In-Mold Coating of PMMA Parts
By using a newly developed coating system, PMMA parts can be coated directly in the mold. The costs are considerably lower than for coating in a post-molding operation.
In spite of their high surface hardness, PMMA parts are given a scratch-resistant coating in many cases. The classical approach to applying this coating entails relatively high costs. For instance, it is first necessary to clean the part’s surface and apply a primer. The newly developed CoverForm coating system makes it possible to coat PMMA parts directly in the mold through use of a reactive system based on multifunctional acrylates. The injection molding machne must first be fitted with a so-called flooding assembly. An injector nozzle introduces the liquid coating into the mold.
Coating of the parts takes place in three steps: first, the plasticated PMMA is injected into the mold; next, mold executes a compression stroke to compress the surface to be coated; the part is then allowed to cool in the mold. In the second phase, the cavity opens slightly (relaxes). This creates a gap at the surface, which is then flooded with the liquid reactive system. Finally, the mold executes a second compression stroke. At the same time, the mold temperature is raised, triggering the curing process in the coating. In the third phase, a robot removes the coated part from the mold and places it in a UV irradiation tunnel, where the coating undergoes final curing.
In contrast to coating in a post-molding operation, the parts are finish-coated upon leaving the production cell. This reduces both logistical costs and the costs associated with the space required for subsequent part finishing. Extensive material investigations of sample parts have shown that the coatings exhibit very good mechanical and chemical resistance.