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Prepress Key to Offset Printing Success
by Staff

Decorating Workshop
April-May2002



There are many factors that go into the success of a multi-color dry offset printing job. When an operator is applying 4-colors or even as many as 6 or 8 colors in one pass, attention to detail is essential to begin and maintain proper registration of the image on the plastic product or container. Factors such as artwork preparation, manufacturing of the printing plates, correctly mounting the plates to the press, the necessity of a high-quality printing system, the quality of the part to be printed, and, of course, the operator are all important elements. The bottom line is that it must start at the beginning to ensure success. This article will discuss how prepress and other elements in preparing for a multi-color offset job are key to quality registration.

The process begins with the artwork. When preparing the artwork, all multi-color printing processes require a degree of commonality between the color separations. This is especially important with the dry offset printing method because all of the colors are transferred to the product simultaneously. It is essential when preparing the artwork that each color is matched to a common X-Y axis or registration point to make sure they register on the printing press. The registration points guide the sequence of turning the artwork from electronic files to film negatives to printing plates, and finally to the printing plate registration on press. Skewed artwork or printing plates and/or printing plates out of registration will cause different colors of wet ink to touch on the offset printing blanket, which will eventually blend the colors and feed them back through the print heads. The spacing or trap required between colors that prohibit this from occurring will vary depending on the substrate being printed and the specific application.

Once the artwork preparation and color separations are prepared properly, the next important stage is the manufacturing of the printing plates. The plates are usually made of photopolymer, metal back photopolymer, or metal. To maintain uniformity in registration between each of the colors, the reference marks that were used in the creation of the artwork and negatives are now built into the plates and are used to identify the color separation locations. At this stage in the process, it is very important that the artwork is completely square with the registration marks, as these marks will be used to correctly affix the plates to the press.

Following the successful manufacture of the printing plates, the plates are then mounted to the printing drum. Common methods of mounting the plates include the use of magnetic drums with a combination of pins, offline registration devices using segmented plate drums (either with magnets or double-sided tape), and for less critical registration type systems, magnetic drums or drums that use double-sided tape with simple raised X-Y datum shoulders.

An excellent example of how this type of prepress logistics works is with Apex Machineís caulking tube offset printers. These printers are built to print up to 6 colors on rigid and squeeze type tubes. The prepress starts with a matching set of pins. The negative and photopolymer plates are then made with the matching pin holes. The negative is exposed to the photopolymer plate on the pin fixture. The plate is then mounted to a plate segment through a plate registration unit, using double-sided tape. The segments are then mounted to the drum with a common pin system, allowing six colors to be tightly registered along and around a standard caulking tube. Most rigid and squeeze caulking tubes have smooth clean surfaces that are ideal for the dry offset method. Open and closed-end food and pharmaceutical packages also fall into this category.

There are printing systems available today that eliminate the step of creating color separations and that can go directly computer-to-plate. These types of systems can produce digitally imaged plates and predict ink densities almost perfectly even before a job is set-up on press. Eliminating a step in the process can help with the registration on press, which in-turn can reduce waste and costs. Kammann Machines have utilized this technology with its K15-40 offset printing press for DVDís. The press includes a computer-to-plate system and an inspection station that can verify ident codes, inspects image quality and interrupts printing when a specific number of discs are rejected. The printing plates for the K15-40 are attached to the cylinder, thus allowing for the pre-registered positioning prior to exposing the plate. The exposed plate/printing cylinders can be mounted into the press with minimal downtime as experienced if you when utilizing some traditional plate systems.

Other areas must be analyzed carefully as well to ensure a quality registered job. The design and consistency of the molded plastic part can have a dramatic impact on the final results. The product must have a uniform thickness and surface, especially when printing high-tolerance multi-color artwork. The dry offset printing technique places a thin film of ink on the surface of the substrate, so a part that has pores, textures, or varying thickness can create challenges with dry offset printing. These challenges may be less of a problem with processes such as screen printing were the ink deposit is much heavier.

Of course, there are many other factors that contribute to a successful registered offset printing job. A quality operator is key. He or she must find a balance between the appropriate amount of fresh ink for each color that is equal to the amount being printed on the substrate. In addition, there are many on-press circumstances that may result from a 4 or 6 color job. An experienced operator and quality service from the machine manufacturer are important elements. However, without properly preparing for the job from the start with quality prepress, the best operator in the world cannot be successful. Prepress and having quality plastic product to print are keys to low amounts of rejects and high output rates.

Plastics Decorating would like provide a special thanks to Todd Coningsby of Apex Machine 954-771-5115 for his contribution to this article and to Steve Gilbertson of Kammann Machines 630-513-8091 for his assistance.