|
|
 |
WHY IS THE STENCIL
BREAKING DOWN DURING PRINTING ? |
| |
|
|
| |
 |
| EMULSION
IS UNDER CURED |
| |
Light source needs checking - old
or broken.
Old emulsion.
Sensitiser not mixed into the system correctly.
Not exposed enough — used exposure calculator. |
| INCORRECT
COATING TECHNIQUE |
| |
Not enough emulsion coated onto the
screen, it may need an extra coat on the print and/or
squeegee side.
Not dried print side down.
Emulsion not 100% dried before exposure. |
| INCORRECT
CHOICE OF EMULSION |
| |
Emulsion too fast or too slow.
Emulsion not resistant to the system being printed —
water / solvent.
Solid content of emulsion not high enough. |
| SQUEEGEE
PRESSURE TOO HIGH |
| |
Reduce pressure.
Change screen to higher count.
Reduce viscosity of ink.
Check that open stencil or image completely washed out
(no emulsion left in mesh)
Mesh tension too low. |
| FLOOD
PRESSURE TOO HIGH |
| |
Reduce pressure.
Change screen to higher count.
Reduce viscosity of ink.
Check that open stencil or image completely washed out
(no emulsion left in mesh)
Mesh tension too low. |
| LOW
MESH TENSION |
| |
Change the screen |
|
|
|
 |
|
WHY IS THE STENCIL
BREAKING UP DURING WASHING OUT ? |
| |
|
| EMULSION
UNDERCURED |
| |
Light source needs checking - old
or broken.
Old emulsion.
Sensitiser not mixed into the system correctly.
Not exposed enough - used exposure calculator. |
| INCORRECT
CHOICE OF EMULSION |
| |
Emulsion too slow. |
| WATER
PRESSURE TOO HIGH |
| |
Reduce pressure.
Wet out screen and allow to stand for a few minutes. |
|
|
|
|
|
WHY IS THE STENCIL
NOT WASHING OUT OF THE IMAGE AREA ? |
| |
|
| EMULSION
OVERCURED |
| |
Emulsion too fast.
Incorrect exposure time — check using an exposure
calculator. |
| POSITIVE
DIFFERENTLY PRODUCED |
| |
Exposure times must be modified
depending on the type of positives are used, i.e velum
and clear polyester film will require different exposure
times.
Positive Density needs to be enhanced. |
| INCORRECT
CHOICE OF EMULSION |
| |
Emulsion too fast or too slow.
Emulsion not resistant to the system being printed - water
/ solvent.
Solid content of emulsion not high enough. |
| WATER
PRESSURE TOO LOW |
| |
Increase water pressure.
Wet out screen and allow to stand for a few minutes |
|
|
|
|
|
WHY IS THE STENCIL
NOT WASHING OFF DURING RECLAMATION OF SCREENS ? |
| |
|
| EMULSION
POST HARDENED |
| |
Emulsions can be post treated
with an acid catalyst to create a permanent screen. |
| STRIPPER
CHEMICAL INEFFECTIVE |
| |
Stripper chemical too diluted
— add more active agent or discard.
Stripper chemical contaminated — discard.
Residual ink / tape / contaminants preventing the action
of the stripper. |
| STRIPPER
ALLOWED TO DRY ON SCREEN |
| |
All stripper chemicals are
acidic and will act as a post hardening agent. |
|
|
|
|
|
HOW DO I PREVENT
FISHEYE ? |
| |
|
| EFFECTIVE
SCREEN CLEANING SYSTEM |
| |
Residual cleaning chemicals
left in the screen are the main cause of fisheyes.
Use a complete system without missing steps.
Ensure thorough washing out of the screen with water after
the final degreasing step.
Screens must be dried horizontally and not vertically
so as to avoid drops dripping off the frame onto the mesh.
Screens must not be exposed to contaminants or touched
after degreasing. |
| DUST
FREE DRYING ENCLOSED AREA |
| |
Reclaimed screens must be stored
away from a general traffic area in a dust free enclosed
area. |
|
|
|
|
|
HOW DO I PREVENT
PINHOLES IN THE SCREEN ? |
| |
|
| DUST
FREE DRYING ENCLOSED AREA |
| |
Reclaimed screens must be stored
away from a general traffic area in a dust free enclosed
area. These pinholes are usually irregular in shape and
only appear during the washout phase or during the print
run. This is due to the dust particles being “forced
out” by the water pressure, flood and/or squeegee
stroke. |
| CLEAN
AND DUST FREE GLASS ON EXPOSURE UNIT |
| |
Majority of pinholes originate
from dirty glass. They are characteristic in that they
are well defined and clean holes in the stencil area.
|
|
|
|
|
|
HOW DO I PREVENT
STREAKS IN THE EMULSION ? |
| |
|
| DAMAGED/DIRTY
COATING TROUGH |
| |
The main cause of streaks in
the emulsion coating, ensure that coating troughs are
properly stored so that the edge does not get damaged.
All burs must be removed with steel wool and not sandpaper.
Wash coating trough immediately after use, remove residual
dried emulsion with stripper chemical.
Replace trough if damage is excessive. |
| EFFECTIVE
SCREEN CLEANING SYSTEM |
| |
Residual cleaning chemicals
left in the screen or that have dripped onto the screen
during drying.
Ensure thorough washing out of the screen with water after
the final degreasing step.
Screens must not be exposed to contaminants or touched
after degreasing.
Screens must be dried horizontally and not vertically
so as to avoid drops dripping off the frame onto the mesh.
|
| EMULSION
PARTICULATE SKINS |
| |
Emulsion that has been left
open will dry out and form small to large dried out particles
that act as a bur at the edge of the coating trough when
coating screens. Emulsion must be screened through a filter
to remove dried particles or discarded. |
|
|
|
|
|
HOW DO YOU PREVENT
UNDER EXPOSING THE SCREEN ? |
| |
|
| CHOICE
AND CARE OF EMULSION |
| |
Emulsion too slow.
Exposure times too short. Recheck exposure times using
exposure calculator.
Application of too much emulsion — reduce the number
of coats to the screen.
Solid content of emulsion too high for coating method
on mesh count.
Screens must be dried horizontally and not vertically
so as to avoid sagging of the emulsion towards the bottom
end of the screen.
Screens must be effectively dried in a dust free environment. |
| LIGHT
SOURCE FAULTY / INSUFFICIENT |
| |
Check that all the bulbs are
operational.
Recheck exposure times using an exposure calculator remembering
that this must be correlated to a specific mesh count.
Reduce distance between light source and coated screen. |
| SCREEN
TENSION |
| |
If the screen tension is too
low, the amount of emulsion deposited in the middle of
the screen will be thicker than that towards the edges
of the screen. This will result in slightly under cured
emulsion in the middle of the screen and over exposed
towards the edges. |
|
|
|
|
|
HOW CAN I PREVENT
OVER EXPOSING MY SCREENS ! |
| |
|
| CHOICE
OF EMULSION |
| |
Emulsion too fast.
Exposure times too long. Recheck exposure times using
exposure calculator. |
| SCREEN
ENVIRONMENT |
| |
Screens stored for very long
time after coating - age hardened chemically due to “dark
reaction phenomenon”.
Screen storage area not properly light safe or darkened
effectively.
General working lights not safe - emulsion is being partially
exposed. |
| LIGHT
SOURCE |
| |
Light source choice not compatible
with the emulsion in use — light source is too intense. |
|
|
|
|
|
WHAT DO I DO TO
PREVENT UNEVEN STENCILS ? |
| |
|
| MESH
TENSION |
| |
If the screen tension is too
low, the amount of emulsion deposited in the middle of
the screen will be thicker than that towards the edges
of the screen. This will result in slightly under cured
emulsion in the middle of the screen and over exposed
towards the edges.
Change your frames to retensionable frames to ensure a
constant good tension. |
| COATING
METHOD AND EQUIPMENT |
| |
Coating should be applied evenly
from top to bottom one side of the frame and then turned
around and upside down so that your coating is even.
The coating trough edge must be checked to see if it is
even and has not been sandpapered evenly.
Screens must be dried horizontally and not vertically
so as to avoid sagging of the emulsion towards the bottom
end of the screen. |
|
| |
|
|
