by Jeff Peterson
Pad printing inks are a very delicate matter because of the process in which they are used. There is a very tricky balance that must be obtained between the type of application, the speed of the machine, and the drying times of the ink. Every application can be vastly different – meaning the proper mixing and color can change from job to job. We have called upon a couple of experts in the field of pad printing inks to help us sort through the proper mixing and matching necessary to achieve a quality impression on your pad printing system.
One of the most confusing issues for pad printers to work with is how their inks should be mixed and maintained. The reason for this confusion is simply the great variety of pad printing machine layouts, different plastic substrates, and the multitude of ink brands on the market. Because of all of these factors, it is practically impossible to develop exact guidelines to follow. However, there are many influences involved with the proper mixing of pad printing inks, and a clear understanding of the cause and effect will lead to a better understanding of the process. Hopefully, this will, in turn, help the operator understand how the ink should be controlled within the press. First, let us analyze the components added to pad printing inks and their purpose.
Ink. The type of ink used in pad printing is based upon its ability to adhere to the substrate and resist whatever specifications and tests identified by the end user. To determine the type of ink used, refer to an ink manufacturer application chart or speak with your ink supplier’s technical representative.
Hardener. This component is used with certain inks to increase resistance to abrasion and chemical attack. However, it is not a component of all inks. If the application does require hardener, again, check with your ink supplier to mix the correct amount.
Thinners. This is where most of the confusion lies. In addition to the adjustment of viscosity, thinners are used to control the drying speed of the ink. How much to add and which type to add is a function of the machine type, ink well type and cycle speed of the operation. The amount of thinner added will depend on the viscosity of the ink as supplied by your vendor. The relative drying speed of the thinner must be determined by taking your process into account. Different levels of thinners are available, ranging from very fast drying to very slow drying. Your ink supplier should have at least five or six levels of thinners available.
The best way to understand which thinners to use is to analyze how the pad printing process occurs. Once the wet ink floods the image, a doctor blade or ink cup “doctors” the image, leaving ink only in the image area of the plate where the surface of the ink is exposed to the air. The surface of the ink looses thinner by evaporating and forming a tacky surface. When the silicone pad comes down upon the plate and picks up the ink, it then travels to the location where the part will be printed. The wet side of the ink on the pad is losing thinner by evaporating as it is now exposed to the air and becomes tacky. The addition of thinner helps control this process. A fast cycling machine needs thinner which will evaporate quickly so the ink is ready to transfer to the next element on time. Conversely, a machine with slow cycle speeds requires a slow drying thinner prohibiting the ink from drying too early.
Once you have determined what you believe should be the correct mixture of thinner and/or hardener, the next step is to set up the machine. Each stroke should be set up to operate at the same speed. The pad up and down speed should be very close to the pad carriages front and back speed. This will allow you to have the ink dry at equal speeds at each stage. This is a very important point that many operators do not think about. Next, you can begin printing samples to test if your solvent balance is correct.
- If the ink dries the image, the ink is drying too fast
- If the ink does not transfer to the pad, the ink is drying too slow
- If the ink dries on the pad, the ink is drying too fast
- If the ink refuses to transfer off the pad, transfers incom- pletely, or with poor opacity, the ink is drying too slow
In practice, this simply means that a manual pad printer will need a slow drying thinner to handle the slow cycle times (5 parts/minute), an average semi-automatic press will require a medium speed thinner (15 – 20 parts/minute), and a fast automatic press will require a fast drying thinner (30 – 40 parts/minutes).
Ink Color Matching
Besides the issues of press speed, environmental influences can effect the correct mixing of pad printing inks. A hot/humid environment will require slower drying solvents, while cool dry weather may need a faster drying thinner. Your ink supplier can help with the correct mixing of inks for your specific application. However, for most situations, thinking through the process and the effect of the thinner on performance will lead to the solution.
Besides the issues that must be addressed with the proper mixing of inks, in many cases, a particular color is specified that must be matched exactly by your ink supplier. This might seem somewhat simple, but there are many issues involved with arriving at a precise color match.
Because there is such a small amount of transfer of ink, pad printing inks must be formulated with high pigment contents to improve the opacity. Opacity is a key feature when it comes to pad printing inks and matching a specific color. The color matcher must maintain a balance of pigments and vehicle to not only meet the color quality, but the durability as well. Opaque inks play a key role if the plastic part to be printed is dark or textured, or possibly both. If the ink does not include the proper opacity, the color may look quite different printed on the substrate than what was matched.
One way to battle this challenge is to print the image more than once. Multiple hits, the process of multiple ink transfer of the pad to the cliche back to the substrate, can achieve an acceptable match over dark materials. Finally, a transfer of white can be applied first to regain a brilliant appearance and utilize a more mono or transparent ink.
In the past, matching ink colors has been subjected to the color perception of the customers eye, which could be affected by the viewing conditions of the printed substrate or simply the mood of the customer on a given day. Presently, such devices as spectrophotometers have come into play to help eliminate any variances that may cause doubt of the color accuracy.
A spectrophotometer measures the light reflectance at many points on a visual spectrum, which results in what is known as a colors curve. Each color has its own curve based upon three elements – hue, value, and chroma. Hue is how the color is perceived by the human eye (red, yellow, green, etc.). Value is the degree of lightness or intensity. And finally, chroma is the vividness or dullness of the color. Utilizing the attributes of all three of these elements, a color can be accurately identified and properly matched.
With the advent of computer technology and the help of innovations such as the spectrophotometer, color matching has become more of a science. The numerical values used as guidelines or tolerance limits can be set and stored to help control color consistency from lot to lot. However, working with an experienced color matcher is still vitally important to make sure you receive a quality pad printing ink with the exact color specified by your end user.
Plastics Decorating would like to thank Robert Chadwick of Comdec Inc. (978-462-3399, [email protected]) and Dave Buchanan of Trans Tech America Inc. (630-752-4000, [email protected]) for their contributions to this article.