Pre-treating plastic bottles or containers prior to printing is sometimes required in order to get good ink adhesion as often measured by the tape test. Several methods are used for improving the surface tension of the substrate, including open flame exposure, corona discharge, and chemical application. Generally, flame treatment gives the best result that will last the longest. Simple marker pens are available to show standard levels from 30-60 dynes/cm. The flame gives the bottle a licking, so the ink keeps stickin’. In other words, some substrates do not have the right surface tension to allow the ink to soak in and bond correctly. Just like water beads off a recently waxed car, ink needs a surface that has a high dyne level in order to adhere. Both low- and high-density polyethylene are most often treated after the molding process. Normally, in the case of HDPE bottles, the manufacturer will pass them through a treatment process of some kind. One common way to handle this operation on bulk bottles is corona discharge.

The corona system uses electrical energy to, in effect, spark or arc to produce a hallo of energy needed to alter the surface tension of the substrate. The bottles pass through this electrical discharge area normally on a conveyor. Another option is flame-treating. It is not the heating of the bottle that is needed. The flame corona will alter the surface as described above. Once flamed or corona discharged, the HDPE surface will hold the flame for up to a year or more. Many things can affect the length of time the surface will hold its flame, including climatic conditions, jostling the boxes around, and static electricity caused by opening bag liners in the boxes of bottles.

It is interesting to note that even labels will not adhere well to un-flamed bottles. Many automatic decorating machines will have an inline flamer station to treat the bottles just prior to printing. This not only increases the dyne level for adhesion of ink but also cleans the surface of dust and dirt, as well as eliminates static electricity that can cause spider webbing of the ink. The heat transferred to the screen also will thin the viscosity of the ink sometimes for improved flow and gloss.

What advantages are achieved by screenprinting a cylindrical plastic part as opposed to other decorating options? 

Various methods are commonly used to decorate containers. Cylindrical and slightly tapered containers are labeled or printed in most cases. This classic choice seems to change with the times based on numerous factors: speed required, how many colors, environmental and recycling issues, and most often, cost per unit analysis. One application to consider would be 5-gallon pails or buckets. These are sometimes sold empty or reused once empty of its original contents like chemical cleaners, detergent, and paint. These pails sometimes are labeled with a pressure-sensitive sticker, a paper, or a plastic label bonded with cold or hot glue. Large, good quality labels such as this can be expensive, have long lead times, and are difficult to apply on a tapered pail. Applying them straight and without bubbles can be a challenge. A quality issue is apparent if the product contents get on the label and make it illegible.

Another way to decorate the 5-gallon pail is by direct printing, which entails printing directly on the container. This requires only one labor operation as opposed to the label application method, which would require two operations. Two common methods of direct printing pails can be observed in the marketplace. Most large-volume runs are produced by dry offset printing. This is a high-speed rotary process using rubber blankets to transfer ink from engraved plates, either in single or multiple colors at the same time. However, the offset process ink thickness is thin and weak compared to screenprinting. Dark colored pails like black, printed in offset with white ink may appear grey. The screenprinted pail will have an opaque look with heavy ink coverage and good detail. When screenprinted and cured with ultraviolet ink, the detailed, high-gloss decoration produces a very high quality look. Bold reverse print and blocks of color look bold and can contain fine details such as UPC codes as well. Despite the quality argument, screenprinting pails with modern UV equipment, especially one color, can provide a cost savings argument over labels in the final analysis.

What are some of the recommendations to keep in mind when ordering and setting up screens for a multi-color screenprinting job?

Ordering and setting up screens for a multi-color decorating job requires attention to details. Registration or lining up the bottle to the screen each time is the main issue. The screens used to UV print multi-color, 16-ounce HDPE shampoo bottles, for example, at first glance are similar to screens used in textile and t-shirt printing. Wood or aluminum, 1" to 2" square frames are stretched with a fine mesh 350, 390, or in some cases 420. Some screenmakers will shoot the image on a 22 degree bias to reduce the saw-tooth effect. Once the image is burned into the screen, it must remain under good, consistent tension. Realistically, 10 to 15 Newtons is common to observe in the field.

Good multi-color registration in this application is primarily a function of the machine and tooling used and how well it matches the bottles. Basically, the screen moves and strokes horizontally. The squeegee remains fixed over the center or apex of the bottle. The squeegee centering is a critical part of container printing. Soft squeegees may fold over and lose this ‘center’. Normally, as the screen moves left to right, a mechanical linkage through a rack and gear drive (turn) the register pin hopefully engaged in the lug or ramp in the base of the bottle. The register pin is connected to the screen movement and should have no mechanical slop or backlash. Printing in registration implies this pin is really engaged in the base lug of the bottle, even though this can’t be seen. The friction of the screen movement will turn the bottle, but this is not printing in register for multi-color.

Problems observed with out-of-register printing will be left to right shifts in color and rarely vertical or up and down variations. Artwork preparation must take into account this left/right color variation and design space or enough trap to compensate. Normally the final color is created to trap or overlap existing cured ink and not butt-register colors side to side. A final note would be to put any color that is difficult to cure, such as opaque black, dark green, and others, in the press before the last station to be exposed to the UV several times. Unlike other inks and processes, UV printing containers is a repetitive process of register, print, cure or dry, then the second color by register, print, cure or dry, etc.

Newman Printing Equipment Inc. specializes in new/custom and used/rebuilt screenprinting and curing equipment for all shapes and sizes. Visit www.newmanequipment.com or call (847) 803-8091.