Multi-color Screenprinting for Cylindrical Plastic Parts
Question and Answer
What does a GPS device have in common with printing multiple colors on plastic cylinders or bottles? Precision. Both require a precise starting point and stopping point.
Screenprinting is a developed art form that, when applied properly with the correct printing machine, renders a picture-quality product. This Q&A addresses some of the common concerns and potential recommendations for multi-color screenprinting.
What are the basic areas to consider when starting a multi-color job?
Many variables must be considered when screenprinting a plastic cylinder. These include screen material, screen tension, screen mesh, screen frame, screen height, machine speed, ink viscosity, squeegee material, squeegee hardness, squeegee angle, container material, container density, container roundness or concentricity, container finish, flaming, artwork, registration, and more.
It is critical to make sure the container is round or concentric to specifications. This will ensure the proper squeegee contact and avoid excessive pressure (smearing) or missing print due to gaps under the screen. It also is equally important that the plastic container or bottle has the proper wall thickness to handle air inflation. Most containers require air inflation to support the squeegee in the printed area and occasionally a bottle with a thin wall section will balloon in one spot, causing excessive squeegee contact and defective print.
Is it important to know the type of plastic that will be screenprinted?
Whether printing a plastic cylinder with a single color or multiple colors, it is critical to know the type of plastic to be printed. Most plastic containers are made of LDPE, HDPE, PP, or PET, which have a natural skin film surface created in the mold that protects the container and gives it strength. This surface has to be opened to allow the inks to penetrate the plastic and bond. To achieve proper adhesion, the plastic container must be pre-flamed or corona-treated to open the pores of the plastic and allow the ink to adhere to the surface. After a short period of time the surface returns to its original state, so it is important that the container is screenprinted soon after treatment.
Are there specific on-press recommendations for multi-color screenprinting?
Once the screen is prepared and mounted in the machine, the operator must utilize a flat screen to print a round cylinder. Actually, this is far less complicated than it sounds. If the container is a single color, the process is relatively simple as long as the location of the print can be located randomly on the container. However, when a specific position is needed or there will be multiple colors in registration, a registration tool is required to guarantee each color has perfect repeatability. When this is required, there should be a molded registration device located somewhere on the container. Normally this is a small ramp located on the bottom. This allows an operator to position the container into the tooling of the printing machine at a pre-determined location. Envision a 12 hour clock face with a starting point at 2 oclock. The container will be engaged by a tool in the machine that drives the container under the screen to a pre-determined position. This is repeated at each station of print to guarantee all colors are in registration.
Printing a single- or multiple-color plastic cylinder requires the proper equipment and a basic understanding of the variables mentioned above. As any decorator can attest, any one of the above conditions can lead to frustration. Understanding these variables and how to deal with them will help the job run much smoother.
What specific areas are important in preparing artwork for container printing?
Difficulties and problems with container decoration can oftentimes be traced back to improperly prepared artwork. Too many times, a design is made for full coverage beyond the limits of the process. When this happens, production problems occur and a quality print becomes hard to achieve.
Artwork (or the decoration to be screenprinted) should be laid out inside the ends or edges of the cylinder to be decorated. Keeping the design away from the extreme edges of the container will increase the life of the screen and present fewer print problems. Squeegee blade pressure against the edges wears the screen, causes color build up and smearing, and distorts any straight line printed parallel to the cylinder edge.
Container decorators should prepare (or obtain from the container manufacturer) art specification sheets for all the containers they decorate. These sheets usually contain a line drawing or other illustration of the container with dimensions relating to its height, diameter or depth, material, style, capacity, color, and maximum screenable area. The type of closure also can be helpful in certain circumstances. Since the maximum screenable area is discretionary, careful consideration should be given to establishing the perimeters within the limitations of profitable container decoration.
The printable area of a container is determined by wrapping the container with a piece of paper and marking the circumference dimensions. Simply overlap the paper and mark where the paper intersects. A no-print area should be established one quarter of an inch (6.35 mm) from all label protector areas. If the container does not have an indentation for the label, then the no-print area should be at least 3/8 inch (9.525 mm) from the top and bottom of a straight-walled container.
To determine the stopgap area, draw a circle equal to the printable diameter of the container. With some containers, especially those with label protector areas, the diameter of the container and diameter to be printed are different. Make sure you use the printable diameter. Next, draw a chord 1/16 inch (1.588 mm) inside the diameter and then draw radii from the center to the intersection of the chord with the diameter of the circle. The stopgap or no-print area will be equal to one half the chord length.
Using this method will prevent wet ink from rolling around and contacting the under side of the screen during printing.
Always use these methods to check the printable area of the containers before developing artwork. Preparing the artwork within the tolerances and specification of the container is much preferred to trying to explain why customer-approved artwork does not fit. n
Plastics Decorating would like to thank Dutch Drehle, senior technical associate with the Specialty Graphic Imaging Association (SGIA), and Barney Hanrahan, national sales manager with Kammann Machines, Inc., for their assistance with this article.
SGIA, (703) 385-1335, www.sgia.org
Kammann Machines, Inc., (630) 513-8091, www.kammannmachines.com