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Perhaps you can share your code and we can suggest improvements?

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Below is the code I use.
When I change the line 24 union into difference.
The rendering take only 8 seconds but my screen becomes white. The object
does not appear in the screen or very slooooooow.

//bass clarinet bell
// version 3
// J.G.Jonker
// date: 6-12-2015
$fn=40;
maxheight = 150;
d_btm_inside = 41;
d_btm_wall =17.5;
d_top_inside = 138;
d_top_wall = 2;
t_top = 60;
rotatie_top = 45;
mildfactor = 0.5;
h_tenon = 5;
//---------------
astraal = maxheight/sin(rotatie_top);

difference()
{
//difference()
union()
{
calyx(buiten=true);
calyx(buiten=false);
}

}

module calyx()
{
delta = 1;
for (height = [0:delta:maxheight-delta])
{
percentage = height/maxheight;
percentage2 = (height+delta)/maxheight;

    d_inside    = d_top_inside 
                - sqrt(sqrt(1-percentage))* (d_top_inside -

d_btm_inside);
d_inside2  = d_top_inside
- sqrt(sqrt(1-percentage2))* (d_top_inside -
d_btm_inside);

    d_wall      =   d_top_wall;
    d_wall2     =   d_wall;

    dx_raw      = (d_inside2-d_inside)/2;
    s_raw       = sqrt(dx_raw*dx_raw+delta*delta);
    dx          = d_wall * delta/s_raw;
    dy          = d_wall * dx_raw/s_raw;

    // bepaal de walldikte

    //--------------------------
    hoek = rotatie_top*percentage;
    hoek2 = rotatie_top*percentage2;
        
    if (buiten == true)
        {                      
            cylinder_pie(hoek, hoek2, 
                    -astraal*(1-cos(hoek*percentage))*mildfactor,
                    -astraal*(1-cos(hoek2*percentage2))*mildfactor,
                    height, height+delta-dy,d_inside/2+2*dx,d_inside2/2

+2dx);
}
else
{
cylinder_pie(hoek, hoek2,
-astraal(1-cos(hoekpercentage))mildfactor,
-astraal(1-cos(hoek2percentage2))*mildfactor,
height, height+delta,d_inside/2,d_inside2/2);
}
}
}

module cylinder_pie(angle1, angle2, dx1,dx2,dy1,dy2,r1,r2)
{
// draw top and bottom cylinders
// rotate the cylinder
// hull the cylinders
//
d=0.1;
hull()
{

             translate([ 0,dx1,dy1])
             rotate( angle1, ([1,0,0]))
             translate([0,0,-d])
             cylinder(d,r1, r1, center=false);
           
             translate([ 0,dx2,dy2])
             rotate( angle2, ([1,0,0]))
             translate([0,0,d])  
             cylinder(d,r2, r2, center=false);
      }
}

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Some pictures:

http://forum.openscad.org/file/n15000/Bell_2.jpg
http://forum.openscad.org/file/n15000/kelk.jpg

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Sent from the OpenSCAD mailing list archive at Nabble.com.

Using import of stl is a very good idea.
It works for $fn=40 and results is a fast object.

http://forum.openscad.org/file/n15001/kelk_solved.jpg

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The stepsize for each ring is called Delta in your calyx module.
If you change this to 5 you will see more clearly that the hull() is not
working quite right.
Each step is not the same as the Delta+1 step.
http://forum.openscad.org/file/n15007/bell-hull.png

Mnay ways to refactor but possibly you could refactoring so that you pass in
the thickness of the wall as an arg to the module and calculate the next
ring(for the hull) on basis of +1 to Delta.

So calculation is simple for the inner and outer shapes for the difference.

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