diff --git a/busbar_export.py b/busbar_export.py
index 602e4a7..d2d8a03 100644
--- a/busbar_export.py
+++ b/busbar_export.py
@@ -46,6 +46,7 @@ from build123d import (
     Sketch,
     add,
     export_step,
+    export_stl,
     extrude,
 )
 
@@ -299,8 +300,29 @@ def to_svg(payload: dict) -> bytes:
         return path.read_bytes()
 
 
+def to_stl(payload: dict) -> bytes:
+    """STL with the busbars extruded to a printable plate.
+
+    STL is inherently 3D, so extrusion is forced regardless of the payload
+    flag. Thickness defaults to 2 mm when missing or <0.5 mm (slicers can't
+    do 0.2 mm sheet — that figure is for the 'thin nickel strip' STEP case).
+    """
+    busbars, extrude_flag, thickness = parse_payload(payload)
+    if thickness < 0.5:
+        thickness = 2.0
+    shapes = build_shapes(busbars, extrude_flag=True, thickness=thickness)
+    compound = _as_compound(shapes)
+    with TemporaryDirectory() as tmp:
+        path = Path(tmp) / "busbars.stl"
+        # build123d's export_stl takes a single shape; the Compound carries
+        # all busbars together so the slicer sees them as one job.
+        export_stl(compound, str(path))
+        return path.read_bytes()
+
+
 WRITERS = {
     "step": (to_step, "application/step", "step"),
     "dxf":  (to_dxf,  "image/vnd.dxf",     "dxf"),
     "svg":  (to_svg,  "image/svg+xml",     "svg"),
+    "stl":  (to_stl,  "model/stl",         "stl"),
 }
diff --git a/static/index.html b/static/index.html
index 1ccbd91..c8ee782 100644
--- a/static/index.html
+++ b/static/index.html
@@ -115,7 +115,23 @@
         <label>Slit width (mm)   <input type="number" id="p-slit-width"   value="1.0"  step="0.1" title="Width of each cross arm"></label>
         <label>Neighbor factor   <input type="number" id="p-neighbor-factor" value="1.15" step="0.05" min="1.0" title="Bridge two cells if distance ≤ factor × shortest pair distance"></label>
         <label class="checkbox"><input type="checkbox" id="p-extrude"> Extrude solid</label>
-        <label>Thickness (mm)    <input type="number" id="p-thickness"    value="0.2"  step="0.05"></label>
+        <label>Thickness (mm)    <input type="number" id="p-thickness"    value="0.2"  step="0.05"
+               title="Used only with 'Extrude solid' above (STEP export of a thin nickel/copper plate). FDM print uses its own thickness below."></label>
+      </div>
+      <div class="fdm-block">
+        <h3 class="fdm-title" title="3D-printable plate for verifying dimensions on a real printer. Forces extrusion regardless of the 'Extrude solid' checkbox.">
+          FDM test print
+        </h3>
+        <div class="fdm-row">
+          <label>Thickness (mm)
+            <input type="number" id="p-fdm-thickness" value="2.0" min="0.5" step="0.1"
+                   title="Layer-printable wall thickness. 2 mm is a safe starting value on a 0.4 mm nozzle.">
+          </label>
+          <div class="fdm-buttons">
+            <button id="btn-export-fdm-stl" type="button" class="primary" title="Extruded STL ready for the slicer">STL</button>
+            <button id="btn-export-fdm-step" type="button" title="Extruded STEP for CAD import">STEP</button>
+          </div>
+        </div>
       </div>
     </section>
 
diff --git a/static/js/app.js b/static/js/app.js
index f03b378..4ea9aad 100644
--- a/static/js/app.js
+++ b/static/js/app.js
@@ -157,6 +157,16 @@
   $("btn-export-dxf" ).addEventListener("click", () => Exporter.exportFormat("dxf",  state, params));
   $("btn-export-svg" ).addEventListener("click", () => Exporter.exportFormat("svg",  state, params));
 
+  // ---- FDM test print -----------------------------------------------------
+  // Forces extrusion at the FDM thickness, regardless of the 'Extrude solid'
+  // checkbox above (which keeps owning the thin nickel-strip case).
+  function _exportFdm(fmt) {
+    const t = Math.max(0.5, +$("p-fdm-thickness").value || 2);
+    Exporter.exportFormat(fmt, state, { ...params, extrude: true, thickness: t });
+  }
+  $("btn-export-fdm-stl") .addEventListener("click", () => _exportFdm("stl"));
+  $("btn-export-fdm-step").addEventListener("click", () => _exportFdm("step"));
+
   // ---- viewport init ------------------------------------------------------
   Viewport.init($("viewport"), state, params, {
     onCellClick: (cellId, mods) => {
diff --git a/static/styles.css b/static/styles.css
index 24e9c0b..7ad5de2 100644
--- a/static/styles.css
+++ b/static/styles.css
@@ -420,6 +420,36 @@ label.checkbox {
   color: var(--text);
 }
 
+/* FDM test-print block inside the Params panel */
+.fdm-block {
+  margin-top: 10px;
+  padding-top: 10px;
+  border-top: 1px solid var(--border);
+}
+.fdm-title {
+  font-size: 11px;
+  font-weight: 600;
+  color: var(--muted);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  margin: 0 0 6px;
+}
+.fdm-row {
+  display: flex;
+  align-items: end;
+  gap: 10px;
+}
+.fdm-row label {
+  flex: 1;
+}
+.fdm-buttons {
+  display: flex;
+  gap: 4px;
+}
+.fdm-buttons button {
+  min-width: 50px;
+}
+
 input[type=number], input[type=text], select, textarea {
   background: var(--bg);
   color: var(--text);