Preventing silvering of overlaminates

by carly_mchugh | 14 June 2023 12:34 pm

By Jay Kroll

Decal applications commonly require an overlaminate film to provide protection for a graphic, extend its useful life, offer some specific functionality, or enhance the overall look and feel of the material. Each project can call for slightly different products, depending on finish, durability, and application.

On very rare occasions, these overlaminates will be applied by hand, but the most common and effective procedure for applying an overlaminate film to a graphic is by using a dedicated laminator. Typical methods include liquid lamination; hot lamination, which uses a film with a heat-activated melt or adhesive layer; and cold lamination, which uses a pressure-sensitive adhesive to achieve a bond with the base material. The discussion below will focus on the latter process, cold lamination. This method is accomplished by running the graphic and laminate between two rolls under pressure, sometimes with low heat, to assist with adhesive flow.

Laminators can vary in width up to 1,651 mm (65 in.) and some can accommodate rolls of laminate more than 304.8 m (1,000 ft) in length. Operators can compensate for thickness by controlling the speed and tension of the individual film rolls travelling through the laminator. The media chosen will determine the gap needed between the rollers or nip setting. The speed and tension—and on certain laminators, the pressure—will need to be adjusted accordingly.

What causes silvering?

Lamination is a mechanical process and adjustments must be made continually, depending on the topography of the graphic. Most problems with lamination are a function of tension, pressure, heat, or speed. One common issue is air becoming trapped between the graphic and the laminate, which can manifest itself as large, visible bubbles in the worst case, or fine entrapment of air at the micro level. This phenomenon is called “silvering.” When large bubbles are present, it is often a function of tension and pressure, and it is very visible, which makes the problem easier to observe and be corrected quickly. However, since silvering is very fine, it can be difficult to detect and can go unnoticed during a laminating run, because of the subtlety of the small bubbles.

During the lamination process, the adhesive of the overlaminate must come into direct contact with the contours of the printed surface of the graphic. Total contact with the surface allows for an unobstructed view of the fine detail and colour of the graphic through the overlaminate to the observer’s eyes. When small pockets of air are trapped between the adhesive of the overlaminate and the printed surface of the graphic, these create small disturbances in the image that will distort the graphic to the observer. This disruption slightly scatters the light waves, creating a white appearance. It is easily observed against a dark background as a flecked or salt crystal form, hence the term “silvering.” It can exist against a lighter-coloured background, but without the colour contrast, it is much more difficult to observe.

What types of graphics are most likely to induce silvering?

Graphics with rough surfaces are more likely to exhibit silvering over graphics with smoother surfaces. A rough surface can be caused by something as simple as the texture of ink on the media, which is typically found with ultraviolet (UV) cured inks. When printing with a large quantity of ink for a graphic that requires a higher saturation, roughness may occur, because more ink dots are required to achieve certain colours. All digital printers use a minimum four-colour printing process, which means any high-ink-density print has the potential to develop silvering. For example, four-colour screenprints have comparatively large ink dots, so the process can also be particularly susceptible. To eliminate all micro air during lamination, the pressure-sensitive adhesive has to penetrate all the valleys between the ink dots. The larger the ink dot or the rougher the surface of the print, the harder it is for the adhesive to reach the valleys between ink dots, resulting in air entrapment at the bottom of each valley.

How can operators reduce silvering on their graphics during the laminating process?

Temperature

A warm pressure-sensitive adhesive will flow over a surface easier than a cold one. Most laminators have a heated top roller to improve adhesive flow. When employing this method, moderation is the best practice.

Using too much heat on a roll of laminate will cause shrinkage for certain films. Additionally, inks may start outgassing and adhesive can break down. It can also induce sheet curl to the graphic’s liner.

The optimal temperature for pressure-sensitive laminates is between 32.2 and 43.3 C (90 and 110 F), and the maximum level of heat is 54.4 C (130 F). The operator should allow the laminator’s temperature to stabilize after temperature adjustment, before starting production. This may take 10 to 15 minutes. During warmup or cooling, it is important to turn the laminating rolls for uniform roll temperature.

Pressure

As the name implies, a pressure-sensitive adhesive will flow better as the pressure increases. Most commercial laminators have hydraulic or air cylinders to apply pressure between the laminating rolls. Similar to temperature changes, moderation is required. The optimal laminating pressure is 275.7 to 551.5 kPa (40 to 80 psi). A general rule of thumb is to use less pressure with thicker overlaminate films, as opposed to thin films.

Tension and speed

Tension and speed are interdependent in the laminating process. Air bubbles are an indicator of insufficient tension on the laminating film in relation to the graphic. Silvering means fine amounts of air are getting trapped between them, so tension on the lamination film should be one of the first variables investigated. Another general rule is that as lamination speed increases, tension will need to increase, to maintain a flat lamination.

Further, as film thickness increases, tension will also need to increase, to maintain a flat lamination. An immediate curl off the laminator is a primary indicator the tension is drastically unequal to the speed, with one being too high in comparison to the other. Silvering in a flat lamination indicates the tension is close to optimal, but the pressure and the speed should be adjusted. Slowing down the laminating speed will increase the time the adhesive has to flow under pressure and squeeze the last bit of air out of the valleys of a rough print, which will reduce silvering. The optimal laminating speed for pressure-sensitive laminates is 0.6 to 1.5 m (2 to 5 ft) per minute.

It is important to keep in mind the guidelines above are general operating settings. For any specific recommendations, an operator should consult with their laminator’s manufacturer.

Conclusion

With the large variety of print methods and the ever-expanding creativity of graphic designers seen today, it stands to reason that print service providers and laminator operators will continue to encounter an infinite combination of challenges when it comes to laminates and silvering. When a print has a rough surface finish and predominately consists of dark or saturated ink coverage, there is a high possibility the image will exhibit some silvering. Fine amounts of silvering may dissipate from 24 to 48 hours after lamination, as the pressure-sensitive adhesive continues to wet out the print surface. However, moderate to heavy initial silvering will require changes to the process and potentially reworking the whole project. With this knowledge, the operator really needs to inspect the first laminated pieces for silvering and, if it is detected, follow the recommended troubleshooting measures above. These recommendations will help to control costs, eliminate rework, and produce the smoothest, silvering-free lamination, so companies can deliver the best possible product to their customers.  

Jay Kroll is a product manager at General Formulations, a manufacturer of pressure-sensitive solutions for the graphics industry. His focus is on cut vinyl, transit, and wall graphic applications. Kroll has been designing and developing various manufactured solutions for more than 20 years in the medical, aerospace, specialty tape, and graphics industries.

Source URL: https://www.signmedia.ca/preventing-silvering-of-overlaminates/

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