Top 9 Causes and Control of Coating Particles of Powder Coated Aluminum Profiles

Top 9 Causes and Control of Coating Particles of Powder Coated Aluminum Profiles

Top 9 Causes and Control of Coating Particles of Powder Coated Aluminum Profiles

Powder coating is a new technology that has developed rapidly in the coating industry in recent years.

It is characterized by environmental protection and energy saving, high production efficiency, rich and beautiful colors, and excellent physical and chemical properties of the coating.

Now it is almost everywhere in household appliances, automotive and aluminum profiles and other industries, and it is the development trend of the coating industry in the future.

In the process of electrostatic powder coating of aluminum profile, coating particle defects are a common problem with complicated causes.

Based on the work experience over the past few years in the aluminum industry,

Brightstar Aluminum Machinery summarizes the causes and control measures of electrostatic powder coating particles as the following.

Process flow

The electrostatic powder coating process is as follows:

Profile pre-inspection→degreasing→water washing→plasma water washing→chromizing→water washing→plasma water washing→water control→drying→purging→spraying powder coating→curing→inspection→packaging.

Causes and Control Measures of Coating Particles

1. Particles caused by the burrs of aluminum profile itself

The big burr is mainly because the oxide skin is not completely removed when the aluminum billet is extruded, and the extruding speed is slightly faster,

All leads to the appearance of burrs, it is obviously feel thorny when touched.

The inclusion of black ferrosilicon and silicon carbon particles is mainly due to insufficient heating,

Which leads to the incomplete and uniform diffusion of the internal components of the aluminum billets,

So that such defects appear after the aluminum material is extruded.

Most of these particles have a particle size of about 0.1 to 0.3 mm, which cannot be removed by pre-treatment.

It can be removed by aluminum profiles surface defects brushing and cleaning machine before the pre-treatment and powder coating process.

It is necessary to treat the alkaline etching tank before the surface treatment section.

The control measure is to strengthen the control of the aluminum profiles production process to avoid such problems.

2. Particles caused by the chips of aluminum profile sawing

After aluminum extrusion, the chips in aluminum profiles are not blown clean,

They are brought into the tank after washing, and are dispersed and adhered to all surfaces of aluminum profile under the action of circulating and stirring tank liquid.

During chromizing, the chromated film formed at the same time becomes one,

And it is tightly attached to the chromed film and cannot be removed by purging, resulting in the appearance of large protruding particles (0.3~1.0 mm large particles) in the coating.

Most of these particles are in the form of flakes, during the process of filing the particles with a rasper, it can be found that the particles are wrapped with silver-white aluminum chips.

3. Particles caused by residues at the end of aluminum profiles

3.1.When the aluminum profile is extruded and cut, the splashed coolant is mixed with aluminum chips and adheres to the end of aluminum profile,

And the operator does not wipe it clean in time, which causes the coolant to dry into glue spots after aluminum profile are aged, which cannot be removed by pre-treatment.

3.2. Labels are sometimes attached to aluminum profiles to mark them after extrusion.

Sometimes it is attached to the main decorative surface, and after the label is removed during pretreatment, there is a little adhesive left on aluminum profiles.

The pre-treatment tank liquid cannot remove and clean the things adhered to the ends of aluminum profiles.

After drying with hot air, they have firmly adhered to the profiles.

If the operator cannot find them in time, it will cause local particles of the coating to be densely distributed.

One face of the head is distributed over an area the size of a matchbox.

It is necessary to strengthen the quality control of the mill finish aluminum profiles and strengthen the management of the sawing process.

Using small clips to clamp the labels at the end of the profile can completely solve such problems.

4. Particles caused by dust in the workshop

There is a lot of dust floating in the air, after the profile is chromed or the reworked material is placed, a layer of dust will fall on the surface.

If the dust cleaning is not clean, dense and uniform small particles will appear in a large area.

The dust particles on the floor are very hard and can embed in the rework paint film or scratch the coating when sanding on the floor.

If it is not detected and cleaned in time, it will cause larger particle defects.

When cleaning and mopping the floor, the mud spots splash on aluminum profiles, forming wet mud dots, which are tightly adhered to the surface of aluminum profiles after air-drying, very hard and dry.

If they are not removed in time, they will cause large particle defects after dusting.

When this type of defect occurs, the dust component is SiO2, and its surface tension is quite different from that of the powder coating,

So the formed particles are concave into a small pit with a small protruding point in the middle, which is easy to identify, but not easy to repair.

It is required that the chromed aluminum profiles and aluminum profiles that are not coated temporarily must be covered with a cotton cloth to prevent dust from falling on the surface, as well as mud and water spots to be stained.

5. Particles caused by floccule

5.1 Linear defects caused by filaments of cotton work clothes, cotton threads, or suede gloves.

In production, the contact between the filaments and the sharp corners and burrs on the end of aluminum profile will produce a wire drawing, which will be attached to aluminum profile.

5.2 During the curing process of the powder coating film, the surface tension between the coating films will shrink, resulting in linear defects after powder coating.

Therefore, operators are required to wear rubber gloves or thin hard canvas gloves and wear synthetic chemical fiber clothing that is not easy to produce fiber filaments.

Require pre-treatment operators to scrape off the stubble and burrs at the end of aluminum profile to prevent the generation of silk threads from hanging on the clothes in the coating process to avoid such defects.

The volatiles produced in the solidification of the powder is cooled and crystallized into catkins at the inlet and outlet of the solidification furnace, fall on the powder coating, and then melt and solidify together with the powder.

Due to the inconsistency of leveling between both, the powder coating surface produces small clumps of large particles, which are crunchy and easy to scrape off with fingernails.

It is required to regularly blow and clean the flocculent volatiles at the mouth of the curing furnace and the inner wall of the furnace, and keep the furnace path clean to avoid such defects.

6. Particles caused by poor cleaning of the hanger

In electrostatic powder coating production, the grinding and cleaning of the hanger is a very important process, which mainly ensures that the hanger has good electrical conductivity.

When the hanger is cleaned and polished, a lot of paint slag, iron powder, and debris will be produced, and because the powder coating is resin, it will generate static electricity when it is rubbed and hit, and it will be adsorbed on the hanger.

If the hanger is not cleaned with compressed air in time, the airflow of the coating gun will blow off the next bit of debris and fall on the powder layer during powder coating.

After entering the curing furnace, when the temperature is high, the larger debris and paint slag that loses electrostatic adsorption will automatically fall off, resulting in local coating particle defects on the surface of aluminum profiles.

An obvious feature of such defects is that they are all concentrated on both sides of the hanging tool and the hanging point.

Since the debris is scattered from the hanger, it is basically distributed on the top of the profile, and there are almost no such defects on the bottom, which is easy to identify.

7. Particles caused by rework

During the powder coating process, some aluminum profiles need to be sanded and repainted due to defects.

Coating debris will be generated during rework and sanding.

Some large particles are mixed in the coating, and some debris is adsorbed on aluminum profile due to electrostatic action.

The compressed air can only blow off the surface debris, but not the inlaid debris.

If these adhering things are not carefully removed, it will cause obvious dense particle defects in the rework part.

For this reason, use fine sandpaper to rub lightly when sanding.

After sanding, you should feel that the sanded part is smooth, then wipe it repeatedly with a wet towel, and finally dry it with the wind.

In this way, the adhered sanded objects can be removed to ensure that the rework part is qualified.

8. Particles caused by the equipment system not being cleaned up

8.1 Spray gun head, nozzle, discharge needle, large and small mesh screen, the inner wall of the large cyclone, dense valve cone, casing, powder pipe and powder layer hardened on the powder suction pipe,

If it is not cleaned up in time, small particles will be generated under the airflow during spraying and sprayed on the powder layer, and large particles will be formed on the surface of the coating after curing.

Regular inspection and cleaning are required, especially when the temperature is high in summer, to avoid such problems.

8.2 If the dust and debris in the air filter screen of the curing furnace, as well as the broken fibers generated by the aging filter screen, are not checked and replaced in time, filamentous defects will be formed on the surface of the coating.

The shape of such defects is slender, generally, they can be rubbed off with a little force by hand, or the particles can be gently rubbed off with fine sandpaper.

Rework is also easier and doesn’t require a lot of hard sanding.

8.3 In production, the molybdenum disulfide lubricated by the girder catenary system and the oil sludge produced by the rolling of dust, powder, rail iron filings, etc., is blown away by the hot air in the curing furnace when it falls in the curing furnace and falls on the coating.

This type of sludge contains oil, which floats on the coating surface during curing, and forms small black particles on the coating surface after curing.

The particles are softer and fall off as soon as they are wiped, but the black oil spots stain the area around the spots.

Since the sludge falls from the catenary, it is also distributed on the upward-facing side of aluminum profile.

Use sandpaper to gently rub off the coating at the black spots, and it is easier to rework.

To prevent such problems, it is necessary to clean the girder catenary regularly (weekly), use compressed air outside the furnace to blow off the dirt on the girder oil tank, and blow off the catenary track, oil sludge, iron filings and other dirt on the roller bearings.

9. Particles due to powder quality

9.1 Small semi-cured and compacted particles are mixed in the powder.

The reason is that the screw extrusion rod of the powder coating factory is not cleaned, resulting in a small amount of semi-cured and compacted powder particles mixed into the powder, resulting in large-area particles during powder coating.

These particles can be seen everywhere on the coating, and are evenly distributed on all sides, with basically the same size.

To determine such defects, just clean the powder spraying equipment and the surface of aluminum profiles.

Such defects are rare and do not need to be specially prevented, as long as the judgment method is mastered.

9.2 The powder temperature is not completely lowered before packing, the powder coating itself has poor performance, the glass transition temperature is low, and heavy pressure and moisture during transportation may cause large pieces of hardening.

The use of such problematic powder results in larger particles on the coating, along with defects such as raised powder, build-up, fat edges, orange peel, etc., and rework is difficult.

The preventive measure is to choose a reputable powder manufacturer, grab the powder by hand after unpacking, and check it with a mesh screen with a mesh of 300μm.

Conclusion

The above is the top 9 Causes and Control of Coating Particles of Powder Coated Aluminum Profiles.

In the process of electrostatic powder coating, the form factors of coating particles are more than the above causes. 

As long as we carefully observe, analyze and test the characteristics of particle shape, size, color, texture and formation position, and implement corresponding solutions, we will be able to control such defects to produce high-quality products.

Brightstar Aluminum Machinery provides not only aluminum profile shape corrector, aluminum profile wrapping machine and aluminum profile polishing machine, but also wood grain effect sublimation machine and aluminum profile shot blasting machine for our customers.

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Contact us now for your aluminum project solution!

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February 22, 2023