Fixtures play in important role in maintaining consistent and quality results given the dynamic interaction between the silicone pad and the product during ink transfer. At its core, good pad print fixture design is all about providing proper support to the printed areas of the part while allowing for repeatable registration and accounting for loading, unloading, and other processes taking place during the print cycle such as shuttle movement, pretreatment, and vision system verification.
Intricate parts with tighter tolerances will naturally require more highly engineered solutions, but the most complex fixture isn’t always the best fixture. Designers must also take in to account the usability of the tool in the production environment.
The complexity of the fixturing necessary for any given project will be driven by the following factors:
- Part size and geometry
- Number of colors or images being printed
- Number of print locations
- Print tolerance
- Part variants (graphic variants vs physical variants)
- Pre/Post treatment requirements
- Load/Unload methods
- System throughput
Consider these projects:
Simple part geometry
Single color, 1” x 3” print area
Several graphic variants
Moderate print location tolerance
No pre/post treatment needed
Complex part geometry
Multicolor, multi-location printing
Two physical part variants
Tight print location tolerance verified by camera
Plasma pretreatment required
It is possible that both projects could share the same type of plate, pad, ink, and even pad print machine model, but the fixturing solutions should look different while adhering to the same general principles. Since an operator is loading the product in project A, the tooling needs to be optimized for human interaction. This may take the form of a machined nest to hold the part consistently in registration with proper support of the print area that is as easy as possible to load and unload quickly. A simple poke-yoke may be built in to ensure that the fixture can only be loaded in the proper orientation.
Project B has more variables to consider. Mechanical tooling may be needed to facilitate load/unload via robot. The fixture may have to interact with more part features than the part in project A in order to achieve the tighter printer tolerances. Given the volumes and pretreatment required, multiple fixtures set on an indexing table may be needed to allow processes to work simultaneously. The design needs to account not only for the interaction of the part with the print head, but also the pretreatment head and presentation to the vision system post-printing. The tooling may also need to have pinned base plates for ease of changeover between part variants.
Insufficient consideration of the factors listed above could lead to unacceptable printing results or a fixture that is over-engineered for the task at hand. Fixturing that does not engage the part well and does not support the print area at the appropriate angle could lead to print distortion, shadowing on double prints, and poor registration, among other print quality issues. Cycle times can also be negatively impacted when the fixture is too difficult to load and unload, and the same goes for changeover time if all part variants are not taken into account in the design.
If a part is worth marking or decorating, then it is likely worth engaging with a team with experience designing and fabricating tooling specifically for pad printing applications. It could make the difference between the success and failure of your project.