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Stuck on the idea of direct-to-glass
Plasma arc blown onto the surface with compressed air
We are in a world where buying decisions are lifestyle choices. Customers purchase because it makes them feel good. A product’s looks and perceived value are crucial factors. Just look at a supermarket’s ‘bargain’ shelf to see bottles of wine reduced because the label is torn or twisted. This demonstrates how fickle customers can be.
One of the busiest industry sectors is glass bottle printing. Having used vast numbers of PET labels on bottles, designers are recognising the caché of direct printing onto glass bottles is having on their products. Just look at premium glass cosmetic bottles: all directly printed. Designs are either fired into the glass or printed with UV inks designed for the purpose.
Traditionally, it has been possible to achieve a degree of permanency with glass based enamels fired-on at elevated temperatures. These are ceramic enamels (mixed with a thermoplastic material) that become liquid when placed onto electrically heated screens. When they contact the surface of the cold glass container, the mixture solidifies immediately. The printed
glass is passed through a lehr and fired at temperatures between 590 to 650°C to fuse the enamel to the glass. The glass is then annealed by slowly cooling it to ambient. This can take several hours. The process is expensive regarding energy usage, the lehr’s capital cost and the fact it slows the printing process.
The lehr can be 30m long, not a proposition to be taken on lightly. The advantages are that high temperature fired enamels allow the use of high gloss metallic and inorganic pigments. Resistance to abrasion and chemical attack is excellent as the pigments are bound to the glass by the melted glass particles in the enamel. There are issues regarding the toxicity of some enamels. Those containing lead or heavy metals are restricted.
UV printing inks
Printers have always wanted to avoid these systems but inks with organic pigments have not been particularly successful as their mechanical/chemical resistance and glass adhesion has never matched inorganic fired systems. Improvements in UV curing ink systems have made this substitution in all but the most arduous applications. UV systems have adhesion promoters that decrease pot life but improve performance. Some, also recommend post curing at 140°C for 20 to 30 minutes.
An alternative is also to spray the glass with a UV curing coating that accepts the UV ink. Probably the best solution is to pre-treat the glass to provide a surface suitable for UV ink adhesion. Ink manufactures suggest flaming the glass before printing but the most successful system is plasma treatment. Commonly used to improve adhesion, plasma is the forth state of matter, using electrons and molecular debris to alter a material’s surface structure.
Plasma pre-treatment
On contact with the substrate, the plasma’s added energy is transferred to the surface, altering the surface to accept an ink. Plasma will treat virtually any material and its effect is to raise the substrate’s surface energy enabling it to be wetted by the ink. Glass, silicone rubber and PTFE are all difficult to stick to. Plasma treatment allows adhesion.
Although the plasma temperature is 300°C the surface temperature is raised by less than 20°C. Thus the process suits delicate substrates. Flaming is a lower capital cost alternative but it isn’t as effective. Plasma not only increases the substrate’s surface energy but also removes contamination, leaving a clean surface.
Irregular shaped containers
Bottles and containers are no longer just cylindrical. Some are square, others are triangular and printing systems are available to print around a square profile with round corners. It this instance, all movements are servo driven using CNC technology, letting the squeegee and flood coater assembly rise and fall in sequence with the stencil that also moves to keep the off-contact distance stable.
Such systems make it possible to program movements for a range of bottles and recall them instantly. Systems are sold as single or multicolour units. The technology offers printers tremendous versatility in applications involving a wide range of ‘difficult to print’ bottle profiles.
Basic container printing
With little degree of sophistication flat, cylindrical and oval bottles and containers can be printed. Regardless of shape they must be wettable by the ink which means at least 42 Dynes cm. Generally, flaming is the cost effective solution.
When printing cylindrical items one of the keys to success is ensuring the squeegee is aligned top-dead-centre of the cylinder. Squeegee pressure should be just sufficient to keep the stencil in contact with the bottle over the complete print area. To print multi-colours, a positioning ramp in the bottle’s base is ideal.
Following the principle of the stencil maintaining contact with the bottle throughout its image area, if the bottle is deliberately uneven the squeegee should be shaped to suit. There is no need for special machinery but it is sometimes better to use a Nylon (polyamide) mesh which is more flexible than polyester.
An oval is printed like a cylinder except the printed surface moves on the circumference of a circle in the opposite direction to the stencil.
Printing a cone needs a different machine mechanism. This swings the bottle while it rotates. It is possible to get away with a few degrees of taper on a bottle with a conventional mechanism but obviously conical shapes need the additional movement in the printing process.
Screen printing 3D objects can be interesting. The same fundamental screen printing principles apply. Be consistent setting up and operating the machine; produce good quality stencils; make sure components are clean and wettable by the ink. Given the variety of container materials and contents, inks must be carefully selected. Using four-colour process is particularly challenging as it is difficult to measure the printed result. Even so, some wonderful effects are achieved. When buying perfume, take a look at the bottle and consider how it was printed. When you pick up that expensive hair conditioner, admire the edge definition and fine detail. Don’t worry you won’t get arrested.
