STEPS IN INK PRODUCTION-
BLENDING THE INK BASE
The ink production process starts with the varnish (also known as the binder, carrier or vehicle of the ink). The main components of the varnish are resins, solvents or oils, which can be in the form of liquids, powders, beads or granules (see Figure 5.1). This component provides the basic functionality of the ink and acts as a carrier, or vehicle, for the other additives which add color-specific properties at a later stage.
Figure 5.1 Resin beads
These components are dispensed and weighed into a mixing vessel, then blended and mixed to form the ink base (see Figure 5.2).
Heat is generated during this process by the sheer forces on the materials working against the equipment, and the temperature is managed using water cooling systems built into the mixing vessels.
Figure 5.2 Blending head
Correct temperature control and mixing time are vital at this stage to ensure a quality finished product. The process plays an important part in defining the final properties of the ink relating to viscosity and flow rates.
For an offset or letterpress ink, the formulator is looking for a high viscosity, slower flowing product. The flexo and UV flexo process, by contrast, requires a high flowing, lower viscosity ink able to run through the print units without first being broken down by multiple rollers.
The next stage of the process is pre-dispersion, where the color powder pigment is introduced to the varnish and further diluents are added to produce a homogenous mixture. This process is called wetting – the object is to wet the powder pigment uniformly. Further dispersion additives are introduced at this point.
Sawtooth blades rotate at high speed and draw in the product from around the mixing vessel, above and below the mixing blade, creating a shearing action within the mixing head (see Figure 5.3). The object is to achieve an even pre-dispersion ready for the next stage of the mixing process. As the pre-dispersion is refined further, it develops color strength, brightness and gloss.
Figure 5.3 Sawtooth mixer blades
There are three main methods of manufacturing pre-dispersion:
Triple roll mill
The most widely used equipment for the dispersion process is the triple roll mill. This consists of three steel rollers rotating in opposite directions against each other with a very small gap between the rollers (see Figure 5.4). The pigment particles are broken down between the rollers, which need to be water cooled to prevent too much heat build-up in the ink. It typically takes two or three passes through the machine to reduce the particle size to specification. On each pass the gap between the rollers is reduced to increase pressure on the pre-mix.
Particle size is measured using a grind gauge.
Figure 5.4 Triple Roll Mill
Another method of manufacturing pre-dispersion is a bead mill, where the pre-mix is passed through a cylindrical vessel equipped with a rotating shaft and agitators (see Figure 5.5). Within the bead mill are a series of zirconium or glass beads which act against the high-pressure product, which is then fed out through filters at the far end. For high viscosity inks, one pass through the bead mill is usually sufficient, but for UV flexo-type inks, it may require longer processing to create the type of viscosity and flow rates the formulator is looking for.
Figure 5.5 Bead mill
The basket mill is more suited to the production of concentrates for intermediate-type inks than the higher viscosity products used in offset printing. The basket mill is the mixing head of an industrial mixing machine and it typically blends up to one tonne batches (see Figure 5.6).
Figure 5.6 Basket mill
The drive shaft, with the bead mill attached, is lowered into the pre-dispersion mix. When it is immersed, the mixer is switched on and the product flows in over the conical top plate, through an impeller inside the basket.
The mixing blades and the basket create a sheering action on the product inside the basket mill.
With the grinding media on the inside of the basket mill, the ink runs around the basket mill and is forced out through the bottom (see Figure 5.7). On the bottom is another sawtooth blade arrangement which forces the product down, creates a vortex in the mixing vessel and provides an even recirculation, resulting in an evenly dispersed product.
Figure 5.7 Grinding beads
Time and temperature limits must be adhered to, because too much heat getting into the product may cause early polymerization. This may become evident while the product is being mixed, but may not become evident until the ink later reaches the customer.
The next stage is blending, which follows the dispersion operation and quality control checks. The final properties of the ink are achieved through the blending process (see Figure 5.8). At this stage further resins, varnishes, monomers, photoinitiators, acrylates, oils, waxes and drying compounds can be added to impart color strength, viscosity and flow to the product (see Sidebar_Additives).
Again, this operation needs to be carefully controlled to avoid excessive heat and overworking. Higher shear operations generate a lot of heat and pressure, so are not used at this stage. Instead, paddle stirrers and lower speed blades are used to achieve a final blend. Once blended, this is considered a finished ink.
Figure 5.8 Blending
The finished ink is passed onto the quality control department to check against the specification and retained samples of the product.
A grind gauge is used for this operation. This tool has already been used during pre-dispersion, when the pre-mix is checked to make sure the pigment particle has been reduced to the correct size.
Now, a small deposit of the product is placed at the coarser end of the grind gauge, then a metal scraper is used to draw the sample down over the grind gauge. The position on the grind gauge – where we start to leave trails of pigment – indicates the fineness of the grind or the fineness of the dispersion.
If the pigment particle size needs to be taken down further, processing time is increased at the earlier stage of pre-dispersion. Following this operation, the ink goes through a series of further checks, depending on the end use requirements of that batch. The checks confirm the color strength, viscosity, flow, drying and adhesion properties and any other specific requirements. They include a side-by-side drawdown comparing the new batch with a retained sample of a standard product.
Using a drawdown bar – a steel bar wound with very fine wire – a drawdown is conducted over a whitepaper background, checking opacity among other parameters. A side by side visual check backed up by spectrophotometer readings confirms whether the new product is in the same specification compared to a standard. Other mechanical testing equipment may also be used at this point, for example a UV curing rig on a print proofing machine (see article on Testing, proofing and quality assurance).
Once the ink batch has passed the QC process, it moves on to packaging. The production works order indicates the packaging specification. For example, products with matting agents, silicones or waxes may be prone to some separation between the manufacturing blending operation and packaging operation and will need to be packed into the final containers reasonably quickly.
The dispensing station that pours the inks into the containers is equipped with calibrated scales. A container label is applied prior to dispense showing the product description, part number, unique batch reference number, date of production and any appropriate hazard declaration symbols. Risk phrases appropriate to the product are displayed in the customer’s local language.
UV inks and varnishes are packed and stored in opaque black containers to stop any UV light starting an early polymerization of the product.
Once the product is packed, it is moved on to the warehouse until required for picking and transportation.
GMP FOR FOOD PACKAGING COMPLIANT INKS
It is the ultimate responsibility of the seller of packaged goods to ensure that the packaging complies with relevant legislation and guidelines regarding migration. In recent years, the packaging chain has worked hard to minimize the potential for migration of packaging substances into foodstuffs.
These components include substrates, inks, adhesives and coatings.
Ink manufacturers have taken steps to ensure food packaging compliant materials and GMP processes are used working closely with the other members up and down the supply chain,
While products may be termed ‘food packaging compliant’ by ink suppliers and packaging manufacturers, this terminology must always be qualified and shown to be compliant with the local guidelines where the final product will be sold. Examples include the Swiss Ordinance, European Commission Plastics Regulation (EU) 10/2011 and Nestlé Guidance Note for Packaging Inks.
For European ink manufacturers these guidelines are integrated into GMP (good manufacturing practice) set out in the EuPIA (European Printing Ink Association) guidelines for food contact materials. The guidelines are audited within the ISO 9001 quality management system certification to ensure that food packaging compliant products are manufactured from suitable materials and in a controlled and audited manufacturing environment.
Ink suppliers will also carry out their own process risk assessment. This starts at the initial formulation and scaling up stage of the product and shows up any potential issues attributable to storage, manufacturing processes and potential for cross-contamination.
Possible sources of contamination include: the wrong product getting into the ink or coating; microbiological contamination (only likely to affect some water-based products); and physical contamination from the factory, which could be paper, glass or plastic contamination.
The process method used is FMEA (Failure Modes and Effects Analysis), which looks at the severity, the probability and detectability of a problem, along with the counter measures when a problem arises.
This generates a risk priority number, which leads to action to remove the problem or potential problem from the process. GMP also takes in the monitoring and documentation of general site hygiene for employees and visitors, including hand washing and eating, and smoking guidelines for appropriate areas.
Consideration must be given to cleaning agents, auxiliary materials and lubricants used in the production process to ensure no traces contaminate the finished product.
Identification and traceability of components is another essential element of GMP. Raw material batch numbers are recorded during delivery to the ink maker and through the manufacturing process. The raw material supplier’s batch numbers allow the ink maker to link the raw material to the manufactured batch and then on to the end customer.
Two-way traceability allows the ink maker to trace back through his raw material supply chain. In the event of a product recall, there is a documented procedure that has to be followed with specific roles and responsibilities for the people involved.
The QC specification for all products – including food packaging compliant inks and coatings – is set up at the manufacturing stage by the formulator. This details the checks which need to be done and includes color shade, color strength, drying and resistance properties, all compared to a standard sample.
The results of the testing are documented and a retained wet sample of the batch is kept for future reference.
Packaging storage follows EuPIA's GMP guidelines, which include dedicated storage facilities for both raw materials and finished products of food packaging compliant inks and coatings.
The products are kept in cool, dry, indoor conditions, where the temperature does not fall below 5 degrees C or above 25 degrees C, ensuring stability of raw materials and finished products. All the containers used for primary packaging are previously unused and stored in covered, clean and dry environments.
Responsible ink manufacturers demonstrate commitment to GMP by establishing and communicating the GMP policy, regularly reviewing its effectiveness and striving to continuously improve processes, manufacturing environment and product quality.
As stated earlier, GMP is not only about what happens within the ink manufacturing operations – it is about quality across the supply chain, including the inks, manufacturing and testing equipment, substrates and peripheral products.
Liquid inks manufacture