One can have the most advanced computer systems with updated graphics programs, perfect art, flawless separations, tight, cleanly-stretched screens and perfect print practices on press, but it is all for nothing if the ink doesn’t cure properly on the shirt.
The truth is, the last piece of equipment in the screen print process is, without a doubt, the most important. The ink has to cure; it’s the final step to seal all that hard work. If it fails, the job fails. It truly makes or breaks the job.
The oven’s task is to create an environment in which the ink will reach a recommended cure temperature. This sounds simple enough, but great care and monitoring is required to keep this piece of vital equipment in check. Monitoring the ambient and ink temperature during the curing process is the only way to ensure that the oven is doing its job correctly and efficiently. By keeping a record of cure curves and oven readings, one can be assured that the ink is reaching cure, is not over-cured and that the dryer is running correctly.
Doing so requires the use of temperature monitoring tools available in the marketplace. The most common tools that are available are temperature tapes, a laser gun and a thermo-probe (also known as a donut-probe). These tools are important, but equally important is how they are used.
It’s vital to know what the measurement data gathered from these tools means. The sole purpose of monitoring is to assure that the ink is reaching a recommended temperature at the “point of contact” with the fabric. Ambient, belt and surface temperature readings are only relative to the actual cure point. With this in mind, let’s look at the recommended use of these tools.
Temperature strips are designed to darken as they hit certain temperature ceilings. For instance, when a strip has a range of 320°F to 400°F, the strip will darken up to the point of which temperature is reached. This is a good measurement, but keep in mind the important “point of contact” is where the ink touches the fabric. It is best to either place the tape on the back side of the fabric in a spot where the ink is actually printed. Or, print the ink directly onto the tape that is attached to the top of the fabric. Once the garment makes it through the oven, the tape can be checked on the underside of the garment or by scratching away the ink to view the reading. The downside to this tool is its inability to give you real-time exposure curves. It does not indicate how long the ink has been exposed to the displayed temperature.
Laser temperature gun
This tool has one function: reading surface temperature. This makes it is an excellent tool for reading flashes or platen temperatures. It does not, however, read ink cure temperatures. In regards to cure temperature, the best use for this tool is as a quick gauge for a relative surface reading. That is, once the oven settings to reach cure have been established with the use of the temperature strips or a thermo-probe, the gun can read the surface temperature of garments exiting the oven.
If the ink reaches a full cure of 325°F, chances are the surface is hitting a temperature of 340°F or higher. By knowing what the surface temperature should be, the gun can assist in some quick monitoring of the ovens during the day. If its readings change, it is a good idea to use the probe or the strips to double-check the output.
Thermo-probe (donut probe)
This tool is the most efficient and accurate way to test cure temperature. Not only does this equipment record the temperature, it also allows the user to record time increments during monitoring. To complete an ink’s full cure curve analysis, you’ll need two people, a thermo-probe, a stop watch, and pen and paper.
One person holds the stop watch and thermo-probe unit, while the other is ready to write down the temperature readings. The cross-hairs of the thermo-probe donut are placed face-down directly into a wet ink film/print. As the garment and donut enter the oven, the first person watches the stop watch. At five second intervals, this person calls out the temperature and the second person writes the information down. This continues until the garment and donut exit the oven.
Create a graph with a temperature axis on the side and a time axis on the bottom. Mark the temperature points in the appropriate space on the graph. Once the points are created, draw a line to connect the dots to show the curve.
The important information to look for in this curve is what temperature was reached, the amount of time that the ink temperature is above the cure temperature, and consistent nature in which the curve bends. Of course, if the ink never reaches the cure level, the oven either needs to be slowed down or the temperature needs to be increased. Conversely, if the ink is in the cure level too long, the belt can be sped up or the oven turned down. Either way, this puts the oven exactly where it needs to be to cure ink without wasting energy.
An uneven or stair-step curve can be an indication of a faulty oven. This can tell the operator that either a blower is going out or an element is failing—which makes this a very good preventative diagnoses for the machine. It is recommended to repeat this procedure throughout the week to keep good records. Also, a change in weather can affect the readings, so it is good to know how the oven is performing during those times.
Oven mapping is a very easy way to check the performance of the oven and to assure good cure of inks. The peace of mind aspect alone is worth the trouble and knowing the limits and abilities of the oven gives confidence that the ink is cured correctly, efficiently and permanently.