Member of the Academy of Screen Printing Technology

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Edwina Barber
5 years ago
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1 Member of the Academy of Screen Printing Technology

Printable Coating Technique : EOM/Rz Mesh/Tension Ink/Substrate/Squeegee

2 Durable Stencil Selection (Resistance) Exposure Accurate Stencil Selection (Resolution) Film Quality Exposure & Washout Variables Printable Coating Technique : EOM/Rz Mesh/Tension Ink/Substrate/Squeegee Consistent Repeatable Procedures/Process Trained Personnel Maintained/Adjusted Equipment

3 Offset 85 line 133 line 150 line 1-2 microns Flexo 3-6 microns Screen 8-10 microns

4 Smaller Thread Diameter = Thinner Mesh Thickness = Less Mesh Interference = Better Ink Transfer = Higher Resolution

5 10% Emulsion Over Mesh (EOM) ratio is the general industry recommendation, but proof is in the print. Example: Fabric = 40 microns X.10 = 4 microns EOM EOM adds ink deposit height to small images i.e. halftone dots.

6 40 microns 80 microns The finest detail should be twice as wide as the total printing thickness. Why highlight dots are so difficult to clear. Must Consistently Print the Dots!

7 Ink transfer becomes more difficult when the surface area of the screen height is more than the surface area of the detail diameter. Thinner Stencil & Mesh More Detail

8 Dot Skipping Black Skipping Cyan and Black Skipping

9 Dot Stacking

10 Ink Flow Rough Stencil Surface Smooth Stencil Surface

11 Too thin Correct Too thick Squeegee Direction

Face coating fills in the stencil valleys and

Use a sharp edge coater to face coat the

12 Face coating fills in the stencil valleys and scrapes over the hills, producing a smoother stencil with minimal additional stencil build-up. Use a sharp edge coater to face coat the substrate side of the screen, after the initial coating is dried. Face Coating Dried Base Coating

13 87 lpi Film Face Coats Face Coats Screen Printing Technical Foundation 2007

14 Screen Printing Technical Foundation 2007

15 Screen Printing Technical Foundation 2007 Face Coating Adds Only Minimal EOM

16 Screen Printing Technical Foundation 2007 Face Coating Lowers Stencil Rz

17 Wet on Wet Face Coating

380/31 PW Mesh : Initial Coats 2-2 : Printed with 75/90/75 Squeegee Film 74 lpi 10%, 25%, 50%, 75%, 90% 0-0 Face Coat EOM = 3.4 Rz = 11.

18 380/31 PW Mesh : Initial Coats 2-2 : Printed with 75/90/75 Squeegee Film 74 lpi 10%, 25%, 50%, 75%, 90% 0-0 Face Coat EOM = 3.4 Rz = Face Coat EOM = 4.0 Rz = Face Coat EOM = 4.1 Rz = Face Coat EOM = 3.9 Rz = 7.2 Screen Printing Technical Foundation

19 Squeegee Side Face Coating Reduced Squeegee Wear Reduced Squeegee Friction More Uniform Ink Shear CAUTION : Adds EOM

20 Coating Trough Fill Level Hydrostatic Pressure

image reproduction Even more critical for fast exposing emulsions Quick test: Expose 50%

21 Uniformity of light hitting stencil area Based on lamp type, distance & placement, reflector design and lamp age Great enough inconsistencies impact stencil exposure and image reproduction Even more critical for fast exposing emulsions Quick test: Expose 50% halftone pattern film in entire image area and print screen to check for uniform dot size/shape

22 Map exposure light profile Identify hot spots in exposure area Fine tune lamp placement Serve as independent integrator Monitor lamp life UVA Spectral Response UVV range useful for SBQs

Read radiant energy density (dose) for 60 unit exposure (where 60 units is approximately 60

23 1. Sectionalize exposure area based on size 2. Warm lamp prior to test and ground vacuum frame 3. Attach probe to glass in center and draw vacuum 4. Read radiant energy density (dose) for 60 unit exposure (where 60 units is approximately 60 seconds.) 5. Repeat 5X in center and average for baseline 6. Measure remaining sections 7. Divide each reading by the center baseline average number.

24 Acceptable range: % max variance in stencil area Note emulsion exposure latitude Screen Printing Technical Foundation 2004

Hot spots can cause halftone highlight dots to close-up from overexposure, resulting in color shifts on the print. Improper lamp distance can cause a hot spot in the center.

25 Hot spots can cause halftone highlight dots to close-up from overexposure, resulting in color shifts on the print. Improper lamp distance can cause a hot spot in the center. 3 KW Exposure Unit 36 x 48 Radiant Energy Density (Dose) in mj/cm 2 Series1 Series2 Series3 Relative Percentages Center = 100% Series Series5 Screen Printing Technical Foundation 2004

26 90 Units of Integrated Exposure 100% of Light Intensity 74% of Light Intensity 33% of Light Intensity 380/34 with 18 micron Capillary Film

27 Vacuum Pressure in. Hg of pressure More is not necessarily better Vacuum Blanket Type of material Condition : holes, age, buildup Use cord for speeder air evacuation

28 Newton Rings Vacuum Draw Time Effected by blanket condition (leaks), size of area, etc. Allow full draw before turning on lamp

for safety Low-iron tempered float glass good option 81%-88% UV Transmission Mark

29 Glass UV Transmission Rating Seed-free and inclusion free glass Verify high UV Transmission rating the higher the number the more UV light Tempered vs plate glass for safety Low-iron tempered float glass good option 81%-88% UV Transmission Mark Coudray, Screenprinting, 2007 Use Graphics Grade Glass Cleaner No Ammonia No UV blockers

need for a tonal curve for most screen printed lpi So if you need a curve anyway, why not

30 Intentionally underexposing the stencil to open up the highlight tonal areas The simple truth: You cannot open up the small highlight to mid-tone dots enough to eliminate the need for a tonal curve for most screen printed lpi So if you need a curve anyway, why not expose the stencil for full durability and use more curve to adjust the highlights and mid-tones?

31 Post Exposure: SBQs benefit from 1-2X post exposure on squeegee side Dual Cures require 6-10X original exposure time to improve resistance by 15% Diazo not effective at all But how can you post-cure underexposed emulsion that has been rinsed away during washout?

32 80% of all Industry Stencils are Underexposed!!! Weighing the High Costs of Underexposure Stencil breakdown on press Pinholes Poor edge quality lost dots Reclaiming difficulties Scumming Use an Exposure Calculator

Determine which step has no color change on the dried screen.

33 Neutral Grey Types Double the estimated exposure time. Expose the calculator on the stencil at that doubled time. Determine which step has no color change on the dried screen. Look at resolution of screen at steps above and below that point. Multiply the selected step s factor by the exposure time to determine the correct exposure.

34 Washout technique makes a significant difference in stencil halftone images

35 For properly exposed stencils ONLY! Benefits Opens image detail more effectively than garden hose Better edge definition Cleaner mesh bridging Can reduce moiré Improved screen to screen consistency Less operator variation Simplified training

Stick with 2000 psi and lower Maintain a distance of 12-18 inches from tip of wand to screen Use a fan spray nozzle, not a narrow nozzle Start pressure washer away from the screen and

36 Stick with 2000 psi and lower Maintain a distance of inches from tip of wand to screen Use a fan spray nozzle, not a narrow nozzle Start pressure washer away from the screen and then move onto stencil Do not use pressure washer on underexposed screen, soft SBQ emulsions or low quality emulsion products. Only use a pressure washer the substrate side of capillary films.

37 Film 90 lpi Printed Images: Correct Exposure Garden Hose: Conventional Pressure Washer: Conventional Pressure Washer: Half & Half 10% 11% 12% 15% Target 18% 50% 59% 60% 63% Target 68% 4 pt. Text Screen Printing Technical Foundation 2008

38 A fully exposed screen produce the same tonal results as an underexposed screen when a pressure washer was used for development - missing shadow dots Screen Printing Technical Foundation 2008

39 Film 90 lpi Printed Images: Correct Exposure Garden Hose: Conventional: Short Pressure Washer: Half & Half: Short Garden Hose: Conventional: Long Pressure Washer: Half & Half: Long 10% Target 18% 11% 14% 13% 16% 50% Target 68% 59% 62% 62% 63% 4 pt. Text Screen Printing Technical Foundation 2008

40 Longer washout times open up more highlight detail no matter what washout pressure is used in development Screen Printing Technical Foundation 2008

41 Excellent Quality Control Check 21 step Stouffer Scale Checks exposure as well as washout Ensures each screen is correct before entering production 1. Determine ideal exposure time with exposure calculator. 2. Place control check on the next screen just outside image area, and washout using normal production procedures. 3. Read to last hard step still fully attached to screen 4. Use last hard step number as reference for each future screen made with same emulsion & mesh coating variables 5. Place in same spot on glass each time At this point a different hard step indicates something changed in exposure or washout

42 Scales Equally Exposed on Identical Emulsion Coatings Garden Nozzle: Conventional Side to Side Pressure Washer: Conventional Side to Side Referenced Hard Step is Dependant on the Washout Procedure

1. Use an exposure calculator!!! 2. Use an exposure control scale on each screen 3. Develop screen as soon as possible. 4. Use cold to lukewarm water not hot. 5.

43 1. Use an exposure calculator!!! 2. Use an exposure control scale on each screen 3. Develop screen as soon as possible. 4. Use cold to lukewarm water not hot. 5. Flood both sides of screen with water. 6. Direct Emulsion: a. Start on squeegee side with pressure washer for half the total washout time b. Repeat on the substrate side. c. Check hard step d. Use a timer to ensure consistent time

7. Capillary Film: a. From the substrate side only, washout stencil for pre-determined time b. Check hard step 8. Do a final low pressure rinse on both sides to remove debris. 9.

44 7. Capillary Film: a. From the substrate side only, washout stencil for pre-determined time b. Check hard step 8. Do a final low pressure rinse on both sides to remove debris. 9. Examine the stencil edges and highlight dots with 30-50X magnifier 10. Use a screen vacuum to remove water 11. Whatever you do, be consistent day to day 12. Create a written procedure and train employees

45 Automatic Washout Systems Removes human operator variation more consistent results. Changing to automated system will most likely require adjustment of tonal curves each system will give unique profile. Control settings must be documented and kept the same.

46 Measures Dot Area on Developed Stencil Allows verification of exposure and washout before screen enters production Measures Stencils Film CTS images Litho Plates Ripware Ltd.

47 Screen Making Press Time

Screen Coating Techniques

Screen Coating Techniques Direct emulsions offer quality in print, mechanical endurance, solvent & water resistance, and affordability all in one bucket. To use these qualities profitably requires a basic