Elimination of Moiré Patterns Caused by Improper Handling in Screen Printing

In screen printing, improper handling can easily produce two types of moiré patterns: overprint moiré and screen moiré. In color reproduction, it has long been established that the angle between any two strong colors must be greater than 22.5°; if it falls below 22.5°, moiré patterns will appear.

I. Causes of Moiré Patterns

In multi-color halftone reproduction, two types of moiré patterns may arise from improper technical handling: overprint moiré and screen moiré.

II. Methods for Eliminating Moiré Patterns

1. Correctly Matching Screen Mesh Count to Halftone Line Count

The smaller the mesh-to-line ratio, the more severe the moiré. A ratio of at least 3.5:1 is generally required. International practice currently favors a ratio of 4.5:1 or higher. Within this range, any moiré that does appear will be minor and within acceptable printing tolerances. For color halftone screen printing, a ratio of 5:1 or higher is typically selected. At this ratio, moiré will generally not occur with standard straight stretching or straight plate-making.

French expert Michel César conducted extensive experiments on the causes and elimination of moiré in halftone screen printing. In cases where the mesh-to-line ratio was inadequate, he employed an angled mesh stretching method to successfully eliminate moiré. He demonstrated this with two examples:

This method is highly effective, though its drawback is greater consumption of mesh fabric.

2. Mastering Correct Plate-Making Techniques

During mesh stretching, close attention must be paid to the surface tension of the screen frame. Poor stretching can cause the mesh to deform — appearing wavy or geometrically irregular — leading to localized moiré. Mesh must be stretched strictly according to specifications. Two sets of screens (four frames per set) should be stretched at the same time, all conforming uniformly to the required printing dimensions.

• Straight stretching (0° stretching): Mesh threads run parallel to the frame edges. This saves fabric and allows minor registration corrections.
• Angled stretching: Mesh warp and weft threads are stretched at ±4°–8° to the frame edges. All four frames must have matching mesh count and tension. The drawback is greater fabric consumption; the advantage is that it ensures the mesh lines are perfectly parallel and square.

To conserve fabric, straight stretching may be used combined with angled plate exposure at ±4°–8°: during exposure, the film positive is placed at an angle on the emulsion surface and fixed for exposure, or the press bed registration can be rotated by a corresponding angle during printing.

Correct exposure time must also be controlled. Current processes use diazo photopolymer emulsions matched with UV light sources at wavelengths of 365–420 nm. Exposure energy equals illuminance multiplied by time (H = E × t). Both mesh count and mesh color affect exposure time:

• Higher mesh count → thinner emulsion layer → shorter exposure time
• Lower mesh count → thicker emulsion layer → longer exposure time
• White mesh → shorter exposure time
• Dyed (colored) mesh → longer exposure time

3. Selecting the Right Inks and Substrates

Inks should be selected with vehicles compatible with the substrate material. Pigment particle size should be fine — generally, pigment particle size should be no more than one-third of the mesh opening diameter; otherwise, mesh clogging will occur.

Substrates should have a smooth surface that is compatible with the ink. For material quality, double-coated art board is recommended.

4. Correctly Selecting Halftone Screen Angles

The choice of screen angles is critical for preventing moiré, creating visually pleasing dot patterns, and achieving accurate color reproduction. The conventionally assigned screen angles are 0°, 15°, 45°, and 75°.

An alternative approach — used when the mesh-to-line ratio is insufficient — is to employ angled stretching or angled plate exposure. This ensures that neither screen moiré (between the film and the mesh) nor overprint moiré (between color separations) occurs.

• 45° — Best single-screen angle; generally assigned to the dominant (key) color
• 75° and 15° — Assigned to the strong colors
• 0° (90°) — Assigned to the weakest color, typically yellow

5. Selecting the Appropriate Dot Shape

Different dot shapes should be chosen based on the tonal distribution of midtones and shadows in the original image: