diff --git a/src/kiri/layers.js b/src/kiri/layers.js
index e96df07e..8d73a35d 100644
--- a/src/kiri/layers.js
+++ b/src/kiri/layers.js
@@ -85,6 +85,9 @@
         }
 
         addPolys(polys, options) {
+            if (options && options.clean) {
+                polys = polys.map(p => p.clean());
+            }
             if (options && options.flat) {
                 return this.addFlats(polys, options);
             }
@@ -200,8 +203,6 @@
                 const contour = [];
                 poly = poly.debur(0.05);
                 if (!poly) return;
-                poly = poly.clean();
-                if (!poly) return;
                 poly = poly.miter();
                 poly.points.forEach(p => {
                     contour.push(new THREE.Vector2(p.x, p.y));
diff --git a/src/mode/fdm/slice.js b/src/mode/fdm/slice.js
index 5b041f83..70e5aef4 100644
--- a/src/mode/fdm/slice.js
+++ b/src/mode/fdm/slice.js
@@ -484,7 +484,7 @@
 
             if (top.shells) output
                 .setLayer("shells", isSynth ? COLOR.support : COLOR.shell)
-                .addPolys(top.shells, vopt({ offset, height }));
+                .addPolys(top.shells, vopt({ offset, height, clean: true }));
 
             if (top.gaps) output
                 .setLayer("gaps", COLOR.gaps)