As we move into 2018, the pretreatment process for D2 printing is still a major requirement. We have seen a lot of development with inks and a wider range of fabric printability combined with newer pretreatment solutions. The actual process of pretreating a garment prior to printing is something that will probably be around for a long time.
What is pretreat?
When it comes to digital printing, the ink needs to be very thin to spray through the tiny nozzles of a print head. Because the ink is so thin, the material being printed needs a pretreatment to bind it properly. If no pretreatment is used, it just gets absorbed.
Pretreatment serves a few different tasks during the printing and curing process. First, the pretreatment creates a surface that the ink can print on and not be absorbed. When the ink encounters the pretreatment, a chemical reaction causes the ink to gel. The reason for this reaction is to help the ink itself become more stable and rigid for printing on top. A good example would be when printing a white ink underbase on a dark garment, as the white is printed first followed by the color on top. If this process did not occur, the ink would begin to bleed together creating an unwanted swirl of color and white ink. The final stage for pretreatment is during the curing phase. When the print is being heat pressed, the pretreatment becomes a bonding agent that helps bind the ink to the fibers. When the correct temperature, time, and pressure are used, you should have washfastness on par with screen printing or any other garment decoration process.
Once the pretreat has been applied, it must be dried. A heat press is the most common method for this, although higher production needs a conveyor dryer which can be used similarly to screen printing.
Different pretreatment requirements
Generally, you will need to use a pretreatment for light garments and another for dark garments. This can cause some confusion when first trying to differentiate when a garment is considered light or dark. It is not really about the color of the garment, but more so the print and whether you are printing white ink. Whenever you print white ink, you use dark pretreat regardless of the color of the shirt. Whenever you are just printing CMYK color with no white ink, you use light garment pretreatment. Essentially, the only time you use light garment pretreatment is when printing white shirts or light pastel colors.
In some cases, you may not have to pretreat white T-shirts. You can print directly onto a white shirt and get an acceptable print. In the early days of D2, the only time pretreatment was used was for dark garments or whenever white ink needed to be printed. Over the last few years, it has become the standard to use a light garment pretreatment. The introduction of this product showed a major improvement in color vibrancy and detail. When you put a pretreated and non-pretreated garment side by side you can see that it is worth the effort to pretreat.
The latest development has been moving towards expanding the range of fabrics you can print on with D2 printers. Cotton and natural fibers have been the best match for digital inks with a small range of blends with less than 50 percent polyester. Until recently, 100 percent poly and especially dark polyester have been almost impossible to print. This last year we have seen a couple of companies offer a new ink that works with all cotton, polyester, and blends. The same rules apply to this pretreatment when it comes to light and dark although curing times and temperatures are a little different than cotton. So, now when you print on cotton, you use the pretreatments made for cotton, and when you want to print on polyester, you use pretreatment specifically for poly.
This guide shows the path you should follow for even pretreatment application. (Image courtesy OmniPrint International)
Pretreating by hand or machine?
The process of applying pretreatment to a garment needs to be done with a specific amount of precision. The industry standard for a long time has been done using a Wagner HVLP hand sprayer. These sprayers are generally used for professionally painting fences and walls.
Over the last few years, pretreatment machines have become very popular as well. There are a decent amount of brands to choose from with a price range of $800–$5,000. Popularity has grown a lot due to the range of products and decrease in cost to own. The first generation of these machines were thousands of dollars making it hard to justify this cost in some cases.
Both of these application methods have become an area of constant debate when it comes to production capability, application quality, and consistency. This is one of those chicken or egg debates and it really depends if it fits your needs and individual skills. In my opinion, both applications go hand in hand and having the ability to utilize both aspects is a good solution.
Regardless, both processes have their own requirements for optimized use. Both with learning curves.
Hand pretreatment process: Pretreating a garment by hand will give you a direct reference point of how much pretreatment to use in relation to the print quality obtained. The print itself is going to show you if you did not use enough or were not consistent with your overall application. You are going to learn what the result of these errors look like in the print itself. With a little practice and experience, you will know what you did wrong when your print has a pretreatment related issue. Pretreating by hand is going to make you understand just what is needed and what is going on. This by far is the most important aspect of manual pretreating.
Pretreating by hand also requires you to create a specific area or set up for the process. Because you are spraying in the open, a decent amount of overspray mist is going to occur. A basic fold up plastic table will work great to spray shirts on. A hard floor that can be mopped will be best. If the weather is not a common issue, it can also be done outside. Wherever you pretreat, it is critical to never pretreat in the open near your printer. The overspray from pretreating may build up in your printer and cause damage over time due to corrosion.
Pretreatment machine: The variety of machines and price points available have made owning a pretreatment machine much more common these days. The great thing with many pretreat machines is that they are enclosed, so pretreating can be done close to and in the same room as your printer. There are a lot of factors that benefit production capability. Having your heat press, pretreater, and printer closer together will help workflow for starters.
Pretreatment machine (Image courtesy Brother)
The pretreatment amount is adjusted by the speed the platen moves; faster means pretreatment and slower more. You will need to find the sweet spot on the speed dial to get just the right amount of solution. The equipment will have a way of adjusting the amount dispensed. The other adjustments are the width of the spray pattern and the start and stop points for the pretreatment. When you spend a little bit more, you will have a digital screen with actual zones to choose from when spraying.
Just like your printer, a pretreatment machine needs to be cleaned and maintained. After every use, it is very important to keep up with the required maintenance. Pretreatment is similar in some ways to white Elmer’s glue and will get gummy and then dry up quickly if not cleaned. It should only take a few minutes and save you a lot of grief.
Proper pretreatment procedures are critical for outstanding results. Following the steps outlined here will set you on the right path towards achieving washfast, bright, and quality prints.
Pretreatment machine (Image courtesy OmniPrint International)