What is UV litho printing?
Lithography/lithographic/offset printing — otherwise known as litho printing — is a printing process which was developed in 1796 in which the printed image is achieved through a fine chemical balance of water and inks.
‘UV’ relates to the curing process of the ink. Once the ink is transferred to the material, it then needs to be dried or ‘cured’. In conventional litho printing onto paper and board, the conventional inks are dried with heat. In UV litho printing, UltraViolet lamps are used on the printing press in conjunction with special UV-reactive inks so that the UV inks ‘cure’ quickly when exposed to UV light (‘UV curing’).
Essentially UV litho print tends to excel when the following attributes are required:
- High quality – accurate colour reproduction of fine detail.
- High speed printing
- Large volume printing
- Extensive choice of materials and thicknesses.
UV Litho tends to be more cost effective than digital printing when printing in larger numbers, and whilst it takes longer to set up than a digital printer, it is faster and more cost effective when printing large quantities of high-quality repeated items.
The UV Litho Printing Process, in more detail
Firstly, the image to be printed is first created on a printing plate via direct laser imaging in a Computer-to-Plate (CTP) device known as a plate-setter. This is typically carried out by ‘pre-press’ department or team. The blank portions of the emulsion are removed by a chemical process (although advancements in technology mean that plates have become available that do not require such processing). The ‘positive’ image area – which is the area to be printed – remains on the plate after imaging in the form of an emulsion.
To commence printing, firstly the press needs to be setup. To create a full colour image, typically 4 plates are used, each carrying a different colour – Cyan, Magenta, Yellow and Black (CMYK). This conversion is known as four colour process. To setup, plates are fixed to cylinders on the printing press. Dampening rollers on the press feed water to cover the blank portions of the plates, however the water is repelled by the positive image area. Hydrophobic litho printing ink is then fed in by the inking rollers, which is repelled by the water and only adheres to the emulsion of the positive image area. The plate cylinder then rolls against another cylinder covered with a rubberised ‘blanket’, which squeezes away the water, picks up the ink and transfers it to the paper with uniform pressure. The material to be printed then passes between the blanket cylinder and an impression cylinder and the image is transferred. This transfer of consumables to various cylinders forms the name ‘offset lithography’ or ‘offset printing’.
The sheets pass through the UV curing system, which can be at the end of the press or between the colour units (‘interdeck’ UV curing). The lamps can use traditional or LED curing bulbs (UV LED).
After the colours are printed, a UV varnish can also be applied to provide protection and various finishes typically gloss, matte or satin. These varnishes or coatings can be applied across the whole sheet (know as ‘flood’ varnish) but also special ‘spot UV’ finishes, whereby varnish or coatings are only applied to a portion of the sheet to create a special area of effect.
While the press is running, even with assistance from modern technology, this process requires a great degree of skill to maintain the balance balance of water, ink, speed and pressure.
UV litho printing creates vibrant colours and sharp images and is preferred for fast and repeatable high quality and higher volume print production. This makes it the preferred method for large print runs of products such as packaging, catalogues and brochures, and marketing collateral. Due to the relatively higher setup costs, digital print is more cost effective for smaller volumes of products, whereas litho printing can handle thicker materials and is cannot be matched in the reproduction of detail.
Litho printing can also be used to print on more surfaces than just paper, such as plastic. When UV litho printing onto plastic, the knowledge, skill and processes required are even more specialist, as the ink must be robustly fused to the surface of the material and protected from scratching throughout its journey.
As such this UV litho print onto plastic creates a high quality and highly durable printed product which proves to be cost effective for relatively medium to higher volumes.
What plastics can be used?
‘Plastic’ is a general term and in fact there are many different types of plastic polymers and resins, each having their own attributes and specialism. When printing on plastic, it is therefore vital to select the optimum material for the product or project at hand, for example relative cost, durability, weather-resistance, adhesion, recyclability etc. Specific types of plastics are preferred over other materials typically when a durable printed product is required.
When printing, plastics are not as porous and papers and boards and therefore need different dot gains to ensure optimum colour-matching. Some types of plastic create better results and more sustainable print that others.
General plastic use (and therefore printing on plastic) can be a complex and often misunderstood landscape. There are many different types of plastic resins and blends which gives a wide array of attributes and uses; however, it is typically recognised that polymers are split into seven types as per the Resin Identification Code (RIC) as below. This helps the UK residents and waste management system to sort post-consumer waste at kerbside as well as to sort and collect post-industrial waste:
Recycling in the UK
Waste is dealt with typically by local council contractors who will take it to the nearest facility, be that landfill or Material Recovery Facility (MRF), a Plastic Recovery Facility (PRF), an Associated Waste Management facility (AWM), or directly to the local incinerator. Recycling is classed a subsector of the waste management industry and is incentivised by the commercial gains from reprocessing and recycling material for sale. Recycling is highly dependent on material separation, which are then usually granulated and resold to make recycled products. Mixed waste vastly reduces the chance of recycling.
General waste tends to go to incineration or landfill as there is so little recoverable value in it.
MRFs’ manually sort materials while AWMs employ automated sorting e.g. IR separation, (water) density separation, magnetic separation. Some MRF’s have all technologies, some have partial technologies. MRFs, PRFs and AWMs then sell the separated waste (e.g. to mills) for use in recycled products. All centres much prefer easy to separate and volume waste – this is reflected in the market value of the waste products. The majority of this recycled material is uncoated and coated paper and board (fibre products), plus plastics such as PP, PET, HDPE and LDPE.
In the UK, there is a greater focus on using recycled and recyclable materials rather than using biodegradable or laminated rigid plastic products. This is because of the waste management infrastructure that we have, and using non-recyclable products increase the potential for pollution of existing recycling streams.
Printed plastics are also split into two general types: rigid and flexible plastics, which are also considered in the waste management process. Rigid plastics are often used in posters, pots, tubs and trays. Flexible plastics are often used for created pouches and wraps. It is important to seek advice on colouration and transparency of different material types before production.
Below is a list of the most common types of plastic used in UV litho printing.
Polypropylene (PP & rPP)
Printing on rigid recycled polypropylene (PP) is recommended for most retail and display applications in the UK because it is relatively cost-effective, durable, lightweight and widely recycled at kerbside. It is available in many colours and can be frosted or transparent. PP can be recycled multiple times before end of life and recycled polypropylene is also known as ‘rPP’. At end-of-life, when incinerated as waste-to-energy, PP has a high calorific yield and low emissions.
Polyester (PET, rPET)
Highly durable due to its relative density, PET is frequently used in food and drinks packaging, however it this can also make it relatively expensive when purchased in rigid sheet form. Clear PET is often called ‘Bottle Plastic’ and recycled as such, however care must be taken once printed with conventional inks, as PET does not follow the same waste stream and typically ends up in the ‘Jazz’ mixed waste stream.
Polyethylene (PE)
Two major types of PE are typically used in packaging and are suited to a wide range of applications from heavy-duty damp proof barriers for new buildings to light weight and flexible bags and films. LDPE (Low Density) is generally used for trays and heavier duty items such as long-life bags and sacks, polytunnels, protective sheeting, food pouches etc. HDPE (High Density) is used for thinner carrier bags, fresh produce bags and some bottles and caps as well as for milk, shampoo and detergent bottles.
Polyvinylchloride (PVC)
UV printing on vinyl can create products which are lightweight, cheap, tough & long-lasting, PVC has been used for a wide range of presentation products such self-adhesive stickers. Comparatively PVC is not widely recycled in the UK or easy to deal with at end-of-life. Although recycled PVC is widely available, printing on polypropylene is typically preferred in the UK where possible due to the relative recyclability of PP.
Polystyrene (PS, rPS)
Printed in its rigid, high-impact polystyrene form, PS is lightweight but can be relatively brittle and easily torn due to a pronounced grain direction. PS is not widely recycled in the UK and is not often recommended.
Special Orders
It is possible to source special blends and arrange makings direct with manufacturers, if required, to create a bespoke blend for specialist purposes, such as horticultural plant labels. Printing on plastic requires a careful matching of material, surface, ink and consumables in order to produce repeatable quality.
Print Finishing
As well as printing on plastic, it is also important to consider finishing techniques. Specialist producers have familiarity with cutting and finishing polymers, which by definition are often resistant to such processes. For example diecutting using a heated bed & cutting rule, to achieve the best finish without burrs or hairs.
Machine operators need to skilled with stripping and waste removal to ensure fast processing.
It may also be necessary to use bespoke hand stripping facilities to deal with more challenging products.
How to choose the right plastics for your project
When printing on plastic, organisations should seek to use the best material for its intended purpose and create the minimum amount of waste throughout the products lifecycle, be it through reduced size and weight, or maximising recyclability before end-of-life.
The full carbon scoping and life cycles of materials can be complex to calculate accurately and approaches to waste management can vary greatly depending on local and international regions. In the UK, plastic use is guided by the UK Plastics Pact, which advises a focus on recycling and recyclability rather than ‘biodegradable’ or ‘compostable’ plastics which, following EC guidance based on long-term studies, have the potential to contaminate existing recycling streams.
The main objectives of the UK Plastics Pact are:
- Eliminate problematic or unnecessary single-use plastic packaging through redesign, innovation or alternative (reuse) delivery models.
- 100% of plastic packaging to be reusable, recyclable or compostable.
- 70% of plastic packaging effectively recycled or composted.
- 30% average recycled content across all plastic packaging.
A good supplier can help you sort through all the options to find the optimum solution for your printing on plastic project.
What legislation do you need to be aware of?
As legislation comes into place across the UK and Europe, the polymer supply chains are adapting, and price and availability will continue to change to support more recyclable polymers. The two initiatives below are currently in effect and are also being adapted over time:
The Plastics Packaging Tax
https://www.gov.uk/guidance/check-if-you-need-to-register-for-plastic-packaging-tax
NB This tax only applies to plastics that contain LESS THAN 30% recycled material. If you choose a material that the supplier can declare to contain more than 30% recycled content then the tax does not apply.
Extended Producer Responsibility
According the gov.uk:
The way UK organisations responsible for packaging must carry out their recycling responsibilities has changed.
If you’re affected by extended producer responsibility (EPR) for packaging, you will need to report your packaging data.
Packaging definition
Packaging is any material that is used to cover or protect goods that are sold to users. It makes handling and delivering goods easier and safer. It also includes anything that’s designed to be filled at the point of sale, such as a coffee cup.
Packaging also makes goods look appealing for sale and may display a company’s logo or brand. ‘Goods’ could include raw materials or manufactured items.
What you may need to do
You may need to:
- collect and report data on the packaging you supply or import
- pay a waste management fee
- pay scheme administrator costs
- pay a charge to the environmental regulator
- get packaging waste recycling notes (PRNs) or packaging waste export recycling notes (PERNs) to meet your recycling obligations
- report information about which nation in the UK packaging is supplied in and which nation in the UK packaging is discarded in – this is called ‘nation data’
- What you need to do depends on whether you’re classed as a ‘small’ or ‘large’ organisation.
This is based on:
- your annual turnover
- how much packaging you supply or import each year
What to look for in a supplier?
A good supplier of UV litho printing onto plastic will be an expert in all of these factors and will therefore be able to advise you on the optimum material use for your product and needs. For example they could help to advise on:
- layout for yield / production, artwork production, graphic design, print spec, colour management, cutting options, finishing solutions, supply chain solutions, deadlines etc
- bespoke colour management, online proofing and workflow. ISO12567/2 can be applied to rigid plastic sheets, taking into account the discrete issues related to printing on plastic, such as the different dot gains.
They will consider the relevant factors such as cost, availability, recyclability, legislation in order to select this optimum. They will also keep abreast of emerging trends and legislations to be able to advise as the goalposts shift over time.
They will also be able to refer you to key third parties in order to create a broader and consistent set of references which are relevant to the target customers, market and supply chain.