Trouble Shooting GuidE for label printing using Sicura UV inks
2 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 experience 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 overcoming any difficulties that you may encounter. The contents are divided up according to the various printing processes.
Trouble Shooting Guide UV label printing Introduction UV drying Unlike 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 process, 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 perspective, UV technology is one of the cleanest printing processes. Characteristics of UV inks Shade, color intensity and transparency of a printing ink are dependent 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 specifications (printing process, purpose for which it will be used). Tack Tack 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. 3
Introduction 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 printing inks. It is advisable to use agitators in the ink fountain to ensure optimum ink distribution. Good flowability thixotropic Printing rollers Due to their high polarity and solubility values, materials composing 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 materials, 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. 4
Trouble Shooting Guide UV label printing Introduction Health and safety Components 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. 5
Simple test methods 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 (= 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. Poor pre-treatment Good pre-treatment 6
Trouble Shooting Guide UV label printing Simple test methods Adhesion testing 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. 7
Simple test methods Scratch resistance There 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 drawing 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 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. 8
Trouble Shooting Guide UV label printing Simple test methods Product durability 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. 9
Combination printing 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. 10
Trouble Shooting Guide UV label printing Combination printing 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. Silicone contamination Clean print 11
UV-specific problems and their solutions 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. 12
Trouble Shooting Guide UV label printing UV-specific problems and their solutions 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% 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. 13
UV-specific problems and their solutions Unsatisfactory mechanical properties Reduce layer thickness, if necessary by the choice of a suitable printing 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. 14
Trouble Shooting Guide UV label printing UV-specific problems and their solutions Inadequate hot-foil or thermal transfer printability 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. 15
UV-specific problems and their solutions Poor die-cutting performance 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. 16
Trouble Shooting Guide UV label printing UV-specific problems and their solutions Gearwheel stripes 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. 17
UV-specific problems and their solutions Cracking of the ink film 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. Bleeding of inks with fanal pigments 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. 18
Trouble Shooting Guide UV label printing UV-specific problems and their solutions Insufficient Light fastness 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. 19
UV-specific problems and their solutions Thermal printability 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. 20
Trouble Shooting Guide UV label printing UV rotary screen printing Area of application: solid areas, half-tone and line work. Ink usage: approx. 6 20 g/m 2. Construction of a screen Squeegee side Squeegee side Print side Print side 21
UV rotary screen printing Inadequate adhesion / drying 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. 22
Trouble Shooting Guide UV label printing UV rotary screen printing 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, printing machine and the printing speed. Unsharp print For example for fine typefaces. The ink is too thin: Add 10% Thixpaste. 23
UV rotary screen printing 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. 24
Trouble Shooting Guide UV label printing UV flexo printing Area of application: solid areas and half-tone work. Ink usage: approx. 1.5 6 g/m 2. 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. 25
UV flexo printing Bad laydown on UV screen printing inks 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. 26
Trouble Shooting Guide UV label printing UV flexo printing Faults in the printed image 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. 27
UV Offset // Waterless UV Offset Area of application: half-tone and linework Ink usage: approx. 1.2 1.6 g/m 2. 28
Trouble Shooting Guide UV label printing UV Offset // Waterless UV Offset Consumables: Blankets 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. Offset plates 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. 29
UV Offset // Waterless UV Offset Ink build-up Check the ink/water balance. Check the printing process. Check the ink tack and reduce if possible. Ink misting Ink 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. 30
Trouble Shooting Guide UV label printing UV Offset // Waterless UV Offset Inking roller blinding 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. 31
UV Offset // Waterless UV Offset Picking at the back edge of the print 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. 32
Trouble Shooting Guide UV label printing UV Offset // Waterless UV Offset Faults in the printed image 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. 33
UV Offset // Waterless UV Offset Fibre picking 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. Mottling Reduce the quantity of fountain solution being applied to the plate. Check the printing pressure. Replace blanket if it is too hard. Scumming Viscosity and tack of the ink too low. Check the ink/water balance. Check ph value of fountain solution. 34
Trouble Shooting Guide UV label printing UV Offset // Waterless UV Offset Dot gain 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. 35
UV Letterpress Area of application: Half-tone and linework, ink usage: approx. 1.8 2.2 g/m 2. 36
Trouble Shooting Guide UV label printing UV Letterpress Dot fringes 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. Cliché clogging If negative typefaces clog up, add 5 10 % Thixpaste. 37
UV Letterpress Fibre picking Substrate exhibits bad picking resistance. Lower the tack of the ink used by mixing in anti-tack paste. Ink misting Ink supply too high. Reduce the quantity of ink or use stronger ink systems. Check the ink roller adjustment. Replace unsuitable rollers. 38
Trouble Shooting Guide UV label printing
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 information 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. 40
Trouble Shooting Guide UV label printing Imprint Printed March 2013 Photographs: Siegwerk Druckfarben AG & Co. KGaA Printing ink: Siegwerk Tempo Elite Siegwerk and the Siegwerk logo are registered trademarks of Siegwerk Subject to technical and other changes Siegwerk Druckfarben AG & Co.KGaA Alfred-Keller-Str. 55 53721 Siegburg Germany info@siegwerk.com 41
www.siegwerk.com