Indirect transfer dye-sublimation printing is the technique where a digital plotter is used to print a design onto a special paper and then a calander or a flat press is used, operating at temperatures in excess of 180°C to transfer the ink onto the fabric via sublimation.
Before we continue, it may be useful to explain the natural phenomenon of sublimation, using a simple example: after freezing water, we can turn the ice directly into steam using heat. We can then take the gas, transfer it to a different place and turn it back into ice by cooling it down, without it ever passing through its liquid state.
This is exactly what happens with transfer printing.
In more detail: the paper (called transfer paper) undergoes a special surface treatment so that it accepts the sublimation printing ink (usually water based). Once the ink has dried and solidified on the paper (the ink being the ice in our previous example), it turns into gas due to the temperature in the calander (like the steam) and moves onto the fabric where it is cooled down rapidly and turns back into a dry, solid ink (like the ice) without every becoming liquid (like the water).
This is a linear phenomenon that can lead to some practical issues during production.
Digital plotters have revolutionised traditional printing on transfer paper (which previously only used analogue rotary or screen printing systems) enabling the reproduction of photographic images and small/medium printing runs.
In any case, the design has to be mirror-printed backwards on the paper, so the software (that gives free rein to creativity) has to have a “mirror printing” function so that the design will be the right way round after it has been transferred onto the fabric.
The following has to be decided, taking into account the definition of the design, the fabric used and the intended application:
- the correct calibration of the ink.
- the best weight for the paper
- the speed and number of passes for the printing heads.
All plotter manufacturers recommend installing the plotter in rooms where there is constant temperature and humidity control, for rapid drying of the ink.
This is because paper swells up when it is very wet, running the risk of being touched by the heads on the printing carriage, which would completely ruin the job.
It is also advisable to leave the rolls of paper for at least 24 hours in the room where they will be used, possibly in a vertical position for them to acclimatise to optimum room conditions.
Calander and Flat Presses
Calanders or flat presses are used for transfer printing in order to transfer the ink from the paper to the fabric.
The basic parameters for best results are:
- Cylinder/press pressure
Compared to plotters, it is vital to consider the type of fabric used because its properties will have a considerable impact on the settings of the machine.
As discussed in previous articles, the best fibre for dye-sublimation printing is POLYESTER because it offers the best reaction to the thermal shocks caused by the high temperatures that can reach up to 215°C and the residence time that can often reach well over 60 seconds.
It is important to follow the manufacturer’s instructions and advice with due care when handling or moving the equipment on account of the high temperatures and potentially dangerous operating conditions that demand your complete attention throughout the production process.
The calander enables roll to roll fabric processing and the use of practically any type of fabric including ORTHOGONAL/SHUTTLE woven or KNITTED fabrics, as well as stretch yarns, non wovens, RUGS with low pile. For higher piles, such as CARPET, flat presses (especially aspired versions) produce more in-depth absorption of the ink. They are generally used for transfer printing on fabric pre-cut to the required shape, and this is called “garment-laid print”.
Bear in mind that EVERY fabric has its own properties and must be tested to define how it should be worked, and that EVERY batch can give a different end result. Therefore, it is ALWAYS wise to do a quality test run before starting actual production, even if you are using the same fabric/design.
There are also some precautions for handling the rolls before and after use.
Always store the pieces of fabric laid out horizontally, overlapping them in length and never crossing them over.
If only a part of the roll is used, try not to unroll more fabric than is needed to avoid having to roll back up the unused part, as kinks and creases may form that could cause difficulty and faults when the remaining fabric is used later.
To guarantee the fabric’s dimensional stability, they are HEAT SET during the finishing stage at the dye works, i.e. they are stabilised using heat but if the thermal shock they will be exposed to when passing through the calander or flat presses exceeds the level of shock received at the dye-works, this may lead to shrinkage on the warp (lengthwise) and on the weft (width-wise). This risk should be checked with tests carried out prior to production and calculated for the printing on the paper in order to add the necessary “extra” amount so that the final measurements are correct at the end of the job.
These are some basic indications regarding transfer printing using calanders or flat presses and it is important to remember that there are many unexpected variables and issues to be resolved for this type of processing. But this is the case in any job, and experience and hard work are the best way to learn the techniques and methods needed to improve.
All of the steps involved are quite delicate before, during and after printing, both as regards the raw materials and the equipment to use.
Which technology to buy depends on a lot of factors that only an in-depth analysis of your strategies and your specific market will help identify.
What’s important is to not become overwhelmed by any of the probably unavoidable mishaps that are the key to professional growth.