The DTG Column


It’s all in the cure, or is it? - DTG and durability in the wash


This month Colin Marsh, managing director of Resolute DTG explains the direct to garment curing process and the importance of correct procedure.


L


ast month the DTG column was about storing your inks correctly to get the best performance from them. This month I am going to be giving tips on how to get the best possible durability in the wash from the curing process.


Pigmented textile inks used in inkjet DTG printers all require specific curing to perform correctly. This can change with each brand of ink but the curing process for all is to achieve good durability in the wash.


What does the cure do?


Behind the scenes nano particles in the ink and also in the pre-treatment are hard at work making sure your prints last as long as possible. Strict guidelines must be followed or you could find your prints looking dull after only a few washes. The curing process has two main parts. The first one is to remove all the water from the ink that has been printed onto the shirt. This will of course air dry in time but it does not prepare the garment for general washing, in order to do this the second part must be activated. The heat introduced into the curing process activates something called a binder. These binders do exactly as the name would suggest, when heated to the correct temperature for the right amount of time they bind themselves to the pre-treatment which is already bound to the fibres of the fabric. The process is called polymerisation (see figure 1), once


both the binders in the pre-treatment and the binders in the ink are activated by temperature, cross linking takes place.


The cross linking process is when two parts become one. My description here is a very rough generalisation of what is actually quite a complicated process. Both temperature and time are critical in making sure the cross linking is done correctly. Too much heat or too little time will render the process weak and the durability is affected.


Figure 2 below has the full technical explanation provided by Dr Andy Hancock.


Now the curing process is explained


letʼs cover what is used to perform the cure.


Heat press


In most cases a heat press will be used for DTG printing. This can be used to dry the pre-treatment and also cure the DTG prints.


Regular checks for cold spots and an even temperature should be performed to make sure you are getting good repeatability with your curing. A simple heat strip can be used but this has limitations.


Most good quality heat presses will have a Teflon coating allowing you to use a laser thermometer. This can be an easier way of detecting cold spots, these normally appear on older well used presses but can be present on brand new equipment.


Pigment particles


Ink layer Textile substrate Binders/ pre-polymer Water, humectants and co-solvents evaporate


Pigment particles


Cross linked binder


Ink layer Textile substrate Binder to substrate adhesion Figure 1


A typical DTG ink will contain water, humectants/ co-solvents, pigments (to give the colour) and binders (to give the wash integrity)


Figure 2


During the heating (curing stage), the water and co-solvents evaporate. Also, a polymerisation reaction occurs, which cross links all the binder chains, forming a cross linked matrix. The cross linked binder sticks to the fabric and also protects the pigment particles – giving the wash fastness required. Under cure of a garment will not only mean trapped oils and water in the ink film, but will also result in a lower cross linked density, both resulting in poor wash fastness


| 72 | October 2017 www.printwearandpromotion.co.uk


Example of poorly cured print after washing Tunnel drying


With some DTG inks now only requiring a 35 second cure, smaller tunnel dryers more suited to the DTG environment can be used. These are not so well suited to drying pre-treatment, without pressure the fibres are free to stick up making for a poor print.


Using a heat press for drying


pre-treatment and a small tunnel dryer for ink curing is probably the most productive method that can be used.


Key points to remember ● Always perform regular wash tests yourself.


● Check your heat press for cold spots. ● Make sure your temperature and times are correct.


● Keep doors closed when using a tunnel dryer to avoid under cure.


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