Direct Stencils (Capillary & Emulsions)
Fault finding guide
1
Detail filling in
2
Detail filling in
Cause
Remedy
•
•Use dyed mesh and increase exposure
Use of white mesh
UV light
UV light
3
Detail filling in
Cause
Remedy
•
•Improve vacuum
•Check positive is right reading
•Ensure backing sheet removed
Poor contact between positive and
stencil
Positive in good contact
with the stencil
Undercutting caused by
positive in poor contact with
the stencil
Positive
UV light
UV light
4
Fine detail filling in
Cause
Remedy
• Light undercutting on high Rz
stencil
•Use Capillex
•Use higher solids stencil
•Add additional wet on dry coats
Positive
Positive
UV light
UV light
5
Detail filling in
Cause
Remedy
•
•Increase lamp distance
•Reduce reflected light
Poor light source geometry
UV light
Light more perpendicular to the stencil
6
Detail filling in
Cause
Remedy
•
•Ensure positive and stencil are clean by
using a clean room wipe or Teknek roller
Dust between positive and stencil
Dust
Positive
UV light
7
Detail filling in
Cause
Remedy
•
Over exposure
•Reduce exposure
•
Poor wash out
•Many stencils benefit from a final wash
with a high pressure gun at 1m distance,
especially water resistant stencils
No resolution improvement at different exposures indicates a washout issue
8
Detail filling in
Cause
• Diazo decomposed by heat or
age
Good diazo
Remedy
•Ensure diazo is stored between 3oC
and 20oC and is within 2 year shelf
life
Badly decomposed diazo
9
Difficult to washout
Cause
Remedy
•
Gross over-exposure
•Reduce exposure
•
Excessive drying temperature
•Dry coated screen at max. 40°C
•
Coated screen fogged by light
•Store coated screen in dark or
under safelight
•
Low density positive
•Re-make positive
10
Pinholes - Emulsions
Cause
Remedy
• Air entrapment during coating •Ensure emulsion fills the mesh
during print side coatings
• Air entrained in the emulsion
•Ensure the pot has fully degassed
before coating
• Air developing in the trough
•Reduce coating speed
• Bits in the emulsion
•Metal stirrers gouge the sides of
the pot on sensitising. Use the
wooden stirrer provided
11
Poor Adhesion
Cause
Remedy
•
•Dry coated screens thoroughly.
•Do not dry unexposed screens in the
same cabinet as wet screens
Inadequate drying prior to exposure
Strong stencil
with all diazo
sites cross
linking PVOH
Correctly dried
stencil
Diazo
UV
Weak stencil
with some
diazo sites
reacted with
water
Underdried stencil
12
Poor Adhesion
Remedy
Cause
•
•Use Autotype exposure calculator to
find correct exposure time
•Ensure has correct UV output and lamp
has not aged
Under-exposure
Intensity vs Wavelength Metal Halide Lamps
2000
1800
Newer lamp
Older lamp
1600
1400
Intensity
1200
1000
800
600
400
200
0
300
320
340
360
380
400
420
-200
Wavelength
13
Poor Adhesion
Cause
Remedy
•
•Increase exposure time
Under exposure
1/4 optimum exposure
EFFECTS: During washout the image would appear
and the stencil would probably start to come away
during washout.
1/2 optimum exposure
EFFECTS: The stencil would probably be damaged
by a strong washout spray. The emulsion appears
soft on the squeegee side.
3/4 optimum exposure
EFFECTS: Emulsion appears slightly soft on squeegee
side. Stencil will scum of not washed out properly.
Durability is compromised. Stencil is harder to decoat.
Optimum exposure
EFFECTS: All the emulsion is fully hardened and the
stencil will provide the optimum resistance and
durability.
14
Poor Definition
Cause
Remedy
•
Poor edge quality of positive
•Remake positive
•
Incorrect exposure
•Expose for optimum time
•
Use of white mesh
•Use dyed fabric
15
Mesh difficult to reclaim
Cause
Remedy
•
Under-exposed stencils are more
difficult to reclaim
•Increase exposure to optimum
•
A and B Hardener used
•If reclaim is required do not use A and B
Hardener
•
Stripper incorrect concentration
•Check stripper is correct concentration 24%, for very water resistant stencils err on
the higher side, so 1:25 stripper
concentrate should be used at 1:15
dilution
•
Stencil fusing
•Change solvents used with the stencil
16