Multiple color capability is probably the most popular option. Obtaining this capability can be as easy as purchasing a multiple color (split) ink cup or cliche' holder, or as difficult as completely retrofitting an existing machine.

A few manufacturers offer small (60.0 mm) two or three color, split ink cups. These cups allow you to print multiple color side by side, or in some cases wet on wet. The limitation is size versus cost, as these cups are very expensive. I have seen a two color ink cup that costs $1,800.

Multiple color inkwells can allow you to switch from one to as many as four colors in open systems. In addition you would also need a set of multiple color accessories, like an ink spreader, doctor blade assembly, and pad holder.

Most multiple color systems are designed as such, rather than modified into being. In open systems for example, it is better to have four independently adjustable cliché holders than one multiple color ink well, as artwork and cliche' layout, and machine set-up is much easier when you have the additional adjustability.

The same holds true to multiple colored closed systems. Having two, three, four or six independently adjustable cliché’s is much better than trying to put several images on one cliche'.

Having semi-automatic or automatic multiple color capability requires a method for moving the parts from color to color without changing nests. (It is not recommended that you print one color, then move the part to another nesting fixture for subsequent prints.)

This can be accomplished by means of a part shuttle, a rotary indexing table, or a conveyor of some type.

Part shuttles can be pneumatic, or electronic. Electronic shuttles are more expensive than pneumatic ones, but last longer and are typically more accurate. The nice thing about either one of these two options is that you only need to build one nesting fixture.

Rotary indexing tables can be either electronic or pneumatic as well. Here again, the more expensive electronic versions offer better accuracy and durability. Some manufacturers stock modular rotary systems that allow you to use up to four machines either simultaneously or individually. The number of nesting fixtures needed depends upon the size of the rotary table and the parts you're printing.

Conveyors can be "racetrack" or "over-under" type conveyors. Racetracks are usually pneumatically driven. Many times these are standard equipment on multiple color machines. Over-under conveyors are less popular. Operating like an oval shaped paddle wheel, these conveyors require that the parts be dried prior to falling off their nests at the end of the line.

Drying parts is as easy as attaching a hot-air dryer to the outbound side of the printer on a rotary indexing table or conveyor, or by having a separate self-standing tunnel dryer like those used in the screen printing industry.

One process limitation that I mentioned was the requirement that the substrate have a higher surface energy than the pad. Certain low surface energy materials can be pre-treated to increase surface energy.

Pre-treatment can be accomplished using flame treating, corona treating or plasma treating equipment.

Flame treating is the most popular method. The equipment uses specially designed flame treating heads to carefully control the amount of surface treatment. Since heat can cause damage to some materials, flame treatment is not always applicable.

Corona and plasma treating equipment both use electricity to deposit an ion rich discharge on the surface of the substrate. Corona is more useful on large, flat parts, whereas plasma is more useful on smaller, three-dimensional parts.

Ink pumps and thinner metering devices are used to control ink viscosity during long production runs. Ink pumps are used to add thinner to the ink in open, recirculating systems. Thinner metering devices are used to add thinner to closed ink cups.

Ink residue removal systems or "tape-offs" are used for cleaning ink and debris off of the pad surface during the print cycle. Using a slide mechanism, these systems position an adhesive covered "tray" beneath the pad after a print stroke. The pad is then compressed onto the tray where the adhesive removes ink and debris. The tray then retracts, and printing continues.

Nesting fixtures are many times taken for granted. Until you gain a knowledge of pad-print-specific requirements with regard to designing and building nesting fixtures it is highly recommended that you allow the pad printing equipment manufacturer to build your nests.

Pad shuttles are an option useful in certain applications where your ink cup or cliche' isn't large enough for the image you want to print, and you don't want to buy a larger machine.

Safety equipment other than that which comes standard is an option you may want the pad printing equipment manufacturer's advice on. I have seen a number of perfectly good machines rendered almost useless by the addition of poorly designed safety enclosures and / or equipment. The people who design and build pad-printing equipment know what is practical, and what is not.