by Steve Costner, Tampoprint International
Pad printing can seem like a very involved process with many different variables to consider. However, once you plan your job out correctly and know what to look for when troubleshooting, much of the “mystery” can be taken out of it. Here are a few common questions and challenges that occur on a regular basis when running a pad printing job.
Q: When printing film on a laser printer for polymer plate exposure, can transparencies be used?
A: The answer is no. The toner in laser printers will not bond very well to the transparency material to give a dense enough image for exposure. Many companies try to save money on plates by producing films and polymer plates in-house. There is nothing wrong with trying to save a little money; however, when it comes to pad printing, going the most inexpensive way can sometimes affect the final outcome of a print. Just remember that whatever the image looks like on the film is exactly what will end up appearing on the plate.
If you are using a laser printer to produce your films, the best material to use is polyester laser film. Always be sure to print your films emulsion side down for pad printing. It is still not as good as using film produced from an image setter or camera, but it is possible to get adequate results for polymer plate exposure.
A good way to tell if a film will be acceptable for exposure without testing it on a plate and wasting material is to hold the printed film up to a fluorescent light or over a light table. If you can see light through your image, then it is not going to expose properly and your plate will turn out shallow.
Since the quality of polyester film is not as good as that of an image setter or camera film, there still can be problems with light passing through an image. Here are some tips to help improve the image density:
1. After printing a laser film, heat the emulsion surface with a heat gun or a blow dryer. This will help the toner from the printer bond to the polyester material better. Some polyester film manufacturers also make processors for their laser films. The film is still printed through a laser printer, but it is then run through a chemical bath to further aid the toner in bonding to the polyester material. Heating the film with a blow dryer again after this step will also help.
2. If you are printing an image with a fairly large area, you can go over the light spots with an opaquing pen or dark marker on the non-emulsion side. The biggest problem with polyester films is printing dot patterns or line screens. Line screens just do not work very well on most polyester laser films; therefore, in order to get good results, these films should be produced from a camera or image setter.
Q: What causes dimples or small recesses in polymer and thin steel plates when printing?
A: This is usually a result of debris between the plate itself and the magnet holding plate. Always make sure your plate is clean of any dust or debris on the bottom as well as on the top. The same goes for the magnet holding plate. If anything gets between the two, it can cause the doctoring ring or blade to dig into the plate and leave small areas that could eventually pick up ink. Not only that, but if there is debris on the surface of the printing plate, it could result in serious damage to the doctoring ring or blade.
Q: Why is there a difference in durometer (hardness) for silicone pads?
A: The durometer of a pad is determined by the surface of the part on which the image is printed. Pads can range from very soft to very hard and come in many different shapes and sizes as well. If you are printing on a textured surface, for instance, you would need to apply more pressure with a harder pad to insure that the ink gets down into the peaks and valleys of the textured surface. On the other hand, if you are printing on a very smooth surface, then a softer durometer pad is required.
Most pad manufacturers carry a wide variety of shapes and sizes for many difficult or odd applications. The shape of the surface you are printing on can determine the shape of the pad you should use. Using the wrong pad can result in distortion on the final print. There are also different types of silicone. If a pad is made with a lower quality silicone, it will wear out much faster than higher grades of the material. In addition, the length of the run can have an impact on the grade of silicone you use.
Q: What causes ink to splatter when printing?
A: There are a few reasons why ink can splatter in the pad printing process. If it is too thin, it is definitely going to splatter. A good test for getting the right mixture of ink and thinner is letting the ink drip off of the mixing stick. If it flows smoothly and evenly into your mixing cup without splashing or piling up, then it is the right viscosity.
If the ink is too thick, it will leave a spider web effect on your print. This is the result of the ink being pulled back up when the pad lifts off of the part you are printing on.
Another cause of splattering can be a plate that is etched too deeply. When a plate is too deep, it will fill up with ink and be pushed out by the pad compression.
The proper depths of thin steel and thick steel plates are as follows:
- Sealed Ink Cups: 18-22 microns without line screen
- Open Ink Wells: 25-30 microns without line screen
- Polymer and Water Wash: 40-45 microns with line screen
If you have to use a line screen for the thin or thick steel, increase the standard depths by 10-15 microns.
Q: How do I control depth when producing polymer plates?
A: This can be done by a few different adjustments in the etching process. Most exposure unit manufacturers have set times for exposure. Generally, there should be no reason to stray from these settings. However there may come a time when you need to go deeper or shallower. Normally, if you increase your exposure time, the plate will turn out shallow. If you decrease your exposure time, it will be deeper. The reason for a deeper plate with a shorter exposure time is because the plate is not curing as much. Therefore, the material is softer and more of it will wash away in the washout process.
Another way to change the depth is to adjust the washout time. If you increase the washout time, the plate will be deeper. If you decrease the washout time, the plate will be more shallow. Usually, there is no need to change the depth for polymer and water wash plates. They are easy to produce and should turn out fine for printing every time if developed at the proper time settings on the exposure unit.
Of course, these are just a few common challenges operators come up against. If you have the proper training and know where to get the answers, your pad printing operation can run smoothly with little downtime. Inadequate preparation is when you can experience trouble. Make sure you have a good contact from your pad printing equipment manufacturer and/or supplier of ink, pads, etc., to help you through a challenging job.
Steve Costner has been in the pad printing and laser industry for over 6 years and is a graduate of the University of Florida. He is the manager of Graphic Arts, Cliché, and Laser Departments for Tampoprint International Corporation in Vero Beach, Florida. He can be contacted at (800) 810-8896.