Making a Mold questions

Hi All,

I am planning on making a mold for a swim fin blade with the Shapeoko XXL that I just received.
The material I am planning on using is 1" thick MDF sheet that I have to glue and stack up to get 2" thickness. The part bounding box is 2.010", but the other side of the 2 part mold will need some of that thickness/depth.
Recommendations for glue type?

I will surface both sheets before gluing.

The smallest detail is a 1mm Radius, but I plan on using a 1/16" cutter (yet to order) that has 1 degree angle, and the client is OK with that.
Is an an Upcut bit with 2 flutes going to work well?
How much of the bit is held in the collet of the router? is 3" tool length going to work, where 1" is held tight by the router?
I’m guessing that I will need to define this tool in the library (Fusion 360 or Meshcam)?

I’d like to hog out with the included 1/4" router bit then change to the 1/16" cutter, but I haven’t done a tool change in the middle of cutting before, so that will be new for me too. I think that I will need to be very careful inserting the tool into the router at the correct depth.

What other issues should I consider?

1 Like

use gorilla polyurethane glue, no need to face them before gluing. You will need to consider flute length. On a 1/16" cutter flute length is probably only .375 at best. tool change is pretty simple just be sure not to machine off your original zeroing point. G/L Ray

Thanks fiero1! Thanks for the info on the cutter flute length (max .375") and the tool change technique.
I am assuming that is cutting depth into the stock? If the OD of the cutter is .25" but the cutting tip is tapered to 1/16" in the last 1/2" or so, then could depth be increased without the router slamming into the stock? I know that you are unfamiliar with the part. My current plan is to rough / hog out as much material I can with the provided .25" cutter, although maybe a bullnose would be better. and then change over to the 1/16" for finishing??

Are you planning to laminate the two sheets of MDF together - i.e. are you spreading glue over the whole surface and then clamping? For this kind of work I would use wood glue, and lots of it. MDF will take it very well. I personally don’t like gorilla glue much because it expands a lot when it cures.


Yes, that is my plan, unless there is a better way. I searched for 2 1/4" MDF locally in the size sheets needed, but nothing is available, so I will utilize material I have already. Glue is cheap in this instance, but I was concerned about the glue gumming up the cutter. I will need to surface the top of the laminated sheets, so that operation could possibly mitigate the Gorilla Glue expansion issue? Or does it continue to expand over time/temperature too? Any insight? Thanks!

I was thinking about using this:

Once it cures it stops expanding, so there’s no worries about that. I just prefer wood glue for mdf and other wood or wood-like products. My feelings towards gorilla glue are more just personal preference - although you do need to dampen the surface it’s going to adhere to, which may not be ideal for MDF…
For a mold cavity you’ll want the MDF to be bonded uniformly across the layers. I’ve had gorilla glue fail - though it was on styrofoam - and when it pulled apart I could see that the glue had not spread out in a consistent layer across the bonded surfaces, but had just stayed in more or less the same places I’d squeezed it on. I was too thick to spread evenly. I suggest a test, to make sure you’re not going to end up with a gap between your layers.

That molding result must have been a very unpleasant surprise, to say the least.
I was planning on using a paint roller to get even coverage of glue, then evenly distributing heavy weights across the top sheet of MDF. I do appreciate your feedback and hope to share my success with the forum.

I made this by gluing (2) pieces of 3/4" MDF together with gorilla glue. You will want to use a white foam roller to roll it out, it works perfect on MDF.

G/L Ray


That project seems to have turned out well.

I wouldn’t try the tool change like that, I’d split the job into 2 tasks. There may be a better way but I would home the machine if you have limit switches, that gives you an independent repeatable point on the machine to reference. I then ‘zero’ before moving it to my actual stock and then I note the location offset in carbide motion before zeroing it for the job. So I zero it twice by my logic is I have the ‘stock zere’ as a reference from the machine’s limit switch based zero. Hope that makes sense, that way if the machine dies or resets or whatever I can easily get back to the zero I need to set for the job.

With that information in place, you can have 2 jobs, one to hog out material and one to do a proper cut. I’ve been playing with Fusion 360 (though NOT run a job based on it’s output) and I know it supports roughing passes, which is what you’re talking about.

Programming 2 jobs makes sense, it’s easier to trouble shoot and modify, even though zeroing out (twice) to calibrate is a few more steps.
Now need I to get some clamps and also assemble the Shapeoko.