One of the typical advantages of the screen process is the possibility to print onto nearly all possible substrates. This process does not only meet the requirements regarding colour and adhesion but also various demands regarding ink resistances. The first generation of UV-curing ink systems was introduced to the visitors of DRUPA in 1977. However then applications and properties of these systems were very limited and their use required a great deal of attention. UV-drying and overprintability of the UV-inks was limited and there were also limitations regarding adhesion properties on various types of substrates. This first introduction of UV-inks took place more than 25 years ago and there has been a great deal of progress since then.

In reference to their physiological properties UV-curing have become significantly less hazardous. New production methods are the reason the reactive raw materials of UV-inks like oligomers and monomers are now more pure and therefore show a lower skin irritation potential. Thus the odour of the inks themselves as well as smell of the prints is also significantly reduced compared to the beginnings of this technology.

Today many UV-curing screen printing inks show extremely high reactivity values thanks to new developments in the field of reactive compounds and photoinitiators. Therefore UV-inks can be cured on quick running multiple colour lines. It is also possible to cure UV-inks with low energy values thus applying low heat values on the materials resulting in improved registration of multiple colour prints.

Nowadays UV-curing ink systems are offered for almost any substrate, again because there is now an increased choice of raw materials. However, at this point we want to point out that the substrate itself also has to be suitable for UV-drying, i.e. the substrate has to be resistant to heat application to prevent deformation in the UV-dryer. On the other hand suitability of a UV-ink for a certain type of substrate is not only determined by adhesion and scratch resistance but also by the requirements the ink has to meet during further processing of the material. After printing usually there are further production steps such as cutting, folding, forming or deep drawing, thus the ink system has to meet special requirements in respect to elasticity.

The better question to ask now is: UV-inks for all printing applications?

With the exception of a few limitations, today you can basically substitute solvent based ink systems with UV ink systems.

Why are UV-curing Screen Inks so Important for Screen Printing?

There are many pros favouring the use of UV-curing inks.

UV-curing ink systems are almost free of solvent emissions, thus environmental impact such as odour in and outside of the printing shop are reduced. This also means that increased consumption of printing inks will hardly result in increased emissions and expensive investments to avoid solvent emissions can be prevented.

Use of solvent-free UV-curing ink systems will result in significant productivity improvements for a screen shop. There are no standstill and cleaning times as UV-inks do not dry in the screen enabling printers to continuously print large numbers without the risk of colour shifting. If properly used there are also no drying problems. Printing from roll-to-roll or rack drying is no problem at all.

Especially the quick drying properties offer the possibility to use printing stations in quick running machines. There is e.g. the label industry, which would not have experienced such a quick increase without UV-inks, even though screen printing in this field is only one printing method among many others.

Concepts for large format multiple colour printing also can only be effective with UV ink systems.

In the following we will take a closer look and give you some details about the different types of substrates.

PAPER, CARDBOARD, PAPERBOARD

Generally for screen printing this group of native substrates is loosing its importance. Although several years ago large-format posters were still produced by screen process, they are now mainly offset printed in Germany. This development is not due to the bad quality of screen prints but due to the tough price competition between the two printing processes. New offset machine types significantly reduced set-up times and now smaller amounts are increasingly offset printed .

Cartons are also mainly offset-printed, corresponding prints are often laminated. This basically also applies for printing of cardboard.

Despite the above various UV-ink systems are offered for these substrates. For example water dilutable UV-inks like Harmony HRN or 100% UV-inks for regular UV-drying with mercury lamps or for flash curing with xenon lamps such as our ink type Multistar MLS/MLF.

High gloss varnishes for spot coating used to mark parts of offset prints like UV 70/L and UV 70/495 have been used for paper materials for a long time.

PVC

We basically know three types of PVC substrates:

PVC self-adhesive foil
textile-reinforced poster foils
rigid PVC as boards or foil

For decades these three substrates have been well-known materials to screen printers. At first change from solvent based ink systems to UV-curing inks basically did not show any disadvantages. In respect to adhesion and resistances UV inks like ink type UVN exhibit excellent suitability for PVC.

However, problems can still occur when printing these materials with UV-inks.

Embrittlement of rigid PVC-foils
Sometimes the impact strength of UV-printed rigid PVC-foils deteriorates to a large extent. This embrittlement of printed foils may even cause glass-type splintering of foils during stamping or transportation of printed foils. The reason for this is not insufficient UV-stabilisation of the foil but more hardness and brittleness of the UV-ink used. Latest developments in this field are products causing low degree of embrittlement (e.g. Multistar MLS/MLF).

Edge Curl of PVC-adhesive Foils
Ends and edges roll up and are thus more or less sticking up from the substrate. The edge curl is mainly due to a small degree of ink shrinkage during the polymerisation process.

The more foil surface the ink covers the higher the edge curl. Direct die-cutting will also increase the risk of edge curl.

There are new UV-ink developments with a very low degree of edge curl (e.g. UV 650051). These ink systems require the use of so-called high premium cast foils with a very good adhesive.

However the edge curl problem may also occur using solvent-based ink types, especially when using low-priced foil materials.

There are further development possibilities for UV-inks for PVC substrates. We would like to point out that some ink manufacturers succeeded in developing UV-inks for credit card printing, e.g. Solarsmart SSM. When pressing the foils these inks result in a good lamination.

POLYOLEFINES

Polyethylene and polypropylene are mainly used for package printing applications. For direct screen printing suitable UV-inks have been available for some time now, which, following corresponding pre-treatment will show good adhesion properties. PE and PP are also often used for label printing. Such foils are mainly coated and inks like UVN or UV-650051 will show good adhesion. There is a tight association between the upswing of the label industry and the development of UV technology due to the quick curing properties of UV-inks.

During the last years environmental reasons have caused users to change from PVC foil materials for large format posters to PP foil materials. However, so far the printability of such materials has been considered to be quite problematic. The market offers solvent inks resulting in excellent adhesion, however due to their slow drying they are not suited for large format multiple colour equipment. The development of special UV formulations like NEU UVPO offer practical solutions.

POLYCARBONATE

The thermoplastic material polycarbonate is another crucial substrate gaining more and more importance for screen printers.

Polycarbonate foils are mainly used for production of car displays like speedometers and in-mould articles. Furthermore it is often used as high-quality transparent plastic material for injection-moulded panels or mobile phone displays and keys.

During the development of UV-inks for polycarbonate in the past ink manufacturers had to deal with some kick-backs. Initial optimum adhesion of the substrate often deteriorated. This problem was due to amine compounds contained in the ink, which were used to speed up drying but were the cause for adhesion loss. The latest ink formulations for polycarbonate are therefore free of amine compounds.

Car displays are still mainly produced with solvent-based inks like CX. Use of UV-inks (such as UVE), however, is increasing, often in connection with solvent-based inks. However, the high adhesion values required in this field are still only achieved using solvent-based ink types.

For printing of fine details and process colours, however, UV-inks are used. Especially highly opaque whites and blacks are being offered.

For anti-reflective coating of glass materials UV-curing structure and mat varnishes have been offered for years.

IMD technology uses polycarbonate foils for the production of printed form parts.

Solvent-based IMD inks such as Decomold Classic are well known on the market. On the other hand some ink manufacturers are already offering UV-curing inks for the IMD technology in combination with special varnishes. This development will continue and UV-inks suitable for direct moulding will be available for polycarbonate.

In the past polycarbonate foils were often used for keyboard foils. Lately primed polyester foils are used instead of polycarbonate foils. Meantime special UV-inks like Panel Display PD have been developed for printing keyboard foils.

When printing polycarbonate its sensitivity to UV radiation has to be taken into consideration. For curing so-called dotted UV-radiators have to be used in order to reduce yellowing and ageing of the foils will be reduced to a minimum.

POLYSTYRENE and PMMMA

Polystyrene and PMMA are classic screen substrates.

These materials do not only require inks with good adhesion and weather resistance when used for advertising applications (e.g. UVP and UVE). Polystyrene and PMMA are also suitable for production of formed parts.

A few years ago forming and deep drawing applications of these plastic materials were considered to be not or hardly possible with UV-inks. Meantime, however UV-formulations have been developed which also allow deep drawing of printed material. Such systems like e.g. Vacuplast VAC are already being used and future production of formed parts with UV-inks can be considered to be unproblematic.

GLASS and CERAMICS

Glass and ceramic are materials becoming more and more important substrates for screen printers. Main screen printing applications are printing of flat glass materials for the furniture industry and also flat glass for various industrial applications of e.g. gaming machines.

Glass used in the packaging industry is also screen printed directly. Various ink manufacturers offer UV-inks for different applications. Formulations are often developed for one single application such as UV/GL for furniture glass or Crystal GLS for packaging materials. Often adhesion and resistance requirements are very high, then only 2-component ink systems such as Z/GL are suitable.

METALS and COATED SUBSTRATES

Even today metals and coated substrates are mostly printed with 2-component solvent-based ink systems. Despite this special UV-formulations like UV/K or UVP are offered, which partially achieve good results. Prior to using UV-inks, however, weather resistance requirements and further processing need to be evaluated. We recommend close co-operation with the ink manufacturer to find the most suitable screen ink type.

TEXTILES and other ABSORBENT SUBSTRATES

Use of UV-inks for textiles and other absorbent substrates is quite problematic. Often an even ink layer cannot be applied to the structured surface. The main problem absorbent substrates present, however, is the fact that monomers absorbed by the material cannot be cured sufficiently and the printed inks therefore do not show the required properties. In addition due to non-polymerised monomers they have a high irritation potential. Non-coated wood materials show similar conditions.

COMPREHENSION

After this description of the most common substrates for the screen process we can say that meantime UV-inks are offered for almost any material. Naturally screen printers want to print as many materials as possible with one ink type. For graphic applications the UV-inks available today meet this demand. Common materials like paper, cardboard, PVC and polystyrene can be printed with one ink type, e.g. Multistar.

When printing other materials and if resistance requirements are high special UV-formulations have to be used. Thus the number of UV-ink types will constantly increase mostly with inks especially developed for one application.

It is a fact that innovations are important for us to keep taking advantage from the growing potential of this flexible printing technology. Innovations of printing machines, innovations of ink production. Innovations, above all in reference to UV-curing screen printing inks.

Possible applications of our UV-curing inks for various substrates (pdf-file, 52 kB)