for label printing using sicura uv...

Trouble ShooTing guidefor label printing using Sicura UV inks

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Siegwerk is one of the most dynamic and innovative manufacturers of UV technology for the graphic arts industry. This handbook on the theme of UV label printing provides an overview of the experi-ence that Siegwerk has so far accumulated in this field. Our aim with this booklet is to pass on tips from practical use, possible solutions and self-help ideas. We hope that you will find it useful in over-coming any difficulties that you may encounter. The contents are divided up according to the vari-ous printing processes.

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Trouble ShooTing guide ∙ UV label printing

UV dryingUnlike conventional printing inks or liquid inks that are based on an

aqueous or solvent based carrier, UV inks do not require a solvent

or thinner that has to be eliminated by evaporation or absorption.

All reactive components become involved in the polymerization pro-

cess, that is to say the hardening of the ink/varnish film. 100% of

the ink/varnish transferred during the printing process remains on

the substrate. As a result, from the environmental protection per-

spective, UV technology is one of the cleanest printing processes.

Characteristics of UV inksShade, color intensity and transparency of a printing ink are depen-

dent first and foremost on the properties of the pigments used. UV

inks are most commonly formulated with organic pigments, many of

which are also used for other printing processes, so they generally

have the same colorimetric properties as other print processes. On

special request and following laboratory investigations, pure shades

(e.g. fanal inks) may be available. For this purpose, however, it will

be necessary to provide the original substrate and detailed specifica-

tions (printing process, purpose for which it will be used).

TackTack is the measure of the force required to pull a wet ink film apart.

It affects the transfer of the ink at all stages of the print process,

from the ink fountain up to its transfer onto the substrate.

Introduction

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Viscosity and flow (thixotropy)These affect the ability of inks to travel from the ink fountain and

roller frame and thus the ink’s distribution through the printing

machine. Depending upon the pigment employed, UV inks are

generally more compact and have less flow than conventional print-

ing inks. It is advisable to use agitators in the ink fountain to ensure

optimum ink distribution.

Printing rollersDue to their high polarity and solubility values, materials compos-

ing the UV vehicle system may cause swelling of certain elastomers

from which conventional rollers and blankets are made. Materials,

such as polyurethane, should be replaced UV-compatible materi-

als, such as butyl or nitrile rubber. These materials exhibit minimal

swelling and can still be used with conventional inks on the same

roller frame. If the machine is used exclusively for UV inks and UV

varnishes ink rollers made from UV-resistant EPDM plastic should be

used.

Introduction

Good flowability thixotropic

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Health and safetyComponents in UV products, such as acrylates, monomers and

photoinitiators may cause irritations, allergies or hypersensitization

among sensitive persons if repeated skin contact occurs. The EUPIA

information leaflet „Guidelines for the Safe Use of Energy Curing

Printing Technology”, safety datasheets and information leaflets

(published by local trade associations) contain precise details on the

risks of irritations and allergies as well as essential guidelines for

the safe handling UV inks and varnishes.

Introduction

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Testing the Corona pre-treatment During the Corona pre-treatment, an electric charge is generated

over the synthetic film substrate by means of conductors carrying a

high voltage. This changes the surface tension of the film. Details

of the ideal surface tension for good ink/varnish adhesion can be

obtained from your film suppliers; it is quoted in mNm (= millinew-

tonmeter). In the case of polyolefines (PE, PP) a surface tension of

at least 39 – 45 mNm is recommended. Testing the surface tension

of a film is carried out with the aid of a graduated series of test inks.

Poor pre-treatment

Simple test methods

Good pre-treatment

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Adhesion testingThis testing is used to check the adhesion of UV inks and UV

varnishes to the print substrate. Cut off a length of adhesive tape

measuring approximately 10 cm in length. Place the adhesive tape

slowly on the printed surface and press down with the fingers,

ensuring that no air bubbles are trapped between the adhesive

tape and carrier material. Pull off one end of the tape as far as the

middle with a short, sharp tug, then the remainder with further

short, sharp tug. Examine the adhesive tape and the print and as-

sess the adhesion.

A more severe form of the adhesive tape test is the cross-cut adhe-

sion test. Here the kiss cutting, and therefore the 'injury' to the ink/

varnish film is simulated. The ink film is scored crossways, and then

the adhesive tape test is carried out on the ink film, which has been

damaged in this way. Suitable cross-cut adhesion test apparatus is

available from suppliers of graphic arts laboratory equipment.

Simple test methods

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Scratch resistanceThere are no standards for this simple test, and it calls for a degree

of experience on the part of the printer. It is carried out by draw-

ing a fingernail over the printed surface, using light pressure. If the

surface is damaged, it indicates inadequate curing or an excessively

soft ink film. In this case the addition of wax or abrasion protection

pastes - or varnishing over the print in the case of high demands - is

recommended.

Testing the drying of UV varnishes This test makes use of the fact that an aqueous solution of potas-

sium permanganate (KMnO4) will oxidize any unreacted acrylate

components in UV varnish, causing them to darken in color. The

optical density of the surface spots caused by the solution is

measured using a densitometer. The darker the spot, the poorer

the drying and the slower the curing of the varnish film is said to

be. The result of the measurement is compared to a measurement

carried out on a well-cured UV varnish of the same type. Compari-

sons between two different varnishes are meaningless. Using a

pipette, apply a drop of a 5% aqueous solution of KMnO4 onto the

varnished area directly after printing and curing. Allow the solution

to react for 5 minutes. Dab off the excess solution with absorbent

paper (do not wipe). Measure the optical density of the spot and

record the figure.

Simple test methods

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Product durabilityIn view of the large number of product durability requirements,

the variety of different test methods and the widely varying

substances used, it is impossible in such a guide to give a general

recommendation with respect to the specific UV inks and varnishes

that are to be used.

Please check the exact requirements, test conditions and substances

before carrying out a production run. Our application technicians

will be pleased to offer you their assistance and help you to find a

solution to any problem.

Simple test methods

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Combination printing has developed to become a totally routine

process in modern label production and it has become an indispens-

able part of this industry. Nevertheless, the combination of different

printing processes poses stiff challenges for the printing ink systems

employed.

Due to the different viscosities and surface tensions of the ink

systems being used together in combination, problems may be en-

countered with the flow or lay down of the ink during overprinting.

A major step forward in solving this problem was made in the

development of silicone-free ink systems.

Combination printing

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However, since even very small quantities of incompatible flow

additives (such as silicone for example) may lead to imprintability

problems, it is of paramount importance to adhere to the follow-

ing parameters before using silicone-free inks for the first time:

· Fit clean new screens. · Clean the anilox rollers and storage containers thoroughly during any changeover.

· Use only washing agents and cleaning cloths that have not come into contact with other ink systems or silicones.

· Thorough cleaning of the machine, pumps and hoses, and in the case of UV screen printing and flexo printing, the doctor blade

system.

Silicone contamination Clean print

Combination printing

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During daily production of labels with UV-inks and varnishes, some

problem can occur regardless of the applied printing method. On

the following pages, we would like to show solutions in order to

solve these problems easily and efficiently.

UV-specific problems and their solutions

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Inadequate adhesion / drying · Check the UV lamp output

Excess inking, especially with dark colors, opaque white or metallic

inks.

· With problematic plastic films, add 2 – 3% hardener or 5% adhe-sion promoter.

Untreated print substrates:

· For polyolefines (PE + PP), an inline Corona pre-treatment to 39 – 45 mNm is necessary or the substrate has to be pre-varnished

with an appropriate primer.

Bad intercoat adhesion: the ink is over-hardened.

· Reduce the output of the UV lamp.


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Unsatisfactory mechanical propertiesReduce layer thickness, if necessary by the choice of a suitable print-

ing process in combination printing

· Print darker ink colors at the start, if possible. · In flexo or screen printing reduce the ink film thickness by using finer anilox rollers or screens.

· In the case of prints that will not be varnished, add 3 – 5% rub protection paste to the last ink layer to improve the mechanical

durability of the print.


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Inadequate hot-foil or thermal transfer printabilityPoor quality embossing, muddy printing:

· Use a suitable, if possible additive-free ink system. · Additives containing wax or silicone, such as flow agents, rub protection pastes or lubricants may not be used.

Ink is over-cured:

· Change print sequence or reduce UV lamp output. · Correct setting-up of the thermal transfer or hot stamping dies are essential, as is the right hot stamping temperature and use of

suitable hot stamping foils.


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Poor die-cutting performanceOver-hardening (ink flaking at the cutting point), mainly with light

colors:

· Change print sequence or reduce UV lamp output.

In the case of poor die-cutting performance of UV-screen opaque

white:

· Use a softer opaque white. · Check the sharpness of the die-cutting tool.

Please contact the application engineers at Siegwerk about appro-

priate UV ink types. We will be pleased to help you and provide you

with support in solving your problems.


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Gearwheel stripesCheck the settings of the ink rollers to the oscillating roller.

Check the settings (pressure) of the rollers to the printing plate.

· Decrease the tack of the ink in small steps. · If feasible use an ink system with an adapted rheology.

Gear marks are most often caused by mechanical problem of the

printing press. Please contact your press manufacturer.


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Cracking of the ink filmThis phenomenon appears most currently if the substrate used is

highly stressed due to temperature changes. If the printed substrate

is stretched and then shrunk again, the dried ink film cracks. This

problem appears as well in card printing, when that cards are lami-

nated under heat and pressure.

If feasible use substrates with a higher temperature resistance, you

may possibly use special inks that are more resistant.

Bleeding of inks with fanal pigmentsWhen using fanal inks, the print can bleed if it comes in contact

with solvent based products or even UV-laminating adhesives

containing amines.

· In this case please use ink system with resistant pigments.


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Insufficient Light fastnessThe light fastness of a printing ink depends mostly from the pig-

ment used in the formulation.

This resistance is indicated according to the wool scale (WS) and is

in between 1 (worst) and 8 (best).

· If the print fades quickly under sun light, you may use inks with a higher light fastness.


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Thermal printabilitySet-off or migration of waxes or silicone-acrylates can influence the

reaction of the thermal coating negatively.

The darkening during the thermal printing can therefore not be

guaranteed anymore.

· Use of appropriate additive-free ink systems. · Reduce winding pressure if necessary.


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Area of application: solid areas, half-tone and line work.

Ink usage: approx. 6 – 20 g/m2.

UV rotary screen printing

Construction of a screen

Squeegee side

Squeegee side

Print side

Print side

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Inadequate adhesion / dryingExcess inking, especially with dark colors, opaque white or metallic

inks. Screen printing inks are intended to be applied at a thick-

ness of approx. 6 – 15 μm. Thick applications may lead to adhesion problems.

· Use a finer screen if possible. · Add blending varnish. · Add photoinitiators.

Bad intercoat adhesion: the ink is over-hardened.

· Reduce the output of the UV lamp. · Do not add flow agent to the ink which is printed first. Use thinner to optimize the flow.


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Bad lay down of the ink · If possible, blend mixed shades with a proportion of opaque white (up to 10%).

· Add thinner in careful steps of around 1% up to a maximum of 15%, until the desired result is achieved.

· The combination of 2% flow agent and 5% thinner usually shows good results, but depends on the substrate, surface tension, print-

ing machine and the printing speed.

Unsharp printFor example for fine typefaces. The ink is too thin:

· Add 10% Thixpaste.


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Screen clogging · Protect screen from sunlight and other light sources. · When using inks and varnishes with hardeners or adhesion agents, clean the screen immediately after printing or in the event

of a machine stoppage.

· With metallic, pearlescent and special effect inks, a screen mesh aperture that is 2.5 times the pigment size must be used.


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Area of application: solid areas and half-tone work.

Ink usage: approx. 1.5 – 6 g/m2.

The ink thickness is governed by the choice of anilox roller. Here

approx. 35% of the theoretical volume drawn is actually transferred

to the print substrate.

The use of the anilox roller must therefore be matched to the print

subject.

UV flexo printing

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Bad laydown on UV screen printing inksAs a result of incompatible surface tensions between different ink

systems, UV flexo inks may cause flow problems on the pre-printed

UV screen printing opaque white.

· Pre-print using a suitable screen printing white, additive-free if possible.

· Adjust the flow of the flexo inks using suitable additives that are compatible with those of the screen printing white.

· If possible, increase the travelling distance between printing unit and UV lamp.

UV flexo printing

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Faults in the printed imageThe printing may take on a ‚"cloudy" appearance according to the

nature of the surface and absorbency of the print substrate.

· If possible, blend the mixed ink with opaque white instead of diluent / transparent white.

· Pre-print with a suitable transparent white or varnish to seal the pores of more absorbent substrates.

UV flexo printing

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Area of application: half-tone and linework

Ink usage: approx. 1.2 – 1.6 g/m2.

UV Offset // Waterless UV Offset

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Consumables:

BlanketsThe layers of a blanket are almost identical to the construction

of the elastomer rollers. Swelling of the blanket can result in an

increase in pressure and thus lead to inconsistent ink transfer

between plate/blanket and blanket/print substrate with a direct

effect on printing quality. It is therefore necessary to use appropriate

blankets when using UV inks.

· Please contact your blanket supplier about this.

Offset platesThe photopolymer coating on positive plates is not always resistant

to UV cleaning agents. If this type of plate is being used, the

coating must be baked in accordance with the manufacturer’s

instructions. A positive plate treated in this way is resistant to the

majority of UV cleaning agents. When using negative plates, baking

is not generally necessary.


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Ink build-up · Check the ink/water balance. · Check the printing process. · Check the ink tack and reduce if possible.

Ink mistingInk supply too high.

· Reduce the quantity of ink or use stronger ink systems. · Check the ink roller adjustment. · Check the ink/water balance. · Replace unsuitable roller material. · Reduce machine speed if too high.


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Inking roller blindingIn the offset process due to emulgated dampening solution.

· Check the quantity and composition of the dampening solution.

In waterless UV offset printing due to separation of the printing ink at

low press temperature (Silicone is separating from the printing ink).

· Stir the inks well, allow the machine to warm up. · If necessary, increase the inlet temperature of the plate or friction cooling.


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Picking at the back edge of the printBack-splitting of the printing ink from the substrate to the blanket.

Therefore heavy ink build-up on the blanket.

Depending on the surface tension and on the surface smoothness

of the substrate.

Tack of the printing ink is too high:

· Reduce the tack of the ink. · If necessary, increase the temperature of the plate and friction cooling.


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Faults in the printed imageDue to migration of softeners or antistatic agents to the surface

of the substrate (especially with soft PVC) a uniform print is made

impossible.

The way to check if this is the case, is to clean the surface of the

substrate with alcohol and to make a short print trial with the

cleaned samples. These samples shall then show a uniform print

again.

· Change the substrate.


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Fibre pickingSubstrate exhibits bad picking resistance.

· The tack of the blanket may be too high. Check blanket quality. Lower the tack of the ink used by mixing in anti-tack paste.

MottlingReduce the quantity of fountain solution being applied to the plate.

· Check the printing pressure. · Replace blanket if it is too hard.

ScummingViscosity and tack of the ink too low.

· Check the ink/water balance. · Check pH value of fountain solution.


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Dot gainIn UV offset, dot gain is slightly greater than with conventional

offset. This is dependent, among other things, on the vehicle system

being used and may vary from one ink system to another.

Adjustment of the printing reference line should compensate for

the more pronounced dot gain.

· Check the ink roller adjustment. · Check the printing pressure. · Check the thickness of the rubber packing. · Fit harder rollers or blankets.


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Area of application: Half-tone and linework, ink usage:

approx. 1.8 – 2.2 g/m2.

UV Letterpress

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Dot fringesIf the ink is too soft, drying problems may be experienced on ac-

count of the greater layer thickness at the edge of the printed area.

As a rule, this leads to drying problems, particularly in the case of

dark and/or metallic inks.

· Use the ink undiluted, without additives if possible. · If necessary, use ink with a higher concentration.

Cliché clogging · If negative typefaces clog up, add 5 – 10 % Thixpaste.

UV Letterpress

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Fibre pickingSubstrate exhibits bad picking resistance.

· Lower the tack of the ink used by mixing in anti-tack paste.

Ink mistingInk supply too high.

· Reduce the quantity of ink or use stronger ink systems. · Check the ink roller adjustment. · Replace unsuitable rollers.

UV Letterpress

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In view of the differences in printing substrates, processing conditions and test criteria, we are able to provide the advice contained in this guide for informa-tion only and without responsibility.

All information reflects our current knowledge. It is based on our practical experience.

We accept no liability for damages in respect of applications for which our products are not intended.

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Imprint

Printed March 2013Photographs: Siegwerk Druckfarben AG & Co. KGaAPrinting ink: Siegwerk Tempo EliteSiegwerk and the Siegwerk logo are registered trademarks of Siegwerk

Subject to technical and other changes

Siegwerk Druckfarben AG & Co.KGaAAlfred-Keller-Str. 55 · 53721 Siegburg Germany [email protected]

www.siegwerk.com

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