Today I would like to share some tips about the quality inspection of pre-painted aluminum metal coil/sheet for your better learning about this kind of material. Here we go!
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Mainly 2 aspects but including some small points.
--Physical Tests
the physical test is important to check that the paint has been applied and cured properly so that it will perform well during the end-use.
--Visual Inspection
Visual inspection is important to confirm that there is no unacceptable visual defect because coil coated metal must meet the aesthetic requirement as well as the one associated with performance.
Followings are the tips of various tests and their importance for assuring the quality of color-coated aluminum coils/sheets.
Physical Tests:
1. Film Thickness:
Having the right film thickness is critical for the long-term performance of the paint film. For the primer coat, the low film could result in premature corrosion or poor adhesion. For the topcoat, the low film could result in premature failure due to weathering. The high film is also a problem, you know, as it means an added cost, which affects the profitability of the product.
2. Color:
One of the main reasons for buying pre-painted products is to provide a specific colored appearance. As usually customer matches the pre-painted metal to other components of a building or product, so matching the color standard where specified is very important for the great appearance of the final assembled product. For repeat colors, matching the standard on each run ensures that there will be color consistency over time.
Color must be consistent across the width of the strip, as well as between coils of an order. Many factors can affect the color: Paint Formulation & Quality, Film Thickness, Substrate Color, Cure Temperature (PMT), and Dwell Time in curing ovens or even the Contamination with Other Paints.
3. Gloss:
Consistency in gloss is important to provide a uniform appearance to customers. The final product may be made up of several components made from different coils. If the gloss is not consistent, the final product’s appearance will be poor and looks not good.
4. Adhesion:
There are several tests conducted to check the various aspects of adhesion, including T-bends, Impact, and Various Scratch Tests.
Pre-painted aluminum coil/sheet may undergo several types of forming to convert it to the final product, like the types of bends in construction of siding or roofing, embossed garage doors, residential doors, ceiling grid, home & office furniture, embossed refrigerators, beverage cans & lids, etc.
One of the key advantages of the pre-painted coil is that it can be formed after painting, so these tests are important to make sure that the paint will not be damaged or removed during the following processing.
Factors that can affect adhesion include Paint Formulation, Film Thickness, Cure Temperature (PMT), Line Speed, Treatment Quality, Aluminum & Steel Quality, and Cleanliness.
5. Cure:
The cure is a term that refers to the degree to which the material is converted from its initial state to the final or cured state.
All coil coatings are baked and 2 things happen during the baking cycle:
1) The solvent in the paint evaporates and is burned in the afterburner.
2) A chemical reaction in the paint film causes the resin molecules to join together (polymerize or cross-link) to form a strong, cross-linked, solid paint film.
Usually, there is no direct way to measure the degree of cross-linking. But a combination of tests can indicate whether the paint is under cured or over cure, such as MEK Rubs, Pencil Hardness, and Adhesion.
Under cured paint will tend to be soft, and will not withstand forming or weathering in its end-use. Over cured paint will tend to be brittle, and may be damaged during forming or weathering.
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Visual Inspection:
1. Why Need This Inspection?
From the end customer’s perspective, one of the most important aspects of pre-painted aluminum is its appearance. No doubt that the customer expects a uniform, consistent appearance without any unacceptable defect.
Visual defects can be caused by The Aluminum Substrate, The Paint, or The Method of Application & Curing.
2. Pay Attention to Different Appearance Standards
Because of the wide variety of customers and products in different industries, there is also a fairly wide range of specifications for visual appearance. For example, the appearance standards for refrigerator, which is seen up close every day at consumers’ home, are much stricter than the appearance standards for roofing product, which is only watched from a distance.
Ideally, we would like to have a perfect appearance on all products, but that is not always cost-effective, so some level of variation may be acceptable to customers.
3. Several Defects & Bad Influences
Defects, which are identified during a visual inspection, may also affect the paint performance & life during the end-use. For example:
Craters are not only an appearance problem but also may result in premature peeling or corrosion at that position.
Differences in Textures across the strip or between coils may be an indication of film thickness inconsistency, which will have an effect on the product lifetime.
Dirt Lines are visual defects and can cause low film thickness – premature corrosion or peeling could appear at that spot.
4. Benefits of Visual Inspection
Visual defects may start and stop suddenly during the painting process, so thorough visual inspection is important to minimize or avoid the defective material produced. The sooner a defect is found, the earlier it can be corrected, minimizing scrap and customer returns.
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Over 800 million tons of coil-coated metal are produced and shipped annually in North America alone. Coil coating (see Diagram I) is a very efficient way to produce a uniform, high quality, coated finish over metal in a continuous automated fashion. Coil coating is also referred to as pre-painted metal, because the metal is painted prior to, rather than after, fabrication.
In the coil coating process, the metal coil is first unwound, cleaned and pre-treated, applied on a flat continuous sheet, heat cured, cooled and rewound for shipment. At the fabricator, it is then cut to the desired size and formed into its finished shape. Versus most other application methods, coil coating efficiency is nearly 100%. Application is at very high line speeds as modern coil lines can run at speeds as high as 700 feet per minute and cure the applied paint in 15 – 45 seconds. As opposed to a spray-applied coating, for example, a coil-coated, formed surface offers uniform film thickness rather than the thicker films on edges, corners and bends that is more typical of spray-applied coatings.
Topcoats are applied by reverse roll coat in which the applicator roll travels in the reverse direction of the strip and thus provides a smoother film with fewer defects. Primers and backers are normally applied by direct roll coating. Some lines also apply coil coatings using an extruder or via a solid block of paint with a softening point such that it can be applied smoothly when heated.
The types of paint curing employed in the coil industry include thermal, infrared, induction and UV cure. By and large, the vast majority of coil coatings are cured using gas-fired ovens. Accordingly, the remaining portions of this article will focus on thermal-cured coil coatings. Coil primers and backers are normally applied much thinner than spray-applied liquid or powder coatings, dip or electrocoat paints. Applied primer dry film thicknesses are normally in the range of 4 – 6 microns in thickness, whereas topcoats are normally applied to provide a dry film thickness of 18 – 20 microns.
Special consideration when formulating coil coatings include adjustments in the resin technology, crosslinking, solvent system, catalyst, surface modifiers and lubricants to accommodate proper cure, flow and leveling, and minimization of surface defects. Once cured, coil coatings can offer excellent handling, physical, chemical and environmental properties. From a paint usage standpoint, polyesters make up the bulk of coil coatings. Table II details typical materials utilized in a polyester coil coating.
In summary, coil coating technology has grown more rapidly than more conventional application technologies due to a variety of benefits that include cost savings, versatility, environmental, performance and quality issues.
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