Red Ink?
Pierre Morel
Engineering management | Materials & Mechanical Engineer | Materials testing | Additive Manufacturing | Strategic Problem solver | Invention & Enablement WG
NO, I am not broke, but just testing ink on paper!
Putting ink on paper
I had a crash course on printing when it was time to order some invitations! First, the paper then the ink! I had no idea that inks were so different and could vary so much in price! This almost got me in the red (pun intended).
Interesting how something as simple as ink on paper never crossed my mind as a complex system with many variables to make it last and shine.
As usual, my engineering brain started firing up and I decided to investigate this problem. I realized that inks are all around us in products (not just in the infuriating printer that needs a new toner!). So, I picked 3 paper / ink systems that I found on packaging and brochure and decided to run a few tests on them to differentiate them and see if the shiny brochure ink was worth the higher price tag!
I wanted to look at three characteristics for each system: coefficient of friction (related to touch feeling), permanent deformation (visibility of scratches) and surface cracking resistance (visual aspect and “bend-ability”).
Preliminary information
Talking to some ink specialists, I discovered that my 3 samples actually had ink plus a coating on top. Here is the info I gathered:
·???????Sample A: Blue ink with UV coating
·???????Sample B: Purple with mat coating
·???????Sample C: Red with mat coating
Results
Rtec Instruments SMT5000 with a lambda profilometer was used for the following tests.
Friction
Many studies have shown the coefficient of friction to be related to the touch-feel perception of products. So, for the first test, I run a linear reciprocating tribology test on each of the samples. Using a 4 mm sapphire ball, the test applied 50 mN (light touch!) to the top of each sample and ran for 500 cycles at 1 Hz.
Despite the roughness of the paper substrate (creating some noise on the data), sample A exhibits the lowest coefficient of friction (this was the fancy brochure!).
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Permanent deformation
When ink is used on packaging products, it needs to be resistant to wear and scratch as the product is transported from the production site to the store and to the customer. Therefore, I wanted to see how “visible” some scratches would be on each of the ink/paper sample. I tried to reproduce a situation that those products could easily see in transport: small particles scratching the surface of the ink. Using a 50 μm diamond tip, I scratched the samples using an increasing force up to 500 mN.
Both the depth and the width of the groove generated on the samples were measured to compare the samples. The results show sample A as the lowest values. In other words, the ink system on sample A is the most “resistant” to permanent deformation.
Surface crack resistance
Beyond the groove size in the case of a scratch happening on these ink/paper systems, I wanted to quantify the resistance of each sample to cracks that could happen when bending the paper substrate or due to the previously made scratch. Using increasing force scratch, the critical load (force at which the first crack is observed) is a well-accepted parameter to differentiate these samples.
The summary of all the scratches reverses the ranking seen so far: sample A exhibits cracks at smaller forces that samples B and C. Sample C is the most resistant to cracking.
Conclusion
I was not expecting such stark differences between those ink samples. Sample A was the best until I looked at?the cracking of the top layer.
The requirements for a brochure are different than for a product that will travel and needs to still have a good aspect on the shelves of a store. Mechanically, I understand the need for an ink that resists deformation and cracking, but I was surprised that the coefficient of friction wasn’t more differentiated.
I made me feel a little better when I had to pay for the ink we picked, knowing that it will certainly look better and last longer!
More information about the instrument and applications:
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