Screen Printing: What Makes a Good Screen?

Every tradesman knows that poor preparation and substandard equipment lead to substandard work. We have a print department mantra which is exercised each day at Shirtworks where ‘good printing starts with good screens’.

Here is some basic and not so basic information about what constitutes a good screen. There are two basic components in a screen; The frame and the mesh. The frame systems which are used at Shirtworks are Fixed frames and Self-tensioning frames.

 

Fixed Systems Screens

stretched-screen

These are rigid frames where the mesh is glued into position onto the Frame. As with anything, there are advantages and disadvantages with this system.

Advantages

1. You can achieve a safer and more controlled stretch as most clamp systems allow you to stretch 'off contact' from the frame which reduces the possibility of screen/frame friction which can reduce the life of the screen.

2. It is possible to achieve higher tensions with lower risk of breakages.

3. It is possible to fit the mesh at an angle which means reduces the risk of moire ( an unwanted pattern in your print where your halftone clashes with your screen count) and allows for sharper print edges on line work. For more info on the moire effect.

4. Shirtworks has worked with the leading screen tensioning company in the UK to achieve the highest tensions in the industry. We are regularly achieving super high tensions of 28/cm newtons where other screen printers are only achieving 20/cm newtons. This often allows us to achieve higher quality/definition printing. Achieving these high tensions is a slow process over a longer period of time but the extra and cost effort pays off significantly.

5. Fixed frames come in varying quality. Shirtworks uses Enjell Weldon frames, which are widely considered as the best frames in the industry. These are better frames because they contain aluminium-strengthened bars running inside the frame, which provides 40% higher frame strength which enables reach the 28/cm newton stretch.

Disadvantages

Once the tension has gone out of the mesh, that screen has to be ‘destroyed’ by having the mesh removed and a new mesh glued in place. This is in contrast to self-tensioning frames where a slightly slack mesh can be restored to life by adjusting the tension on the movable frame.

Self-tensioning Screen Systems

self-tension-screens

These consist of movable frame edges, which allow the mesh to be tensioned along the warp and weft axis.

Advantages

1. You can re-tension as the mesh ages or becomes slacker through use.

2. Screens can be meshed up quickly ‘in house’ should you need a particular mesh count in a hurry.

Disadvantages

1. Uneven tensions will distort the aperture, which naturally exists between the warp and weft and this will affect the ink deposit.

2. Screens need to be checked regularly to ensure that the warp and weft tensions are the same in both directions. It is possible to end up with different tensions resulting in rectangular apertures at the cross section of the threads instead of square apertures. This will affect how your fine halftone work is reproduced.

3. Costs are quite higher with more wastage with mesh offcuts than when sending away to dedicated screen tensioning company like Alpha Print Solutions.

4. It is Impossible to put your mesh at an angle on self re-tensioning screens.

5. There is a training and time cost for in house re-tensioning which makes the unit more expensive.

Screen Printing Mesh

In the past, screens were made of silk, hence the word ‘silk screen printing’, but for the last 30 years, printers have been using a polyester fibre in their screens. Under a microscope you would see the usual warp and weft configuration of a standard fabric and it is the gaps in the cross sections which provide the ‘hole’ for the ink to pass onto the garment.

These holes vary in size depending on the size or thickness of the fibres and their relative positioning in the weave. The thicker the fibres, the smaller the holes, the less ink will be deposited unless the spacing of the fibres is increased.

We use a standard numbering system on our screen to determine the ‘mesh count’. A 43T mesh count has 43 threads running down and across each square centimetre, a 55T has 55 and so on. The smaller the number the bigger the gaps are in between the threads, the bigger the gaps the more ink will be deposited on the shirt.

Generally speaking the higher the mesh count, the more detail can be achieved within the print and the less ink is deposited. White and light coloured inks need more ink to deposited on the shirt because the print need to be opaque.

As you get into the higher mesh counts, the actual diameter of the thread decreases to accommodate the increased number of threads and so it would be typical to have a 31 micron diameter thread in a 140T mesh while there would be a 35 micron diameter thread in a 43T mesh-count.

Higher mesh count screens also tend to be a different colour to lower mesh count screens. We use amber (orange) mesh for high detailed work which is not uncommon. The reason that amber meshes are used for higher detail is to mitigate the refraction and reflection of the ultra violet light from the lightbox during the exposure process.

Light will always travel in straight line unless it is deflected off an object and during the exposure process; the ultra violet let emitting from the light box makes contact with the screen after it has passed through an acetate or vellum, which has the artwork printed onto it.

This contact will invariably and minutely affect the path of the ultra violet light and cause it to ‘bend’ through refraction and reflection around the edges of the artwork at a micro level. If that light then hits the mesh, which happens to be orange, the ultra violet light is then partially converted to a red light wave, which becomes a safe light source against the photosensitive emulsion coating the screen. If you reduced the ‘creep’ of light around your artwork, you can obtain finer detail.

As anyone knows from their days in exposing photographic paper in a dark room knows, red light is safe light and so the principle carries with screen exposure. White mesh is normally found on the courser mesh counts of 62T and lower.

This is because you would be using these counts for ‘block work’ that lacks the type of detail required for finer mesh counts and therefore the requirement to hold better detail through the use of amber mesh.

We use NBC meshes that are widely considered the best in the industry because they offer better reclaiming properties with improved washout that means better and cleaner screens. They also come with variable thread diameters, which can improve print results.

Shirtworks has a quality control test in place to examine at what point the fibres in the mesh have become flattened and therefore closing the aperture and reducing ink deposit. After a screen has been subjected to 10,000 prints, it is cleaned and examined under a microscope to check if any damage or flattening has occurred. It is then replaced or put back into service where appropriate.

Screens and mesh are one of the important variables in determining good screen printing. Use this information to check that your printer knows his stuff before placing your order.

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Thursday, 23 January 2020

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