Life through a pinhole

What specifically causes pinholing when printing on an underbase and how do you best avoid it?" Pinholing is a strange phenomenon, do we really know what causes it? The short answer is no.

Let's take a look at what we mean by pinholing, the snowflake effect of gaps in the ink when printing onto a white base. The ink is pushed through the mesh and leaves a pattern of gaps that look like a snow storm on the freshly printed ink. As with everything in our industry, it’s never one deciding factor. We must go back to the five fundamentals 

1. Mesh
2. Blade
3. Speed
4. Pressure
5. Angle

My first one of these key factors to check is Speed!

If we are printing too fast we can trap air in the corners of the mesh and create small pockets that do not print. A dull squeegee passing over the top of the freshly printed base can also leave air pockets behind. Slow the print stroke down and let the ink pass through the mesh cleanly.

Secondly I would start to check the other variables, first we look at the mesh, if this is too high, again we can experience the strange snowy phenomenon. 

I like to assess any problem on press into two categories,

• Too much ink
• Not enough ink

Pin holes fall into the ‘not enough ink” category.

Increasing the pressure to apply more ink can solve this issue, but we must balance this with the effect it has on the print.

When we print a sticky white ink onto the valleys and troughs of the fabric, we create an uneven landscape for the next colour to populate.

Often we must look at the base white, is the white printing clean? Or did we sacrifice that little bit of clarity for a few more prints per hour? Keeping the boss happy by speeding the print stroke up will definitely get you on his Christmas card list, he may even let you run all the difficult jobs now! But did it create another issue?

The base white exists purely to hide the nasty dark colour of the shirt. But it also lays the foundation for the next colors.

If the base is thick and sticky, printed with a ‘favorite ‘ blade on a mesh that’s seen more action than the strange new screen goblin working in reclaim, then the chances are we are creating troughs and valleys here that we don’t see until it’s too late.

If we are using a high mesh for the base then we can use a little of the additive that I hate telling people about, the dreaded ‘CURABLE REDUCER’ 

Curable reducer was invented purely because we as screen printers simply cannot be trusted! Plastisol thinner was readily available in the early days, it had some real cool names like ‘viscosity buster’ and it was a clear liquid sold by the snake oil pedlars of the day, it  resembled the oil that sits on top of that pot of ‘My Little Pony Purple” that has sat at the back of the ink room shelf since the purchasing lady was young!!

This clear liquid was superb at thinning down plastisol, it would release the bonds holding together the toughest of white plastisol inks like a long lost plasticizing super hero!, however…

The clear liquid was sold with clear instructions to NEVER break the 1% rule, did we listen to this strict scientific advice? Nah, we are screenprinters! We chucked a good glug in the top and mixed it in with an old stick we found lying around outside!

The ink would relinquish all hope of staying thick and printable, and with the addition of  2 fingers (approx.) of this wonderous oily liquid, our heroic ink would have the consistency of  a choc ice on a summer day. This would solve the on press problem but would awaken the kraken that is the QC lady. Shouts of ‘stop stop stop” quickly emerge from the wash test room (canteen) as she puts on her hi vis waistcoat and informs you that the ink is no longer on the shirt but is sat at the bottom of the twin tub with her new polyester weights.

When we break the 1% thinner to 99% binder ratio, we didn’t make the ink any thinner, we just made it stay wet forever!

The solution was simple. 

Either, Drive around every print shop in the Uk and remove the oily ‘viscosity buster’ that sits in its now rusting gallon tin in the ink store just behind the plascharge sample that no one ever used!!

Or

Mix some Viscosity Buster into some soft hand base, at the exact ratio that does not upset the delicate balance of the known universe and sell this new product as Curable Reducer, no matter how heavy handed us knuckle dragging self-proclaimed mad scientist printers are, we cannot break the main binder ratio in the ink.

This new fangled solution to all things print related is responsible for a myriad of print issues, but it can help to achieve a soft feel and can actually help print through higher mesh counts. The problem comes when we overuse this snake oil! A thin ink will also leave pin holes when printed onto a base, but the kick in the teeth is, so can an ink that’s too thick! We must assess the consistency of the coloured ink and use judgement to make small changes.

Adding Curable Reducer to the base in small amounts (I’m going to regret this advice out in the field) can also help reduce the pin hole issue.

A roller squeegee will create a super smooth surface to print onto, eliminating some of the peaks and troughs in the base layer.

Pressure and angle can be fine tuned to create a ‘more ink” solution to the problem.

The screen itself can also be a factor in this unwanted snowstorm effect, if the tension is too low it will leave small deposits of ink in the mesh on not on the shirt, if the screen has been reclaimed by the new screen goblin, the one with the breath that can make a cat puke from 10 yards away, then this can also create blockages in the mesh that look like pinholes.

In summary, we need to apply more ink to a smooth white base and don’t be tempted by the snake oil salesman. 

This issue has plagued all of us printers at some point and I would like to call on all of you to let me know your solutions! Let us know on the images SM pages or DM me and I will share your wisdom. 

 

Article written by Tony Palmer, Palmprint
Tony has more than 30 years experience in garment decoration ranging from manual screen printing on hand carousels to the operation of multi-color automatic presses.  Specifically Tony is an expert on MHM Automatics, Tesoma, Exile Spyder, Douthitt CTS, Zentner, and numerous manufacturers of textile decorating equipment.