Grafica News - February 2018
The quality of your print begins… before you print!
It is always good to repeat some essential facts !
Some reminders, with the best products and best press the best printer, it will be impossible to make a good job if the screen is not good !
- Choice of the fabric for the screen (according to the works and the effects)
- Choice of the frame (wood, alu, steel, self-stretching)
- Tension (the very high tension)
- Technique for thick stencilling, or fine one (direct, capillary, direct - indirect)
- Quality of image on screen § EOM (thickness of emulsion on fabric)
- Rz Value Choice of the fabric for the screens
Choice of the fabric for the screens
To control if the choice of the fabric for the screen done is the good one, it is necessary obviously to know exactly which fabric is needed for the application in question : "normal" printing, for the lines, relief, halftones, indexed colour, special effects, etc. The first rule is so to know what it is necessary for what!
Here are some examples :
- special Effects as relief (3-D), glitter, metal ink : Fabric S (fine threads) or T (average threads) between 10 and 43 sons / cm.
- Puffing inks : HD 29 in 49 / cm
- On dark T-shirts, for the white of undercoat : T 43 in 90 / cm
- For line or solid colours : T from 120 to 140 / cm
- For the halftone, classical or stochastic and the indexed colours : T 120 to 150 / cm.
- For the flock, Braille or printing averagely thick : HD 20 to 43 / cm.
- Four colours in UV or water based UV, T 140 or 180 / cm, calendered if possible
The weaving can be either « twill wave » (TW) or « plain weave » (PW), the best but not always possible in very fine fabrics such as 180 or 200/cm.
The colour of the mesh must always be yellow, to prevent light dispersion in the thread, then a lack of definition.
Choice of the frames (wood, alu, steel, self-stretching)
If one takes into account my recommendation (to see farther) very high tension of screens,
the choice of the frame is very important.
It is obvious that the good old wooden frames are completely to ban.
The choice stays, when it is possible (financial reasons in investment), among the aluminium frames (rather the steel or iron because not oxyding at short-term) and the « self-stretching frames » screens.
- Aluminium frame : these should be rather in profiles " Slop ", more resistant to the bending.
- a very good solution remains the the best example by being those of Stretch Device (USA), the « Newman Roller Frames », with their system of fast semi-automatic tension Screen-Stretcher.
Their advantage number one is the capacity to re-stretch them at any moment, what allows to maintain them in the chosen optimal initial tension, and allows so solving the problems of the slow loss in time owed to the change of the number of the threads connected to the inevitable loss of tension of polyesters day after day. I recommend these frames, when possible to use them, especially when a precise register is needed and moiré problems solved « at a certain tension = number of threads at a certain moment ». Their reliability, in spite of their price obviously higher, than that of the frames for the « stretch and glue ». There are copies on the market, but they are far from being worth the original.
Tool of indispensable control : Tensiometer, electronics if possible (Sefar), system of counting of the threads by photographic film on the fabric « in the thread near » (Garnaud “Mesh Calc” ).
Here, I want to be very clear : I always advise in screen-printing to use the very high tensions ( more than 25 or even 35 Newtons / cm – when possible ) = 25 in 30 Newtons / cm from 140 to 180 threads / cm) 25 – 35 Newtons/cm for 77 to 120 cm.
The advantages of the very high tension are fantastic in graphic screen-printing, textile and industrial (except the case of certain packaging, containers, phials or bottles which ask or more flexible fabrics, such as polyamides.
- Weaker deformation of image (thanks to reduced off-contact),
- Higher speed of printing,
- Quality of halftone image or fine line much higher (sharpness, dots shape),
- More precise register,
- More constancy from the beginning to the end of the run, so fewer adjustments in the course of printing,
- Better transfer of the ink (ink flow)
- Less penetration of the ink in the substrate,
- Less wear of squeegees, of the stencil and of the fabric of the screen.
- Consumption of ink reduced,
- Much better quality control of the printed image,
- Faster setting and adjustments from a colour to the next one,
- Repetitiveness in case of reprinting or replacement much superior,
- At the end, strongly increased productivity.
For the systems of tension, the selfstretching frames do not need it in priori, but Roller Stretching System " Roller Master " makes gain a lot of time, repetitiveness and constancy.
For the «stretch and glue » frames there is on the market a heap of mechanical or pneumatic devices of tension of different origins and prices. Verify that the manufacturer has this type of system or makes its frames stretched by a supplier who has it.
Manual tension completely disadvised obviously!.
Before stencilling, whatever the stencilling technique in use and this includes the diverse CTS techniques, the fabric must absoloutely be very well prepared to receive the stencil. This indispensable step is too often neglected by too many screen printers. A good adhesion of the stencil to the fabric, the elimination – when a screen is re-used of any trace of inks, emulsion or ghost images – must be total and perfect.
As a chemical preparation is generally not sufficient with a new fabric, the fabric must be slightly (and of course not too much) prepared with a mechanical abrasion using preferably the special products supplied by the manufacturers of fabrics or of chemical for screen printing.
A careful washing with high pressure water must then be done : the polyester must become « hydrophilic ».
Reclaiming the screen with high pressure washer
Michel CazaFrance Past President. FESPA,
Recipient of Howard Parmele Award from SGIA, USA.
Michel is a great supporter of DMI since its inception and his technical articles are published in Grafica News India.