How to identify the quality of color-coated aluminum coils ...

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:

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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.

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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