Author: Peter Rose
Date Published: 2007-06-01

Choosing a textile dryer

Peter Rose, of Active Screen Europe, discusses the factors that go toward achieving correct ink cure,and provides some invaluable tips on identifying the right dryer for your shop…

Let’s get one thing straight - the purpose of a dryer is not actually to dry your prints, it’s to cure them. Drying and curing are not quite the same thing. Curing is when all the ingredients of the ink are fully cross-linked to provide a print with optimum longevity and washfastness. Drying is when your customer phones you up a month later and tells you all his prints are fading or washing off (and he didn’t ask for the ‘distressed’ look!)

Choosing the correct dryer and learning how to use it effectively are vital to achieving a correct cure, and thus the long-term success of your business. So let’s run through some basic parameters you will need to consider in order to identify the right dryer for your printshop.

Measuring temperature

If you don’t have any way of measuring the temperature the ink is reaching in a dryer you won’t have any way of monitoring the curing process, other than doing a test wash after each batch. This is time consuming and usually too late if you have already undercured a job.

Invest in packs of temperature strips, an infrared ‘ray gun’ thermometer, or a digital thermometer with a long enough probe to go through your longest dryer. Place the strips or the probe thermocouple wire both on top of and inside the shirt so you can ensure you are getting a good cure all the way through the ink film. That way, if a manufacturer specifies their ink needs to be at 160°C to cure, you can confirm that it is actually reaching that temperature.

In fact, temperature measurement equipment of one sort or another is an essential tool for all printshops. Cure time varies according to the type and weight of fabric, as well as the type and thickness of ink deposit. You are unlikely to be able to pick a ‘one cure .ts all’ setting for your dryer and expect it to work for all circumstances.

Use the temperature measurement tools to measure cure rates for different jobs and record the optimum settings for future reference. The tools should also be used periodically to check heat distribution in the oven and to see if you have any ‘hot’ or ‘cold’ spots.

Types of heat

You should be aware that there are two types of heat employed in textile dryers – radiant and convection. Radiant heat is generated by infrared elements and the energy is transmitted to the surface of your print by – yes, you guessed it - radiation. A good analogy would be a microwave oven.

There are different types of heating elements found in textile dryers which transmit on different wavelengths – the shorter the wavelength, the fiercer the heat generated on the target, but usually the depth at which the heat concentrates is more specific. For example, in a quartz flashcure unit, the object is not to fully cure the shirt, but to dry the surface very quickly for the next colour to go down. These units will generally have high-wattage medium wave elements (or lamps) which are great for flashing, but you wouldn’t seriously expect to fully cure your prints with one.

Conversely, the longer wavelength radiation typically found in the ‘black body’ type of heater element gives a ‘gentler’ heat that will penetrate further into the substrate and is more suitable for a full cure.

Whilst shorter wavelength elements can theoretically cure plastisol very quickly in a short tunnel, the downside is there is a tendency towards colour-sensitivity. Remember that dark colours absorb heat faster than lighter ones: in a dryer with colour sensitivity, black ink on a white shirt will cure very quickly indeed, but white ink on a black shirt may be problematic, with the black fabric absorbing heat whilst the white ink reflects it back. This is where you can end up with scorched shirts that have undercured prints.

The other type of heat is convection. This is where hot air is blown through the substrate and heat transfers via the air .ow into the shirt and the ink. For textile printing, convection heating is preferable as it is not colour sensitive and gives a much ‘gentler’ cure. This means less possibility of shrinkage or scorching, especially to delicate fabrics.

The downside of convection textile drying is that it takes a lot more energy to heat up the requisite volume of air to the right temperature and then maintain it. This is what has prompted the move to gas dryers for larger units over the last 15 years, as it became cheaper to heat up these large volumes of air with gas burners rather than electric elements. However, with all the recent changes in energy prices, there may now be very little difference in per-shirt running cost between gas and electric models, especially when considering modern well-insulated ovens.

Ink type

Plastisol ink cures by a cross-linking process that occurs at a certain temperature (usually at a point between 140°C and 160°C). This temperature needs only to be achieved throughout the full depth of the ink film for a few seconds for the ink to become fully cured. Water-based inks, on the other hand, have to have all the aqueous content of the binder evaporated out before the curing (or baking) process of the pigment part of the ink can take place correctly. This is why water-based inks take longer to cure than plastisol and, therefore, why they need greater drying capacity – that is, a longer tunnel.

If you are planning on printing mainly with water-based inks, you should be looking at a convection-type dryer with good air .ow: the more cfm (cubic feet per minute) the better. If you are strictly a plastisol printer, you could get away with looking at radiant-only ovens, although bear in mind the caveats outlined in the ‘Type of heat’ section, (above). Do you use special effect inks, such as puffs, suedes, hi-builds, glitters, metallics and so on? This is where a cheap, basic dryer that is .ne for simple work may just not do the job correctly. Inks such as these are far less forgiving in the consistency of cure required. If you plan on using a lot of these inks, it is vital to undertake test prints on your prospective purchase.

Shirts per hour

You have to be realistic about the drying capacity you need. For example, the average modern automatic press can print shirts at a rate of anything up to 840 per hour quite happily, even more if you can load fast enough. In practice, what with stopping to top up inks, remove threads, slow down for .ash curing and so on, your actual output of completed shirts may only be half that per hour. But should you then decide you only need a dryer that copes with 450 per hour, what do you do when you get that massive ‘bash-em-out’ job of ‘six zillion’ white shirts with a single-colour left-breast print that just .ies off the press? Do you want to print more slowly because the dryer won’t cope? The point being, try and budget for a dryer that will deal happily with your maximum theoretical output and then some, leaving you extra capacity for future expansion. Put another way; buy the biggest capacity you can afford!

Garment type

In the ‘old days’, all you had to worry about were bog-standard T-shirts, sweats and polos. Nowadays, in addition to this traditional garment decorator’s fare, you could be printing hoodies, jackets, overalls, bags, and even caps in the average day. Easy to overlook, but ensure that any dryer you are considering has plenty of clearance for your bulkiest items. And not only at the entrance and exit to the oven - what is the height of any radiant panels internally? Can they be adjusted to accommodate unusually large things? Are you printing simple cotton garments or do you deal with Lycra/elastanes, neoprenes and other fashion fabrics? These are more susceptible to scorching or shrinking if the heat is too .erce and really need a convection-type dryer for the best results.

Gas or electric?

Gas dryers are generally available only in the larger sizes for automatic output (1.2 m or 1.5 m belt widths with a 6 - 7 m overall length are common). This is due to the accepted wisdom that it is only in these sizes that the economy of gas versus electric comes into its own. However, energy prices have been in turmoil over the past few years and it is now by no means certain whether gas or electric will offer significant savings in running costs, especially if you are looking to the future. You should check the kw per hour costs with your local energy supplier carefully, and get some realistic figures for genuine energy consumption per hour in production from your potential dryer manufacturer before making your mind up. You may be surprised by your findings.

Don’t forget also that gas dryers need electricity to run the big blowers that circulate the hot air within the oven, the exhaust system, and many of them have infra-red pre-heaters as well. That all needs to be taken into consideration when calculating total running costs.

Dryer size

Choosing the correct size of dryer requires a bit of simple arithmetic. Let’s say you’ve established how many shirts per hour you will be printing. Now you need to know the cure time of the average print. The only way to .nd that out is to make some test prints on your potential purchase at the supplier’s showroom and then do wash tests to verify the best cure.

Let’s say, for this example, that your tests show you can fully cure a plastisol print in 60 seconds and a water-based one in 90 seconds. Now you need to consider the size of the oven chamber and how many shirts will comfortably be lying on the conveyor belt in that area at any one time. Suppose the chamber is 3 m long, and, with a bit of overlapping, you would normally have .ve shirts occupying that length on the belt at any one time in production. With the plastisol cure time of 60 seconds, that means you have a cure cycle of .ve shirts per minute. Which means you have a capacity of 300 shirts per hour.

If the belt is wide enough so you can lay two rows across, your capacity immediately doubles to 600 per hour. If you’re printing kids’ sizes and can squeeze eight shirts per 3 m length, then you’re up to 960 per hour and so forth.

With water-based ink the curing time is 50 percent longer (90 seconds as opposed to 60). So your dryer capacity is correspondingly reduced.

Feature checklist

You should have established by now the type and size of dryer you need for your production. But beyond that, what are the essential extra features you should be looking for?

• Even heat distribution Most dryers measure the heat in the oven by sampling at only one point. Poor heat distribution (or ‘cold spots’) can mean a perfect reading on the control panel, but shirts on one side of the belt not curing properly. Take the temperature measurement tool (see above) of your choice and measure at the extreme sides of the oven as well as the centre to ensure even curing.
• Level of insulation The better insulated the dryer, the less energy it will use to cure your product, and the cooler your shop will be in the summer. Ask your supplier for full details of the insulation used. Or try standing next to your prospective purchase after it’s been running at full temperature for a couple of hours. Could you fry an egg on the casing? Or could you safely leave your cold beer on top?
• Ease of cleaning Running textiles through a dryer all day long leads to a gradual build up of combustible lint, and there have been some spectacular dryer .res in screen printing history, when a stray spark has ignited ovens with longer beards even than ZZ Top. Ask your supplier about access for cleaning; does the hood lift up, for example? Are there .lters to catch the lint? And how easy are they to change?
• Built-in exhaust If your dryer does not come with a built-in exhaust blower, ready to hook up to ducting, then you are looking at extra expense. Bear in mind also that most dryers will not have an integral exhaust that discharges more than 4 or 5 metres. If you have a high ceiling and need to exhaust through that, you may need an extra exhaust blower on the outside, otherwise the fumes will just drift back down the chimney.
• Instrumentation Do the temperature controllers give a good clear display of the set point and the actual temperature inside the oven? Is there any kind of alarm to warn of excessive temperature .uctuation? Does the belt speed control display a digital readout of the actual time in minutes/seconds in the oven, (most useful to humans), or belt speed in feet-per-minute, or is it just a simple knob with 1 - 10 on the dial, (least useful to humans)?
• Portability Dryers tend to be big heavy lumps and if you decide you need to move one two feet to the left, it can be problematic. Some will come with wheel kits as an option that makes repositioning for cleaning or maintenance very easy. This is the sort of apparently insigni.cant feature that can make a big difference in real-world situations.
• Cost of ownership Not quite the same as running costs, (that is, how much you have to put in the meter every day!) Obviously, you should make as detailed enquiries as you can about this - try and .nd out how many BTU of gas and KWH of electricity a dryer actually consumes under full load. When it has to heat up a few kilos of fabric in the oven, it will consume more energy than when it’s idling with nothing on the belt. The maximum rating is always a good guide – for example, you would expect a dryer that has a maximum electrical requirement of 60 amps to be more expensive to run than one rated at 40 amps, but it’s worth checking.

  Beyond the energy costs, try and get an idea of what items need to be regularly changed and what the cost would be. What is the life expectancy of the heater elements and motors? Should you have the foresight to buy a couple of spares for your own stock when you buy the dryer? Lost revenue if there is a delay in obtaining spares to fix a breakdown will usually cost you far more than the part itself.

Peter Rose has been involved in the garment printing industry since 1971, both as a printer and an equipment supplier. He is currently a sales agent for both Hix Corporation (USA) and Anatol S.P.E. (Poland). Tel: 07771 987452.
Email:
Web: www.activescreeneurope.co.uk

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