Thermal Plates -- The Heat is On
Ablation technology plates start with an aluminum layer coated with an ink-receptive layer covered by an infrared-absorbent layer. The top layer (infrared) acts as a primer for the final top coat of PVA or silicon (PVA for dry, silicon for wet printing). The platesetter laser ablates (cuts) the infrared layer to remove the ink-receptive layer below.
Some thermal ablative plates require a quick wash off before or on the press to remove debris after the ablation process is complete. Others can be used without this step, but they require water on the press. With these plates, the washout is done using a fountain solution when the plates have been put on-press.
Bi-metal technology, used by PDI, involves placing a photopolymer layer on top of a thin layer of copper that sits on top of a thin layer of aluminum or stainless steel base. After the plate is exposed to the heat in the platesetter, the developing process removes the copper in the non-imaging areas. In the last development step the polymer stencil is dissolved and removed. The result is a plate with press dot sizes that are the same as the original film.
Plate manufacturers are experimenting with several other kinds of techniques involving phase changes and chemical coatings. The concept is to trigger a physical or chemical change to a substrate to make the plate ink-receptive.
There are two approaches to phase change plates, and both involve a special coating on a metal plate. The first is a thermally alterable coating applied to an aluminum base. On-press, the unimaged layer is removed by a fountain solution to reveal the water receptive aluminum layer. The second is to use a special coating on a metal plate. After being exposed in the thermal platesetter, the plate changes from being water-receptive to ink-receptive.