Powder coating is a solvent free coating technology which utilises a thermoset (requiring heat) dry powder applied through electrostatic spray equipment to a variety of substrates. The powder is a ground resin that looks like coloured dust. Once the item is coated it is baked in an oven to melt the solid particles which flow and cure to create a durable coating.
Powder coat application equipment is generally comprised of a ’fluidised’ hopper to hold the powder. Clean, compressed air is injected into the bottom of the hopper; this aerates the powder so it can be sucked up into the hoses for delivery to the spray guns.
The powder guns generate a negative electrostatic charge of up to 100 Kv and when pointed towards a suitably earthed and grounded part a ’Corona field’ is created in the air between the gun and part to be coated. The field is comprised of negative ions and passes the electrostatic charge onto anything in this field, i.e. the powder flowing out of the guns toward the part. The particles of powder accept the negative charge and are then attracted to the grounded part like tiny magnets. The electrostatic application enables powder to ’wrap’ around an item and coat the sides and back to a degree, this makes the coating of complex parts like wire work relatively easy.
The powder adheres to the part, but it can be easily wiped or blown off until it is placed in an oven. When the powder is baked it melts and flows into a protective film. However the powder needs to ’cure’, that is, the chemicals in the powder need to combine or cross link to provide the chemical and abrasion resistance properties of the coating. To do this the coater needs to bake the parts according to the stoving schedule of the powder. This is generally 200° Celsius for 10 minutes, although some powder systems require lower or higher temperatures for differing times. Special low bake powders for use on plastics or medium density fibre board (MDF) are baked at around 160° Celsius.
If the powder is not cured fully it may look fine but it may not adhere to the substrate and it will be brittle and chip off easily. It may also deteriorate rapidly if exposed to sun light or chemical attack. Conversely if the powder is over baked it may discolour the powder and affect its long term performance.