This is my process for designing and cutting a foam mold for a custom skateboard deck. Starting with a block of foam and an idea use SK8CAD to create the shape you want. Use a CNC router to carve that shape into the foam block. Then follow the next article to vacuum press wood veneers onto the mold.

Requirements:

Materials:

• foam blank 12" x 36" x 3"
• double sided tape for CNC hold down
• end mill for your CNC machine with at least 3 inches of stick out

Tools:

• CNC router with at least 12" x 36" X/Y working area and at least 6 inches of Z travel
• Software for creating 3d tool paths

Details

Foam

For foam I generally use XPS pink insulation foam because its is readily available at local hardware stores. It is not often available in 3 inch thickness so I use multiple layers of 1-2 inch thickness with spray adhesive to stack them together. As long as you give the spray glue time to dry this will work just fine. There are special spray glues made for foam. I have not had success with these. I use normal spray glue 3M Super 77 or equivalent which can melt foam but I have had good luck as long as you spray from at least 1 foot away from the surface and only apply a thin coat.

The downside to the XPS foam is that is compresses during the vacuum press process so the mold is only reliable once. I have used a mold a second time and it works fine but the board will not be as close to the exact shape designed as the original. You can use stiffer foam which will last longer that is available from specialized suppliers or even cut your mold from wood or cast them in concrete if you want to re use them over and over again.

CNC Router and Endmills

I have made boards on several different machines. The Inventables X-Carve Pro 4x2 has just barely enough Z clearance at 4 inches and is a low cost machine that will do the job. I have used 96"x48" and 48"x48" machines from ShopBot and a few other large machines. The Biggest considerations after making sure the X/Y working area will fit your board is the Z clearance. To cut 3 inch thick foam you want a 3 inch long end mill to clear above the blank. If you have that clearance you are good to go. Cutting foam is easy and should not push the limits of any commercial CNC router.

To speed things up I don’t bother with a roughing pass when cutting the 3d shape in foam. I just go straight to a finishing pass. Because we are vacuum pressing wood onto foam any small inconsistencies in the foam won’t show through to the finished wood. To this end it doesn’t make any difference to use a flat or ball nose end mill so I use a flat end mill for both the 3d finishing pass and the profile pass to cut the final size of the mold. I use a 1/2" end mill for both, again to cut as fast as possible. Using a smaller tool will still work just fine it will just increase your cut times.

Design

The process starts the same as making a fingerboard. Using SK8CAD to shape the custom deck you want to build. See the fingerboard project page for a quick rundown of SK8CAD settings or use their Users Guide in the help menu. Choose any of their Example Shapes (except fingerboard) keeping in mind the size of the veneer that you have to make your skateboard. I use 9.5x35" veneers so some of the long board shapes will be too long be default. You can either buy longer veneer or compress the mold length/width to fit your materials. I used length of 35" and width of 11" for my workshops because it is easy to prepare 12x36" stock to cut the molds from. As for the mold height I recommend that you set the value in SK8CAD to 0.1 inches less than your foam block. I used 3 inch thick foam so set the value to 2.9 inches. This will make sure that you cut away at least 0.1 inch from the top of the foam and if surface imperfection dip below 3 inches there will still be some material to cut.

Once the design is ready export the .stl file of the Male Mold and the .svg file of the outline. We don’t need the female mold for the vacuum press process.

Tool Path Generation

I use VCarve for tool path generation. If you use something else I will describe the general process and you can adapt it to your software.

Setup

Setup your workspace. I start with 16" x 40" x 3" material in VCarve. As stated above I start with a 12" x 36" x 3" block of foam to cut the mold. The extra space is to add fixtures detailed below.

Import the .stl file of your male mold. If you followed my suggestion to set Mold Height to 0.1 inch lower than the height of your stock just make sure the bottom of the mold and the bottom of the stock are flush to each other and you’ll be all set. If not you’ll want to offset the height so that the top surface of the mold is set down from the top surface of the stock slightly. I center the mold in the stock. This makes aligning the .svg file with the profile and truck holes easier down the line.

In VCarve I draw a rectangle the same size as the mold. 11" x 35" This will help with setting the machining boundary and some of the fixturing cuts later.

3D Carve

To save time I skip doing a roughing pass and just go straight to a finishing pass. I use a 1/2" flat end mill with 50% step over. Cutting at 300 ipm this will carve a mold in about 20 minutes. The large bit and high step over will result in a stepped appearance to the mold but this will not show through on the finished skateboard. If you have more time or and cutting a stiffer material to reuse the mold multiple times I would recommend using a roughing pass and finishing with a smaller tool and smaller step over. Use the 11" x 35" vector as the machining boundary to just cut the top of the mold. This will save time cutting the profile as a separate operation.

Profile

Use the 11" x 35" rectangle to define a profile tool path to cut the mold out of the stock. If your work holding solution is strong enough I have successfully cut 3" depth of cut in a single pass to cut the profile. That said cutting in 2 or 3 passes will not take very long and won’t risk your nicely carved mold getting pulled off your machine. Set your tool path to cut outside the line. It doesn’t need tabs because both the mold and the scrap should be held down by double sided tape.

Warning

If you do not have a tool that can cut the full depth of the foam in a single pass then you can offset a second rectangle by the tool diameter and create multiple stepped profile cuts to prevent a collet crash. I’ve had to do this when I only had a 1.5" long end mill. See my example VCarve file below.

Fixturing

To cut a lot of molds quickly I developed a series of fixtures to get quick alignment of materials. I start by drawing a 12" x 36" rectangle centered in the workspace. This represents the foam stock. I create a L bracket from the origin to the corresponding corner of the foam block extending 6"-8" in X/Y. I put a dog bone in the corner so that the foam can sit square into the corner.

Create a profile tool path to cut out this fixture. I used some scrap 1 inch thick foam so I could cut the fixtures just as fast as I was cutting the molds and then I don’t need a tool change. Any material that will align the stock will do.

This image shows the fixture is the full height of the 3" stock. This is just a quirk of doing tricky things in VCarve. I cut it from 1" tall material and then add the 3" tall material for the mold.

Cutting

Fixture

Start with the fixture. I use 1" thick scrap foam for the fixture. It will need a piece approximately 12" square depending on the size you made it. I use 2" double sided carpet tape to hold the foam to the bed of the CNC. Make sure the bed and the foam are free of loose dust so it will stay in place through multiple mold cuts. Zero the X/Y axis close to the corner where there is plenty of space to place the mold stock later and then cut the fixture.

This should leave a foam L shape on the bed read to receive your mold stock.

DO NOT CHANGE YOUR X/Y ZERO FROM THIS POINT ON!

Mold Carve

I used 2" double sided carpet tape to hold the foam to the bed of the CNC. Make sure the bed and the foam are free of loose dust. I apply 3 stripes of tape the length of the stock and place the stock into the fixture cut in the last step. Alignment is not critical on this step but practice doing it will because it becomes more important later. If you cut your fixture with the same tool you are going to use to cut the mold then you don’t need to re zero anything. If you have to do a tool change make sure you only re zero the Z axis. Cut the mold. This will generate a lot of foam dust. Unless your dust collector is amazing it will get everywhere and the foam become statically charged and will stick to everything. You can try to ground the moving parts of your machine better to fix that but I haven’t tried that yet. Have a vacuum on standby for clean up as soon as the cut finishes.

Profile Cut

Other than clearing out the foam dust you shouldn’t need to do anything to cut the profile as long as you are using the same tool. Run the profile cut and your mold will be done. Remove the mold and the scrap from the bed. If you are cutting more than one mold then be careful to not remove the fixture. Get your next piece of stock ready and as long as the tool path for subsequent molds were setup with the same stock size and work area the fixture should line right up and be ready to go.

Finishing

It will be helpful for later if as soon as you take your mold of the CNC you mark it with which end is the tip and tail. A simple big arrow pointing toward the tip will do or you can label each end. This will prevent mistakes later when you cut the profile of the deck and the truck holes.