What Are Optimum Drying Conditions?
Question #1
We try to keep the temperature in our screen coating/drying room around 75 degrees Fahrenheit, and use fans and a dehumidifier to dry our screens. However, we've noticed that during the summer, our busy season, screens take a long time to dry.
Question #2
Also, we would like to switch to using a dual-cure emulsion, because we like the better detail and easier reclaiming, but these screens seem to take forever to dry. We have even had some get stuck down to the glass in our vacuum frame.
Solution
The optimum drying conditions for coated screens are the same, regardless of whether you are using dual-cure emulsions, diazo emulsions, photopolymer emulsions, or even capillary film for that matter.
The reason you have had a problem with dual-cure emulsions is not really that they are any more difficult to dry than other types. It's simply that prior to exposure, they have a softer surface finish than coatings made with other types of emulsion, due to the plasticizing effect of the photopolymer component.
The other factor could be excessive heat generated during exposure, which bakes the screen onto the glass, particularly when you are shooting a lot of screens and things really start to heat up. Or in your case, since you have already determined that you have a drying problem, the other factor is more likely to be residual moisture trapped in the screen. (Refer to "Determining Stencil Moisture Content" tech tip.)
Regardless of what type of emulsion you are using, residual moisture trapped in the coating prior to exposure can cause a whole host of problems. Stencils made with diazo emulsion may not be sticking to the glass in your exposure frame, but you are probably experiencing some other problems down the line. It may be excessive pinholes, premature stencil breakdown, or even poor reclaiming problems that you may not normally associate with a screen drying problem.
What are optimum drying conditions for any type of emulsion? Unless you are located in an environment like Arizona in the summertime, we would recommend that you split your coating and drying operations between two adjoining rooms. Degreased screens can still be dried in your coating room, but you should really have a separate environment to dry your coated screens. Ideally you should use a drying chamber, although how feasible this is depends on the size and number of screens you are coating.
When drying coated screens, there are two important aspects. First, from a productivity point of view, you want screens to dry quickly. Second, from a quality point of view you want the screens to dry thoroughly.
By thorough, we mean drying down to a very low equilibrium moisture content. De-humidified air, being drier, is obviously more efficient at removing moisture from your coated screens than normal room air. But by itself, it is not necessarily the most effective method. The relationship between relative humidity, temperature, and drying capacity is fairly complicated, but fortunately very well understood. Heating and air-conditioning engineers routinely use psychometric charts, which relate these varying parameters, when designing climate-controlled environments.
Thus, what we would like to do is try and explain the basis of drying in terms of these basic parameters. First, we would like to introduce the concept of vapor pressure. Vapor pressure exists when you turn a liquid into a gas. When the liquid is water, then vapor pressure is the driving force responsible for evaporation and condensation.
When you put a wet screen in a drying room, the bottom line is that in order for the screen to want to dry out, there has to be a difference in vapor pressure. A difference has to exist between the vapor pressure of the moisture in the coating, and the vapor pressure of moisture already in the air. If there is no difference, then the screen will never dry. It is the size of this difference which controls moisture flow, and is important in determining: first, how fast your screens will dry, and second, how dry they ultimately become. (We should add at this point, to beware the salesman who tells you that his emulsion has a higher vapor pressure than anyone else's!)
The vapor pressure of the moisture in a wet coating is basically dependent on temperature, and the higher the temperature the higher the pressure. The vapor pressure of the moisture already present in the air is dependent on the dew point. This is the temperature at which condensation starts to form. Removing humidity from the air lowers the dew point, and hence lowers the vapor pressure.
The question is, what is the most effective way to maximize the difference between these two vapor pressures? Is it to raise the temperature of the coating? Or is it to dehumidify the air?
Let's start with, for example, a drying room at 75°F with a relative humidity of 60%. Just say we coat some screens and leave them to dry. Now if the fans being used to dry the screens change the air in the room to prevent moisture building up and saturating the air, the screens will eventually dry. A look at some charts, and a quick calculation, tells us that we have been working with a vapor pressure difference of 0.17 psi.
Now let's add a dehumidifier to the room, and say we are able to lower the relative humidity from 60%, and hold it down to a nice dry 20% (which requires a good de-humidifier). Consulting our charts, and performing the same calculation, shows that the vapor pressure difference is now 0.34 ps. This means we have twice the drying capacity, in terms of the evaporation load that the air is able to carry away from a wet emulsion coating. Screens should dry in about half the time, and equilibrium moisture content should be lower too. If however, we allow the relative humidity to creep up, due to a heavy load of screens for example, then drying efficiency falls off quite quickly.
Instead of using a de-humidifier, let's say we decide to use a heater to warm the air entering our drying room. If the air coming in was at 75°F and 60% relative humidity, and we warm it up to 100°F, although the amount of moisture in the air hasn't changed, the relative humidity drops to around 25% since warmer air has a much greater drying capacity. We are now working with a vapor pressure difference of 0.7 psi (due to the higher temperature of the coatings), and have four times the drying capacity of the original example.
Screens will dry faster and harder with the application of a little heat, than with a lot of de-humidification. In this case, we are also better able to cope with adverse conditions, such as a heavy load of screens to be dried, or even an increase in relative humidity of the outside air.
De-humidification does start to make more sense with very warm and damp conditions. But even dealing with temperatures of 90°F and relative humidity of 75%, you still get greater drying efficiency from a 10 degree rise in temperature, than from a de-humidifier able to remove 50 grains of moisture/lb. of air.