Shoe Sole Molding

I worked at a shoe factory that only manufactures lady's footwear and decided to make myself some shoes. We didn't have any mens soles, so I decided to make a mold and design my own. I did find a mens size 10 last which made the whole process easier for me.

A shoe last is the foot shaped plastic part that the upper is formed on and provides the solid backing when the sole is glued to the upper.

A shoe is made on a last, leather (in this case) is pulled tight over the last and glued onto insole board (which is stapled to the bottom of the last). The sole is then glued onto the insole board and leather, and the last is removed.

Designing the upper is another project altogether and I will not be going into that in any detail in this Instructible.

This is very simplified and not the only way to do it, but this is how I made the soles for my shoes.

The sole HAS to be made specifically for the last you are using. The bottom pattern of the last is used for the insole board and the sole. My sole will have a randing (the little lip that sticks up around the edge), this allows the last and your upper to drop in slightly giving you a cleaner looking finish. If your sole does not match the last you will have gaps where the sole and the upper don't meet nicely.

So lets get into it...

I used the Shoemaster shoe design package and a Zund CNC cutting table. It can be done without these it will just be much more labour intensive and time consuming.

- 1mm and 2mm Insole board. This is simply because I had access to it and I could cut it on the CNC cutting table. It is very porous, so is not actually ideal as a mold.

- Contact adhesive for gluing the insole board

- Modelling clay

- Cold Cast Urethane rubber. I used 30A shore hardness which is very soft and flexible and probably wont last too long.

- Sealant. This seals porous material for better detail and stops the rubber from penetrating into the material.

- Release agent. Stops the rubber from sticking to the other materials used.

The last 4 items can usually be bought from the same place (search for Molding, Casting or Composites companies).

Extras

- Nuts and bolts

- Soft brush for the sealant

- The rubber can be colored, but this was too costly for me.

I had limited time and money so I had to plan very carefully (and then still had to redo stuff).

Visualize and sketch your design.

Visualize and sketch the process you will follow.

Sole Design

My design was a fairly flat sole, 11mm high at the heel down to 7mm at the front. I used 1 layer of 1mm insole board for the design on the bottom of the sole, thus the odd numbers. I also had a 2mm high randing all around, so the total height of the sole is 13mm at the back and 9mm at the front. The bigger you go the more materials you need and more layers in the mold.

The bottom pattern of the Last is the starting point. Your insole (which the upper gets glued to) will be exactly the same as the bottom pattern.

The top surface of your sole needs to be about 2mm bigger all around. This allows for the thickness of the leather and lining when its folded around (Shown in image).

My randing was 2mm wide and 2mm high (1 layer of 2mm insole board)

Those are the basic measurements, from there just use your imagination. The images on the first page show the design I made, its a fairly basic design.

Mold Design

I decided to design the mold with the sole upside down. This means that there is a 'deep' point at the back sloping up towards a large opening. I designed the sole as I was working on the mold. With the software you can show all the layers at once, thus you can easily put a curve or detail into one layer and translate it onto other layers as required.

My mold was split into 3 parts, but it could have been done in 2 parts. I made it 3 layers to make it easier to pour the rubber into the mold. The idea being to put the bottom 2 sections together, pour to the top then put the top layer of the mold on and fill the mold through the small air outlets.

It can be done with 2 sections. Just add slightly larger holes to pour the rubber into.

Start with your base layer and work your way up, remembering that you are designing the cavity for your sole not the sole itself.

Unfortunately, I lost the original images from my actual design. I made a quick example on Solidworks to try and help explain the process and design.

1. Check everything before you start gluing. The mold must be able to open up at (or very near to) the widest part of the sole so that it can be removed from the mold.

2. Start gluing layers together, make sure everything lines up properly. I had the bolt holes around the edges to help with lining everything up. I used contact adhesive (apply to both sides let it dry etc.)

The top layer and design for the bottom of the sole were the most difficult to glue. Put glue on all the pieces you want to come off but leave the surrounding area so that it can be pulled off once the glue is dry. My design had some fairly small pieces that had to be repaired and in one instance replaced with clay as can be seen in the photo. The whole design can thus be made in clay if you want.

3. Once all your different sections are glued together you can start the finer finishing off of the mold. I used modelling clay to smooth out the steps formed by the board. Also to round off sharp corners on the randing.

My modelling clay didn't dry overnight and I didn't have time to wait any longer so I just carried on with it still wet. It didn't seem to matter much.

4. Read the instructions on your sealant and seal the mold wherever it will make contact with the rubber. The insole board is very porous and even multiple coats of sealant didn't seal it completely.

The last step is to apply a releasing agent but this is done right before the mold is used.

Follow the instructions of your particular rubber

1. Get everything you need ready and close at hand. If you can get someone to help, do it.

2. Spray the mold with the releasing agent and put it together. Because the mold has an overhang I had to spray it first to get a good coat everywhere.

3. Measure out your rubber and mix it well (follow instructions on bottle), you have about 30 mins but it becomes difficult to pour before then, so don't waste time.

Over filling is better than not pouring enough, any extra bits can be cut off easily.

5. Let the rubber dry and extract your fantastic new soles... hopefully.

Note: I took too long to pour the rubber through the small holes I had made. This meant that by the time I got to the second shoe it was too viscous to flow into all the cavities and my one sole was a disaster.

I had to repair the mold and cast one sole again. Luckily the mold was not too hard to repair and the second sole came out well.

You should now have a nice pair of soles

Enjoy...