Mineral lenses consist of many individual components that are melted into a glass mixture. In the first step, the raw materials, also known as “batch”, are melted into liquid glass to achieve a homogeneous blend. During the refining process, gas bubbles trapped in the batch are removed from the glass melt. The melt is then pressed into a minus or plus lens mould and left to cool down. The lens is then ground and polished, starting with the front face. This semi-finished product is also called a blank. It is then stored until an order is received for it. Only then will the back face be treated. Several grinding and polishing steps are necessary to match the required prescription.
Plastic lenses are made of fully synthetic plastic material. The two production processes involved are chain polymerisation and condensation polymerisation. Chain polymerisation is a process of reacting monomer molecules in a chemical reaction to form long polymer chains. Condensation polymerisation is a process by which monomers carrying at least two functional groups are joined to form a macromolecule. During the process, simple by-products are given off, such as water.
Two different processes, casting or injection moulding, can be used to form the lenses. In the casting process, the liquid material is poured into a mould consisting of two halves and then allowed to solidify. In the next step, the semi-finished product is ground and polished as required. In contrast to the casting process, injection moulding can be used to produce large quantities. The molten material is injected under high pressure into a mould. After a short cooling time, the lenses have attained their shape with high dimensional accuracy. This process is mainly used for sunglass lenses since the quality of ground and polished cast lenses is far better.