by Henry L Newman
The wide variety of containers used in the packaging industry present screen printing experts with many unusual and seemingly difficult decorating requirements. The most simple geometric- shaped items can be printed by using a few basic techniques. Some examples of 3-D objects that are commonly screen printed include bottles, pails, drums, tubes, cups, mugs, jars, pens, bats, sticks, CDs, and much more.
There are several advantages of the direct screen process for container decorating. The ability to apply an easy label image or color changes via the inexpensive screen is ideal for short runs. The rich opaque color unique in the printing industry is due to the ink layer thickness pushed through the stencil or screen mesh is also a benefit, as well as its ability to cover the base color of the substrate. For many applications, objects are printed or labeled in one or more colors. Labels, stickers, decals, and other pressure and heat applied graphics are the competition to the screen printer. Because of the reasons mentioned above and the possible cost advantage over pre printed labels, screen printing on industrial items is everywhere.
Historically, manufacturers of plastic containers were not as concerned with the adhesion or legibility of the label as they are today. But if the label comes off or is torn off over time, how can the identification of the product or its contents be identified on a permanent basis? This can be of utmost importance when working with toxic or other environmentally concerning material.
Although screen printing cylindrical objects is somewhat basic in nature, it can create certain challenges for the operator. Let us take a look at a couple of specific examples of screen printing challenges and how to solve them.
CHALLENGE # 1 55 gallon HDPE drums used for oil recycling
55 gallon plastic drums are used worldwide in many chemical packaging applications. One use is for the recycling of motor oil in service stations. In this requirement, the closed top drum needs to be printed with various full size, one color graphics. The challenge is flame treating and screen printing the large diameter area of the drum.
Several operations are required starting with flame treating of the HDPE plastic. Not to be confused with heating the plastic, direct exposure to an open flame changes the surface tension and allows the ink to adhere. It is easy to verify this crucial first step in decorating or even labeling HDPE by using the water test. Simply place tap water on the surface to be printed, if it beads up like a waxed car – beware. It may require flaming. If the water flows into a sheet, you can have confidence the surface is good. Pens and more exact dyne testers are available. After molding the HDPE drum, it is placed on the flamer. The drum is automatically lifted to the 12 ribbon burner and rotated for a pre-set time to allow the entire printable surface of the drum center exposed quickly to direct flame a few times. It does not melt, just warm to the touch. Flame treatment on HDPE should last about 6 months or a year depending on conditions. Corona or electric arc pre-treatment are also used in some situations.
To decorate the circumference of the 23 inch diameter drum, an 80 inch screen stroke was required. If it were a flat surface like a label, the squeegee would stroke 80 inches against a stationary screen. In this case, the squeegee remains over the center of the turning drum driven by the friction of the screen traveling horizontally above it. The drum is sitting on four rollers that look like inverted casters. In operation, the press lifts the drum to the screen on the roller carriage, the squeegee comes down to the drum pushing down the screen from above and does not move during print. The third operation is the horizontal stroke of the screen from left to right. This stroke turns the drum on the rollers clockwise and the ink is deposited in a line contact under the squeegee pressure. After the print stroke, the drum comes down for unloading and the flood blade or second squeegee comes down slightly off the ink surface of the screen to push back the ink or flood as the screen returns to the right to left home position. Special conventional inks and thinners for HDPE are needed. An epoxy air dry ink was used in this example. The screen mesh used was 230. An 80 durometer, 3/8 x 2 V shape squeegee blade was used. Print cycle 5 seconds excluding load/unload time. Three operators can run the flamer printer line as follows: 1- load flamer to front, 2- unload flamer from back, feed printer to front, 3- unload printer, upright to conveyor.
The third and final step in the decorating process is drying or curing. The previous industrial HDPE decorating example used the basic air dry process of doing nothing further. After an hour the ink is dry to the touch and after 24 hours it is finished drying. If heat and forced air circulation is provided, it will dry much quicker. Heat dryers for items this large are not practical. To keep up with the output of the line, a dryer of almost 50 feet long would be needed. When decorating smaller items, conveyor dryers help, but are still somewhat large and inefficient. Ultraviolet curable ink and related ink curing units are one of the most significant developments in the screen printing industry in the last 25 years. UV curing equipment is desirable although expensive. Ultra fast cure speed, small size, energy savings, and low VOCs, are some of the reasons using ultraviolet inks and equipment may be desirable.
CHALLENGE #2 16 ounce Polycarbonate measuring cup, 2-color
Printing a cylindrical 3 inch diameter tube can be a fairly simple operation. Even manual machines where the operator pulls the screen across by hand can print simple rounds. However, if you are needing high speed, two color printing (one front, one back), that is dishwasher water resistant, with no mechanical notch or register point, then you have a few more things to deal with. Many multicolor automatic screen printing bottle printers are available that print and cure with UV systems and many colors at high speeds. This equipment is expensive and bottles generally have a register notch or lug in the base to assist mechanical register. Photo eye register systems are also available but speed and accuracy issues make mechanical register more reliable. Also, screen printing on plastic surfaces generally requires one color at a time and always requires a drying or cure cycle between the colors. This print/dry, print/dry, etc. multicolor process requires multiple dryers or UV cure equipment, which can be quite expensive.
The solution for this application was a used automatic twin head printer that was retrofitted with a single Fusion F300 UV curing system. The line runs right to left with a roll ramp in-feed, first color print is red with a chuck and counterpoint tooling using direct gear rack drive. The tube is printed random with the first color red with a 0-180 degree stroke and released carefully onto the transport chain to the second color. A second print station lifts the tube into the chuck from its last position and continues the other 180-360 degree side in another color. Now the two wet sides are transported on the chain carriers to the UV station where one spin in lamp focus cures both colors together. They then roll of the left end of the machine for further assembly. Labor saving automation in container printing must consider the drying of the ink between colors. Inline ultraviolet systems provide almost instant ink curing.
Henry Newman is President of H.L Newman Equipment Co., Inc. He can be reached at 847-831-0220.