Potential Techniques Reviewed
In this article we will consider how to get good high opacity prints onto dark backgrounds with the help of master screen printer, Douglas Grigar.
Printing a light colour ink onto a dark background can be challenging if you don’t know how. However, with expert guidance, printing white ink onto black shirts could become a speciality.
Generally there are three approaches to obtaining an opaque layer of ink onto darker garment substrates.
- Use of an under-base or primer layer of ink
- The Print-Flash-Print (PFP) technique *
- The Heavy-Flood-Single-Pass (HFSP) technique
* Since the print-flash-print technique is to some extent similar to the use of an under-base or primer layer (the print colour is acting as its own under-base) we will review and compare the Print-Flash-Print (PFP) technique vs the Heavy-Flood-Single-Pass (HFSP) technique here.
If you are interested in printing a single pass onto a darker garment substrate (HFSP) or just increasing opacity with the usual print-flash-print technique, the following points should prove very helpful to you.
How To Get Good High Opacity Prints Onto Dark Backgrounds
The Main Factors
1. Ink Selection
The HFSP technique absolutely requires the use of a high opacity plastisol ink for best results. If you have to use a standard opacity plastisol ink, you will have to print an under-base (primer layer of ink) first and it would not be possible to achieve a high opacity print using the HFSP technique.
2. Mesh Selection
It is important to use a mesh count that is within the range suggested by the ink manufacturer if you are to obtain the best opacity, so always refer to the technical data sheet for the ink you are using. Over time, the choice of mesh count will become “second nature” depending on technical demands and desired finished results.
Here are a few further pointers on mesh selection –
- Very Coarse mesh counts: 23–31 TPC threads/cm in “T” or medium thickness threads (60–80 TPI / threads/ inch)
If you have taken sufficient care to get a really good, thick stencil, you may be able to “get away” with the Heavy-Flood-Single-Pass technique using these mesh counts. The trade-off can be a slightly rougher texture to the finished print and a much thicker perceived layer of ink with a “harder hand feel”.
The ink layer is most likely to sit on top of the surface fibres of the garment and do not penetrate deeply. This is a visually pleasing result and is aimed at maximum “shelf appeal”. The trouble is that such garments can be less resistant to laundering and physical abuse (lower wash fastness).
The HFSP is often derogatorily referred to as the “sloppy spatula plop” for its thick layer and tendency to “orange peel” on the surface. Practice makes perfect is the mantra you need for this technique.
Because mesh count is a major variable in the ability to hold and print detail, many designs are simply not able to be printed using this technique. This is because the thicker threads and lower mesh counts cannot hold finer detail.
- Medium low and higher mesh counts: 43-61 TPC (110-155 TPI)
Go any finer than the mesh counts already mentioned as being suitable for the HFSP technique and you will need to use PFP or Print-Flash-Print technique. Overall, your results will be a much smoother, flatter finish, softer hand, thinner perceived thickness of ink and higher penetration of ink into the substrate giving a more durable and washable product. We highly recommend the print-flash-print technique for these reasons.
3. The Stencil
Another key step in how to get good high opacity prints onto dark backgrounds is to have sufficient emulsion over mesh (EOM) or a thick stencil. Larger diameter threads that are also naturally further apart gives a higher volume of ink in the deposit.
In all our screen printing classes, we teach the shine method of screen coating. This method of screen coating gives a much higher than normal EOM than is used commonly among screen printers.
Read more about this technique in our article entitled How To Coat A Screen For Best Results
Always fully dry the screen. A screen drying cabinet can dramatically cut-down on drying time and greatly reduces contamination of the screen.
If you are aiming for a HFSP onto dark background, you may need to add further to the thickness of the stencil by coating the bottom (face) of the screen with another coat of emulsion and allowing the screen to fully dry once more (often “upside down” or “face up”).
This is an advanced technique of screen coating referred to as “face coating”. Face coating can be repeated by adding additional coats of emulsion onto the bottom (face) of the dried stencil layer, plus additional drying time for each added face coat. The technique would be coat-dry-coat and so on, until the desired stencil thickness is achieved.
4. The Squeegee
Use a sharp edged squeegee of around 60-70 durometer and just enough squeegee pressure to contact the mesh to the garment. This minimised pressure on the squeegee is designed to help prevent the ink from being forced to penetrate the fabric to such an extent that the fibres move into the ink layer and become visible.
When this happens, it makes the final print look as if there is a decided lack of ink opacity. Learning the right amount of squeegee pressure will be key in how to get good high opacity prints onto dark backgrounds.
5. Mesh Tension & Off-Contact
Ensure off contact is correct. This is a variable that changes from ink to ink and mesh to mesh. Off-contact is affected by the squeegee, artwork and desired ink layer!
If the screen has sufficiently tight tension, this off-contact can often start around 3mm (1/8”) but will need to be increased at lower tension levels.
Slack mesh tensions will often exclude the possibility of successfully using the HFSP. If you want to attempt this technique, we highly recommend the use of retensionable screens to obtain optimal screen tension. The higher mesh tension screens will “snap-off” much easier and result in a better quality print.
One way you can set up the correct off-contact is to make a print with the screen touching the garment. Turn the knob that increases off-contact until the mesh snaps out of the ink layer deposited on the garment. Now test print once more. Your aim is to get “snap-off” IMMEDIATELY behind the squeegee blade as you print.
Mesh tension is measured in Newtons and we would suggest 30-35 Newtons. If you don’t stretch your own screens, discuss your requirements with your supplier and try not to use old, slack screens. You will save yourself a lot of headaches.
Unfortunately mesh loses tension over time as the screen is used and requires additional tensioning until the mesh reaches it’s stable tensioned level. This is where high-tension retensionable and the hybrid retensionable frames easily outperform standard static frames. These products are invaluable if you want to attempt the HFSP technique.
Mesh tension is measured with a tension meter. If you stretch your own screens, start with the mesh manufacturers higher level tensions. The mesh will relax to the mid-high level (or go for relax/tension/relax/tension methods for advanced high level tensions).
6. Print Speed
Generally speaking a much, much slower stroke and a steeper angle on the squeegee results in a higher ink deposit.
The thicker more opaque ink required to produce the HFSP cannot move quickly and If you print too fast, the mesh will not properly snap-off and out of the ink layer from the garment. This could result in an even rougher print surface.
The mesh should peel immediately as it trails the squeegee.
7. The Print Technique
- Thoroughly stir the ink before loading it onto the screen as this will help it to flow better*.
- Do a complete fill stroke to ensure your mesh is properly loaded with ink.
- Print using a push rather than a pull stroke. It is a superior technique for a wide variety of reasons we won’t go into here.
*(Plastisol ink is thixotropic which means that it gets very thick when stored. Stirring it prior to use will lower the working viscosity and make the ink flow and cut easier, in effect easing some of the issues that plague this technique.)
8. Emulsion Choice
It is a good idea to carry at least two types of emulsion in your screen printing shop. A high solids content (around 50%) fast exposing pure photopolymer emulsion for coarse mesh and a general purpose diazo dual cure liquid emulsion (≥35% solids).
The pure photopolymer emulsion will make it very easy to build up a good level of EOM on the back of the screen. The diazo dual cure emulsion will be better for higher resolution applications with halftones and fine detail.
All of the factors discussed here are equally as important whether you are printing t-shirts, fleece-wear, polo shirts or athletic wear. The smoother the fabric, the more successful this technique will be to produce a visually opaque ink layer.
We hope this article is of assistance to you. Please send us your questions if you are facing challenging print jobs, we are happy to help. We regularly run screen printing classes to help you not only with problems like this, but to keep your overheads low and frustration levels in your shop to a minimum.
|Emulsion||Diazo Dual Cure (min 35% solids) or Pure Photopolymer (± 50%)||Pure Photopolymer (±50%)|
– Dry face down
|S*+2 or more – Dry face down
(optional face coating)
|Mesh Choice||43-61 TPC / 110-155 TPI||31-43 TPC / 80-110 TPI|
|Screen Frame||Static Aluminium or Retensionable||Retensionable|
|Printing Speed||Slow||Dead Slow|
|Ink Type||High Opacity||High Opacity|
|Squeegee||60-70 duro||60-70 duro|
|Off Contact||Normal||Normal to High|
|Washability||Normal to Excellent||Reduced|
|Artwork||Low to Medium detail||Very low detail|
*S = Shine method. Please read our full article on How To Coat A Screen For Best Results using the shine screen coating method