Each month, leading manufacturers and DTG print shop owners share their know-how in our new DTG column. This month, Geoff Baxter from M&R’s digital division gives the lowdown on the least popular yet most crucial part of the DTG process: pretreatment
There are several misconceptions about pretreating garments prior to printing on direct-to-garment digital printers. Some think it’s an optional step. Others think there’s little difference between manual and automated pretreatment.
So, what exactly is pretreatment – and why does proper application play such an important role in the quality and durability of the images produced on DTG printers?
Pretreatment is a chemical spray that reacts with white direct-to-garment ink, causing the white ink to gel on contact. That gelling (also known as crashing) is the result of a binding agent found in pretreatment solutions – and the subsequent application of heat from a heat press or appropriate conveyor dryer.
Pretreatment chemicals not only enable white ink to adhere to black and dark garments, they also enable it to achieve maximum opacity so that none of the garment’s colour shows through the ink film. It’s not the binding agent that causes the ink to gel; it’s the chemical reaction with the salts in the pretreatment chemical. But while the binding agent aids in adhesion and wash-fastness, the key role of pretreatment is providing opacity.
Pastel shirts may require a light pretreatment if a white highlight print is being used. However, as a rule, the pretreatment solution used on dark garments should not also be used on white shirts. Pretreatment may enhance the initial appearance of the garment, but it will nearly always detract from wash-fastness, and the printed image may even yellow over time. That said, there are a number of pretreatment solutions on the market specifically designed to increase image durability and colour vibrancy on white shirts. But these are optional, and garment decorators will need to decide for themselves if any resultant improvements in quality and durability are sufficient to justify the additional time and expense of pretreatment application on white garments.
The ultimate purpose of pretreatment isn’t to create a patch of white ink in the finished print; it’s to allow the true colours and crisp edges of the designer’s image to be seen. A properly affixed white underbase enables low-viscosity DTG inks to sit on top of, instead of being absorbed into, the garment’s fabric: when DTG inks are allowed to soak into the garment, the result is muted or muddy images instead of vibrant colours that stand out from the dark colour of the fabric.
The pretreatment application process is another crucial element in obtaining optimal results. The process can be as simple as filling an electric or pneumatic paint sprayer with the solution and hand-spraying it on garments. Unfortunately, while it’s an inexpensive approach to the task, there are two major drawbacks: control and consistency. There is simply no way manually-applied pretreatment can compete with automatic pretreatment application. It’s impossible for an operator to uniformly cover the area in need of pretreatment, and there’s even less consistency from operator to operator. One employee may lay it on too thick, while another may lay it on too thin. In some areas – often the centre of the designated image area – the solution will be too thick; in other areas – usually the edges – it will be too thin. And while it may seem that applying a thick layer would be the answer, it can result in unintended printing issues – and it’s hardly cost effective to waste expensive pretreatment chemicals. Waste is also what you get by spraying outside the target zone. Finally, the inconsistent application of pretreatment solution makes it harder to troubleshoot printing problems. If some images in the same print run come out looking better than others – or don’t last through as many washings as others – how can you tell if pretreatment is the issue if there’s no consistency in its application?
High-quality automated pretreatment equipment solves these problems by allowing operators to dial in a precise amount of solution, apply it evenly and consistently, and confine the treatment area to the area where it’s needed. And by keeping close tabs on the settings used for each print run, operators will have the information necessary to fine tune the amount of solution necessary to achieve optimal results.
Another advantage of automated pretreatment systems is that they limit the amount of chemical spray that the operator – and anyone else in the area – is exposed to, not to mention the reduced need to remove overspray from adjacent surfaces.
Both pretreatment options – manual and automatic – should be followed by heat curing prior to digital printing. Times vary based on ink and fabric types, but 30 seconds under a heat press is a good starting point. In some cases, the operator can position the heating element over the substrate to drive off some of the liquid prior to pressing the substrate. In order to minimise time spent on trial-and-error testing, operators should keep track of what works best on various ink/substrate combinations.
Despite the benefits inherent in automatic pretreatment, I don’t want to discourage people from getting into direct-to-garment printing just because they can’t afford an automatic pretreatment system. With close attention to detail, they can achieve acceptable – if not exceptional – results from manually pretreated garments. But I would recommend investing in a high-quality pretreatment system just as soon as it’s financially feasible.