Understanding Screen Printing Exposure & Calculating Optimal Times
Accurate exposure is critical in screen printing. It determines the stencil's durability and resolution. Overexposure can lead to detail loss and clogged screens, while underexposure results in weak stencils that may wash out during printing or break down quickly. This calculator helps you find optimal exposure times using a common method: the step wedge test.
The Science Behind Exposure
Photosensitive emulsions contain chemicals that react when exposed to UV light. This reaction hardens (cures) the emulsion, making it durable. Different components affect how the emulsion reacts to UV light:
Mesh Count: Finer meshes (higher counts) generally require slightly longer exposure than coarser meshes because they hold less emulsion.
Emulsion Type: The sensitizer used in the emulsion dictates its light sensitivity.
Diazo emulsions are common and offer good durability.
Pure Photopolymers are generally faster to expose but may not be as durable as dual-cure options.
Dual-Cure emulsions (like standard ones) combine Diazo and photopolymer for excellent durability and good exposure latitude.
Emulsion Thickness: Thicker emulsion layers require more UV energy to cure through.
Exposure Unit Intensity & Spectrum: The wattage and bulb type of your exposure unit significantly impact exposure time. This calculator assumes a consistent unit.
Environmental Factors: Ambient light and temperature can play a minor role.
The Step Wedge Test Method
The step wedge test is a practical way to determine the ideal exposure time for your specific setup. You create a stencil with varying exposure times on different sections of the screen. A common method uses a calibrated step wedge (a physical object with different densities) or simply exposing sections sequentially.
This calculator simplifies the process. You input your variables, and it provides a range of exposure times based on a starting point and increments.
How the Calculator Works
The core calculation involves determining the exposure time for each step of a test pattern.
Base Exposure: This is your starting point – the time you estimate or know is a good starting exposure for a single, un-incremented block.
Density Increment: This is the additional time added for each subsequent step in your test. It represents how much more UV energy is needed for each density level or incremental exposure.
Steps: The number of distinct exposure durations you want to test.
The formula is essentially: Exposure Time for Step N = Base Exposure + (N-1) * Density Increment.
The calculator adjusts the units (seconds or minutes) based on your selection. The results show a series of exposure times, typically corresponding to sections on your screen. You would then print a test pattern on your screen, exposing each section for the calculated time, develop, and check for the best resolution and durability.
Example Scenario:
You're using a 150 TPI mesh with a Dual Cure emulsion. Your exposure unit is consistent, and you've found that around 10 seconds is a good starting point for a solid block. You want to run a step wedge test with 5 steps, increasing the exposure by 2 seconds for each step.
Mesh Count: 150 TPI
Emulsion Type Factor: 1.0 (Dual Cure)
Exposure Unit: Seconds
Base Exposure: 10 seconds
Density Increment: 2 seconds
Number of Steps: 5
Calculations:
Step 1: 10 seconds
Step 2: 10 + (2-1) * 2 = 12 seconds
Step 3: 10 + (3-1) * 2 = 14 seconds
Step 4: 10 + (4-1) * 2 = 16 seconds
Step 5: 10 + (5-1) * 2 = 18 seconds
You would expose your screen in 5 increments, lasting 10, 12, 14, 16, and 18 seconds respectively. After developing, you examine the stencil for the step that shows the finest detail retention without any signs of undercutting or wash-out, and that section represents your optimal exposure time.
function calculateExposure() {
var meshCount = parseFloat(document.getElementById("meshCount").value);
var emulsionTypeFactor = parseFloat(document.getElementById("emulsionType").value);
var exposureUnit = document.getElementById("exposureUnit").value;
var baseExposure = parseFloat(document.getElementById("baseExposure").value);
var densityIncrement = parseFloat(document.getElementById("densityIncrement").value);
var steps = parseInt(document.getElementById("steps").value);
var resultDiv = document.getElementById("result");
var baseExposureUnitDisplay = document.getElementById("baseExposureUnitDisplay");
// Update the unit display based on selection
baseExposureUnitDisplay.textContent = exposureUnit.charAt(0).toUpperCase() + exposureUnit.slice(1);
var resultsHtml = "
Exposure Times:
";
if (isNaN(meshCount) || isNaN(emulsionTypeFactor) || isNaN(baseExposure) || isNaN(densityIncrement) || isNaN(steps) || steps <= 0) {
resultDiv.innerHTML = 'Please enter valid numbers for all fields.';
return;
}
var adjustedBaseExposure = baseExposure;
// Apply emulsion type factor – simplified approach, real-world might be more complex
// For this calculator, we'll assume the factor modifies the base exposure directly or is a general guideline.
// A common approach is to use the factor to adjust a known baseline.
// Let's assume the base exposure is for a standard dual-cure, and others are relative.
// If emulsionTypeFactor is 1, slower.
// A simple multiplication might not be physically accurate for all emulsion types, but serves as a calculator logic.
// For this example, let's not directly modify `baseExposure` by `emulsionTypeFactor` unless specified,
// but acknowledge it influences the *choice* of `baseExposure`.
// However, if the prompt implies a direct calculation adjustment:
// adjustedBaseExposure = baseExposure * emulsionTypeFactor; // Uncomment if a direct factor is intended
for (var i = 1; i <= steps; i++) {
var currentExposure = adjustedBaseExposure + (i – 1) * densityIncrement;
// Ensure non-negative exposure times
if (currentExposure = 60) {
displayTime = (currentExposure / 60).toFixed(2);
unitLabel = "Minutes";
} else {
displayTime = currentExposure.toFixed(0);
unitLabel = "Seconds";
}
} else { // minutes
displayTime = currentExposure.toFixed(2);
unitLabel = "Minutes";
}
resultsHtml += "Step " + i + ": " + displayTime + " " + unitLabel + "";
}
resultDiv.innerHTML = resultsHtml;
}