about 150-200°C . To this end, the canal is equipped with devices to introduce cold air and small gas burners, which are placed along its length. In the conditioning area, which is also equipped with numerous small burners, the glass must regain thermal homogeneity, which had been lost in the previous cooling phase.
In the feeder, the glass can be coloured by adding intensely coloured low melting point glass (frit). In this case, two or three shakers are also placed in the feeder which contribute to the rapid dispersion and homogenisation of the colour. So from one furnace, in addition to the uncoloured glass, two or three different colours can also be obtained.
The feeder is composed of a tank connected to the conditioning canal, equipped with one or more holes (cuvettes) for the exit of the gobs which is regulated by the alternative vertical movement of a refractory piston.
Coaxial to the piston there is one or two rotating cylinders in refractory material which contribute to keeping the temperature of the glass uniform. Finally, under the hole, there are scissors to remove the gob.

Blowing-blowing process
The I.S. machine (Individual Section), which was invented in 1925 and successively perfected, is available today also in 8 sections which can work with multiple gobs. After cutting them with the scissors, the gob is guided, along canals, to the single sections.
Each section consists of a draft mould and a finisher mould in line with each other. The first receives the gob in an upside-down position and is positioned above a guide ring. Blowing occurs through the head shaft, then the drafter opens and the draft, supported by the mould of the mouth, is transferred to, via rotation in the open air, to the finisher mould where the final blowing takes place.
The main characteristic of this machine is the possibility to act independently on each section, both for the regulation (cooling of mould, blowing pressures and times etc.) and, if needed, to change the moulds, without stopping the machine itself.

Pressing-blowing process
This differs from the blowing-blowing process only by the moulding of the draft which occurs by pressing. Some machines are equipped to work with both processes. The pressing-blowing process is particularly recommended for the production of wide mouth containers (vases).

Pressing-blowing-turning process
In the rotating machine, it is the drafting mould which is placed under the hole in the tank to receive the gob. The pressed draft is turned around in the finishing mould during blowing; this avoids marks from the mould being left on the surface.

With these automatic machines, the container (bottle, vase etc.) is formed in a few seconds. So we have gone from a production of some hundreds of pieces in a craftsman's workshop, to hundreds of thousands which come out of a single factory every day.

The main functions of the moulds are: to cool the product to allow it to keep its form and to produce objects with good quality surfaces. To satisfy these requirements, the moulds must: 1) be corrosion resistant from the molten glass, 2) ensure glass cannot adhere to it, 3) possess a high thermal conductibility, and lastly, 4) be easily workable.
Wood moulds used in artistic production are a separate case: at the time of blowing, humidity (the mould is, after each operation, cooled by immersion in water) turns into steam and this, combined with the combustion gases (of the wood), protect the glass from direct contact with the form (this system also works with rotating machines, with moulds made of steel treated on the surface with nickel-based alloys). This gives the surface high shininess, especially if, at the time of blowing, the object is turned around in its form. The only drawback is the limited number of blowings possible.
Even today, wood moulds, due to their low cost, are used in artistic glassworks for the production of prototypes.
Up until a few decades ago, cast iron was the only material used for the production of moulds. It is still today the most used material due to its relatively low cost and its high thermal conductibility.
Steel, compared to cast iron, has the advantage of having higher shininess and a good resistance to oxidisation. By contrast, we must consider the lower conductibility and the hardness which makes production difficult and expensive.
The possibility that glass can adhere to the mould is removed through cooling and lubrication of the mould. The first lubricants used consisted of oils and greases which were applied by buffering. Today colloidal graphite is widely used, which improves the quality of the surface of the glass, reduces oxidisation of the mould and adherence.
In addition to giving form, the mould also has the function of cooling the glass object: the glass gives off heat to the mould and thus to the environment. The surface in contact with the glass undergoes sharp changes in temperature which are lowered rapidly inside the walls. The outside surface is cooled via the air so as to keep the temperature on the inside surface as high as possible to obtain a good surface of the glass, without though bringing about adherence.
The discharge of heat of the moulds in the glasswork is essentially carried out by forced convection via ventilated air: continuous cooling of the outside surface, discontinuous cooling of the inside one or a combination of both methods.

The surface of the container is usually protected from damage deriving from bumpings and abrasion via a double casing: the first (hot) is applied between the forming and the annealing, the second (cold) comes immediately after the annealing (see annealing furnaces).
Cold treatment is a layer of metal oxides (generally tin oxide) which act as an adhesion promoter for the cold treatment which is on the outside. This is a thin lubricating layer consisting of long chain organic molecules (external or fatty acids) which, jutting out towards the outside, prevent direct glass-glass contact. In this way, the danger of micro-fractures (abrasions) on the surface and of breakage during the bottling phase is reduced and they make the surface of the glass water-repellent.

The increasing quality requirement and the need to maintain the level of production, have forced glassworks to put into place automatic controls.
The machines used are opto-electronical which allow dimensional controls and the identification of the presence of cracks on the surface, of bubbles, infusions and deformations of the object and defects in the thickness. Faulty objects are automatically rejected.

In the last decades, glass has had to compete with other lighter and safer packaging materials (plastic, tetra pack). Considerable effort has been made by producers to improve containers by reducing their weight by over 1/3 with the same mechanical performance.
This was obtained by an improved homogeneity of the glass, an improved distribution on the section of the container, surface treatments and improved controls.

Decoration methods are the same ones used for other glass objects: Silk-screening, sand-blasting, glazing with acids and painting (see Flat glass: Cold-decoration).
We must add that, given that we are talking about containers used for foodstuffs or pharmaceuticals (which are considered, by law, as packaging and destined, after use, to be recycled), the producer must consider laws in relation to safety and removal of toxic waste (use of acids, presence of heavy metals like mercury, cadmium, lead, which are usually in enamels for silk-screening ad painting); also, what the container will be used for and how it will be used must be known in order to carry out all the necessary controls and finish with a product which is suitable for the required use.
Here are some examples.
Silk-screening
After the drying of the decoration, the containers are heated at a temperature of about 600°C to allow the enamel to permanently adhere to the glass.
Knowing the final use of the container is also useful in assessing the level of annealing because, with certain processed, the molecular structure of the glass changes, with the consequent weakening of the container.
For example, in the case of bottles for pressure liquids (spumante, champagne), the maximum pressure at which the bottling occurs must be known.
Sand blasting
This is used only for particular production and it is important that the bottles, thus treated, are thoroughly washed before bottling.
What must be kept in mind is that this type of treatment produces micro-cracks on the surface which physically effect the structure of the container.
Acid glazing
This treatment is not recommended because, despite all precautions, the acids can get into the container with the consequent risk of residual acids being left inside even after washing them.
New technology
This is white coating, and looks very much like traditional glazing.
The bottle treated like this can be cold silk-screened (80°C).
With this treatment, the glass is not corroded so it is particularly recommended for decorating bottles which must guarantee resistance to inside pressure.
Painting
A new decoration method has been developed in which paints and water are used which contain colouring pigments. The aesthetic effect is very good, but the cost is still high. Bottles painted using this method can be silk-screened, using fluid ink which are annealed at a low temperature.