Glass container industry : Hot end

Furnace

The hot end of a glassworks is where the molten glass is formed into containers. This begins when the batch is fed at a controlled rate into the furnace. The furnaces are natural gas or fuel oil fired and operate at just below 1600ºC (this is the temperature at which the refractory roof bricks begin to melt). Glass furnaces typically operate an energy recovery scheme known as regeneration. The hot exhaust gas flow back over one of two piles of loosely packed bricks, called regenerators. These bricks become hot and every 20-30 minutes the flow of the combustion system is changed over so that the combustion air, which is mixed with the gas, is drawn through the heated bricks, and the combustion exhaust flows through the other pile of bricks. The batch melts inside the furnace which is maintained as a great pool of molten glass, perhaps 1200mm deep by 50 to 150 m². The molten glass flows from a subducted channel known as the furnace throat into the refiner and forehearth channels. These channels, 1200mm wide and 400-150mm deep transport the glass to the glass bottle forming machines. The role of these channels is to cool the glass very precisely so that the glass at the forming machine is of a uniform and exact temperature.

Forming process

These days, there are two main methods of making a glass container - the blow and blow method and the press and blow method. In all cases a stream of molten glass at its plastic temperature (1050ºC-1200ºC) is cut by a shearing blade to form a cylinder of glass called a gob. Both of the processes start with this gob falling by gravity and guided by troughs and chutes into the blank moulds. In the blow and blow process, the glass first is blown from below into the blank moulds to create a parison or pre-container. This parison is then flipped over into a final mould, where a final blow blows the glass out in to the mould to make the final container shape. In the case of press and blow, the parison is formed by a metal plunger which pushes the glass out into the blank mould. The process then continues as before, with the parison being transferred to the mould, and the glass being blown out into the mould.

In both cases the moulds are in two halves, and open to allow the parison or final container to be transferred out. The break between the two halves is responsible for the characteristic lines you will see running lengthwise on a container. So containers are not, as some may think, made in two halves and stuck together.

Forming machines

While the huge furnaces are a glasswork's fiery heart, the forming machines undoubtedly make the brain. These complex and highly engineered machines contain the mechanisms that hold and move the parts that form the container. Generally powered by compressed air, the mechanisms are timed to coordinate the movement of all these parts so that containers are made.

The most widely used forming machine arrangement is the individual section machine (or IS machine), invented in 1903 by Michael J Owens in Illinois, U.S.. This type of machine is arranged as a bank of 8-12 identical sections, each of which contains one complete set of mechanisms to be able to make containers. The section are lined up in a row, and the gobs feed into each section via a moving chute, called the gob distributor. Sections are engineered to make either one, two, three or four containers at once. (Referred to as single, double, triple and quad gob respectively). In the case of multiple gobs, the shears cut the gobs simultaneously, and they fall into the blank moulds in parallel.

Annealing

As glass cools, it simultaneously shrinks and solidifies (by the way, it does not flow below the liquidus temperature - see Glass). If the cooling is uneven then stress caused by thermal shrinkage will set inside the glass, and this will cause the glass to be weak. The forming process, occurring over a short period of time (6-20 seconds), leaves the container with internal stresses which then need to be relieved through annealing (See Annealing (glass)). (The stress is compression on the outside and tension on the inside. A typical material properties demonstration involves an unannealed container being used to drive a nail into a piece of wood - the container is then scratched on the inside and it breaks easily or just falls to pieces. Also see Prince Rupert's Drops). An annealing oven (known in the industry as a Lehr presumably after the eponymous German manufacturer of the same) first heats the container up to 580ºC then cools it, depending on the glass thickness, over a 20 – 60 minute period. Annealing ovens have a huge steel mesh conveyor a bit like a pizza oven on steroids.

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