Glass Manufacturing Process - Soda Lime

Glass is a rigid, brittle, and hard under cooled Amorphous substance and posses high viscosity to reduce crystallization physically; It is usually transparent, opaque and sometime translucent.

 Chemically Glass is a fused mixture of silicate and alkaline or alkaline earth metals and other glass constituents that  such as CaO from limestone (CaCO3), Na2O from Soda Ash(Na2CO3), Al2O3 from Feldsper etc. 

 Glass production involves two main methods – the float glass process that  is used to produce sheet glass like window sheet, and glassblowing that produces bottles and other containers; Both follow 

The glass – float glass as we know - is manufactured by the PPG process. This process was invented by Sir Alistair Pilkington in 1952 and is the most popular and widely used process in manufacturing architectural glass in the world today.

It consists of the following steps:

Glass Sheet Making with float method

Stage 1- Melting & Refining:

Fine grained ingredients closely controlled for quality, are mixed to make a batch, which flows into the furnace, which is heated up to 1500 degree Celsius.

The raw materials that go into the manufacturing of clear float glass are:

• SiO2 – Silica Sand (Former)
• Na2O – Sodium Oxide from Soda Ash (Fluxing agent)
• CaO – Calcium oxide from Limestone  
• MgO – Dolomite
• Al2O3 – Feldspar

The above raw materials primarily mixed in batch helps to make clear glass. If certain metal oxides like  are mixed to this batch they impart colors to the glass giving it a body tint.

Examples of such metal oxide are:          

• NiO & CoO – to give grey tinted glasses (Oxides of Nickel & Cobalt)
• SeO – to give Bronze tinted glasses (oxide of Selenium) Some time used to mask ion (III) in flint glass making.
• Fe2O3 – To give Green tinted glasses (oxides of iron which at times is also present as impurity in Silica Sand which is the main former of glass)
• CoO – To give blue tinted glass (oxides of Cobalt)

Apart from the above basic raw material, broken glass  (cullet), is added to the mixture to the tune of nearly 25% ~ 30% which acts primarily as flux. The flux in a batch helps in reducing the melting point of the batch thus reducing the energy consumed to carry out the process.

Stage 2 - Float Bath:

Glass from the furnace gently flows over the refractory spout on to the mirror-like surface of molten tin, starting at 1100 deg Celsius and leaving the float bath as solid ribbon at 600 deg Celsius.

Stage 3 - Coating (for making reflective glasses):

Coatings that make profound changes in optical properties can be applied by advanced high temperature technology to the cooling ribbon of glass. Online Chemical Vapour Deposition (CVD) is the most significant advance in the float process since it was invented. CVD can be used to lay down a variety of coatings, a few microns thick, for reflect visible and infra-red radiance for instance. Multiple coatings can be deposited in the few seconds available as the glass flows beneath the coater (e.g. Sunergy)

Stage 4 - Annealing:

Despite the tranquillity with which the glass is formed, considerable stresses are developed in the ribbon as the glass cools. The glass is made to move through the annealing lehr where such internal stresses are removed, as the glass is cooled gradually, to make the glass more prone to cutting.

Stage 5 - Inspection:

To ensure the highest quality inspection takes place at every stage. Occasionally a bubble that is not removed during refining, a sand grain that refuses to melt or a tremor in the tin puts ripples in the glass ribbon. Automated online inspection does two things. It reveals process faults upstream that can be corrected. And it enables computers downstream to steer round the flaws. Inspection technology now allows 100 million inspections per second to be made across the ribbon, locating flaws the unaided eye would be unable to see.

Stage 6 - Cutting to Order:

Diamond steels trim off selvedge – stressed edges- and cut ribbon to size dictated by the computer. Glass is finally sold only in square meters.

Blow and Blow Method for Glass container(Hollow Glass) Making.

Broadly, modern glass container factories are three-part operations: the batch house, the hot end, and the cold end. The batch house handles the raw materials; the hotend handles the manufacture proper—the forehearth, annealing Lehrs, and forming machines made up of individual sections(IS.); and the cold end handles the product-inspection and packaging.

 Hot end

The following table lists common viscosity fixpoints, applicable to large-scale glass production experimental results.

log10(η, Pa·s)	log10(η, P)	Description
1	2	Melting Point (glass melt homogenization and fining)
3	4	Working Point (pressing, blowing, gob forming)
4	5	Flow Point
6.6	7.6	Littleton Softening Point (Glass deforms visibly under its own weight. Standard procedures ASTM C338, ISO 7884-3)
8–10	9–11	Dilatometric Softening Point, Td, depending on load[2]
10.5	11.5	Deformation Point (Glass deforms under its own weight on the μm-scale within a few hours.)
11–12.3	12–13.3	Glass Transition Temperature, Tg
12	13	Annealing Point (Stress is relieved within several minutes.)
13.5	14.5	Strain Point (Stress is relieved within several hours.)

Batch processing is one of the initial steps of the glass-making process. The batch house simply houses the raw materials in large silos (fed by truck or railcar) and holds anywhere from 1–5 days of material. Some batch systems include material processing such as raw material screening/sieve, drying, or pre-heating (i.e. cullet). Whether automated or manual, the batch house measures, assembles, mixes, and delivers the glass raw material recipe (batch) via an array of chutes, conveyors, and scales to the furnace. The batch enters the furnace at the 'dog house' or 'batch charger'.

 Different glass types, colors, desired quality, raw material purity / availability, and furnace design will affect the batch recipe Eg. Flint glass batch is made up of silica sand (SiO2) Limestone(stabilizer), Feldspar(Binder) soda ash(fluxing agent), salt cake (Purifier) and cobalt and selenium(iron (III) oxide ) masking agents and cullet another fluxing agent. 

Georgia green glass batch is made up of silica sand (SiO2) Limestone(stabilizer), Feldspar(Binder) soda ash(fluxing agent), salt cake (Purifier), cobalt  and Iron chromite source of (iron (III) oxide ) that would be complex to green coloration of the finished glass.

Amber glass batch is made up of silica sand (SiO2) Limestone(stabilizer), Feldspar(Binder) soda ash(fluxing agent), salt cake (Purifier), cobalt  charcoal for amber coloration of the finished glass.

Furnace- Hot end

Batch feed (doghouse) of a glass furnace

The hot end of a glassworks is where the molten glass is formed into glass products, beginning when the batch is fed into the furnace at a slow, controlled rate by the batch processing system (batch house). The furnaces are natural gas- or fuel oil-fired, and operate at temperatures up to 1,575 °C (2,867 °F).

 The temperature is limited only by the quality of the furnace’s superstructure material and by the glass composition. Types of furnaces used in container glass making include 'end-port' (end-fired), 'side-port', and 'oxy-fuel'. Typically, furnace "size" is classified by metric tons per day (MTPD) production capability.

Forming process

There are currently two primary methods of making glass containers: the blow & blow method for narrow-neck containers only,

Narrow neck Glass Wares

 and the press & blow method used for jars and tapered narrow-neck containers.

                                                   

           Glasss Jar

Figure2:  Image of IS machine during bottle production

In both methods, a stream of molten glass, at its plastic temperature (1,050–1,200 °C [1,920–2,190 °F]), is cut with a shearing blade to form a solid cylinder of glass, called a gob. The gob is of predetermined weight just sufficient to make a bottle. Both processes start with the gob falling, by gravity, and guided, through troughs and chutes, into the blank moulds, two halves of which are clamped shut and then sealed by the "baffle" from above.

In the blow and blow process, the glass is first blown through a valve in the baffle, forcing it down into the three-piece "ring mould" which is held in the "neckring arm" below the blanks, to form the "finish", [The term "finish" describes the details (such as cap sealing surface, screw threads, retaining rib for a tamper-proof cap, etc.) at the open end of the container.] The compressed air is blown through the glass, which results in hollow and partly formed container. Compressed air is then blown again at the second stage to give final shape.

Containers are made in two major stages. The first stage moulds all the details ("finish") around the opening, but the body of the container is initially made much smaller than its final size. These partly manufactured containers are called parisons, and quite quickly, they are blow-molded into final shape.

Referring to the mechanism, the "rings" are sealed from below by a short plunger. After the "settleblow" finishes, the plunger retracts slightly, to allow the skin that's formed to soften. "Counterblow" air then comes up through the plunger, to create the parison. The baffle rises and the blanks open. The parison is inverted in an arc to the "mould side" by the "neckring arm", which holds the parison by the "finish".

As the neckring arm reaches the end of its arc, two mould halves close around the parison. The neckring arm opens slightly to release its grip on the "finish", then reverts to the blank side. Final blow, applied through the "blowhead", blows the glass out, expanding into the mould, to make the final container shape.

In the press and blow process, the parison is formed by a long metal plunger which rises up and presses the glass out, in order to fill the ring and blank moulds.[5] The process then continues as before, with the parison being transferred to the final-shape mould, and the glass being blown out into the mould.

The container is then picked up from the mould by the "take-out" mechanism, and held over the "deadplate", where air cooling helps cool down the still-soft glass. Finally, the bottles are swept onto a conveyor by the "push out paddles" that have air pockets to keep the bottles standing after landing on the "deadplate"; they're now ready for annealing

Internal treatment

After the forming process, some containers—particularly those intended for alcoholic spirits—undergo a treatment to improve the chemical resistance of the inside, called internal treatment or dealkalization. This is usually accomplished through the injection of a sulfur- or fluorine-containing gas mixture(1.1.1.2 tetrafluoroethane gas) its a non flammable gas into bottles at high temperatures. The gas is typically delivered to the container either in the air used in the forming process (that is, during the final blow of the container), or through a nozzle directing a stream of the gas into the mouth of the bottle after forming. The treatment renders the container more resistant to alkali extraction, which can cause increases in product pH that can lead to alkoxide formation  and in some cases container degradation (Blooming).

Annealing

As glass cools, it shrinks and solidifies. Uneven cooling causes weak glass due to stress. Even cooling is achieved by annealing. An annealing oven (known in the industry as a Lehr) heats the container to about 580 °C (1,076 °F), then cools it, depending on the glass thickness, over a 20 – 60 minute period.

Cold end

The role of the cold end is to spray on a polyethylene coating for abrasion resistance and increased lubricity, inspect the containers for defects, package the containers for shipment, and label the containers.

Inspection equipment

Glass containers are 100% inspected; automatic machines, or sometimes persons, inspect every container for a variety of defects. Critical  defects( Defects that can affect life of the user) include Bird swing/bird cage, stocked glass in the container, blank or blow  mold sharp seem on the container glass , while other defects that can be categorize as major or minor defects are checks and foreign inclusions called stones which are pieces of the refractory brick lining of the melting furnace that break off and fall into the pool of molten glass, or more commonly oversized silica granules (sand) that have failed to melt and which subsequently are included in the final product.

 These are especially important to select out due to the fact that they can impart a destructive element to the final glass product. For example, since these materials can withstand large amounts of thermal energy, they can cause the glass product to sustain thermal shock resulting in explosive destruction when heated. Other defects include bubbles in the glass called blisters and excessively thin walls. Another defect common in glass manufacturing is referred to as a tear. In the press and blow forming, if a plunger and mould are out of alignment, or heated to an incorrect temperature, the glass will stick to either item and become torn. In addition to rejecting faulty containers, inspection equipment gathers statistical information and relays it to the forming machine operators in the hot end. Computer systems collect fault information and trace it to the mould that produced the container. This is done by reading the mould number on the container, which is encoded (as a numeral, or a binary code of dots) on the container by the mould that made it. Operators carry out a range of checks manually on samples of containers, usually visual and dimensional checks.

Secondary processing

Sometimes container factories will offer services such as labelling. Several labelling technologies are available. Unique to glass is the Applied Ceramic Labelling process (ACL). This is screen-printing of the decoration onto the container with a vitreous enamel paint, which is then baked on. An example of this is the original Coca-Cola bottle. Absolut Vodka Bottles have various added services such as: Etching (Absolut Citron/) Coating (Absolut Raspberry/Ruby Red) and Applied Ceramic Labelling (Absolut Blue/Pears/Red/Black).

Common defects at this are poorly backed ware, bleeding print, rubb off print, fallen wares etc.

Packaging.

Glass containers are packaged in various ways. Most popular ways are bulk pallets with between 1000 and 4000 containers each. This is carried out by automatic machines (palletisers) which arrange and stack containers separated by layer sheets. Other possibilities include boxes and even hand-sewn sacks. Once packed, the new "stock units" are labelled and warehoused.

Coatings

Glass containers typically receive two surface coatings, one at the hot end, just before annealing and one at the cold end just after annealing. At the hot end a very thin layer of tin(IV) oxide is applied either using a safe organic compound or inorganic stannic chloride. Tin based systems are not the only ones used, although the most popular. Titanium tetrachloride or organo titanates can also be used. In all cases the coating renders the surface of the glass more adhesive to the cold end coating. At the cold end a layer of typically, polyethylene wax, is applied via a water based emulsion. This makes the glass slippery, protecting it from scratching and stopping containers from sticking together when they are moved on a conveyor. The resultant invisible combined coating gives a virtually unscratchable surface to the glass. Due to reduction of in-service surface damage, the coatings often are described as strengtheners, however a more correct definition might be strength-retaining coatings.

Ancillary processes – compressors and cooling

Forming machines are largely powered by compressed air and a typical glass works will have several large compressors (totaling 30k–60k cfm) to provide the needed compressed air. Furnaces, compressors and forming machine generate quantities of waste heat which is generally cooled by water. Hot glass which is not used in the forming machine is diverted and this diverted glass (called cullet) is generally cooled by water, and sometimes even processed and crushed in a water bath arrangement. Often cooling requirements are shared over banks of cooling towers arranged to allow for backup during maintenance.