As an important part of children's playground equipment, the color and protective film of metal surface of slides are very important.The powder spraying technology has unique advantages.It is widely used in the metal slide painting.This article will analyze the content objectively and technical key points of above process, so as to help buyers to understand the physical principle of coating process.

1. Surface treatment

Before the metal part is sprayed, it should undergo strict surface treatment.It is very important for the long-term adhesion of the coating.Firstly, the burrs and old paint film should be removed by grinding.Then, the metal workpiece should be mechanically blasted by sandblasting or shot blasting to remove the oxide scale, rust and impurities from the metal surface not only can clean the metal workpiece, but also need to obtain a uniform rough structure on the metal surface, and the roughness should be controlled in the range of Ra 3.2-6.3μm on the microstructure.The rough structure of the metal surface is like countless of "anchor points".The mechanical interaction between the coating to be applied later and the metal substrate is greatly enhanced. This physical basis ensures the durability of the coating.

Sandblasting and polishing of metal parts for slides

2. Electrostatic Spraying

The key to powder coating is that the powder is electrostatically adsorbed.The environmentally friendly powder coating should be selected, which is not only environmentally friendly, but also can pass the strict test of children's safety standard RoHS, EN71-3, GB 6675, and there is no phthalate in the paint.The color can be customized to produce, and the outdoor anti-fading ability of the paint film can be improved by adding lightfast pigments.

Electrostatic powder coating of metal parts for slides

In the process of spraying, the powder is negatively charged under the action of the electrostatic spray gun, and the metal workpiece to be plated is grounded as a positive electrode. Under the action of 60-90kV high voltage electrostatic field, the charged powder is uniformly adsorbed on the workpiece. The process parameters should be controlled accurately: the current of the spray gun is 10-20µA, the distance between the gun and the workpiece is 15-25cm, the particle size of powder particles is 10-100μm, the powder spraying amount is 300-600g/min, and the atomization pressure is 3-5×10⁵Pa. The spraying sequence should follow the principle of "difficult first and easy later", that is, the difficult parts such as corners and welds should be sprayed first, and then the flat parts should be sprayed. "Cross spraying method" should be adopted (spraying horizontally and vertically alternately) to ensure that there is no missed spraying. The thickness of the powder layer formed after spraying should be controlled within 50-120μm to reserve leveling space for the subsequent curing process.

3. Curing and Shaping

The workpieces adsorbed with powder should be moved into the spray booth for hot melt cross link. Generally the temperature is controlled at 180-200℃（the temperature of each kind of powder can be slightly adjusted）. The powder particle should be melted and leveled, and then cured and cross-linked. It can be found that the originally loose powder is turned into a continuous hard coating which is dense and hard as a polymer. The requirement of uniform temperature inside the furnace is ±5℃ to avoid the local parts being overheated and yellowed or insufficiently cured. The thickness of cured coating should also be controlled at 60 to 120μm. If it is too thin, the base will be corroded when it is exposed; if it is too thick, it will be cracked due to internal stress.

4. Quality Inspection

The performance of coating should be tested by series of strict tests. The grid method is adopted to test the adhesion. The coating shall not fall off. It shall not peel off after long-term use. The requirement of mechanical strength test is that the 1kg hammer should impact the coating at 50cm height. There should be no cracks or peeling on the coating. The pencil hardness is 2H, which is equal to that the coating can be scratched by a daily key without leaving any mark. It shall be able to resist the impact of daily children climbing and colliding on the children slide to avoid the danger of children swallowing the parts where the coating of slide is peeled off.The weather resistance test requires that it can resist the salt spray for 500 hours and rust; it can resist the ultraviolet rays for 1000 hours and not fade. It can resist the outdoor weathering for a long time and anti-corrosion.All the above test data are the key to judge whether the coating can pass the safety long-term use required by the outdoor slide.

5. Technology Advantage.

Powder coating shows its value in the coating application of metal parts for kid slide. One is that the coating can present rich color effect, and can present various gloss effect such as dullness, semi-gloss and high gloss. The other is that the color is bright and durable and can meet customers’ various requirements of design. Besides, the coating has high hardness and is hard to wear and scratch. The coating with excellent resistance to ultraviolet rays and corrosion can prolong the service life of playground slide and decrease the cost of maintenance. In addition, the performance of environmental protection is very outstanding. There is no solvent in the powder coating, and there is no VOC emission in the production process. It is completely in line with EN71 standard for heavy metal migration limit of children’s toy, and can ensure that the product used is safe from the source.Economic advantage. The powder that is not adhered in the spraying process can be recycled and used again, and the loss of material is approached to zero. Meanwhile, there is no wastewater or waste residue, and the long-term comprehensive cost is more reasonable.High quality coating layer is presented for the metal part of slide by powder coating, which is safe, durable and environmentally friendly. Through the scientific and rigorous surface treatment, powder coating can make the electrostaticly adsorbed film present precise and controllable curing process, and then form strict performance standard. The technology is mature and the performance is comprehensive.