1. Introduction: The Role of the Film Positive

In screen printing, the Film Positive (often shortened to “film” or “sep”) is the physical intermediary between your digital artwork and the finished print. It is a high-contrast, black-and-white transparency (usually printed on an imagesetter or inkjet film) that is used to “burn” the stencil onto a mesh screen.

The Critical Rule: The film positive is not a “picture.” It is a photographic mask. Its only job is to allow UV light to pass through in some areas (where there is no design) and block UV light completely in other areas (where the design will print).

Therefore, the design file for a film positive is fundamentally different from a file intended for digital printing (inkjet/laser) or offset lithography. It operates on a binary principle: 100% Black (Blocks UV) vs. 0% Black (Transparent).

2. File Format: Vector-First, No Exceptions

2.1. Mandatory Vector Formats

You must submit or produce files in vector graphics format. Raster images (JPEG, PNG, TIFF, PSD) are highly discouraged unless they are extremely high resolution (see Section 8).

Acceptable Formats:

• .ai (Adobe Illustrator) – Industry standard.
• .cdr (CorelDRAW) – Common in garment and industrial shops.
• .eps – Universal interchange format.
• .pdf – Acceptable if saved with “Preserve Illustrator Editing Capabilities” or as a high-resolution vector PDF.
• .svg – Rarely used directly but acceptable.

Why Vector?

• Infinite Scalability: Film is output at 1:1 scale. Vector math ensures perfect geometric edges.
• No Aliasing: The edge of a vector line is mathematically straight. The edge of a raster pixel is jagged (stair-stepping). A jagged edge on the film creates a jagged edge on the screen, which prints as a jagged line on the product.
• Separation Logic: Vectors allow perfect color grouping and spot color definition.

2.2. The Golden Rule: Convert Text to Outlines

Before submitting any file:

1. Select all text objects.
2. Create Outlines (Illustrator: Type > Create Outlines; CorelDRAW: Arrange > Convert to Curves).
3. Delete unused swatches that may reference missing fonts.

Consequence of Failure: If you do not outline text, the RIP (Raster Image Processor) software at the film output bureau will substitute your font with a default font (often Courier or Arial). Your carefully kerned logo will become a misaligned, incorrect mess.

3. Dimension Requirements: The 1:1 Rule

3.1. Absolute Scaling

The artboard size in your document must be the exact final print size. If you are printing a 50 mm x 50 mm logo on a plastic enclosure, your artboard is 50 mm x 50 mm. Do not draw it at 10 mm and ask the operator to “scale it up 500%.”

Why? Stroke weights (line thickness), halftone dot sizes, and trapping values are calculated in absolute units (mm or mils). Scaling up a 0.2 mm line to 1.0 mm fundamentally changes the physical properties of the stencil.

3.2. Bleed and Safety Margins

• Bleed: If the design prints to the very edge of the substrate, the art must extend beyond the cut line. Minimum bleed: 2 mm (0.08 inches). This accounts for screen shift during printing.
• Safety Margin: Keep all critical text or logos at least 1.5 mm away from the edge of the product to prevent them from being cut off or falling off the edge of a curved surface.

4. Color Specification: Spot Colors (Pantone)

4.1. No Process Colors (CMYK)

Screen printing does NOT use CMYK process mixing on the film (with the rare exception of 4-color process sim process, which is advanced). You cannot use C=50, M=0, Y=100, K=0 to make green.

The Rule: One color = One layer = One screen = One film.

4.2. Pantone Matching System (PMS)

Every color in your design must be assigned a Pantone Solid Coated (or Uncoated) spot color.

• Correct: PANTONE 186 C (Red).
• Correct: PANTONE Black C.
• Incorrect: R=255, G=0, B=0 or C=0, M=100, Y=100, K=0.

Why? The screen printer buys pre-mixed Pantone ink. If you specify RGB red, they will guess the ink, and it will likely be wrong. A Pantone number is a global standard.

4.3. The “Solid Black” Convention for Film Output

When you output the film, the RIP software typically converts all spot colors to 100% Black (K=100) on the film itself.

• Exception: If using a pure black ink (PANTONE Black C), the film is printed 100% K.
• Exception for White Ink: White ink requires a specific film layer. On screen, you represent white as PANTONE White C. On film, this layer prints as 100% Black.

The Film sees only two states:

• Black (K=100): The UV light is blocked. Emulsion stays on the screen. (This becomes your print).
• White/Transparent (K=0): UV light passes through. Emulsion washes away. (No print).

5. Geometric Constraints: Physical Limits of Mesh

This is the most critical section for engineers and industrial designers. Screen printing is a physical extrusion process. Ink is pushed through holes. If your geometry is too small, it physically cannot pass through the mesh.

5.1. Minimum Positive Line (Stroke) Width

• Absolute Minimum: 0.15 mm (approx. 6 mils or 0.006 inches).
• Recommended Minimum for Production: 0.20 mm (8 mils).
• Safe for Textile (Fabric): 0.50 mm (20 mils) – because fabric absorbs ink and causes spread.

Physics: A 0.15 mm line on the film corresponds to a 0.15 mm open channel in the emulsion. The mesh thread count (e.g., 305 mesh) has openings of roughly 0.045 mm. The ink must pass through the mesh AND the 0.15 mm channel. If the line is 0.10 mm, the mesh threads physically block the channel, resulting in broken lines or no ink deposition.

5.2. Minimum Gap (Isolation / Reverse Space)

• Minimum Gap: 0.15 mm (6 mils).
• Recommended: 0.20 mm (8 mils).

Scenario: You have two parallel lines 0.15 mm thick, separated by a 0.10 mm gap. The gap is a “transparent” area where UV light should pass. However, due to the mesh knuckles and emulsion surface tension, the 0.10 mm gap will often “bridge” (remain closed) during exposure or washout. The result: two lines that print as one thick blob.

5.3. Minimum Font Size

• Sans-serif fonts (Arial, Helvetica): Minimum cap height = 1.0 mm (0.04 inches).
• Serif fonts (Times, Garamond): Minimum cap height = 1.5 mm (0.06 inches).
• Script/Decorative fonts: Avoid below 3 mm.

Rule of Thumb: For every 1 mm of font height, you can reliably print that many characters without filling in. A 2 mm tall word “TEST” is risky. A 4 mm tall word “TEST” is safe.

6. Gradients and Halftones (Simulating Continuous Tone)

Screen printing is binary (ink or no ink). To simulate a gradient (fading from red to white, or black to gray), you must use Halftones.

6.1. Halftone Principle

A halftone converts a continuous gradient into a pattern of tiny dots. The dots are 100% black (full ink). The size of the dots changes to simulate lightness or darkness.

• Large dots = More ink = Darker appearance.
• Small dots = Less ink = Lighter appearance.

6.2. LPI (Lines Per Inch) – The Halftone Frequency

• Standard Screen Printing LPI: 35 LPI to 55 LPI.
• High detail (posters, dials): 65 LPI to 85 LPI (requires very high mesh count, e.g., 355 mesh).
• Textile (fabric): 25 LPI to 45 LPI (coarser because fabric wicks the ink).

The Critical Relationship: LPI must match the mesh count.

• Formula: Mesh Count ÷ 5 = Max LPI.
  • Example: 305 mesh ÷ 5 = 61 LPI maximum.
  • If you use 85 LPI on a 230 mesh, the dots will fall through the mesh holes or bridge, causing a muddy, undefined print.

6.3. Dot Shape and Angle

• Dot Shape: Elliptical or Round. (Avoid “Line” or “Square” as they cause harsh transitions).
• Dot Angle: 22.5 degrees (Standard for most RIPs to avoid Moiré patterns). Never use 0°, 45°, or 90° for overlapping colors, as this creates a checkerboard interference pattern.

6.4. Minimum Dot Size

• Film output limit: 2% dot at 55 LPI.
• Printing limit: 5% dot. Any dot smaller than 5% will likely wash off the screen during development or be blocked by mesh threads.

7. Trapping and Overprinting (For Multi-Color Jobs)

When printing multiple colors sequentially, slight misalignment (registration error) of ±0.1 mm to ±0.3 mm is inevitable. Trapping prevents white gaps from appearing between colors.

7.1. Spread (Choking) – The Solution

• Rule: The lighter color should “spread” into the darker color.
• Application: Create a 0.2 mm to 0.35 mm overlap.
  • Example: Yellow circle next to Black circle. Extend the Yellow shape 0.2 mm into the Black area. If registration is off by 0.15 mm, the Yellow still touches the Black, leaving no gap.
• How to implement: In Illustrator, use Effect > Path > Offset Path with a positive value (Spread) on the lighter color layer.

7.2. Knockout vs. Overprint

• Knockout (Default): The top color cuts a hole in the bottom color. Risky for misregistration.
• Overprint (Preferred for screen print): The top color prints directly on top of the bottom color.
  • Requirement: The top ink must be opaque enough to cover the bottom ink.
  • Application: Use Window > Attributes > Overprint Fill for the top layer.

7.3. No Trap for White Underbase

When printing on dark substrates (e.g., black plastic), you print a White Underbase first, then colors on top. Do not trap the underbase. The underbase should be 2% to 5% smaller (Choke) than the top colors to prevent a visible white “halo” around the edge.

8. Raster Images (Bitmaps) – The Exception

If you must use a photograph or complex texture (cannot be vectorized), follow these strict rules:

8.1. Resolution

• Minimum DPI: 300 DPI at final output size.
• Recommended DPI for film: 600 DPI to 1200 DPI (The imagesetter exposes at 2400+ DPI, but the source image needs sufficient data).
• Do not use 72 DPI web images. Scaling a 72 DPI image to 300 DPI just makes blurry pixels bigger.

8.2. Bitmap Mode (Not Grayscale)

Convert the image to Bitmap mode in Photoshop:

1. Image > Mode > Grayscale.
2. Image > Mode > Bitmap.
3. Method: Halftone Screen.
4. Frequency: 45 LPI (or match your screen mesh).
5. Angle: 22.5 degrees.
6. Shape: Round.

Result: A pure black-and-white dot pattern with no gray pixels. This is the only raster format suitable for film output.

9. Annotation and Layer Management

A professional film file is not just artwork. It contains manufacturing data.

9.1. Layer Structure

Organize your file with strict layers (from top to bottom):

• Layer 1: “Registration Marks” – Crosshairs outside the art area. One in each corner. Use a 0.1 mm stroke, 5 mm long.
• Layer 2: “Labels & Notes” – Text notes for the printer (e.g., “PANTONE 186 C – 55 LPI – Angle 22.5° – Mesh 230”).
• Layer 3: “Artwork – Color 1” (e.g., White Underbase).
• Layer 4: “Artwork – Color 2” (e.g., Red).
• Layer 5: “Artwork – Color 3” (e.g., Black).

9.2. The Information Block

Include a text block outside the print area (e.g., at X=0, Y=-20 mm) containing:

text

复制

下载

JOB NAME: [Project Name]

DATE: 2026-04-20

SCALE: 1:1

COLOR 1: PANTONE White C (Underbase – Shrink 2%)

COLOR 2: PANTONE 186 C (Red – 55 LPI / 22.5°)

COLOR 3: PANTONE Black C (Trapping: +0.2mm spread on red)

MESH: 230 for colors, 305 for underbase

EMULSION: Dual Cure

FILM RESOLUTION: 2400 DPI

10. Output Settings for the Film Writer (RIP)

When generating the final file for the film output device (e.g., an AccuSet or inkjet film printer), use these settings:

10.1. For Imagesetters (Professional)

• Resolution: 2400 DPI to 3600 DPI.
• Screening: Stochastic or AM Halftone.
• Negative/Positive: Positive (Most common. Black ink = Block UV). *Note: Some old-school shops use Negative film, but 95% of modern shops use Positive film with UV-blocking black ink.*
• Mirror/Reflection: Read Right (When looking at the emulsion side of the film, the text should read correctly. The film is flipped for exposure).

10.2. For Inkjet Film Printers (Desktop)

• Density (Dmax): The black must have an optical density of >4.0. Standard office inkjet black (Dmax ~1.2) is transparent to UV light. You must use specialized film positive ink (pigment-based or UV-blocking black).
• Media: Clear matte polyester film (0.1 mm thickness).

11. Common Failure Modes (Checklist)

Before sending your film file to production, verify:

Parameter	Acceptable Value	Failure Mode
Text	Converted to Outlines	Font substitution, wrong characters
Scale	1:1 exact size	Print is too large or too small
Stroke width	≥ 0.15 mm (0.006″)	Lines disappear, broken print
Gap width	≥ 0.15 mm (0.006″)	Ink bridges, fills in, smudges
Halftone LPI	35–55 LPI (Mesh/5)	Moiré patterns, lost dots
Color mode	Spot colors (PANTONE)	CMYK separation error, wrong ink
Black density	K=100% only	Film is translucent, pinholes in stencil
Trapping	0.2 mm spread (light into dark)	White gaps, misregistration
Bleed	2 mm beyond cut line	White edges, floating design

12. Conclusion

Designing for screen printing film is an exercise in constraint-based engineering. You are not creating a “pretty picture” for a screen; you are creating a physical stencil that must withstand UV exposure, washout, mesh tension, and squeegee pressure.

The three pillars of a perfect film file are:

1. Absolute binary contrast (100% black or 0% black).
2. Geometric tolerance (respecting the 0.15 mm minimum feature size).
3. Color separation (one Pantone spot color per layer).

If you follow these specifications, the film output will produce a crisp, durable stencil, which will yield a professional, repeatable print. Violate any of these parameters, and the print will suffer from broken lines, pinholes, misregistration, or complete washout failure.

Final Pro Tip: When in doubt, request a Test Film on cheap paper before outputting to expensive polyester film. Hold the test film against a bright light. If you see any light coming through areas that should be black (e.g., thin lines or halftone dots), your black density is too low or your design is under-minimum width.