Our sandblasting process uses mechanical abrasion to remove surface contaminants from parts, as well etching the substrate, so that the powder can key into for it for better adhesion.
There are a few main reasons why we sandblast instead of using chemical pre-treatment:
If items are too big to fit into our chemical pre-treatment plant,
if the items are designed in such a way that they entrap chemicals (closed welded pipes etc.),
If the material is not suited to the chemical pre-treatment process.
It must be noted however that when mild steel is sandblasted, it does not have any protection against corrosion. It is therefore not recomended that sandblasting alone be used for suface preparation when these items/products are used for exterior or coastal conditions. We have various options available to increase corrosion resistance for sandblasted items.
7 stage zinc-phosphate pre-treatment
There is nothing more important than surface preparation when it comes to powder coating. It stands to reason that if powder is applied to oil, dirt or rust it will not remain on the part for long. Similarly, if rust occurs under the coating, it will aslo delaminate. A coating should therefore only be applied to a part that is perfectly clean, as well as one that treated so that it is impervious to corrosion.
Our 7-stage pre-treatment does just this; firstly cleaning the material of all organic and inorganic substances, and secondly, by creating a conversion coating on the substrate so that is corrosion-resistant.
But not all conversion coatings are equal. Iron phosphates will give you an amorphous (formless and unstructured) conversion coating that is only suited for interior applications. At Twilight Powder Coating we use zinc phosphate which has a tight crystalline structure that forms incredibly strong bonds with the substrate and prevents corrosion. Furthermore, this structure provides excellent hooking points for the melting powder to key into, giving you superior powder adhesion. For this reason zinc phosphated parts are suitable for outdoor applications and we use it to meet the high standards that are placed on us by our automotive clients.
Once the substrate is perfectly clean and converted, we apply powder to the parts using state of the art applicators, which pneumatically propel the powder while simultaneously charging it with 100 000 volts. The charged powder cloud is electrostatically attracted to the parts and is uniformly distributed across the surfaces.
Next, an endothermic reaction is required in order to transform the loose powder granules into one solid, fused coating. To acheive this, parts are conveyored through our 35m digitally-controlled oven, the heat from which produces the reaction, with four distinct phases:
The powder melts.
The molten powder flows. This phase is critical for adhesions as the flowing powder now keys into the all the hook points created by the crystalline structure of the zinc phosphate, as described above.
The powder starts to gel, solidifying into its final position.
The powder cures by cross-linking at a molecular level, creating a fused structure that is athstetic and highly durable.