NSFW : something personal for my GF ... cast in silicone

NSFW : something personal for my GF … cast in silicone

Overview

This is a continuation of a project I started 2 months ago : The first part can be seen here : imgur.com/a/wlsJ5

Time to start the silicone casting process. After designing and prototyping the form in maple, I began researching silicones and refreshing my casting knowledge. This post is an overview of the second phase of the prototype, molding a prototype positive, and then casting using a variety of silicones.

Step 1 : Mold making container

First step was to make a container to hold the mold making material. I used some plywood drop that was lying around the shop, cut the parts of the vessel to allow for about 1/2" of clearance in all dimensions to allow for a durable silicone mold.
Step 2 : sealing things up

I used a couple coats of shellac to seal the ply; hot glue was used to seal the seams… hopefully preventing the mold making silicone from penetrating the small voids.
Step 3 : preparing positive for 2 part mold

In an attempt to get a symmetrical 2 part mold with an even parting line, I measured the mold box, and then cut spacer blocks that would allow me to suspend the positive in the exact center. I used gaff tape to affix the form to a couple of pieces of wood that would act as miniature joists to allow for easy positioning within the mold box.
Step 4 : positioning the form within mold box

Step 5 : Part 1 of Silicone mold

After researching materials and speaking to product reps, i decided to use a platinum cure mold making silicone by smooth-on ( mold max – 16 fast). The product has a decent pot life of 6 mins and cure time of 30 mins… this allowed me to make a completed 2 part mold within a couple hours. To determine the amount of silicone needed, I first calculated the volume of my mold container, then subtracted the volume of the positive form to be molded, and then finally divided this amount in half… as it would end up being molded in 2 steps. Smooth-on has a really handy set of calculators to assist in this process as much of the calculations are also dependent upon the specific gravity of each of their materials
Step 6 : Cutting registration keys

after the first pour, I waited for the molding silicone to cure, then cut keys to help in the re-alignment of the mold for future castings. I was careful not to disturb the positive form, and applied a mold release agent specifically made for silicone to silicone casting… being incredibly careful to follow the instructions as silicone bonds to itself very easily (and permanently).
Step 7 : mold part 2

second pour of the mold max 16 was not as easy as the first…. i think the temperature in my shop (90+ degreed F) accelerated the cure time… was nervous about the results….
Step 7 : cont'd

After waiting for a complete cure, I separated the two parts of the mold to inspect the results…everything looks good to start casting!
step 8 : pour spout

completely forgot about casting a pour spout into my mold, so i used a razor knife to cut one…..
Step 9 : mixing and tinting silicone

For the first test, I decided to go with smooth on dragon skin 30 : also a platinum cure, skin safe silicone. Using the material estimator, I calculated the volume of the object and used a digital scale to weigh out parts A and B. I also wanted to tint the silicone slightly…. when the package says a little goes a long way, they mean it…. A dime sized drop of tint turned 20oz of silicone jet black
step 10 : Degassing silicone

I improvised a vacuum degassing chamber using a vacuum bag/press that we use in the shop for laminating and veneering material…. its not as strong as a traditional degassing chamber, the hope being that it helped eliminate some of the possible bubbles.
Step 10 : cont'd

as you can see, the baby vacuum pump pulls about half of the recommended vacuum to degas viscous silicone…. it also took about 5 minutes to get to this point…something that would become an issue in the next step.
Step 11 : demolding + first attempt failure

Sooooo….I learned that the ambient temperature is kind of a big deal when casting. On this day, the shop was about 92 deg. F , which apparently cut my pot life in half….. this compounded by difficulties in pouring the silicone in a relatively small opening, with no vents to allow the air inside the cavity to escape lead to a complete failure…… the good news was that the silicone cured, and the surface finish of the final part looked to be pretty nice.
Step 12 : alterations to the mold

before casting again, i widened the pour spout and added a couple of small vents to allow trapped air to escape the mold.
Step 13 : attempt 2

before casting a second time, I put the 2 part silicone kit in the fridge for 30 minutes before mixing, added half as much tinting compound, and had a syringe handy to assist in making sure the mold was completely full. I skipped the degassing step in hopes that i could get a full cast…second test was with dragon skin 20… a little bit softer than 30…
Step 14 : first successful cast

Step 14 : cont'd : results/ evaluation

the first cast came out clean, but there were a good number of medium sized bubbles trapped near the top of the mold/ pour spout … the Dragon Skin 20 turned out to be way softer than the first cast, when lifted by the handle, the J shape would droop to the ground…
step 15 : try try again

Back to dragon skin 30, again cutting the tinting compound in half and this time degassing the mix in an air conditioned space…. waaayyyyy easier to pour than previous attempts : praying for fewer bubbles.
conclusions / results

After testing out several combinations of material from 20A-30A in shore hardness, the final results still need tweaking. I think the next step is to make a dual durometer cast…. using an internal form that is much more rigid than the 30A silicone so that the form does a better job of maintaining its shape, then casting a super soft silicone around the outside ( like dragon skin 10 ). This whole process has been an education in materials and techniques….Just sent the best of the four attempts across the country…hoping my efforts are well received.
Source : conmuchogusto from Reddit