Claw Mold Making

By Scott Sanderson

Preface: This page details the process of making molds to cast resin claws. So many times when making a new part you are rushed and do not find the time to document the process and take pictures, but this time I made a conscious effort to document entire moldmaking the process. I hope this gives a little insight on how much work and time it takes to create a simple mold. 

Day 1: First step is to create a highly detailed and refined master. A little bondo, and some sanding and the claws were ready to go! Time: 4 hours

Day 2: Mold making process is started by mounting the master on a sheet of plexiglass so that it can be divided in half using molders clay to form the first half of the mold. Special consideration must be given to where the mold part line will be and also where the mold will be poured from (in this case the very end of the claw). Time: 3 hours

After the claw has been fully sectioned off the area where the clay meets the master must be cleaned to create a sharp edge free of debris and all clay residue must be removed form the master. Sides must be added to the mold box define the width and thickness of the mold. All gaps must be filled in to prevent silicon from leaking out of the mold.

Once the mold box is finished its time to add dimples to the mating surface so that the mold registers correctly.

Completed mold box.

An impression coat of silicon is brushed onto the master. This eliminates bubbles and ensures full silicon coverage without voids or air bubbles.

The mold box is filled up the rest of the way with silicon. Notice because of the angled tip on the claw this process will require 2 pourings. The silicon is allowed to cure overnight.

Day 3: The moldbox is tilted at an angle and the remaining portion of the mold is filled with silicon and allowed to cure overnight. Time: 15 minutes

Day 4: After the silicon has cured the clay moldbox is carefully removed. It is very important to remove all the clay, but critical that the silicon is not peeled away from the master. If the seal between the master and mold is broken it will allow silicon from the second pouring to seep between the mold and master. Once all the clay is removed the mold and master is cleaned with a brush and water to remove any clay residue. Time: 2.5 hours

The mold is then remounted to the plexiglass and a new moldbox is created from clay to form the second half of the mold.

Once the moldbox is completed for the second half, mold release must be brushed onto the silicone to prevent the second half from bonding to the first half. Mold release is only applied to the silicon surfaces and not the master.

Once again the mold box is filled with silicon. Because of the anlge on the claw, this side will also require two pourings. Note: It is possible to do the mold in one pouring, but this will unnecessarily add to the thickness of the mold and increase the cost of the mold (silicon costs about $115 a gallon!). The mold is allowed to cure overnight.

Day 5: Once the silicon has cured clay barriers are added to create a smaller moldbox on top of the first pouring to cover the remaining portion of the claw. This is allowed to cure overnight. Time: 20 minutes

Day 6: After the silicon has cured the claw is removed and the mold halves are separated from the each other and the master. The halves are then cleaned to remove any remaining clay residue. The first mold is completed and will sit for a few days to fully cure. Time: 30 minutes

Day 6-7-8-9: Repeat steps from Day 2 through Day 6 to make the mold for the second claw half. Note: the second claw mold could be start in parallel with the first, however because of the time required and amount of clay needed it was not practical. Time: 6 hours, 35 minutes

Day 9+: Now its finally time to make the first set of claws and remove the flashing from the mold part line. Completed set of resin claws pictured below. Note: without careful planning and execution you can waste 9 days, a gallon of silicon, and still not have useable molds! Time 1.5 hours

Total hours to make the molds: 17 hours, 10 minutes.
Material cost: $115 (one gallon of silicon), plus clay, clay tools, etc.