What is pattern coating?

SPECIALTY COIL FINISHES WIN EXTRA BUSINESS FOR COIL COATING

In the evolving world of coil coating, precision and innovation go hand in hand — and pattern coating is one of the most advanced techniques pushing the boundaries of what’s possible in the colour coating process. Using high-performance roll coaters and some extra equipment manufacturers can now apply decorative patterns directly onto metal strips with exceptional accuracy.

Pattern coating integrates seamlessly into both aluminum & steel coil coating lines. It enhances the appearance and performance of coated coils, making it ideal for architectural, appliance, and automotive applications. In this article, we explore the meaning of pattern coating, how it fits into the broader coil coating process, and why leading coil coaters are adopting this advanced solution.

The coil coating process is highly adaptable, allowing manufacturers to produce a wide variety of special finishes beyond a standard colour coating. These finishes can add visual appeal or enhance functionality, making coated aluminium coils or painted steel suitable for a broader range of applications. From metallic pigments to textured effects, these enhancements offer unique characteristics that elevate the value of the final product.

Many coil coaters and colour coating line operators find that customers are willing to pay a premium for these special features. This added value not only improves product appeal but also strengthens supplier-customer relationships — supporting consistent utilization of installed colour coating lines and roll coater machines.

Some special finishes can require extra equipment — such as systems to apply patterns — others just need specially formulated coil coating paints with additives like metallic pigments for a unique appearance.

This article explores how these specialty coil finishes, including pattern coating, are effectively integrated into the coil coating line, demonstrating how today’s advanced coating lines and roll coater machines support both performance and design flexibility in modern coil coating operations.

PATTERN COATING

Pattern coating involves the application of a pattern ink, either as a single or multiple colours, over a uniform base colour. Most commonly the patterns are woodgrains so that panelling made from the coated steel gives the appearance of wood. However, imaginative creations have lead to a wide range of patterns including brick patterns, rock patterns, stars, clouds, slate, marble or abstract geometric designs. The inks are a contrasting colour to the base coat. They require an extra protective layer of a clear coating to ensure that the pattern is not worn off by abrasion nor prematurely eroded away on external exposure.

There are two means of applying patterns in the conventional coater heads of a normal continuous coil coating line. Both these techniques have been adapted from conventional printing used to apply ink to paper or similar for newspapers, wallpaper lolly wrappers etc.

These techniques are:

• a) flexographic printing
• b) offset rotogravure

a) Flexographic – of the two techniques this is least used among coil coaters. It requires a three roll V configuration where the pattern is cut into a polyurethane roll. Ink is picked up by a normal pick up roll, metered on that roll by a slow or reverse moving roll then the ink is transferred to the patterned roll which runs in the same direction as the base coloured strip. This system can best be described as being similar to rubber stamping, often used in offices, ie raised rubber stamp picks up ink from an ink pad which is then transferred to the paper through applied pressure. In the case of coil coating the applicator roll is laser cut to give the pattern. As with all printing the pressures used between all the rolls and the roll to the strip are extremely light. To allow very fine pattern definition the polyurethane roll material needs to be specifically developed. This application technique can also be used to apply striping down the length of the strip.

b) Offset Rotogravure Printing – this technique is the most widely used by continuous coil coaters. Modification of the coater head is relatively simple and economical. Unlike the flexographic approach the pattern is engraved into the pick up roll. Ink is picked up out of the paint tray by the patterned roll, excess ink is removed by a thin steel doctor blade; thus leaving ink retained within the engraving on the chromed steel pick up roll. Light pressure between the engraved gravure roll and a normal polyurethane applicator roll, allows transfer of the ink to the polyurethane roll. Again with further light pressure the ink then transfers from the applicator roll to the based colour painted strip. Accurate adjustment of the roll speed of the pick up roll is made to be sure that excellent definition of the fine lines of the pattern is maintained;

Dry film coating thickness of the different layers needs to be understood. If primed the normal 5um is used. The base coat is applied either directly to the pretreatment (internal uses only) or the primer and is commonly 10-15um. The ink coating, obviously varies within the pattern but in the heaviest areas is unlikely to exceed 1-2um. The clear coating is applied at 10-12um.

Dependent on the coater head configuration in a coil line the coil may need to be re-passed through the line a number of times to apply primer (if required, as occasionally internal applications do not demand primer protection), base colour, coating the pattern and the clear overcoat. If the coater head arrangement is favourable the clear coating can be applied wet on wet at the same time as the pattern.

The technology for quality application of a well defined pattern involves techniques commonly used in gravure printing in other industries. An example of this is that the application angle of the doctor blade is critical to ensure that the correct amount of ink remains in the engravings of the gravure roll and is not unnecessarily dragged across the higher section of the roll. This will lead to an unacceptable appearance called hazing which results in a pattern which is not distinct and clear. In figure 3 the important angle of the doctor blade to the gravure roll is shown.

As mentioned above most woodgrain patterns are a simple two colour prints. This means that the base coat is generally a light colour, eg light brown while the ink colour is much darker eg dark brown or black. However there are examples of up to three colour inks being added to give a more three dimensional appearance to the final pattern. Multiple colour patterns, to be effective and reproducible, however, require that each colour overlay is consistent so each gravure roll needs to be indexed one to the other. It is also possible using a U Wrap coater within a normal coil paint line to create a two ink coloured pattern. Three dimensional effects can still be achieved by especially skilled manufacture of the gravure roll such that the more expensive approaches to be mentioned are unnecessary.

Another means of achieving multiple overlays of ink to achieve more colourful three dimensional effects is to purpose build a pattern coating line that takes base coated input feed and consists of three or four standard applicator heads following each other. This approach is expensive, compared with using the normal facilities of a standard coil coating line and results in a line that is inflexible and only used for patterning.

c) Alternative Pattern Application Technique – in addition to these two application techniques a very special technique called rotary screen printing has been adapted and applied within a coil line. This technique is unique and the screen printing heads are solely used to apply a pattern. Again ink is applied wet on wet such that final cure is achieved simultaneously, in conjunction with the clear overcoat. Screen printing heads are well engineered. Ink is forced from the inside of the circular screen, through holes that dictate the pattern, and on to the substrate. Again this technique and equipment has been adapted from the wallpaper and vinyl flooring industries. As these heads are highly specialised, once again these are very expensive. This means that high volumes of patterned product are needed to sustain the expenditure on such equipment.

d) Paint Types for Pattern Coating – there is generally no restriction on the paint, ink and clear coating types that are used for pattern coatings. Polyester base coats, inks and clear coat are the most commonly used in the building and construction industry. However, where extra performance is required better performing systems like PVDF can equally be used.

SPECIAL PAINTS

Quite a number of systems, primers and top coats, have been developed that result in appealing visual effects or differing properties. Unlike pattern coating where special rolls are needed, these systems can be applied using the normal coater head equipment included on coil lines. Although normal coater heads are used it needs to be understood that special conditions often apply which will be mentioned here.

a) Metallic Paints – these are especially visibly appealing. Special pigments, usually aluminium powders or mica flakes are added which give this metallic appearance. As these pigments are heavier than normal pigments and can agglomerate in the drums during application so thorough mixing is essential. As well the flow of the paint in the head trays needs to be constant so that the heavy pigment particles do not separate then deposit on the strip as a heavier, uneven dark line. (This defect is known as pigment streaking). Although this defect can occur with normal paint if flow in the pan is not even, metallic paints are especially prone to it. Close attention by coater operators is essential. Nip feeding or application of 3 roll heads can assist in reducing the likelihood of pigment streaking.

b) Wrinkle Finish Paints – these are again especially formulated by the paint companies but are readily applied in normal 2 roll coater heads. The paint contains chemicals which give separation within the paint during curing, essentially a controlled de-wetting of the paint but at a microscopic level.

This type of finish was first developed in Nordic regions where users were concerned by slipperiness of the painted surface when erecting roofs. The wrinkled surface had the effect of increasing the coefficient of friction, making it safer for roofing contractors.

Two levels of “wrinkle” can be supplied; one at a coarse wrinkle, the other a micro wrinkle. The coarse wrinkle has the additional effect of increasing the surface hardness so that damage resistance is improved. The micro wrinkle has a side effect of giving a low gloss (matte) finish and is often used instead of controlling gloss with other matting agents in the formulation.

c) High Build Primers – these systems are used where extra corrosion resistance is needed. Instead of the 5 um applied normally, extra resistance can be achieved by applying 25 um of primer. These primers are commonly polyurethane based. In Europe the top coat can be given extra damage resistance during roll forming and transport by the addition of small beads (polyamide) which toughens up the surface. These are called PUPA, standing for polyurethane, polyamide. The total dry film thickness of paint is therefore 45–50 um which, in addition to the galvanic protection of the metallic coated base, gives barrier protection. The polyurethane top coat is also an excellent performer with respect to colour and gloss retention externally. Another high-performing coating, PVDF, is also used in conjunction with high build primers. Logically, if a coating is to perform for a long time as a barrier, it also needs to retain its appearance over that time.

d) Plastisol – this is liquid polyvinylchloride (PVC). This is another means of achieving higher corrosion resistance through increasing the film thickness. As with high build primers, the extra thickness forms a barrier to the environment. In this case the top coat (plastisol) is applied at 100 um or 200 um. The lower thickness is applied using 2 roll technique but results in an uneven surface, “ropey” in appearance. It is used where the appearance is not important and the sole consideration is the barrier protection. The majority of plastisol is supplied at 200 um thickness. Liquid PVC is high in solids (95%) so is quite thick and must be applied using a three roll head system. Three roll V arrangement is used. While pan feeding from a conventional facing head (i.e. head B on a U wrap or the head on an S wrap) is possible, nip feeding on the opposite head (head A or first head) of a U wrap is a little easier to achieve both uniform surface and film thickness.

The thicker dry film thickness is always embossed at the exit of the oven with water cooled rolls, while the film is still approximately 180 degrees C. Embossing is used here to guarantee that the surface is uniform. The emboss is usually a leathergrain pattern. The depth of the emboss is restricted to less than 1/2 of the thickness of the plastisol film thickness. Obviously this is another case where the special finish requires equipment especially included in the coil line design.

Extra Special Finish
A further special effect that can been used on coil painted material is metal embossing. Unlike the emboss carried out on plastisols, the base metal is embossed completely. The metal is actually deformed. The purpose of this embossing is less about visual appearance and is more about increasing the strength and stiffness of the base metal such that lighter gauge material can be used. Metal embossing can be carried out inline by a hydraulically driven stand (usually more substantial than the plastisol embosser). A metal embossing stand is installed just prior to the exit recoiler while a plastisol embosser is installed at the end of the oven, and in front of the water quench. It is not unusual for metal embossing to be carried out off line in a simple independent, purpose-built line consisting of an uncoiler and recoiler with the stand in between. The most frequent “pattern” used for metal embossing is a stucco pattern which in addition to adding strength and stiffness reduces reflectivity. Other patterns like diamonds or leathergrain can be used. It should be noted, however, that the directionality of leathergrain dictates that light levelling is needed after embossing to give a flat sheet.

CONCLUSION

As demonstrated, the coil coating process offers the flexibility to achieve a wide range of unique visual effects and finishes. These effects enable producers to differentiate their offerings in a competitive market, helping to attract and retain market share. While some finishes can be accomplished using specialty coil coating paints, others — such as pattern coating or metallic effects — require additional equipment.

When planning a new color coating line or upgrading an existing coating line machine, it’s essential to consider the full range of products and finishes intended for production. This ensures that the necessary coil coating equipment, or at least the structural provision for it, is integrated during installation. A well-designed coil coating line not only supports current demand but also allows for future adaptability — reinforcing the value of investing in advanced coil processing solutions.

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