Polypropylene sheet UV printing issue

[viswanaathan_r]

Hi,
We received a complaint from our customer on white spots (ink picking) on the printed cartons - please see the attached photo. The substrate is Polypropylene and the process is UV printing and Aqueous sealing. Swear the problem was not present during production and inspection before despatch. However the problem was observed at our customer end. We are sure the problem appears after the consignment shiped and manifests only after transit. Also, the problem is localized to one particular batch. The PP supplier claims 38+ Dyne.

Seek help from the forum on how to go about in analyzing the defect and finding the root cause.

Thanks in advance.

 

[Cornishpastythighs]

Do you perform 'Tape Tests' at the press to check for adhesion? Are you checking the Dynes level for yourselves?
I have heard of people printing a 'Size' on plastic in one pass and then putting the print on top in a second pass, I know its not very efficient but they never get adhesion issues ever. Is the issue really adhesion and not ink scratching or being worn off in certain areas either by your process of at your customers.

 

[tamale]

In my expereience aqueous sealing is rarely efficient for polypropylene. We have had great success with cationic uv varnish, its tougher than nails.

Tamale

 

[viswanaathan_r]

Thank you Cornishpastythights.

The tape test is not applicable for PP printed sheets. This is what our PP supplier says. Both the defective cartons as well as the OK cartons fails on tape test. However, on the items printed on the rough side of the PP (some other job), the tape test passes. In this particular job, we have to print on the smooth (front) side.

No, we have not been testing the Dyne. Shall test and come back. In the meanwhile, shall appreciate if you can furnish more information on the "Size" coating. I am desperate and would like to try.

I feel the problem is adhession, I could see traces of ink on the rear side of the sheet that was on top of the defective carton.

RV

 

[danprtr]

Lot's of variables you can try changing to eliminate problem but might be easier to address symptom by: 1)Slipsheet. 2)If warm from UV curing, let sheets cool in small lifts before stacking. 3)Build crate allowing sheets to be stacked on edge instead fo flat. 4)Add tiny amount of fine spray powder. Good Luck.

 

[Cornishpastythighs]

The reason that a company would apply a sizing to the sheet is usually because the Corona/flame treatment has a shelf life. So if the material is treated and the Dyne level is correct when it arrives at your plant if you do not use the material for a length of time the treatment will degrade over time. Thats why it would be a good idea to invest in some Dynes pens to check.

 

[viswanaathan_r]

Thank you danprtr for the valuable insights. I shall try all the points and revert. I do not understand what is Slipsheet. Can you please explain?. Thanks.

 

[REYES1377]

Thank you Cornishpastythights.

The tape test is not applicable for PP printed sheets. This is what our PP supplier says. Both the defective cartons as well as the OK cartons fails on tape test. However, on the items printed on the rough side of the PP (some other job), the tape test passes. In this particular job, we have to print on the smooth (front) side.

No, we have not been testing the Dyne. Shall test and come back. In the meanwhile, shall appreciate if you can furnish more information on the "Size" coating. I am desperate and would like to try.

I feel the problem is adhession, I could see traces of ink on the rear side of the sheet that was on top of the defective carton.

RV

We often print on PP and always perform a tape test. I've never heard that it will not pass. In my opinion it looks as if the sheet had oil drips or grease cling to it that will not allow inks to print. One rule of thumb we have is always check dyne level ourselves. If its too low reject it.

 

[viswanaathan_r]

Thank you REYES,

May I ask which side of the PP you print, the smooth side (front) or the rough (back)? My tape test passes on the rough side printed jobs.

What is the low level of Dyne that you reject? The job is printed at a different location, 400 miles away from my factory (and that is why I do not have the Dyne value). I will advise the printer to check the Dyne value as soon as the PP sheets are received. Thanks in advance.

RV

RV

 

[REYES1377]

We print on the smooth side. One thing that is different is that i UV varnish or coat, not AQ coat for protection. They say that a 36 should be pritable, but i will not print on a synthetic unless it is 38 to 40. it's just not worth trying to adhere to the bare minimum dyne level. I have learned some expensive lessons.