emulsion!!!

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bigwhiteboy
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emulsion!!!

Post by bigwhiteboy »

Newbie in need of some help.

I have been having some issue lately with my emulsion (CCI DXP) and it is driving me to drink.

This is my life lately in a nutshell..I coat, I let dry, I expose, I washout, I get mad!

My exposure unit consists of 4 unfiltered high UV spectrum blacklight lamps, my supplier told me to expose for no longer than 5 min, which I have done.

But after washout the inside of the screen is extremely soft. When I go to blot the inside with the chamois it is extrmemly sticky and it pulls up a lot of emulsion on the chamois but the print side is ok and seems solid. Most of the time the image comes out ok but if you accidently use a little to much pressure on washout or use compressed air to dry it off your image is gone.

So the question is, is this normal? or is my screen under exposed or under dried..there has been a lot of high humidity days here in NY.

I am newbie and I hate it....lol

Thanks in advance :oops:
X_Autotype_#1
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Post by X_Autotype_#1 »

At first thought, your symptoms sound like typical under-exposure. Nobody can determine the proper exposure time for your light source other than you under the conditions you are using, considering your mesh color, mesh count, type of positives and their UV density, etc., etc. Your best bet is get hold of an exposure calculator designed to determine correct exposure on your light source incorporating all of your existing variables. These "calculators" are basically a film positive with several UV filters taped onto it which filters UV light at different calculated levels, thus providing you with 5 equilivent exposures in a single "burn". You start by coating your screen in your normal fashion. Using the exposure calculator as your positive, expose the screen to your light source at an exposure that is estimated to be at least 2x longer than you think the correct exposure should be. In your case, I would recommend your exposure test to be about 12 minutes. This will give you the equilivent of a 3 minute exposure in the .25 filter area, a 6 minute exposure in the .5 filter area, a 9 minute exposure in the .75 filter area, and the full 12 minute exposure in the non filtered area.

Wash out your screen by wetting both sides of the screen and washout mostly from the substrate side, but make sure that both sides are washed out turning the screen over consistently during the washout process. Use cold water of about room temperature only. You will see a color change from lighter to darker in the image area where the UV filters were withholding light from the stencil. This color change represents the thickness of emulsion on your screen - the lighter the color, the thinner the stencil. The objective of the test is to determine the point in relative exposure time where the image on the stencil is the both the thickest without image fill-in from over exposure. At some exposure level, the color or thickness will not change, indicating that you have successfully exposed the emulsion completely through the thickness of your coated screen. This will mean that the emulsion on the squeegee side of your stencil is "robust" and fully crosslinked by the UV light during exposure.

You can also achieve a similar result by preforming a exposure step test using any positive you have on hand. Although the manual step test is not as accurate and is more prone to accidental fogging of the screen, it will help you to determine the correct exposure time for your screen and light source. Start by using a 12 min exposure and a black piece of cardboard or other high UV blocking material that will hide the UV light from your stencil. Start your exposure by blocking at least 4/5 of the emulsion area with an image in it. Move the blocking board by 1/5 of the image area every 3 minutes during the exosure. This should provide a similar exposure step on your washed out stencil as the exposure calculator will provide. Look for the color change (thickness change) in the stencil. The most dense color is also the thickest, most exposed area of the stencil. The squeegee side of the stencil should not be slimy after washout, nor should it discharge any color onto your blotting paper.

Both the step test, and the exposure calculator test will save you time in determining your correct exposure for your light source and the other conditions that effect exposure time, including ambient conditions. You could just expose your screens longer and hope you hit it right, but it might take you 4 or 5 screens to determine the correct exposure level. The above method just saves you some time and some expense.

Ask your emulsion supplier if they can provide you with an exposure calculator. There are 2 or 3 manufacturers that make one. Autotype has one that works well, and there are others. Its a good idea to keep this handy in the shop to check your exposure from time to time, as the UV output of all light sources does diminish as the bulbs get older.

Sorry to be so wordy, but determining the correct exposure time for your screens and your light source is one of the most important variables to control in the screen printing process. It determines your capabilities in your image quality, durability of stencil life, ease of ink cleaning of your stencil without damage, and whether or not the screen can be reclaimed easily.

Good Luck!
bigwhiteboy
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thanks

Post by bigwhiteboy »

Thanks so much for the GREAT response...I feel like an ass for not step testing it before posting but I was really getting frustrated with it and the angry customers were not helping the situation either. So likse I said. I really appreciate the help.

If you don't mind another question to put my soul at ease... what would be the symptoms of the emulsion not being 100% dry all the way through before exposure? Reason I ask is because when I originally called my supplier with the issue they told me my screens weren't dry and that was causing it. The last batch of screens I ran sat horz in front a fan for well over 24 hours and still had the same problem.

If possible let me know what you think.

Regardless I am going to step test it and record all my variables from now on and go from there.

Thanks Again!
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Post by X_Autotype_#1 »

Typically, when your emulsion is not dry all the way through prior to exposure and washout, you will notice an image forming during washout, then the entire stencil will wash down the drain. What happens is that the dry areas of the emulsion crosslink and harden during exposure, but when the emulsion nearest the fabric is not dry, there is no adhesion to the fabric and it simply washes away.

During high humidity months, try to dry your screens in a dehumidified area, but make sure that they are never exposed to any UV light sources, even the very small amount generated by regular light bulbs. If you can get a hold of a dehumidifier or room a/c it will substancially speed the drying time.

Blowing air containing 70-80% RH will never dry your emulsion enough, even if they sit in front of a fan for a month! You need to evacuate the moisture out of the emulsion to enable the product to work properly. The dried emulsion should not be tacky prior to exposure.

Good Luck, and don't give up!
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Post by DTGPrinting »

Hey X, you definitely know what you are talking about! Keep up the great information!
Jerid Hill
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http://www.screenerschoice.com
bigwhiteboy
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Post by bigwhiteboy »

ok I have never had that happen yet... :shock:

I will work on my exposure times and give it all I can to make it work.

Once again your help is much appreciated...

you are the man.
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Post by Bored »

hi,

im having a same problem too...on the box of my emulsion says that it expires in 4 months, and my emulsion is a little over a year old now. does this affect anything?
X_Autotype_#1
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Post by X_Autotype_#1 »

Theoretically, in some cases under exposure and shelf life issues could exhibit a similar technical problem in processing the stencil. Obviously, if the product is near the end of its recommended shelf life, or has been stored under adverse conditions for a length of time, it will not perform as it should. In most cases, the sensitizer portion of the emulsion is more susceptible to failure due to aging or adverse storage conditions than the emulsion itself. And then again, there is a totally different “shelf life” of the product once you mix the sensitizer together with the emulsion. I don’t know what emulsion you use or the type of sensitizer system it incorporates (diazo, photopolymer, dual-cure, etc.), but it is always advisable to use the product within the timeframe recommended by the manufacturer. Bicromate sensitized emulsion, for example, must be used within hours after it is mixed. Although bicromate sensitizer systems are rarely used in North America now days, it was common in the industry during the 1980’s and is still used in many developing countries. Diazo sensitizers are much more stable and will likely last (unmixed) on the shelf well over a year if stored in favorable conditions. Once mixed with the emulsion, most manufacturers recommend that it is used within 30 days, less if stored in high heat conditions. Some of the newer sensitizers have extremely long shelf life, either mixed or not.

Check with the manufacturer specifications on the emulsion for the recommended shelf life, and whether or not your emulsion has a recommended “pot life” once the sensitizer is mixed into the emulsion. There are also time restrictions for use of emulsions once it is coated onto the screen. Try to use the product within the time frame that the manufacturer recommends, and always store the product, mixed or not, under the recommended conditions. Many emulsions become unusable if stored in too cold of conditions, and some completely separate solids from liquids when frozen. Heat can have a major impact on the performance of some sensitizers also, so be sure to know the storage requirements of your product. It may save you some money to follow the manufacturer’s directions on storage conditions, especially with the price on some of the higher performing or specialty products.

Best advice is to determine your proper exposure with your light source using a freshly coated screen with fresh emulsion. Once you have determined the proper exposure time for your conditions, you will be able to eliminate the exposure variable in future problems, and concentrate on other variables that may have been overlooked. Screen printing is all about control of the variables, and the more control you have, the better success you will have. Using your product within the recommended time frame is a variable that is easy to control compared to all of the other variables you must deal with in screen printing. You should eliminate that variable as a policy for any time sensitive product used in your shop.

Hope this helps.
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Post by Bored »

X_Autotype_#1 wrote:Theoretically, in some cases under exposure and shelf life issues could exhibit a similar technical problem in processing the stencil. Obviously, if the product is near the end of its recommended shelf life, or has been stored under adverse conditions for a length of time, it will not perform as it should. In most cases, the sensitizer portion of the emulsion is more susceptible to failure due to aging or adverse storage conditions than the emulsion itself. And then again, there is a totally different “shelf life” of the product once you mix the sensitizer together with the emulsion. I don’t know what emulsion you use or the type of sensitizer system it incorporates (diazo, photopolymer, dual-cure, etc.), but it is always advisable to use the product within the timeframe recommended by the manufacturer. Bicromate sensitized emulsion, for example, must be used within hours after it is mixed. Although bicromate sensitizer systems are rarely used in North America now days, it was common in the industry during the 1980’s and is still used in many developing countries. Diazo sensitizers are much more stable and will likely last (unmixed) on the shelf well over a year if stored in favorable conditions. Once mixed with the emulsion, most manufacturers recommend that it is used within 30 days, less if stored in high heat conditions. Some of the newer sensitizers have extremely long shelf life, either mixed or not.

Check with the manufacturer specifications on the emulsion for the recommended shelf life, and whether or not your emulsion has a recommended “pot life” once the sensitizer is mixed into the emulsion. There are also time restrictions for use of emulsions once it is coated onto the screen. Try to use the product within the time frame that the manufacturer recommends, and always store the product, mixed or not, under the recommended conditions. Many emulsions become unusable if stored in too cold of conditions, and some completely separate solids from liquids when frozen. Heat can have a major impact on the performance of some sensitizers also, so be sure to know the storage requirements of your product. It may save you some money to follow the manufacturer’s directions on storage conditions, especially with the price on some of the higher performing or specialty products.

Best advice is to determine your proper exposure with your light source using a freshly coated screen with fresh emulsion. Once you have determined the proper exposure time for your conditions, you will be able to eliminate the exposure variable in future problems, and concentrate on other variables that may have been overlooked. Screen printing is all about control of the variables, and the more control you have, the better success you will have. Using your product within the recommended time frame is a variable that is easy to control compared to all of the other variables you must deal with in screen printing. You should eliminate that variable as a policy for any time sensitive product used in your shop.

Hope this helps.

wow thanks...im pretty sure its the emulsion aging. because ive burned many screens before and this has never happened to me. so i guess ill buy a quart this time, a gallon is probably too much. thank you so much
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