Effects of Drying Temperature on Screen Tension Part 2

Executive Summary

This study looks at the effects of drying temperature on screen tension. The results show that drying at higher temperatures created lower tensions on screens before they were printed. To take the investigation further, a second experiment, identical in all points to the first, was performed to determine what effect printing had on the various screen and temperature combinations. Results from this second test showed that higher temperatures cause permanent tension loss on the screens tested after printing and reclaiming. Recommendations based on this finding are to keep drying temperatures as standard as possible to ensure screen tension stays consistent screen to screen.

Introduction

In an SPTF report published in 2002, the effects of drying temperature on screen tension were explored to a limited level. One question raised from the results of the study was how printing would effect the tension of the screens processed at different temperatures. The theory posed from initial findings was that the higher temperatures could possibly stabilized the screens before printing more effectively than lower temperatures.

The issue was significant enough to prompt SPTF to run another trial. Two additional sets of screens were subjected to the same conditions as the first test, and the experiment extended to include printing each screen with 250 impressions and monitoring tension through these stages. Screens were also reclaimed to see what affect that had subsequent to printing. The goal was to track the screens further in the process to see if stabilization occurred differently based on temperature.

Experimental Procedure

There were two modifications to the experiment intended to speed results while still gaining the needed information. Only the two extreme processing temperatures were tested – ambient 72-74° F (22.2-23.3° C) and 120° F (48.9° C). The dramatic comparison of these two temperature levels was sufficient to prove out any tension differences throughout the printing and reclaiming test.

The second change allowed a single tension measurement in the center of the screen to be monitored instead of five points as before (Figure 1). The decision was made to measure the center point after comparing previous data, which showed the average screen tension around 1N/cm lower than the average center measurement. Based on this, the center measurement would be able to sufficiently track the overall screen tension throughout the test, and produce reliable results.

As before, each set consisted of four screens of different mesh counts. SPTF used the mesh from the same bolt of fabric as the 2002 test for three of the fabrics, and used the same brand and specification for the fourth mesh, 380.33 (150 tpc), as the original bolt was no longer available.

Apart from these adjustments, the screens were processed exactly as in the first test (see SPTF’s report entitled Effects of Drying Temperature on Screen Tension for the detailed procedure.) From that point, the screens were subjected to the following additional processes.

Printing

After sitting overnight in room temperature, the screens were removed from the cabinet and tension was measured and recorded in the warp and weft directions. They were then printed using a semiautomatic press, 70 durometer squeegee and UV process clear ink. The stencil was fully exposed without an image, allowing us to perform an automatic wet cycle print without replacing the substrate.

Reclaiming

The ink was cleaned from the screen using a d-limonene based product and wipes, after which the tension was measured and recorded again. At this point the screen was reclaimed with the following method: (1) apply ink degradent (2) rinse with low-pressure water (3) apply stencil remover (4) rinse with a power washer. After this process, the tension was measured on the wet screen and recorded. The screen was returned to the drying cabinet for one hour, after which tension was measured on the dry screen and recorded. It was returned to the dryer for an additional hour, and tension measured a final time.

Results

The measurement results are listed in Tables 1-4 and are illustrated graphically in Chart 1-4. Each of the vertical blocks on the graphs represents 1 N/cm. The bottom axis represents the processing points where tension measurements were taken. It is important to note that the test results SPTF obtained in this study are limited in scope to the variables and procedures used in the experiment. Results may vary in actual practice, as there are many other variables that could act on tension response not explored in this study.

Several interesting observations emerge from these graphs. First and most important, the theory put to the test seems to be proven false by the experimental results. The 120° F (48.9° C) drying temperature clearly caused a greater permanent tension loss on the screens than the ambient temperature of 72-74° F (22.2-23.3° C). The difference in final screen tension between the two temperature settings ranges from 1.6 to 3.2 N/cm. The data then supports the statement that higher drying temperatures cause a slight but lasting loss in screen tension. Another interesting observation from the graphs is screen tension remains virtually unchanged before, during and after printing on all the meshes.

In fact, in comparing the tension before printing to the final tension measurement taken after the screens were reclaimed and dried, the majority of the tensions remained exactly the same. Only the 110.80 (44 tpc) screen showed a drop of 0.5 N/cm from printing. This shows that when off-contact and squeegee pressure are kept to their proper minimal levels, tension will remain stable. It is often the excessive settings of both these variables that cause tension loss on press.

The addition of the reclaiming step provided another glimpse into the mysterious tension drop when the mesh gets wet. In the original results, a dramatic tension dip was seen in the measurement taken after stencil washout.

The same dip is clearly present in these results. But in addition, a second less striking dip is now seen after reclaiming the screen (measured while still wet.) Closer examination of the graphs shows a tension dip after the degreasing step as well, just less noticeable. All these dips occur while the screen is wet. One difference between them, the larger one takes place while the stencil is on the screen and wet with water. The other two smaller dips happen with nothing on the mesh. Additionally the screen was in the drying cabinet immediately before this step, where the other two events took place after the screen was in normal room environment for some time.

Possible Tension Dip Explanation

A probable explanation for the tension dip seen after washout and reclaiming comes from Professor Steve Abbott, Research & Technical Director of Autotype International Ltd and a visiting Professor at the School of Mechanical Engineering, University of Leeds. He proposed that the 15 parts per million (ppm) / percent relative humidity (%RH) hygroscopic expansion of polyester from the water washout would explain the drop in tension that was measured while the screen was wet versus the returning of the tension after the screen was dry.

Mathematically, the theoretical drop in tension can be calculated directly from the percent expansion, the modulus of the polyester fibers, and the effective thickness (corrected for mesh open area) of the mesh, and is sufficient to explain the observed results.

To prove this he performed a simple test. On a 305.34 (120 tpc) mesh tensioned to 22 N/cm, he was able to repeatedly get the tension to go down by 0.5 N/cm by wiping the mesh with a damp cloth. Subsequently, by drying either in an oven or in a blast of room temperature air the tension returned to its previous level.

The bigger dips seen in the meshes processed at higher temperatures can be accounted for by the fact that higher temperatures create lower percent relative humidty (the 120° F (48.9° C) test produced a RH of 0%.) So the change in %RH when going to washout is bigger and therefore the change in tension is bigger.

The Delta or difference in %RH is 65% for ambient, 80% for 95° F (35° C) and 100% for the 120° F (48.9° C) temperature. So the drop in tension should be 100/65=50% bigger for 120° F than for ambient, and 100/80=25% bigger for 120° F (48.9° C) over 95° F (35° C). Roughly speaking both sets of data support this.

In the case of the degreasing step and reclaiming step, the meshes hadn't been subjected to the lower percent relative humidity so the drop in tension is less marked. While there may be some other factors at play here, the hygroscopic expansion seems to explain things pretty well.

Possible Explanation for Tension Loss from Heat

In regard to the permanent tension loss from processing screens at higher temperatures, an explanation is a little less clear, but Professor Abbott again offers some thoughts. The mere temperature of 120° F (48.9° C) should not drastically affect the polyester, which does nothing much until it reaches 176° F (80° C). So a simple sagging of the mesh due to heat is not a plausible explanation.

However, in looking at the thermal expansion of the elements involved, there may be a possible answer. If we look at the mesh, the thermal expansion coefficient (in degrees) is approximately the same as the hygroscopic contraction effect (in percent relative humidity). So the thermal expansion is more or less exactly offset by the hygroscopic contraction. But, add the thermal expansion of the aluminum frame, which has a similar coefficient of expansion to polyester, and it is possible that the frame causes the mesh to go up in tension slightly, and subsequently sag on cooling.

Effective Screen Drying Principles

The ability to quickly and effectively dry screens impacts productivity and print quality. Of course, it can often create a bottleneck in the process if not completed quickly enough to meet screen demand in production. Shortcuts taken to ‘get the screen to press’ often lead to improperly dried screens being exposed, ultimately leading to a whole host of problems on press. Consequently, the so-called shortcut actually becomes a long costly delay.

There has been a lot of misunderstanding about the optimal conditions for drying screens, so it is worthwhile to cover the main principles we must abide by. There are three issues that interact to create the ability of an environment to dry screens; temperature, percent relative humidity and air flow. Progressively warmer air will absorb and carry more moisture than cooler air. Air that is drier, i.e. lower percent relative humidity, can absorb a greater amount of moisture before becoming saturated because it has less moisture to begin with. Once these two conditions are in place, good airflow can speed things along by replacing the moisturesaturated air with drier air that can pull more moisture from the material being dried. Obviously to increase air temperature a heater must be used.

Evenness of the heat is also important. With homemade cabinets utilizing a space heater, it is easy to create hot spots and wide temperature swings inside the cabinet. The heater must be directed properly inside the cabinet to minimize these effects. No matter what the drying situation, it is wise to check the temperature uniformity by takeing several measurements in different areas in the room or cabinet. Relative humidity is the ratio of the actual amount of moisture in the atmosphere to the amount of moisture the atmosphere can hold. At a relative humidity of 100%, the air can hold no more water. Temperature and relative humidity are linked.

As temperature increases, its ability to hold more water (humidity) increases. So as the temperature in a room goes up, the relative humidity will go down (with all else being equal). Once the maximum drying temperature is reached, the only way to reduce relative humidity is condense it out with a dehumidifier. This small investment should be standard equipment in every screen room. Effective airflow will exchange the air that is loaded with water from the drying/evaporating process with fresh dry air across the drying surfaces.

Another consideration with airflow is the screen spacing. Increasing the space between coated screens will allow more air to move across them, and speed drying. If they are too close together the air cannot circulate and drying times will be extended. Air circulation is often equated to using a fan, usually caked with dirt, directed at the screen. In this case, instead of airflow, you create a dirt blower that will create pinholes in great quantity. In addition, unless a fan is set up to move new air in to replace the ‘wet’ air, it will just blow the same humid air around which will do little to improve drying times.

Creating the proper airflow conditions can be much more effective than just increasing temperature or reducing humidity to dry the screens. Controlling these three factors is best done with an enclosed drying cabinet, whether purchased or home made. A good cabinet will have an accurate thermostat, with tolerances of no more than plus or minus 5° F (2.8° C) to control the heat. Additionally, enclosed cabinets need a good airflow system. If airflow is not sufficient, the moistureladen air is simply is trapped inside the enclosure and the screens will not dry. The intake on such cabinets should draw dry air in and an exhaust should move the wet air out of the space.

Drying cabinets can also be made to filter the air circulating over the wet emulsion, reducing dust contamination. As a final point, the drying environment should be fairly consistent so repeatable exposures are possible.

If the conditions vary drastically, the screens will not be dried to the same point and may not expose properly, potentially causing problems on press. Purchase a gauge that measures both temperature and relative humidity to monitor the conditions in the drying chamber or room. There are remote sensor gauges that can be put inside the drying cabinet while the display is outside if you have an enclosed system.

So to pull this all together, let’s relate these conditions to the common activity of drying one’s hair. If you just let your hair air dry in room temperature, it dries slowly based on the conditions around you. Put a dehumidifier in the room, reducing the relative humidity, and your hair will dry a little quicker. If you turn up the thermostat and heat the room up, the warmer air will dry your hair a bit faster. However, if you blow hot air on it with a hair dryer, your hair dries very quickly because there is a constant exchange of warm less humid air in place of the air that contains the evaporated moisture. The bottom line is the drier you can make the air, and the quicker the wet air is removed, the faster your screens will dry.

Conclusions

Tension is indeed one of the most critical variables in the screen printing process. Raising drying temperatures to excessive levels comes at a cost, not only from stencil performance, but as this study has shown, from screen tension loss as well. We must weigh the need for speed with quality and make equipment and procedure decisions accordingly.

Consistency is also important. If processing variables change from day to day it can negatively affect our repeatability and quality. Paying attention to processing details, such as this, pays dividends of reliability, high print quality and efficiency that can set you apart from your competition.

Recommendations

Based on the results of this study, the following recommendations are suggested.

1. Keep drying temperature as standard as possible so screen tensions will stay consistent screen to screen.
2. Dry screens in 86°F-104°F (30°C-40°C) to maintain screen tension and stencil reliability. While higher temperatures facilitate drying, they also seem to permanently reduce screen tension, so use a lower temperature if possible.
3. Screens should be removed from the heated drying area when they are dry. Storage in high temperatures for two hours or more will degrade the emulsion sensitizer and impair exposure. Dry coated screens should either be exposed immediately or stored in a separate dry area where no wet screens are placed. The addition of wet screens to the area will raise the humidity and create the potential for the dry stencil to reabsorb moisture. It is best to dry and store coated screens in a different area than screens being dried after degreasing, washout and reclaiming.
4. Ensure that airflow throughout the drying area is effective. Create an air exchange system where dryer, less saturated (humid) air is cycled in and the more humid and saturated air is exhausted out. Increasing the spacing between the stacked screens can also allow for quicker drying.
5. Use a dehumidifier to reduce the moisture in the air to a relative humidity of 40% or lower. The more moisture you can extract from the air the faster the screens will dry.
6. Buy a gauge that measures both temperature and relative humidity to monitor the conditions in the drying chamber or room. Record these values each day on a tracking chart to verify continued effectiveness and consistency of the system.

The information and recommendations contained in this report are believed to be reliable and accurate. The authors and publishers make no warranty, guarantee nor representation as to the correctness of this information for any given purpose nor do they assume any responsibility for the use of information presented here, or for results obtained or not obtained, and hereby disclaim all liability in regard to such use and/or results.