So I have been busy the past couple weeks on a number of clients prototypes and I found a way of producing realistic representations of the end product. For one particular project, the client wanted to develop a thin curved form that could be worn under clothing (no futher details so not to compromise the IP). It was determined that the final material would most likely be one of the following:
1) Silicon Rubber
3) TPE (Thermoplastic Elastomer)
I was contemplating investing in some simple CNC-milled mold cavities but, the cost was just too much for the purpose of validating a concept. I then stumbled upon a company that specializes in special effects make up and masks called Smooth-on (www.smooth-on.com). This company makes many different liquid molding compounds that can be used for prosthetic, masks, food, taxidermy and prototypes. In watching a video about how to create a 2-part mold, I was intrigued to say the least.
For me the first thing to do was to make the original model that would be used to create the negative void space within the 2-piece mold. Due to the thin curved nature of the clients product, I decided to work with an air-dry sculpting clay for the main form. Once this was dry, I filled in all of the small imperfections using melted beeswax. I was really impressed with the surface finish I gained using the wax. The final step was to go over the entire model surface with a propane torch to melt the wax completely smooth to the touch. Now I was ready to start with the mold.
To create the mold box, I went and purchased a series of 1/4″ thick acrylic sheet panels that would serve as the bottom and sides of the mold box. Using a hot glue gun and a mold strap, I secured the sides so they were ready to receive the clay and rubber. As noted in the video, I started with a layer of modeling clay on the bottom of the box to about 1″ of depth. I then placed the model into the clay so that the parting line would be across the top of the clay. I then smoothed the entire clay surface and placed some 3/8″ acorn nuts into the clay with the round portion facing up (these will serve as registration keys for the two mold halves).
I purchased the Smooth-on Mold Star 15 which is a Platinum cure Silicon with great elongation properties while also having the finishing properties I wanted. After the clay was ready, I then measured out equal parts of the Mold Star 15 mixture components and quickly mixed them together until a consistent color was achieved. I then poured the Silicon onto the clay and mold slowly to make sure there wasn’t any gas bubbles. After about 4 hours, I was ready to break down the mold box to prep for the second half. By removing the walls of the box I then flipped over the mold and removed all of the clay (making sure not to remove the actual model). I had an old paint brush handy that I used to further remove any trace of clay from the surfaces. The next step was to reassembly the mold box using the now cured first half as the base. Once the walls were secured in-place I sprayed a couple coatings of Ease Release 200 mold release onto the entire cured Silicon and model surfaces. Using a small amount of clay I created the pour spout that would later be used to pour the molding rubber. This can be seen in the video above.
I then mixed and poured the second half of Mold Star 15 onto the top of the mold half. After another 4 hours of waiting, the mold was finally ready to separate to see the resulting cavity. At first, the two mold halves were impossible to part. I then decided to use compressed air to injected through the pour spout which worked very well at parting the two halves. After initial inspection I was very pleased with the results (although not perfect).
Now I was ready to try molding the first sample. I chose to use the Body Double Silicon rubber to create a life-like finish on the clients product. This is also a 1-1 ratio mix that is really easy to use (although the high viscosity may warrant adding a thinner next time to allow for easier flowing into mold cavities). After another coating of mold release I started pouring the Body Double into the cavity through the pour spout. I allowed the sample to cure overnight and in the morning I parted the two halves to see my first molded sample. Not bad.
This was the first time I had ever created a mold using Silicon and I learned alot. First, I created a much larger mold box than needed which cost more in product and also made for a bulky mold. Next, the issues with the two part mold halves sticking may have been from some residual clay that was still on the surface so next time I will try and wash the surfaces. Last, to improved surface finish of the end sample, I will be adding a Silicon thinner to the mix to reduce the viscosity and improve pourability.
If you would like to see some more photos, please message me and I can share.
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