MDF Bottom Mold building by CNC

[falls]

Ok. Planning some new full length non adjustable MDF molds. Multiple identical slices bolted together. Slices cut on CNC router.
Questions:
Anyone see any pros/cons of using 25mm MDF over 18mm MDF in terms of machining accuracy.
(I think I can get enough slices on 2 sheets of 25mm instead of 3 sheets of 18mm, which I think (?) will save machining time and therefore save money, even when extra money for thick MDF is taken into account).
The guy with the CNC I don't think fully appreciates how perfect I want each piece to be. Any thoughts on depth of each pass and how many passes you would think for cutting 18/25mm MDF.
Probably cut with 1/4" bit, the machine is pretty good I think with a proper variable speed spindle.
thanks

[ben_mtl]

I built my last set of tip/tail molds using 1" (25mm) MDF and it was ways easier ! I cut the pieces manually though so that's where I saved a lot of time. I don't see any problem in your case.

One thing I've found though is my tip/til blocks (all of them, not just the last ones) are splitting : I don't know if it's the right expression in english but basically cracks appear on those molds and it's the MDF itself, not the bond, which is cracking. If I can think about it I'll take pics tonight.
For my next set of tip/tail mold I'll try to make a solid block of MDF, glueing pieces of MDF horizontaly, roughly the shape of the tip/tail mold and then bring that to my CNC guy (which I still have to find since I moved :p) so he will machine the block directly.
It might save some machine time, save some material (less waste as I just have to cut rectangular pieces of MDF of different sizes) and it should also be less subject to splitting as when in use I only compress both my MDF pieces and my glue joints, no shear !

[chrismp]

ben, i'm having the same problem...i just filled the cracks with polyurethane glue as much as possible. doesn't seem to affect the end results so far, but i'm also thinking about making molds the way you describe.

[Raccoon]

I build my bottom and top mold with 3/4" thick MDF. cut in 1 pass on the CNC router, 18000 rpm spindle, 600in/min working speed, 1/2 router bit.

Julien
www.evasionkiteboards.com

[threeninethree]

The guy that cuts mine does it in 2 passes, the first offset 1/16" from the finished shape then on the second pass he takes a cleaning pass trimming 1/16" to the final shape. Im using 19mm mdf by the way.

[sir.orange]

i did my moulds in 38mm mdf, we machined it in 3 or 4 passes. the router (8-10mm i forgot)wasn t too sharp, what resulted in a little bending of the router where hitting the material, 0,05mm, not a lot, but noticable.

1/4" router and 25mm mdf - at least 5 passes!? and slow feed rates 1-2 meter/min.
i would be scared by a 1/4" router for a 25mm material anyway

[sir.orange]

i did my moulds in 38mm mdf, we machined it in 3 or 4 passes. the router (8-10mm i forgot)wasn t too sharp, what resulted in a little bending of the router where hitting the material, 0,05mm, not a lot, but noticable.

1/4" router and 25mm mdf - at least 5 passes!? and slow feed rates 1-2 meter/min.
i would be scared by a 1/4" router for a 25mm material anyway

[Brazen]

Raccoon must have a beast machine. Mines not rigid enough to take that depth at that speed. Congrats