Vacuum Web Coating

Optical density question

Here is the question.

I am student of packaging institute presently working in multinational company.please tell me what is the method to find out optical density of metallized film which is laminated to other film and in with print or without print? thank u very much for the information u have display on web.

Answer.

Here are the basic equations used in measuring the optical density

Optical Density    =            log ₁₀ Opacity    =    log ₁₀ 1

                                                                                                  T

Opacity    =     1                                  

                        T

Opacity    =     Incident light                       

                        Transmitted light                                             

Transmittance =    Transmitted light

                                    Incident light

The fact that the light is measured in transmittance means that the OD measured is a combination of the metal coating and the printing.  In order to have a measure of the metal it is necessary to have a measure of the OD with and without the metal present so that the OD of the print can be subtracted from the OD of the combined and thus the OD left will be that of the metal.

The simplest, and most reliable, method is to ask for a sample of the metallized film alone and measure that directly.

I hope this helps you.

CAB

Printing problem on Metallized Paper

Here is a Question that needs help.

Hopefully someone more expert than me can provide a suitable solution.

One of our customers does printing on transfer Met paper for gift-wrap application. While printing 3-4 colours job on 6gsm Met paper he does not find any problem of registration matching but during printing of 7 colours job on Met paper he finds proper registration on guided side & extent of mis-registration increasing as you move from guided side of web to unguided side in the width.

What can cause this to happen?

Is moisture disappearing from the Met paper during printing the cause?

Looking at the printed sample it looks like the mis-registration side edge is trailing & entering into printing nip later compared to the edge where guiding is taking place. Printing is done by rotogravure m/c.

Please advise on the probable cause and suitable corrective actions that might arrest this.

I am no expert on printing however your assessment about the moisture disappearing may be worth following. It is worth checking on the printing process.  Is the ink in a solvent base or water base? Does the web pass through ovens between colours? The metallization will have an effect on radiant heaters, as the metal will reflect more heat than un-metallized.  However the metal will also reduce the rate of water loss from the paper from the metallized surface so there are trade-offs.

Is the printing uniform in area from side to side? If there is more printing on one side than the other may skew the web.

If there is a difference in printed area there will also be a difference in moisture loss from the paper too.

I hope this helps you to a solution.  Hopefully others will add additional comments.