diff --git a/global_defs.scad b/global_defs.scad
index 03a85bc..c9c73f1 100644
--- a/global_defs.scad
+++ b/global_defs.scad
@@ -41,13 +41,14 @@ pp1_colour      = is_undef($pp1_colour)      ? rr_green     : $pp1_colour;// pri
 pp2_colour      = is_undef($pp2_colour)      ? crimson      : $pp2_colour;// printed part colour 2
 pp3_colour      = is_undef($pp3_colour)      ? "SteelBlue"  : $pp3_colour;// printed part colour 3
 pp4_colour      = is_undef($pp4_colour)      ? "darkorange" : $pp4_colour;// printed part colour 4
-
+manifold        = is_undef($manifold)        ? false  : $manifold;        // Manifold library being used instead of cgal
 
 // Minimum wall is about two filaments wide but we extrude it closer to get better bonding
 squeezed_wall = $preview ? 2 * extrusion_width - layer_height * (1 - PI / 4)
                          : extrusion_width - layer_height / 2 + nozzle / 2 + extrusion_width / 2;
 
-inf = 1e10;      // very big
+inf = 1e10;       // very big
+big = manifold ? 1e3 : inf;     // Not too big for manifold
 eps = 1/128;     // small fudge factor to stop CSG barfing on coincident faces.
 
 fa = is_undef($vitamin_fa) ? 6    : $vitamin_fa;          // Used for drawing vitamins, rather than printing.
diff --git a/readme.md b/readme.md
index 8320177..61c47d7 100644
--- a/readme.md
+++ b/readme.md
@@ -7539,7 +7539,7 @@ Original version by Doug Moen on the OpenSCAD forum
 | Module | Description |
 |:--- |:--- |
 | `box(xmin, ymin, zmin, xmax, ymax, zmax)` | Construct a box given its bounds |
-| `clip(xmin = -inf, ymin = -inf, zmin = -inf, xmax = inf, ymax = inf, zmax = inf, convexity = 1)` | Clip child to specified boundaries |
+| `clip(xmin = -big, ymin = -big, zmin = -big, xmax = big, ymax = big, zmax = big, convexity = 1)` | Clip child to specified boundaries |
 
 ![clip](tests/png/clip.png)
 
@@ -7585,6 +7585,7 @@ See [global_defs.scad](../../global_defs.scad) for a list of global constants.
 | `extrude_if(h, center = true)` | Extrudes 2D object to 3D when `h` is nonzero, otherwise leaves it 2D |
 | `hflip(flip=true)` | Invert children by doing a 180° flip around the Y axis |
 | `render_if(render = true, convexity = 2)` | Renders an object if `render` is true, otherwise leaves it unrendered |
+| `render_manifold()` | Render if manifold to work around convexity bug in manifold |
 | `right_triangle(width, height, h, center = true)` | A right angled triangle with the 90° corner at the origin. 3D when `h` is nonzero, otherwise 2D |
 | `semi_circle(r, d = undef)` | A semi circle in the positive Y domain |
 | `translate_z(z)` | Shortcut for Z only translations |
diff --git a/scripts/openscad.py b/scripts/openscad.py
index 1106aca..a39b882 100644
--- a/scripts/openscad.py
+++ b/scripts/openscad.py
@@ -26,6 +26,8 @@ import subprocess, sys
 
 def run_list(args, silent = False, verbose = False):
     cmd = ["openscad"] + args + ["--hardwarnings"]
+    if "-D$manifold=true" in args:
+        cmd += ["--enable"] + ["manifold"]
     if not silent:
         for arg in cmd:
             print(arg, end=" ")
diff --git a/scripts/options.py b/scripts/options.py
index cd11138..8d965a0 100644
--- a/scripts/options.py
+++ b/scripts/options.py
@@ -26,8 +26,9 @@ def check_options(dir = '.'):
     global options, options_mtime
     options = {
         "show_threads": str(os.getenv("NOPSCADLIB_SHOW_THREADS")),
-        "vitamin_fa":   str(os.getenv("NOPSCADLIB_VITAMIN_FA")),
-        "vitamin_fs":   str(os.getenv("NOPSCADLIB_VITAMIN_FS"))
+        "vitamin_fa"  : str(os.getenv("NOPSCADLIB_VITAMIN_FA")),
+        "vitamin_fs"  : str(os.getenv("NOPSCADLIB_VITAMIN_FS")),
+        "manifold"    : str(os.getenv("NOPSCADLIB_MANIFOLD"))
     }
     options_fname = dir + '/options.json'
     try:
diff --git a/tests/polyholes.scad b/tests/polyholes.scad
index 8cf7528..35822fe 100644
--- a/tests/polyholes.scad
+++ b/tests/polyholes.scad
@@ -73,7 +73,7 @@ module polyholes() {
     // Alternating polyholes
     //
     translate([30, -40])
-        alt_polyhole_stl();
+        render_manifold() alt_polyhole_stl();
     //
     // Poly rings
     //
diff --git a/tests/screws.scad b/tests/screws.scad
index 16decc0..638263d 100644
--- a/tests/screws.scad
+++ b/tests/screws.scad
@@ -57,7 +57,7 @@ module screws() {
             }
         }
         translate([20, 60, -15])
-            polysink_stl();
+            render_manifold() polysink_stl();
 }
 
 if($preview)
diff --git a/utils/core/clip.scad b/utils/core/clip.scad
index 1d79ddb..cc1321d 100644
--- a/utils/core/clip.scad
+++ b/utils/core/clip.scad
@@ -41,7 +41,7 @@ module box(xmin, ymin, zmin, xmax, ymax, zmax) //! Construct a box given its bou
          [0,2,3,1]]  // left
     );
 
-module clip(xmin = -inf, ymin = -inf, zmin = -inf, xmax = inf, ymax = inf, zmax = inf, convexity = 1) //! Clip child to specified boundaries
+module clip(xmin = -big, ymin = -big, zmin = -big, xmax = big, ymax = big, zmax = big, convexity = 1) //! Clip child to specified boundaries
     render(convexity = convexity) intersection() {
         children();
 
diff --git a/utils/core/global.scad b/utils/core/global.scad
index 36cebfa..7dbebb0 100644
--- a/utils/core/global.scad
+++ b/utils/core/global.scad
@@ -74,6 +74,10 @@ module render_if(render = true, convexity = 2)      //! Renders an object if `re
     else
         children();
 
+module render_manifold()                            //! Render if manifold to work around convexity bug in manifold
+    render_if(manifold)
+        children();
+
 module extrude_if(h, center = true)                 //! Extrudes 2D object to 3D when `h` is nonzero, otherwise leaves it 2D
     if(h)
         linear_extrude(h, center = center, convexity = 5) // 3D
diff --git a/utils/offset.scad b/utils/offset.scad
index 8c13d3f..97374e3 100644
--- a/utils/offset.scad
+++ b/utils/offset.scad
@@ -47,11 +47,11 @@ module offset_3D(r, chamfer_base = false) { //! Offset 3D shape by specified rad
     else
         if(r < 0)
             render() difference() {
-                cube(inf / 2, center = true);
+                cube(big / 2, center = true);
 
                 minkowski() {
                     difference() {
-                        cube(inf, center = true);
+                        cube(big, center = true);
 
                         children();
                     }
diff --git a/utils/pcb_utils.scad b/utils/pcb_utils.scad
index 3447696..207b960 100644
--- a/utils/pcb_utils.scad
+++ b/utils/pcb_utils.scad
@@ -52,6 +52,6 @@ module cylindrical_wrap(r, h = eps) { //! Wrap a 2D child extruded to height `h`
                             translate([(sides / 2 - i) * -dx, 0])
                                 children();
 
-                            square([dx, inf], center = true);
+                            square([dx, big], center = true);
                         }
 }
diff --git a/vitamins/psu.scad b/vitamins/psu.scad
index 3083667..017a7b8 100644
--- a/vitamins/psu.scad
+++ b/vitamins/psu.scad
@@ -161,7 +161,7 @@ module psu(type) { //! Draw a power supply
                                         fan_holes(fan.z, h = 0);
 
                                         difference() {
-                                            square(inf, true);
+                                            square(big, true);
 
                                             fan_guard(fan.z, thickness = 0, grill = true);
                                         }