Paint application

Good wetting and distribution of the paint on the substrate surface can be achieved when the surface tension of the wet paint is lower than the surface tension of the substrate.

Further, no additives or color-masterbatches should be used in molding the part where there is a chance that ingredients migrate to the surface and reduce the surface tension. This can lead to an irregular paint thickness distribution.

Spraying is the most common painting process for parts, and it can easily be automated with robots. Conventional spraying processes make use of compressed air to force the paint through a spray nozzle, whereas airless processes use a pump. Electrostatic spraying is a process in which the paint droplets and the parts to be painted have an opposite electrical charge as shown in the figure below. Good paint coverage and less over-spraying are the advantages.

The paint droplets must blend together on the surface of the part and form a smooth layer with a uniform thickness. Premature evaporation of the solvent during spraying must therefore be prevented, otherwise an effect called "dry spray" may occur. The climatic conditions, like temperature and humidity, should be well-controlled for a good, reproducible coating result. An undesirable temperature rise during hot days can lead to the dry spray effect.

The paint can also be applied with a brush (stripe painting), a roller, a resilient pad (decorative figures), by dipping or by dyeing (fibers and fabrics). A mask can be used if the surface of a part is only partially to be covered with paint. The painting process can also be integrated with the injection molding process in several ways, avoiding the need for a separate painting line.

Foam molded parts cannot be painted immediately after molding. The gases produced by the foaming agents must first reach equilibrium with the ambient air. This outgassing may require 24 to 48 hours, depending on temperature and humidity. Painting before this equilibrium is reached may cause blistering.