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Technical Tips on Screen Printing by Bhargav Mistry, DMI – #6

April 15, 2017 1 comment

Tips on squeegee blade (squeegee rubber)

  • Maintain a reasonable gap between the squeegee edge and inside of frame
  • Short gap will negatively affect print quality, performance, uniformity of ink deposit; and it also shortens the life of stencil, fabric & squeegee
  • Squeegee should be only 2” bigger than the print area i.e. for printing 30” wide squeegee should be 32” only

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10to12Sep09_0012

  • Keep ends of the squeegee rounded to get good registration and uniform ink deposit

Squeegee with rounded corners


Squeegee with sharp corners

  • Always maintain sharp squeegee while printing and use squeegee sharpener regularly.

squeegee_sharpener


nano-sharpener

  • Storage of squeegees is also very important. Its advisable to store the squeegees vertically (squeegee rubber side up) and ensure no objects are in contact during storage.
  • Good air circulation is also recommended to keep the squeegee dry after cleaning with solvents.

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  • Do not use aggressive solvents to clean the squeegees
  • Do not wipe the ink from the squeegee after printing with any sharp metal or hard materials. This can damage the squeegee edge.
  • Do not print squeegees with one edge for longer period always use both edges after few hours or impressions to maintain the edge sharpness and parallelism

Finally I would to remind you to register for my next Graphics workshop schedule in June 12 to 14, 2017.

This time you will learn all about linearisation right from pre-press to screen making to screen printing including colour management system, digital workflow, ICC profiling and digital proofing on Inkjet Printer before final screen printing. Everything will be displayed live.

Plus for the first time you will also learn all about Monitor Calibration, Digital Proofing & Certification.

Enjoy and have a nice weekend.

Bhargav Mistry

Technical Tips on Screen Printing by Bhargav Mistry, DMI – #5

Screen_Making_Room-7

Screen Dryer

Q – Why is my direct emulsion stencil breaking down during printing?

Underexposure (the most common cause of stencil failure)
Use an exposure calculator to determine proper exposure time.

Screen not fully dry before exposing
Use proper screen dryer after coating emulsion and give enough time to dry before exposing.

Screen not fully dry before printing
After developing let the water to completely evaporate before sending the screen for production. If possible keep screen in dryer for min 20 mins @40℃ after developing

Screen_Making_Room-8

Emulsion Coating Machine & Screen Dyer

Coating technique
Too many coats (wet-on-wet) will increase the emulsion build up drastically and slow down emulsion dry. Reduce number of coats (wet-on-wet).

Improper handling
Use yellow safelights. Insufficient degreasing or degreased screen must have been touch by naked hands/fingers.

Screen Tension too low
Use the screen tension recommended by the mesh manufacturer.

Emulsion thickness too low
Ensure that the emulsion build up is proper apply multiple coats or emulsion solid content too low.

Emulsion not compatible with Inks
Use water resistant emulsion for Water based Inks and use Solvent Resistant Emulsion for Solvent Based Inks.

Too much squeegee pressure
Reduce squeegee pressure and reduce off-contact distance (distance between screen mesh and substrate). To high off-contact also increases squeegee pressure.

DMI_Workshop_20150826-420

Screen is clean to often
Do not let the ink dry in the screen. Flood/Fill the mesh after each print stroke. Do not use fans on the top of the print area. Use less aggressive solvents to clean the screen. If the ink is becoming too tacky this can also lead to emulsion break down. Use screen hardeners to increase the resistance and durability of the emulsion.

Emulsion not stored properly or outdated
Always store the liquid emulsion in a refrigerator temperature between 20℃ to 23℃. Write the date on the can after sensitised. Dual Cure Emulsion has to be consumed within 1 to 3 months depending on storage and manufacturer. Single Component ready to use emulsions generally last for 6 months to 1 year depending on storage and manufacturer.

Improper sensitizing
Ensure that diazo is thoroughly mixed into the added water, and then into the emulsion. Use only recommended amount of diazo. Always stir diazo in the emulsion very slowly and use only after one over night or 12 hours after mixing.

Technical Tips by Michel Caza #4

Q – Why my UV inks break when I cut, fold or die-cut paper, board after printing ?

The main reason for that is an over curing of the UV inks.

There are three main secrets for a successful UV inks curing :

  1. Print as thin ink coat as possible.
  2. Avoid absolutely “over-curing”.
  3. Take care of “post-curing”.

To respect those two items, the mistakes to avoid are the following:

Wrong choice of fabrics for the screen. In UV , the totality of ink leaving the screen remains intact on the substrate : no evaporation of water or solvent.

50% of the ink thickness is related to the fabric. Use finer fabrics, from 140/cm to 180/cm, 27 to 33 µ threads. Or, even better when available “calendared meshes”. Calendared side on squeegee side reduces the ink coat of 25%. (Exceptions can be made for some special effects).

Calendered Fabrics

Calendared Mesh

Poor tension of the fabric:
It’s even more critical with UV inks. Don’t print if your screen tension is below 22 Newton/inch. Best is between 25 and 32 Newton/inch.

Too thick emulsion coating:
Use on single coat of emulsion (substrate side) or 1 than Dry plus 1 more and Dry again.

Triple-Duro-Squeegee

Image Courtesy – Fimor France

Too soft or rounded squeegee rubber : it increases the surface of the ink transfer.
Use 75° or 85° shore hard squeegee rubber. The best for UV Inks is to use triple shore 75/90/75° shore hard. Very well sharpened.

Too thick flood bars.
1 or 2 mm rounded aluminium to be replaced by 500µ blue steel blade (very well and “sweet” sharpened of course). The flood bars determinate widely the thickness of ink deposit. For best result use angled floor bars.

Too thick ink coat deposited on the screen by the flood-coater : simply fill the fabric, no more.

Advanced Flood Bar-Blank

These angled flood bars with sharp hardened Stainless Steel Blade is very useful for printing fine lines & halftones thus it also reduces ink thickness.

Too much pressure on the squeegee = bending of blade, too large ink transfer surface = more ink on the substrate. Use “kiss printing” with a low off-contact distance : max. 2 mm.

Handbook for screen printers by SefarSqueegee angle between 10° to 15° angle. Too much low squeegee angle will increase ink thickness drastically.

All this leads to a too thick ink deposit, bringing the necessity for the printer to increase the UV curing : increasing the power of the lamp and/or reducing the speed of the belt of the UV dryer.

This, added to a natural tendency of the printer to be afraid of “not be “dry” enough” causes an over curing of the ink.

This over-curing is the key of the problem. It must absolutely be avoided.
Logically, a thinner coat of ink needs a faster curing with less power. Why is over-curing so damageable ?

That’s a classical physical-chemical issue : the number of molecular links increases drastically and the tighter those links, the harder becomes the structure of the newly created polymer and its suppleness reduces…. and it becomes more or less highly “breakable” with many types of UV inks.

Then the shock when cutting or folding the printed substrate engenders breaks in the ink coat. It can even also engender difficulties of inter-coat adhesion of inks.

Then, there is the last bad point : the printer forget the “post-curing” of the ink.
Don’t believe that when the print leaves the curing unit, the polymerization (or curing) stops.

Any reaction of polymerisation initiated by a chemical catalyst or by heat, or by UV keeps going during 24 to 48 hours after the printed sheet left the curing unit. Then, if the curing was at its maximum (at the supposed satisfaction of the printer), the molecular structure will become tighter and tighter and the ink of course more “breakable”.

You must simply cure enough to harden the surface of the ink (for easy pilling) and leave the undercoat less cured. A trick : use your nail to scrap the surface. You must have light traces of wet ink under your nail. Try again same place the day after and you will find the ink “hard at heart”. This mean that the polymerisation has finished naturally with the post-curing. Then the UV ink keeps its suppleness.

Wishing all my friends in India. Happy Holi…

Michel Caza

Technical Tips by Bhargav Mistry, DMI – #3

In my previous blog post I had mentioned about Humidity control and I would like to further elaborate on this subject.

Controlling Humidity is very important and essential in screen making department. Few companies do have Air-Conditioned in their screen making department but that only will not help. Split/ceiling mount or wall mount units are all comfort AC’s and do not have humidity control system.

If you want to achieve high quality screens and repetition accuracy then controlling Temperature and Humidity becomes very crucial.

In DMI many times we keep screens till max. 15/20 day unexposed. Obviously under control environment and in total dark condition to prevent from light and to avoid hardening of screens.

To achieve longer shelf life from pre-coated screens do not allow screens to get expose to any kind of light including yellow light of your screen making room.

Once the coating is over and the screen is removed from the screen dryer. Keep those screens inside cupboards to avoid lights.

The main reason we could keep our pre-coated screens in perfect condition till 15/20 days is due to controlled temperature and humidity.

Alway try to keep humidity as low as possible at DMI we maintain between 50% to 55% RH and temperature 23℃ to 24℃
Humidifier_WhiteWestingHouseIf you are thinking about investment and running cost. Believe me its negligible compare to quality.

Currently in DMI we use centralised air-condition plant with inbuilt humidity control system.

But previously in DMI we were using White-Westinghouse Model WDE550. This machine consumes approx. 20 units in 24 hours which costs us max. ₹140/- per day which is very negligible.

It’s worth spending because the quality of screen will improves drastically. Plus you will always get repetition accuracy.

Especially when you use Inkjet films it will help a lot because I have observed people complaining of ink transferring on emulsion after exposing and there after the films can no longer be used.

The main reason for this is humidity or improper time given for emulsion to dry. The use of Screen Dryer is very much essential to achieve proper drying of liquid emulsions.

Investing on a humidity control system is very much essential in screen making department.

These kind of humidifier are available online on Amazon and Flipkart. The one which we use WDE550 cover approx. 550 sq. ft. easily (5500 Cubic Ft.).

“It’s vital that the amount of remaining moisture in the emulsion coat in the screen – when totally dry after coating (before exposing ), and also when dry after development (before printing) – remains under 4%. If it must resist to printing, especially in the part of India and Asia or in any other country when the humidity of external or internal air is above 70%. There are tools available in market which can measure internal emulsion humidity.” Says Michel Caza.

Below I have mentioned few important points if your screen making department has proper yellow lights, no windows to prevent outside lights, plus controlled temperature and humidity.

  1. Repetition accuracy
  2. Image Sharpness
  3. Reduce stencil breakdown during printing
  4. Increases quantity of prints
  5. Avoid inkjet films black ink transfer on emulsion after exposing
  6. Reuse inkjet films
  7. Reduces tackiness
  8. Increases pre-coated screens storage life
  9. Avoid pinhole problems
  10. Reuse screens after short print runs

So if you want to improve your screens, improve your screen making department first.

In my next post I shall highlight the importance each department and process.

Till then good bye.

Bhargav Mistry

Technical Tips on Screen Printing by Bhargav Mistry, DMI – #2

March 9, 2017 4 comments

Q – Important points about screen making room?

  • First of all you need proper screen making room with yellow safe lights
  • Do not allow outside footwear in the room
  • Do not keep any windows in the screen making room
  • Avoid outside light to enter in screen making room
  • Screens should not be made in dusty open area with bright daylight
  • Maintain room temperature around 20℃ to 23℃
  • Maintain consistent humidity around 55 to max 60 RH
  • Clean flooring with vacuum cleaner twice a day
  • Use anti-static clothes in the screen making room
  • Use anti-static gloves where ever possible plus to avoid contact of hand/fingers on screen mesh
  • Use anti-static brush or anti-static soft cloth to clean your positives and screens before coating emulsions
  • Use high quality emulsion coating trough
  • Use double door system to enter in screen making department to avoid outside lights
  • Do not wash screens inside the screen making room
  • Screen washing should be outside the screen making room but just adjoining to avoid hardening on unexposed areas
  • Screen washing should also be under yellow lights
  • Only keep Exposing Machine, Emulsion Coating Machine or Manual Coaters & Screen Dryers in the room.
  • Positive storage, artwork, designing etc. should not be in the screen making room
  • Do not have overhead fans in the screen making room to avoid dust particle circulation
  • Keep machines, tables etc 2 feet aways from the walls so that cleaning of dust within the room becomes easy

Pictures shown below are of our screen making department at DMI… Each room has proper safe yellow lights, no windows and fully air-conditioned with clean flooring.

I will soon post different layouts of a typical screen making department and how should be the optimum area required for easy movement of screen frames.

Always remember screen is the heart of printing, good quality screen delivers good quality prints. But to achieve good quality screens you also need modern & high class screen making department. That’s the most essential.

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Technical Tips on Screen Printing by Michel Caza – #1

March 8, 2017 4 comments

Bhargav-MichelCazaToday I will be posting my first Technical Tips. As my I have written dozens of technical articles on my blog but this new concept is designed to provide short and quick technical informations on screen printing technology.

Since it’s my first post, so I requested my GURU Michel Caza to kick start by posting one short technical tips.

Yesterday evening I requested him to send me few technical articles & tips so that I shall keep you posting one by one.

He was quite helpful to send me one quick article on Screen Making but there are many more to come from us and also from many other technocrats of this industry.

This is just a beginning, you will see many in-depth articles, tips, videos, visual to learn every day with many new innovative ideas.


Q – The small dots and fine lines printed disappear on the screen after stencilling?

Dots and lines too fine on the film
Respect the indications about the maximum possible fineness to reproduce

Dots and lines not black enough on the film
This can happen with films made by inkjet, not dark enough.
The sharpness, resolution, edge definition of the film must be perfect.

Colour of the screen mesh
Always use yellow mesh, avoid white mesh, to prevent light dispersion in the thread

Too coarse fabric or/and too large thread diameter
This prevent to print fine lines or dot : use S or T, thread diameter of 27 to 31 microns
in 120 to 165 thread/cm, plain wave (PW) if possible

Improper emulsion sensibility for fine lines definition and resolution
Choose a good emulsion, diazo (SBQ) or better, diazo polymer (wide latitude when exposing) with high amount of solid content.

Too many emulsion coats
As it must be exposed more, this causes the closing of the fines lines and dots.

Over exposure of the screen
Even with a correct coating of emulsion (1 coat, 1 + 1, or 2 + 1 maximum), respect the indication given by the Exposure Calculator.

Handling of screen under Improper light.
When the emulsion is dry, before and after exposure and beginning of developing with water, it is sensible to the light and can then be hardened, partially or totally. Then use yellow light in stencil room. When wetted on both sides of the screen , the white light can be used.

Poor contact in the exposing unit
To prevent any dispersion of the UV light during the exposure the contact between film and screen must be perfect. Check your vacuum pump or the rubber carpet quality which must be clean and without holes. Use a thread to the screen frame to permit a better drainage of the air.

Use of “UV tubes” light
I do not advise this because the UV light is not punctual enough and the dispersed light is not able to stencil properly line and dots under 100 microns. For fine lines, use a metal halide lamp.

No “light integrator”
Take care of exposure “time” which can drastically change during the day with the variations of supplied electricity. Use a light integrator which automatically measure and supplies the exact “amount of light” needed

Poor development of the screen.
If some emulsion remains in the finest lines or dot that should be totally open, those will be closed and of course impossible to print.

All the best from;

Michel Caza

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Technical Tips, Q&A, Ideas & Hints on Screen Printing by Bhargav Mistry, DMI

March 7, 2017 25 comments

Dear Friends,

After lots of feedback from GRAFICA customers and DMI workshop participants requesting me to post technical tips on screen printing on my blog ( www.bhargav.com ) related to Graphics, Industrial, Textile, Value Addition and many other areas where screen printing play a vital role.

So today, I am please to announce that after several years of my research & development in screen printing I shall soon be posting short Technical Tips, Q&A, Videos & Visual related to screen making, screen printing, UV/solvent inks & varnishes, screen emulsions and chemicals, drying and curing, ideas and concepts on special effects, production & productivity improvement tips and much more…

If you do not want to miss any of my technical tips then on the right side of my blog you will see “Email Subscription Form” where you can enter your email id and click on “Sign me up!” to receive automatic alerts in your Inbox regularly. (Refer to the image given below)

If you also wish to receive technical updates, DMI Workshop & Seminars Schedule and other technical information related to screen printing then send us an SMS form your mobile phones to the following number:

+91 92 932 92 932
and type in your message board
GRAFICA START
(Currently this facility is only available within India)

DMI

It’s my privilege to announce that many International Experts have also shown their keen interest to support the screen printing community. So very soon many experts will also post their ideas and technical tips which will further sharpen your knowledge & skills in screen printing.

Do not forget to rate each Technical Tips and make sure you post your comments and feedbacks in order to serve you better.

All the best and good luck.

Bhargav Mistry
DMI
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