Converting from Plastisols
in Permaset Blog30 Aug 2013 | 0 Comments
When converting from Plastisols to water based inks there are three important things to consider:
- 1) Water Resistant Emulsion
- 2) Keep the ink on the screen wet +
- 3) Ensure that you have adequate curing capacity
EMULSION: You MUST have a water resistant emulsion, the emulsion must be completely dry before exposure (keep a humidity meter in the dark room/ screen storage room) and then thoroughly exposed to achieve full cure before printing. Moisture in the film, even re-absorbed humidity will reduce efficacy of UV cure on exposure and the integrity of the cured film will thus be compromised. With a water based ink, particular on long runs, this will result in premature failure/shredding of the emulsion/stencil image.
The first indications of such failure will be a slight sweatiness on the underside of the screen this can be felt with the back of the hand when the screen is up between prints. The second indication is pin-hole failure of the stencil, resulting unwanted dots appearing on the printed image. If this happens, stop printing immediately. The third indication is that the emulsion starts to shred.
Plastisols are a lot more tolerant in regard to emulsion choice and degree of retained humidity, so greater care is essential.
So, to prevent premature stencil failure:
- 1) Choose a WATER RESISTANT emulsion and block-out
- 2) Ensure that the film is fully dried before exposure and
- 3) Ensure that the film is fully exposed or cured
KEEP INK WET ON THE SCREEN IMAGE: This is arguably the greatest challenge for printers used to plastisols converting to printing with water based inks. First and foremost, keep the image area flooded between prints. With standard (transparent) inks when printing on whites, keeping the screen open is relatively easy.
However, with the much higher solids opaque PERMASET SUPERCOVER inks, significantly more care is required. Before starting your print run, ensure that you have a spritzer bottle of water handy. Give the image area a really good misting over and allow to soak in for 10 minutes. Our theory is that this water is absorbed into micro-pores and other interstices in the polyester mesh fiber matrix. This does 3 things; first, it stops the very limited “free” water in the ink being sucked out of the ink at the start of the print run. Second, it provides a lubricating film of water over the fibres, allowing the ink to flow more smoothly over the mesh. Third, it acts as a reservoir of water from which the ink can draw during the course of the print run.
After allowing the 10 minute soak, wipe off any excess moisture, put ink on the screen and start printing.
Don’t stop whilst printing! If you are interrupted for a relatively short period, flood the image area with ink and spray mist liberally over the wet surface. If the interruption is likely to be lengthier, spray mist liberally over the wet surface and then put cling-film over the wetted surface to minimise evaporation.
DRYING & CURING: With Plastisols, if, having gone through the drying tunnel, the print looks dry and feels dry, then you can be pretty sure that it has cured. With water based inks, the drying and curing steps are separate. First ALL the water has to come out, so that it feels dry and only then will you start to achieve full cure. Remember that a typical curing regime is 3 minutes @ 160°C (320°F). However, the temperature cannot get over boiling point (100°C; 212°F) until all the water has left the film. It has been calculated that 90% of the energy consumed in a drying tunnel goes into water removal; only the last 10% goes into the cross-linking or curing and it is not until the ink is properly cured that the print achieves wash and dry-clean resistance properties.
An associated issue is airflow within the drying tunnel. If air movement is poor or inadequate, then the humidity in the tunnel becomes saturated, no more water can leave the wet ink film and the print doesn’t even achieve full dry, let alone curing. Thus it is imperative to ensure that you have adequate airflow, ideally flowing from the rear to the front, so that the driest air is at the exit end.
In humid climates, cotton shirts in particular may absorb a lot of moisture prior to printing, so it might be worth running the unprinted garments through the tunnel prior to printing, just to reduce the moisture removal load on the tunnel during the actual print run.
If you only have a limited length drier, it may be necessary to run the garments through multiple times. An alternative is to slow the belt-speed right down, but this can result in slowing down the entire print operation. The best option is to invest in a gas drier of adequate length.
This really is Murphy’s Law at its finest. When you don’t want the ink to dry (i.e. on the screen) it does. When you do want it to dry (i.e. in the drying tunnel) it doesn’t. At room temperature, water has quite high vapour pressure, which is why it evaporates so readily from the wet ink surface on the screen. However, at elevated temperatures, more energy is required to get water molecules to dissociate.