What Is Screen Printing Emulsion

Screen printing emulsion is a light-sensitive liquid chemical mixture that serves as the core material for creating templates (also called “screens” or “stencils”) in the screen printing process.

What It Is

From a technical perspective, screen printing photosensitive emulsion is a specially formulated photopolymer. After being evenly coated onto the screen mesh and dried, exposure to ultraviolet light of specific wavelengths causes its photosensitive components to undergo photopolymerization, resulting in molecular cross-linking and hardening of the exposed areas. This photochemical process is the core of screen making and the foundation for precise image transfer.

At the molecular level, the emulsion primarily consists of photosensitizers, photoinitiators, polymer base materials, and additives. When exposed to ultraviolet light, the photoinitiators absorb energy to produce free radicals, triggering cross-linking reactions between polymer molecules to form a three-dimensional network structure insoluble in water. This microscopic change is the essential mechanism for the emulsion’s functionality.

What It Does

The primary function of screen printing photosensitive emulsion is to form a precise, high-resolution, print-resistant stencil on the screen mesh substrate. This meticulously crafted screen stencil precisely controls how ink penetrates through the mesh onto various substrates (such as textiles, paper, plastics, glass, ceramics, etc.). The specific screen-making process is as follows:

• During the exposure process, a high-precision film positive is placed in tight contact with the screen that has been coated with photosensitive emulsion and dried, ensuring perfect contact.
• When ultraviolet light from a professional exposure unit shines through, the transparent areas of the film allow UV light to penetrate, causing the photosensitive emulsion beneath to undergo photopolymerization and harden.
• The opaque image areas of the film effectively block ultraviolet light, leaving the emulsion underneath unchanged and preventing cross-linking reactions.
• After exposure, the screen is washed with appropriate water pressure at a washout station. The unhardened photosensitive emulsion (the parts blocked by the image) is washed away by the water flow, while the hardened parts remain firmly attached to the screen, forming precise openings. These openings become the channels for ink transfer during the subsequent printing process.

The technical parameters of photosensitive emulsion directly determine the key performance indicators of the screen, including resolution (the smallest details that can be reproduced), edge definition (the clarity of image edges), stencil thickness (affecting ink deposit), and durability (the number of impressions the screen can withstand). Professional printing shops typically select photosensitive emulsion formulations with specific performance parameters based on different printing requirements.

Why It Matters

In professional screen printing, the quality of photosensitive emulsion and application techniques directly impact the precision, clarity, and durability of printed products. Without a high-quality emulsion forming a precise screen, ink cannot be transferred accurately according to the design, easily leading to blurred images or loss of detail. Therefore, selecting appropriate emulsion and mastering correct screen-making techniques are key to achieving high-precision reproduction, efficient mass production, and enhancing business competitiveness.

From an economic perspective, while photosensitive emulsion costs are low (typically 3-5% of total costs), its quality can significantly affect 30-50% of production efficiency and reject rates. High-quality emulsion can substantially reduce screen rework, increase printing speed, extend screen life, thereby enhancing a company’s production efficiency and market competitiveness.

Common Types of Screen Printing Emulsions

In the professional screen printing field, photosensitive emulsions are primarily categorized based on their photosensitive chemical components, photosensitive mechanisms, and application characteristics into the following main types, each with specific technical advantages and suitable applications:

1. Diazo Photosensitive Emulsion

Diazo photosensitive emulsion is a historically established two-component screen-making material in screen printing, requiring the diazo sensitizer powder to be mixed with the emulsion base in proportion to activate. Its advantages include low cost, strong resistance to solvent-based inks, high exposure latitude, suitability for medium-detail images, and moderate screen durability.

The mixed emulsion has a shelf life of only 2-4 weeks, slower photosensitivity (exposure time approximately 2-3 times that of photopolymer emulsions), and sensitivity to high humidity environments, which may affect stencil quality. Nevertheless, its cost-effectiveness and operational flexibility make it still mainstream in small to medium-sized printing shops, particularly suitable for commercial and textile printing with moderate precision requirements.

Diazo emulsions are ideal for printing large areas with lower detail requirements, such as garment printing, advertising banners, and plastic containers. Its high exposure latitude makes it user-friendly for beginners. Recommended for use with 35-90T mesh counts to optimize printing results.

2. Photopolymer Emulsion

Photopolymer emulsion (SBQ emulsion) is a single-component, pre-sensitized material for screen printing stencil technology, ready to use straight from the container. Its advantages include fast photosensitivity (requiring only 1/3-1/5 of the exposure time of diazo types), high resolution (capable of accurately reproducing details above 50 lines/cm), sharp edges, and long shelf life (12-24 months).

Photopolymer emulsions require precise exposure parameters with less latitude. Some formulations have insufficient water resistance, necessitating the selection of formulations specifically enhanced for water-based inks. While more expensive (approximately 1.5-2 times the cost of diazo types), they are irreplaceable for high-precision printing applications such as printed circuit boards (PCBs), fine ceramic decoration, and high-end screen art.

They work best with 100-180T high mesh count screens to fully leverage their high-resolution capabilities. Modern photopolymer emulsions offer various formulations including UV ink-specific, water-based ink-enhanced, and high solvent-resistant options to meet different requirements. When using, precise control of exposure energy is necessary, and an integrating exposure system is recommended to ensure consistent stencil quality.

3. Dual-Cure Photosensitive Emulsion

Dual-cure photosensitive emulsion combines the advantages of diazo and photopolymer technologies, requiring the addition of diazo sensitizer for activation. Its features include medium-high speed exposure (30-50% faster than diazo types), good resolution, excellent durability, and broad compatibility with water-based, solvent-based, and UV-curable inks, making it suitable for medium to high-precision printing.

Dual-cure emulsions offer high durability (capable of tens of thousands of impressions), adaptability to various printing substrates, and stability in different printing environments. Priced between diazo and photopolymer types, the mixed emulsion has a usable life of 4-6 weeks. It is widely applied in diverse printing fields including signage, textiles, and plastic products.

Suitable for multi-substrate printing with strong solvent resistance and moderate exposure speed. Often paired with 60-120T mesh count screens to balance detail and ink deposit. Its ink compatibility reduces the frequency of screen changes, improving production efficiency, particularly suitable for multi-ink system jobs.

4. Capillary Film

Capillary film is an innovative technology in screen printing stencil-making, where a thin film pre-coated with photosensitive emulsion of precise thickness integrates into moistened screen mesh through capillary action. Its advantages include consistent emulsion layer thickness (±1 micron), ease of operation, avoidance of manual coating thickness variations, ensuring sharp image edges and uniform ink deposits.

Capillary films are available in various thickness specifications from 15-100 microns, allowing precise control of print layer thickness, high stencil-making efficiency, and excellent durability. While more costly (approximately 2-3 times that of liquid emulsions) and challenging to apply to very large or irregular screen frames, they are ideal for high-precision printing applications such as printed circuit boards, precision instrument panels, and high-end ceramic decoration.

Capillary film simplifies the stencil-making process to “wet-apply-dry,” reducing skill requirements and increasing efficiency. In multi-color registration printing, its thickness consistency ensures matching ink deposits, enhancing color reproduction accuracy, making it suitable for demanding printing tasks.

Selection and Application Tips for Screen Printing Emulsions

In practical screen printing production, selecting the appropriate type of photosensitive emulsion and mastering correct application techniques are crucial for improving stencil quality and printing efficiency.

Here are several factors that professional screen printing practitioners should consider:

1. Printing Precision Requirements: For high-precision printing with line widths less than 0.1mm or fine halftones, priority should be given to photopolymer emulsions or high-quality capillary films; for general commercial printing, diazo or dual-cure emulsions are usually sufficient.
2. Ink System Compatibility: When using solvent-based inks, select emulsion formulations with strong solvent resistance; when using water-based ink systems, pay special attention to the emulsion’s water resistance, and consider using post-hardeners to enhance screen water resistance when necessary.
3. Production Volume and Frequency: For large-volume, high-frequency production runs, select emulsion types with high durability; for small-batch, multi-variety printing needs, ease of use and stencil-making efficiency may be more important.
4. Environmental Condition Control: Regardless of which type of photosensitive emulsion is chosen, strictly control the temperature and humidity conditions of the stencil-making environment (ideal conditions are 20-25°C, relative humidity 40-60%), and use appropriate safelight illumination to ensure consistency and reproducibility of stencil quality.
5. Emulsion Coating Techniques: The coating quality of liquid emulsions directly affects stencil results. Master the correct coating angle (typically 15-25 degrees), pressure, and speed to ensure even distribution of emulsion on both sides of the screen, avoiding bubbles and thickness inconsistencies.
6. Exposure Parameter Optimization: Different types of photosensitive emulsions require different exposure energies. Determine the optimal exposure time through step-wedge exposure tests and regularly calibrate exposure equipment to ensure stability of exposure energy.
7. Screen Post-Treatment Techniques: Appropriate post-treatment after stencil-making can significantly enhance screen performance. For example, treatment with specialized hardeners can increase the screen’s water and solvent resistance; treatment with degreasers can improve ink transfer effects; regular use of screen reclaiming agents can extend screen life.
8. Screen Mesh and Emulsion Matching: Different mesh counts are suitable for different types of photosensitive emulsions. Generally, high mesh count screens (above 100T) are suitable for use with photopolymer emulsions or capillary films; medium to low mesh count screens (40-90T) are more suitable for diazo or dual-cure emulsions. Correct matching can fully leverage the technical advantages of various emulsions.

Through scientific selection and correct application of screen printing emulsions, professional printing shops can significantly improve product quality, reduce production costs, and provide higher-value printing solutions to clients. In the modern screen printing industry, continuous innovation in photosensitive emulsion technology and ongoing optimization of application techniques are driving the entire industry toward higher precision, greater efficiency, and more environmentally friendly directions.