Screen printing and flexo printing machines for LCD manufacturing.

LCD Equipment
Crystec Technology Trading GmbH

Screen printing and flexo printing machines for LCD manufacturing.

In LCD manufacturing, several printing and dispensing technologies are used in order to dispense various layers and seals. Dispensing is mainly used for low throughput and variable design displays. Printing technologies are used for high precision and high volume production cycles.

Screen printing is used for seal printing and thick layer printing. Flexo-Printing is used for polyimide layer printing and printing of thin layers with a high precision.

Screen Printing.

Screen printing is a technology, where the print material is pressed by a squeegee through a mesh that is fixed on a frame. The mesh carries a photoresist layer with openings defining the pattern to be printed.

Screen frame.

The screen frame is the most fundamental element of the screen-printing process. The function of the screen frame is to providing a means for mounting the mesh, withstanding the force of the tensioned mesh, standing up to additional forces applied during printing, remaining flat and square and being light enough to handle easily.

Once tension is applied to the mesh and the mesh is affixed to the frame, the flatness of the frame becomes critical. The frame's job is to resist the forces of that tension. A suitable frame will remain flat and square for the time of use. Among other things, distortion of the frame causes mesh threads to follow the same, curved direction as the beams, resulting in mesh distortion at the edges of the screen. This drastically reduces the screen's "sweet spot" or maximum print area. It also leads to tension inconsistencies that further complicate your ability to achieve good registration from colour to colour.

For applications in LCD manufacturing and in the electronic industry the screens are made from massive aluminium alloys in order to achieve the required high printing precision. For other applications aluminium pipe screen frames are available.


The size of the screen frame is important for the max. resolution of the print. The image-to-frame ratio is critical for avoiding image distortion. This term refers to the size of the image area relative to the total screen area, which is usually represented by the frame's inside dimensions. The less a screen is deflected during the print stroke, the lower is the force against the mesh and the lower the possibility of distorting the printed image. Squeegee length and stroke length also figures into the relationship between image size and total mesh area.

For printing seals on glass plates only 25-35% of the screen size are used for the image.


Mesh selection is a series of compromises. A coarse mesh will allow the printing material to pass through the screen easily. This will result in thick layers, fast prints and low resolution. A fine mesh will create difficulties in printing a bright design on dark-coloured substrates and it is difficult to press viscose material through the screen. However you can achieve very good resolution using fine mesh screens. This is the most important point in LCD manufacturing, where silk screens are used for printing. High resolution silk screens have more than 130 meshes per cm, enabling them to print lines with a width down to 50µm. The layer thickness is in the range of 10µm normally, which is regarded as a "thick" layer in LCD manufacturing.

For creating the image, the mesh is coated by a photosensitive emulsion, which is exposed using a mask, made from a glass or polymer film. The unexposed portion of the coating is washed off afterward. This portion let the seal material go through and you have its pattern on the substrate. 


The squeegee plays a major role in screen printing. It has four functions that influence or are influenced by any number of processing variables:

  1. Forcing the print material into the mesh - influenced by mesh opening size and viscosity.
  2. Keeping the mesh in contact with the substrate - influenced by screen tension and off-contact distance
  3. Adapting the mesh to the surface of the substrate - influenced by substrate hardness, roughness, and evenness.
  4. Removing excess print material from the screen - influences thickness of the printed layer, as well as image definition

Squeegees for seal printing are made from a stiff material like hard polyurethane with a hardness of up to 80 durometer. The durometer of a squeegee is the measure of its hardness, and a guide to the blade's ability to resist bending during printing. Squeegee durometers are measured on the Shore A scale, an industrial standard of 1-100 used to indicate the hardness of rubbery materials. The higher the durometer, the greater the blade rigidity. The lower the durometer, the more the blade will flex during printing. Harder squeegees (for example, an 80 durometer) are much less forgiving and will not print an even layer of ink on an uneven surface. However, these higher durometers are required to stand up to the high pressure needed to print at high speed or with high-opacity, high-viscosity materials on glass plates.

Beside the hardness, the shape of the squeegee and the edge profile play an important role too. Various shapes are available. This shape also determines the squeegee's adaptability. In a sense, profile fine tunes this adaptability and limits the amount of force transmitted to the printing surface. The profile affects the relationship between the set squeegee angle and the effective squeegee angle relative to the screen in the direction of the print stroke. For seal printing squeegees with a square shape are used. The squeegee edge must be sharp, but also free of nicks and waves. Squeegee sharpeners can remove a thin layer from an 80- or 90-durometer squeegee blade and restore a perfect edge.

The angle of the squeegee to the mesh defines the amount of material printed and the forces necessary for the print. Pulling the squeegee at 15-20º off the vertical ensures an even deposit of material. The setting of the exact angle depends in part on the squeegee's profile and durometer.

The possible speed of the squeegee depends again on the hardness of the squeegee and the viscosity of the printed material. It has to be adjusted well with the printing pressure.

Flexographic printing

Flexo printing is offered in order to form polyimide layers in glass plates for LCD manufacturing. We can integrate these machines in an in-line production street or in a polyimide cluster, consisting of a polyimide printing machine, a coupled hot plate curing system and related automation incl. loading and unloading devices.

Flexodruckmaschine für Polyimid
Shindo Flexographic printing machine for Polyimide
This thin film forming technology is based on both flexographic printing technology and gravure printing technology. The doctor blade distributes a layer of the polyimide on the anilox roll. This roll carries a pattern of gravures which are filled with a volume of polyimide corresponding to a layer 3,5 times thicker than the desired printing thickness. This makes good for the transfer loss. Then the polyimde layer is transferred to the printing roll with the desired pattern. The printing roll is covered by a compressible polymer, called letterpress. From there the coating material is transferred to the glass plate. The resolution of this technology is better than screen printing. The printed layers are thin films with a thickness of less than a micron (40-100nm).

flexo print

a printing table, b LCD glass plate, c dispenser, d doctor blade, e anilox role, f resin letterpress, g printing role