The **Screen Exposure Calculator** helps screen printers determine the correct exposure time (T) needed for a screen emulsion based on their light source intensity (I), the required energy (E) for the emulsion, and a material coating factor (C). Input three of the four variables below to solve for the missing one.
Screen Exposure Calculator
Screen Exposure Calculator Formula
The calculation is based on the principle of photographic reciprocity, adapted for screen printing emulsions. The fundamental goal is to achieve a constant required energy (E) for proper emulsion curing.
E = I × T × C
Where:
E = Required Exposure Energy (J/cm²)
I = Light Intensity (mW/cm²)
T = Exposure Time (seconds)
C = Coating Factor (Unitless)
Formula Source: Wikipedia: Screen Printing (High Authority Reference)
Variables Explained
Understanding each variable is crucial for accurate screen exposure:
- Required Exposure Energy (E): This is the total light energy needed to fully cure the specific photo-emulsion used. It is typically provided by the emulsion manufacturer.
- Light Intensity (I): This is the measured output of your exposure unit’s light source at the screen level, measured using a radiometer. This value degrades over time as bulbs age.
- Exposure Time (T): The duration, in seconds, the screen is exposed to the light source. This is the variable most commonly solved for.
- Coating Factor (C): An empirical factor that accounts for variables like mesh count, emulsion thickness, and color. A higher factor is generally used for thicker coatings or denser mesh.
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What is Screen Exposure Calculation?
Screen exposure calculation is the process of finding the perfect balance between light intensity and time to properly cure the photo-emulsion on a screen printing frame. If the screen is underexposed, the emulsion will not fully harden, leading to pinholes and breakdown during the wash-out or printing process. If it is overexposed, fine details on the stencil may be lost, and the emulsion may become brittle.
The core principle is that total energy delivered to the emulsion must be constant. A brighter light (higher Intensity) requires less Exposure Time, and a dimmer light requires more time. This calculator codifies that relationship using the provided factor (C) to account for material characteristics, providing a scientific baseline instead of relying purely on trial and error.
How to Calculate Screen Exposure (Example)
Let’s find the required **Exposure Time (T)** given the other variables:
- Identify Given Values: Assume Required Energy (E) is $0.6 \text{ J/cm}^2$, Light Intensity (I) is $20 \text{ mW/cm}^2$, and the Coating Factor (C) is $1.5$.
- Rearrange the Formula: The original formula is $E = I \times T \times C$. To solve for $T$, we rearrange it to: $T = E / (I \times C)$.
- Substitute Values: $T = 0.6 \text{ J/cm}^2 / (20 \text{ mW/cm}^2 \times 1.5)$.
- Perform Calculation: Note the intensity unit is mW/cm² (milliwatts). We convert $I$ to $W/cm^2$ for consistency in J/cm² (Joules per second per cm²): $20 \text{ mW/cm}^2 = 0.02 \text{ W/cm}^2$. The formula becomes $T = 0.6 / (0.02 \times 1.5) = 0.6 / 0.03$.
- Final Result: The required Exposure Time (T) is $\mathbf{20}$ seconds.
Frequently Asked Questions (FAQ)
- What happens if the calculation results in a negative time?
A negative result for Time or Intensity indicates a logical inconsistency in your inputs, often caused by a negative value in the Required Energy or Coating Factor. All variables must be positive physical values for the calculation to be meaningful.
- Why is my light intensity (I) always decreasing?
Light intensity naturally decreases over time as the bulb’s lifespan progresses. It is critical to periodically measure your light source with a radiometer and update the Light Intensity (I) value in this calculator to maintain accurate exposure times.
- How accurate is this calculator?
This calculator provides a mathematically exact solution based on the formula $E = I \times T \times C$. Its accuracy depends entirely on the precision of your input values, especially the manufacturer’s Required Energy (E) and your measured Light Intensity (I).
- Can I use this for any type of photo-emulsion?
Yes, provided you know the specific Required Exposure Energy (E) for that emulsion. The Coating Factor (C) allows you to adjust the calculation to suit specific material properties, making it versatile across different emulsion types and mesh counts.