Understanding the Value of Pad Printing
The accuracy of pad printing is a vital component to consider as the information that is pad printed is often of critical value. It may be how to install a saw blade or the dosage scale on a syringe. These situations lead to the need for accurate printing.
How is one to achieve accurate printing? This can be done with many different aspects of the process. We can start with the printer itself. The actual framework of the printer is essential in providing accuracy in the print. If the frame of the printer is weak then vibration and deflection will affect how the print is applied and accuracy will be sacrificed.
Most pad printing machines in the market run on pneumatic cylinders for all the motions. While these are generally accurate, they can vary in the performance as they wear out or as air pressure in a factory fluctuates. One way to achieve more accurate printing is by using servo motors with screws to create motion. The motor driven motion is more accurate and repeatable from print to print. Another factor to think about is how the motion is supported. That is, are there linear guide rails, bearings, rods, etc… to support the motion and keep it parallel or square. The higher quality the support mechanisms the more accuracy one will achieve with their printing. It must be mentioned that the longer the support can guide the motion the more accuracy will be gained.
The next focus for accuracy is the tooling that holds the part during the printing process. The tooling must be built for ease of loading and unloading while still being tight enough to keep the part from moving during the printing process. The sticky nature of the silicone print pad can often unnest the part during the print process. Mechanical and vacuum tooling can be used to better engage the part during printing and release for easier loading and unloading. This becomes much more significant when there are multiple colors being printed or if there are multi fixtures being used, like an index table or carousel.
If there are multiple colors being printed the accuracy from color to color can be critical. When this is the case one needs to understand the mechanism moving the part to the colors or the colors to the part. One should also consider what locks that position. It is also important to have tools to register the print. Some equipment can be built with dead on tooling and dead-on plates which allows for very accurate printing.
If we have good printing equipment, good motion devices, and good tooling we need to make sure that the pad is properly selected. An improper pad can distort the art upon application thus causing an in-accurate print. Pad durometer and shape are important, but one must also consider where on the pad to pick up ink and how to best contact the part with the pad for accurate application.
It is critical to not only understand these factors independently, but also note that these factors create a “stack tolerance” which should be considered. When proper steps are taken, pad printing can be a very accurate process for decorating parts with an accuracy of +/-0.02mm or about 0.001”.