Corning Incorporated invents the first complete continuous-production line
World War II brought paralysis to the European glass industry, depriving the allies of German-made optical glass products. Across the challenge, the baton was taken up by Corning Glass Works, now Corning Incorporated, who developed a revolutionary continuous furnace in which the refractory material was largely platinum. The use of this furnace resulted in the production of glass of perfect quality for precision optics and spectacle lenses. Moreover, the automation of a number of stages of preparation and manufacturing accelerated the arrival of continuous production on a large scale. Thus the modern ophthalmic glass industry was born.
Preparing the composition
This is a vital operation, since the composition determines the homogeneity and refractive index of the finished glass. It entails weighing the raw materials and blending them carefully to obtain the appropriate composition for the furnace.
Raw material checking
Raw materials for glass composition arrive from the supplier with an analytical data sheet attached. The suppliers are selected the world over in accordance with the requisite variety of products. Additional controls are applied in the plant laboratories.
Glass components are weighed with the utmost precision to obtain the right proportions of materials required on the weighing sheet. This may entail an addition of 1 part per 10,000 for a composition in which a minute quantity of a particular ingredient can substantially affect the properties of the glass.
The various materials are blended in an industrial mixer. A predetermined time cycle is observed for each glass type. The batch is then transferred to the furnace area for melting.
This is a four-stage process, involving:
- glass delivery
The batch is placed in the portion of the furnace where melting takes place, in mixture with cullet. Cullet is recycled glass of identical composition with the batch, which has been retrieved from a previous operation and crushed.
A certain amount of cullet facilitates the melting operation.
To melt the composition and produce a homogenous result, the temperature of the furnace must be high enough for the glass batch to become liquid.The temperature may vary according to the type of glass from 1,100 to 1,500°C.
Various types of refractory material are used in furnace construction, in accordance with their placement inside the tank and the temperatures they must withstand. Heating to high temperature is achieved using different forms of energy: gas, fuel, electricity, or a combination of the three.
Where electrical heating is used, the glass bath acts as a resistor. It is a lesser-known property of glass that, at very high temperature, it is a conductor of electricity. Glass, which is a perfect insulator at room temperatures, may have a resistivity of several ohms per cm when heated to its melting point.
At the fining stage of production, the temperature is raised to render the glass more liquid so as to allow the escape of gases still present in the melt. This operation is carried out in a second chamber of the furnace called the fining tank.
Because the temperatures involved are so high (up to 1,600°C) conventional refractory materials cannot be used; they would be adversely affected by the heat, and therefore contaminate the glass with impurities, such as colorants.
This is the reason for employing platinum, a material virtually unaffected by hot glass.
On completion of fining, the glass is at too high a temperature to be used for blank forming; it is too fluid and insufficiently homogeneous.
To condition it, a bank of indicator and regulator thermocouples are distributed within the conditioning zone.
In addition, to attain the required optical quality -- that is, total homogeneity -- the glass must undergo non-stop blending, employing a stirring process, Guinandage (named for its inventor, Mr Guinand).
Following the blending and controlled temperature reduction, the glass is ready to exit the delivery tubes to the press at a working viscosity which varies from 100 to 10,000 poises, according the category of glass.
Glass delivery to the press
The objective within this operation is to deliver to the press glass gobs of constant weight. To achieve this, the stream of glass leaving the delivery tube at a stable flow rate is cut automatically by shears made from a special steel; the cycle is synchronized with the rotation of the press.
These gobs of constant weight are also termed "parisons."
Glass gobs of required viscosity are vital to produce good-quality lens blanks. Each parison slides into a mold on the press turntable. Each position of the press corresponds precisely to a phase of the operation: loading, pressing, cooling, and blank takeout.
This continuous pressing system makes possible production rates of several thousand blanks per hour.
The tooling of the press determines the dimensions of the blank.
A press is comprised of four main parts :
- the mold, which determines the overall diameter,
- the valve, which gives the convex curve,
- the plunger, which compresses the mass of glass to form the concave surface, and
- the ring, which closes the whole assembly and determines the peripheral shape of the blank.
Different types of press are used, according to the blank required and the physical properties of the glass.
When the blank leaves the press, it is carried by a conveyor belt into an annealing lehr. For standard glass blanks, the lehr is used to anneal the glass; the aim of this operation is substantially to reduce internal stresses induced by heat. For this purpose the blank is brought up to a temperature ranging from 550 to 700°C, according to the nature of the glass, then recooled at a controlled rate. Stress reduction within the blank makes later lens-surfacing operations easier.
A special case : photochromics
With photochromic lenses, the annealing operation has the additional effect of activating, in potential, the silver-halide crystals, their size and number precisely determined, which will later give the lens its photochromic properties (transmission change, colour, and speed of reaction). The process is carried out in lehrs where the temperature is very closely regulated, taking into account the characteristics required to be developed.
Initial packaging of the blanks into cardboard trays is effected when they leave the lehr. Containers designed to house the trays are used to dispatch the blanks safely to their destination, wherever it may be.