Troubleshooting Guide for label printing using Sicura UV inks

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 (= millinewtonmeter). 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.

This 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 assess the adhesion.

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

This test makes use of the fact that an aqueous solution of potassium 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. Comparisons 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.

In 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.

Combination printing has developed to become a totally routine process in modern label production and it has become an indispensable 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 encountered 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.

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 following 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.

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.

• 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% adhesion 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.

Poor 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.

Over-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 appropriate UV ink types. We will be pleased to help you and provide you with support in solving your problems.

Check 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.

This 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 laminated under heat and pressure.

If feasible use substrates with a higher temperature resistance, you may possibly use special inks that are more resistant.

When 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.

The light fastness of a printing ink depends mostly from the pigment 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.

Set-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.

Excess inking, especially with dark colors, opaque white or metallic inks. Screen printing inks are intended to be applied at a thickness 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.

• 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, printing machine and the printing speed.

For example for fine typefaces. If the ink is too thin, add 10% Thixpaste.

• 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.

As 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.

The 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.

The 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.

The 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.

• Check the ink/water balance.
• Check the printing process.
• Check the ink tack and reduce if possible.

In 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.

Back-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.

Due 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.

Substrate 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.

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

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

Viscosity and tack of the ink too low.

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

In 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.

If the ink is too soft, drying problems may be experienced on account 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.

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

Substrate exhibits bad picking resistance.

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

Ink supply too high.

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