Photopolymers for enamelists and silversmiths
Introduction
The word photopolymer indicates that the substance is sensitive to light, and when exposed will polymerise, i.e. form chains of molecules. Polymerisation affects the properties of a substance, so that it will for example become harder and no longer dissolve in water. We depend heavily on all sorts of polymers in daily life: they form part of paints, are used in ultraviolet-sensitive glues (such as those used to fix false nails or fix damage to windscreens in cars), packaging material such as polyethylene, plastics, stamps made of 'artificial rubber', etc.
Photopolymerisation is achieved by adding a sensitiser to an appropriate ground substance; the sensitiser is activated by light of the appropriate wave length. Photopolymers are extensively used, for example to print newspapers, boxes and the like. They may be applied in printing on flat as well as on curved surfaces, such as tins. Photopolymers are also available to be cast in 3-dimensional shapes in the process of creating molds. This is most often achieved with CAD (computer aided design) and computer-controlled lasers that locally set the process of polymerisation in motion; this process is known as stereolithography.
Photopolymer sheet
Our applications are confined to the use of flat photopolymer sheets that are widely used in printing, such as in rotogravure, printing from cylinders and planography. When fixing the sheet to a cylinder it should be flexible; such sheets are enforced by a thin metal backing. In planography it is of great importance that the sheet is perfectly flat; on that account the metal backing is much thicker. The former can be easily cut with a pair of scissors, the latter need to be cut with a saw. Whilst sheets with thin metal backing (0.1 mm) can be easily cut, they need to be made thicker,e.g. by adhering sheets of acetate to it; this also makes it stiffer, a definite advantage. Since the back is unlikely to be perfectly flat with the thin metal backing, more work goes into sanding and polishing the casting.
Some polymers give rise to rubber-like substances; due to their flexibility they are used for printing on irregular surfaces, such as boxes, textile, and not provided with any metal backing. These sheets, used in flexoprinting, are too flexible for our purposes.
Photopolymer sheets without metal backing are also available that harden and stiffen after UV exposure. These sheets can be washed out from front to back, burn completely at temperature above 100 °C and can potentially speed up the lost-wax method. However, we have no experience with this material.
Exposure
White light is made up of waves of widely different wavelengths, each wavelength being perceived as a different colour: the colours of the spectrum. The spectrum comprises a visible and an invisible part. The visible part of the spectrum can be seen in the rainbow: the wavelength decreases from red to orange - yellow - green - blue - indigo - violet. The part of the spectrum that is not perceived by the human eye comprises infrared (long wavelength ) and ultraviolet (short wavelength). There are also other forms of waves and radiation, such as X-rays, but these are not relevant to the following.
Substances that initiate the process of photopolymerisation are spectrally sensitive, i.e. sensitive to a narrow range of wave lengths. The maximum sensitivity in photopolymer sheets that we use is at about 380 nm (nanometre: 10 to the power -9 metre, or one millionth part of a millimetre), in the range of ultraviolet (UV) light. The material may be briefly exposed to subdued light (not to direct sun light) during cutting or transfer for exposure; when stored in a cool and dry place it has a long shelf life. The material is nontoxic and chemically inert.
| Figure 7 - Put the negative 'face down' on the photopolymer sheet. Close contact is achieved by pressing the overlying glass plate, or by generating a vacuum. Polymerisation is subsequently achieved by exposure to the UV-lamps. Unexposed material is then washed out. Finally the photopolymer sheet is dried, and hardened and made durable ("cured") by again exposing to UV light. |
So we need a source of UV light, preferably in a case in which the polymer sheet is protected from direct light. Use a glass plate to achieve close contact between the 'negative' and photopolymer. Unexposed photopolymer can be washed out with water and a soft brush (figure 7). The resulting mould should be thoroughly dried and then cured by another, longer exposure to UV light. Consult the chapter “moulds for enamel work” for practical details.