blocking issues

I had some problem when printing 2side coated paper with high gloss varnish,i get blocking when the delivery pile is too high (5000sheets paper 115gm). My pile temperature is between 32 and 34°,hot air at 47% and IR at 45%. The press is an Xl75. Thks

 

Are you applying O.P Varnish? Do you use spray powder or not?

 

We do use op varnish and powder spray (20 microns), but still we have this problem. Any other suggestion. Thks

 

This is a common problem in areas where total ink density is high and when you apply O.P varnish, it adds up to this total ink density approx 100%.
Try using bigger micron powder and low piles.

 

turn down the temps and go to a c-350 powder

 

Thanks,will try it.

 

seeing how your listing your tempeture in centigrade as opposed to fahrenheit I'm gonna go out on a limb and assume that your reference to Varnish is in fact what we in the United States call aqueous coating. If this is in fact correct ,then what I would suggest is that you strive for pile temps not to exceed 95 degrees fahrenheit (ill leave the conversion to you) when printing the back side of a job. The 95 degree farenheit temp should be arrived at with at least 75 percent of the heat output coming from your hot air knives, and the remaining 25 percent output coming from your infrared bulbs. The main reason for the bulk of the heat coming from the air knives is so that the moisture from the coating be better evacuated out of the delivery. You will want to be sure that your ventilation of that moist air is not compromised by dirty filters or inoperable exhaust fans. I would be very cautious in using the the dryers sensors in determining pile temp. Invest in a quality hand held infrared thermometer to accurately monitor pile temps, as a variation of as little as 5 degrees fahrenheit high can be enough to block up even a short stack of freshly coated printing. Also as mentioned i would be looking at a spray powder of at least 25 microns and apply it as needed.

 

Thank you, will make some test and let you if it's ok