Heat curing equipment_2

When do you need heat/drying in the screen printing process?

•    Drying emulsion before exposure
•    Drying your screen after washing off the emulsion
•    Touch drying after printing
•    Drying between colours
•    Fixing ink to fabric, The Cure

N.B. For printing posters and paper type flat work, you would use poster ink which is also an air dry ink. Either of the two textile inks may be used for screen printing shirts but the poster ink is used only for flat work and graphics printed on porous or glossy paper materials.

Drying emulsion before exposure

Screen drying cabinet or overnight, must be dust free and dark environment

Drying your screen after washing off the emulsion

You can blowdry it, or wave it in the air outside, or just let it air dry, but you should let it dry before making prints.

Touch drying after printing

Flash dryer, hand curer or heat gun

Drying between colours

Flash dryer, hand curer or heat gun

How hot does The Cure have to be and how long does it take?

Once you have finished your print, you will need to fully cure your garment. Simply drying the print does not mean that your garment is machine washable. In other words – The print will come out in the wash! The temperature must get hot enough to firstly evaporate the waterbased medium, then set the pigments.

This is a guide only
150F [66°C] Water begins to leave the ink
200F [94°C] Binder reaches lowest viscosity and maximum surface contact is made with the fabric
220F [105°C] Water begins to leave the ink rapidly
270F [133°C] Fifty percent of the water is gone and the binder and pigment start to cure
300F [150°C] Most of the water is gone and the binder-pigment combination is partially cured
300F [150°C] [for 30 seconds to a minute] Binder and pigment is cured

If you have used water-based ink:

Although waterbased ink dries to air, it takes a high temperature to cure water-based inks. You can cure the ink at 165°C (some are hotter) by using a flash dryer, tunnel dryer or heat press.  It can take up to 2 minutes to cure waterbased inks.  If you mix a catalyst with the ink you can cure the ink with a heat gun – but remember only really suitable for cold washes.

WPS waterbased inks need to be cured at 165 degrees or above.  To cure your inks, you have a number of options. You can use a flash dryer, hand curer, heat press or tunnel conveyor dryer for this.  If you are using a WICKED PRINTING STUFF flash dryer to cure your print, we recommend you measure the temperature of your print with a digital laser thermometer until it reaches the required temperature. Remember that it is important that the entire ink film thickness reach the specified cure/fusion temperature. Depending on the nature of your fabric it is recommended that you break this up into intervals of approximately 10-15 seconds and repeat 3-5 times as most garments cannot withstand such continuous heat in one go. If you are a busy printer, you will find that a conveyor tunnel dryer is a much quicker, more efficient and reliable way of curing prints.

If you have used Plastisol or other solvent-based ink:

In between the application of the different colours of your design you can use a heat gun to flash dry but to fully cure you should use a tunnel dryer, flash dryer or heat press.  The ink needs to get to 150°C for typically 60 seconds.  Glitter, reflective and white ink can sometimes take longer.
If you are printing more than one coat of ink for the same image, or printing a multi-colour image, you will need to flash cure your ink by using a flash dryer, hand curer or heat gun between each print. Flashing enables you to print one coat of ink on top of another – e.g., a color on a white base. You also might flash an ink to keep wet ink off the back of your screens. Some inks, such as glitters, metallics and high densities, are not designed to be printed “wet-on wet”. They should be “flashed” when printing in sequence. Once your print is dry to the touch you can apply your second coat or next colour.  Most inks will “gel” (flash) when the ink film reaches 220°F to 230°F (104°C to 110°C). There are 3 factors that affect the “gel” or “flash” of the ink: the temperature of the flash, the distance of the flash from the printed image, and the time the printed image is exposed to the heat. As a rule, you want to flash the ink film until it is just “dry to the touch”. Over-flashing inks can cause inter-coat adhesion problems and make the inks very “tacky”. Check your flash cure unit to see if it has temperature and airflow controls. These can help you better control your flash cure process.

 

All Plastisol inks will not air dry and will therefore need to be dried by heat equipment. Plastisol inks will also need to be fully cured at 150 (or above) degrees, which is drying them to the correct temperature and length of time (typically 60 seconds), in order for the ink to not come off in the wash. A flash dryer will dry and cure the inks in 60 seconds and for longer print runs or larger set ups, a tunnel dryer is recommended.

Drying Options

Iron, Hot Air Gun, Flash Dryer, Hand Curer, Heat press, Tunnel Dryer,

Unsuitable methods
Sun or outside clothes line – Can not be used as temperature is not hot enough.
Household clothes dryer – Can not be used as temperature is not hot enough.
Hand held hair drier – Can not be used as temperature is not hot enough.
Commercial Dryer (Laundromat) – Not a recommended method as temperature is not accurate and time limit can vary greatly on garment type. Garments left in too long may shrink or be damaged.

Comparing Curing Technologies

Curing Technologies

Tunnel Dryers


We manufacture the most comprehensive range of tunnel dryers in Europe ranging from the brilliant ‘Mini dryer’ to the top of the range dryers. The Mini Tunnel Dryer is ideal for the smaller workshop using a tabletop or small standalone carousel up to the top of the range WPS Panther Dryers which are ideal for screen printers with automatic presses or digital printers.

 

Choosing a tunnel dryer

Choosing a tunnel dryer

Panther Tunnel Dryer Comparison Chart

Panther Tunnel Dryer Comparison Chart

The Cure

More about Exposure Timings

Have you got problems with poorly exposed screens?

Do you have pinholes, stencil breakdown, or poor quality prints?

Are you wasting production time and materials?

Also see our blog post Creating a screen using traditional exposure methods

Following our post last week about exposure timings taken from our FAQs we were asked for more details. Transfering your artwork onto a screen is the most complicated part of the screen printing process, do it incorrectly and you will create problems for yourself. Here we have an example of a design with pin holes.

Design

The principle of the screen is that there is a mesh with little holes. The ink is forced through the little holes on to the substrate (t-shirt, paper, cap) underneath. We only want the ink to go through in certain places, in this case the red lettering. So we create a stencil where our design is black and the background clear and we use red ink. We put emulsion on the screen and expose the screen using the stencil. The emulsion hardens and blocks the little holes in the mesh where it reacts to the UV light. The stencil blocks the UV light in the areas of the design. If the exposure is not correct we can end up with some of the little holes not blocked at all (pinholes) or not to a correct depth or hardness. Then when the squeegee comes along and scrapes the ink along the screen, as well as forcing the ink through the mesh it also scrapes of a little bit of emulsion and makes more holes. This picture shows some pinholes just above the e.

So correct exposure is the key to getting a completely covered, really hard, good quality screen that will last for a long print run.

Most exposure lights and chemicals give you recommended exposure times but these can require fine tuning. The UV light has to harden the emulsion on BOTH sides of the screen. It is a chemical reaction between the emulsion and the UV light that hardens the emulsion. Where the emulsion is hardened the mesh is blocked. The stencil protects the emulsion from the light so when the screen is washed that part of the stencil allows the ink through the mesh.

If your screen is under exposed there is not enough emulsion is on the screen. Then as the squeegee rubs on the screen it removes the emulsion leading to problems.

In this example above the stencil is the red lettering. Therefore the clear area of the screen must be complete blocked by hardened emulsion. But we have some pinholes which means stencil filler or tape are required to block them. Correct exposure will stop this occurring.

Lamps age and whilst it might look like they are doing their job they are not producing enough UV to properly expose the screen

Most manufactures give timings for your lamp/emulsion but it is worth doing a step test at 2-3 month intervals to check for any degradation and so you can fine tune your exposure times.

A step test is a graduated exposure of your screen to see which time has the best results

You can get a step wedge or a Step Transmission Gray Scale,, but you don’t really need one. You can use a piece of thick cardboard to test exposure time. Start by burning the whole screen 1 minute less than the recommended time for your unit/emulsion. Then cover about an inch of the emulsion with the cardboard. Every 30 seconds, cover another inch of the emulsion by sliding the cardboard forward. Once the screen was fully covered by the cardboard, the test is complete. Wash out the screen to find which exposure time produced the cleanest result. Keep a record of this and then you can compare results in 2-3 months time.

A simple inexpensive unit like the WPS exposure unit has all the features you need.

Light Source: 1000 watt halogen light.  More efficient than two x 500 watt light units, where the light can overlap and cause uneven exposure.

Adjustable Height: For adjusting the exposure unit

For 12″ x 16″ screens – 10 inches and expose for 6 minutes

For 20″ x 24″ screens – 20 inches and expose for 15 minutes

For 31″ x 24″ screens – 23 inches and expose for 20 minutes

Important: Foam is charcoal in colour to absorb the light and not to reflect the light as lighter colours will do. This will prevent sharp toothing on fine line work and text when exposing.

You will need to supply your own sheet of glass. The glass needs to be at least 6mm thick double glazed or 10mm thick either single/double glazed.  The thicker the sheet of glass, the more heat from the lamp it will be able to withstand so you can reduce the distance of the lamp from the screen to reduce your exposure time.

This unit will do the same job as a unit costing £500.00+ it just takes a little longer to expose the larger screen.

For the professional printer requiring larger volumes the Heavy Duty Exposure Unit is more sutable.WPS sells a  range of  heavy duty exposure units manufacturer in the UK which come in three standard sizes and a number of variations to suit customer requirements. The three standard models have all the same features, powerful suction pump, touch up lights, safety switch etc. There are a number of power options available 1, 2, 3, 5 & 6 K/w versions.

Two controller options are available, a timer system which comprises a digital timer, and switch for the vacuum and fluorescent tubes as well as a power switch, or our Magellis PLC control system this incorporates an LCD display, time settings and all programable features are accessed from the screen. This unit can also incorporate a light integrator which compensates for lamp degradation over time and automatically adjusts the length of the exposure.

Each unit is fitted with fluorescent tubes for artwork positioning and stencil touch up. The powerful suction pump ensures intimate contact between artwork and screen giving perfect exposures every time.

Each exposure unit is suitable for direct or indirect films and can be used in an open work room.

Screen Printing History from Wiki

Chodowiecki Basedow Tafel 21 c Z.jpgScreen printing is a printing technique whereby a mesh is used to transfer ink onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen to fill the open mesh apertures with ink, and a reverse stroke then causes the screen to touch the substrate momentarily along a line of contact. This causes the ink to wet the substrate and be pulled out of the mesh apertures as the screen springs back after the blade has passed.

https://en.wikipedia.org/wiki/Screen_printing