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| September 2004 issue of PRINTWEAR |
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How to Build a Screen-Drying Cabinet
by Douglas Grigar, Master Screen Printer
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(A version of this article originally appeared
in the September 2004 issue of PRINTWEAR.)
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Just like the
proverbial red-headed step-child, the screen-drying cabinet seems to be
the single most forlorn and forgotten piece of equipment in the plant,
and that is a shame. Drying the screens after degreasing or coating are
such important steps in the process, yet I am constantly surprised that
such a necessary and available tool remains ignored in both equipment
acquisition and shop use.
A screen-drying cabinet - a real
drying cabinet, that is, not an unused office, an unvented shower tub,
or a closet, but a drying cabinet built and dedicated solely for this
use, designed to prevent contamination and to dry screens as efficiently
as possible - is just as important to screen printing as the emulsion
you choose or the press on which you print.
Needs and Specs
After suffering for years with the same
contamination-prone methods as many others, I must admit I have become
a diehard drying cabinet evangelist. I can think of no more efficient
and clean way to process screens from wet to dry in as short a time
period.
So what makes a drying cabinet as important
as a press or an exposing unit? The simple answer is that we are talking
about screen printing, and no single factor controls more of the printed-product’s
variables than the quality of the screen. What’s more, no subsequent
step in the process can correct a problem with the screen’s stencil.
What should a drying cabinet provide? Manufacturers
suggest that the best drying environment will have temperatures from
80º and 90º F (and no higher than 105º) with a clean-filtered
air flow and a relative humidity of 50% or less. A well-built screen-drying
cabinet should provide these optimum conditions for drying emulsion
on screen mesh.
Three Types of Drying Cabinets
“Open” or
“open-circuit” describes a cabinet wherein
the air is drawn into the cabinet (see Figure 1-A), optionally heated
(Figure 1-B), slightly pressurized (Figure 1-C & 1-D), pushed past
the screens (Figure 1-D, 1-E, & 1-F), then vented from the cabinet
(Figure 1-H). Moisture-laden air should be vented outside of the room
in which the cabinet resides. Open-circuit cabinets are the simplest
and least expensive to construct, but depend on high air flow for speed
drying. Additionally, open-circuit cabinets will not include the enclosed
pathway that contains the items shown in Figure 1-G,1-I, and 1-J, or
the return vent marked 1-K.
“Closed” or
“closed-circuit” cabinets have circulating
air inside of a completely sealed cabinet where internal air is passed
through a dehumidifier (see Figure 1-I), past the screens (Figure 1-D,
1-E, & 1-F), and returned (Figure 1-G). A closed-circuit cabinet
introduces no outside air after the cabinet is sealed (Figure 1-A),
depends on a dehumidifier to dry the screens, and has no external vent
(Figure 1-H). Without a dehumidifier, the closed-circuit cabinet retains
the moisture, thereby increasing the drying time. Closed-circuit cabinets
must be carefully sealed to prevent contamination.
“Semi-closed”
or “semi-closed-circuit” cabinets combine
the best of both types of open and close cabinets by introducing small
amounts of outside air (see Figure 1-A) into the cabinet. Semi-closed
cabinets circulate air around the screens (Figure 1-D, 1-E, & 1-F),
dehumidify the moisture-saturated air (Figure 1-I), circulate this dry
air into the heating area (Figure 1-J, 1-K, 1-B, & 1-C), and past
the screens again, venting small amounts of moisture-saturated air out
of the cabinet (Figure 1-H). Forcing filtered, outside air into the
cabinet (Figure 1-A) creates an internal positive air pressure that
prevents contaminated, ambient air from leaking into the cabinet.
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Construction Concepts
Assuming a knowledge of simple carpentry
and electrical wiring, allow me to review the basic construction concepts,
discuss a few tips, and offer some ideas that can get you started on building
this important part of your equipment assets.
The simplest of drying cabinets can be built
of left-over large sheets of cardboard, a few thumbtacks, staples, and
a glue gun, attaching a small fan with an air-conditioner filter as an
air intake. Building a simple, temporary, makeshift cabinet may take less
than an hour to construct, but unfortunately, it won’t last very
long or be very efficient.
As the goal of a screen-drying cabinet is
for dry, filtered air to flow over the screens evenly (see Figure 1-D,
1-E, & 1-F), a proper intake fan, and a quality air filter is required.
One (or more) inexpensive bathroom ventilation fans can serve this purpose
and are manufactured to run safely in moist environments.
Warming the air to raise its moisture-holding
capacity will require a heating element (see Figure 1-B). Great care should
be taken to shield the heat from any combustible materials to prevent
fires. Follow the instructions included with the heating unit for specific
safety considerations. Always err on the safe side and opt not to heat
the air if you cannot construct the unit in a way that will prevent hazards.
When heating the air, make sure there is
a baffle separating the heating area from the screens that will slightly
constrain the air flow. A constraining baffle will not only build air
pressure and prevent hot air from concentrating on one area of the screens,
but will also create a more even flow of air (see Figure 1-D).
Air movement in a closed or semi-closed cabinet
should not be greater than one complete exchange (movement of the total
square footage inside the cabinet) every two to five minutes. An open-circuit
cabinet, without the benefit of moisture removal from a dehumidifier,
requires much higher air movement.
Vents should be placed where air flow will
prevent hot, moisture-laden areas from forming inside the cabinet (see
Figure 1-H). Bare, untreated wood products will be damaged by continuous
exposure to moisture, so should be painted with a high-quality exterior
paint or be otherwise sealed. Tip: Cabinets keep out contaminates
more effectively when all corner joints are glued and/or caulked, and
doors and intakes are well-sealed.
Final Details
Screens are best stacked horizontally, in
a way that allows air to circulate freely around and past them (see Figure
1-D, 1-E, & 1-F). It is a good idea to always load your cabinet from
the top down and unload from the bottom up to prevent any dislodged contaminates
from falling onto the screen below.
Cabinet frames can be constructed of standard
lumber. An added benefit here is that vent fans are typically made to
fit the cavities created by typical placement of normal lumber products.
Additionally, most open areas created by standard lumber can be used for
air-travel ducts.
Shelves constructed for screen racks should
provide a stable platform for the horizontal placement of screens and
be constructed in a way that helps prevent drying emulsion from cementing
the screen to the racks.
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Three types of holding-rack shelves can be
built cheaply and quickly with materials available from lumber retailers
or home-improvement stores. The first rack-shelf construction
is a simple downward-sloping shelf made of wood or metal (see Figure 2-A).
Shelves can be connected with available metal bracing pieces or angled
slots cut into the rack frame that will fit the shelf material. The second
shelf option is to attach PVC plumbing pipe or metal conduit tubing to
the rack frame, creating a quick platform for screens to slide in to (Figure
2-B). The third type is constructed of downward-slanting
removable dowel pins or rigid tubing mounted into holes in the frame (Figure
1-C). This type of platform will create four very small contact points
to the screen frame and allow for dowel-pin replacement in various lengths
to fit a variety of screen frame sizes.
Creating a door with
a seal will require a compressible foam or rubber gasket on the entire
cabinet-facing edges of the door. An alternate method is to construct
a vertical or horizontal sliding door that sits in grooved brackets to
create an overlapping seal on the edges.
Store-Bought or Home-Made
If you have the tool skills and know-how,
or choose to hire a wood worker, you can construct a cabinet that will
increase stencil quality, cut your drying times drastically, help prevent
contamination problems such as imbedded particles or pinholes, and save
you enough time to cover the cost of the cabinet in just a few screen-drying
cycles. Alternately, a ready-made screen-drying cabinet purchased from
one of several industry suppliers will also pay for itself in improved
screen-making efficiency in not much more time. The benefits, in either
case, will last the life of the cabinet.
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