Home > Technical Tips > Technical Tips by Michel Caza #4

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

Advertisements
  1. No comments yet.
  1. No trackbacks yet.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: