Pouring Silicone to make a mold

Silicone Mold Makes a Surprisingly Accurate Vintage Car Part

A Restoration Challenge

This week I am back working on the carnival ride restoration.  I've been looking forward to tackling this part of the project.  While researching the ride, I was looking through some old brochures. I noticed a decorative item on the front of all of the cars. Digging into it a little bit further, I discovered that they were probably hood ornaments known as torpedos or bombsights.

Vintage torpedo Hood Ornament

Back when they built this ride, I imagine torpedo hood ornaments were a pretty inexpensive aftermarket accessory. They are now highly collectible, and the price skyrocketed. I need to find five matching hood ornaments to replicate that part of the ride for this restoration. That is going to blow the budget.

I was able to find one hood ornament on eBay at a price that I was willing to pay. It was advertised as chrome-plated to fit Ford Chevy Plymouth and Hudson from 1946 to 1947. In pencil, there is a little notation that I believe says it was $4.50. I paid substantially more than that.

Hood Ornament Purchased on eBay

Planning the Molding & Casting

The problem is I need four more matching ornaments, and that is the subject of this project. I plan to make molds and cast copies.
The original hood ornament casting is in three pieces. I want to create silicone molds and then cast copies in urethane. I’ve done molding and casting before, so I don't expect too many problems. Of course, there are always unknowns like temperature and humidity that are tough to control.

3 Piece Hood Ornament Casting

Prepping the Original

The first step of the process is polishing up the original. There are quite a few imperfections on the surface- some blemishes, some corrosion. All of that needs to be polished out, or they will show up in the copies.

The original hood ornament has a threaded hole for mounting. The plan is to create a piece with a stud permanently embedded in it. This configuration will be much stronger. I used Metal Glaze from Evercoat that I mixed up on a disposable mixing pad. The Metal Glaze was a bit overkill for this job. Spot glazing putty would probably be a better choice, but I didn't have any on hand at the time.

 

 

Mixing Metal Glaze on Disposable Pad

Obtaining a smooth finish required a couple of rounds of filling and sanding, then finally priming. After priming, I went back with higher grits of sandpaper to get a more polished surface.

Polishing

I decided to try to buff the part to get a bit more glossy surface. This decision was a mistake- twice. Both times I slipped and scratched the surface with the shank of the polishing buff. I repaired the damage by using some glazing putty (which I had obtained by this point). One thing to keep in mind is that this didn't need to be a permanent repair. It only needed to last long enough to make the molds. If the filler chips out in the future, it just won't matter.

I used Krylon Crystal Clear as a topcoat. That usually leaves a pretty good surface to mold over. I didn't know if the Krylon was going to have any adverse interaction with the Rustoleum primer. I Had to wait and see if the coating crazed or did anything else undesirable.
There was no interaction, and I was happy with the finish. It was very smooth with a visible sheen. It was a good starting point for making our first mold.

Part Sealed with Krylon Crystal Clear

Making the Mold

I had to make some decisions about how to mold this part. There was evidence of a parting line, so I knew the original mold split vertically. I planned to make my mold in a bottom-up, horizontal split orientation. In this position, if any air bubbles are trapped, they're going to rise up and not leave blemishes on the visible top side.

To make the mold box, I used very generic Lego-style building blocks. You can make a box that is form-fitting right around the item that you're trying to mold. You don't end up wasting silicone, and the silicone does no damage to the blocks. When this project is complete, the grandkids can still play with them.

I need to put down a layer of clay in the bottom of the mold. I bedded the hood ornament into the clay about halfway. The clay I used was a Chavant product called sulfur-free Plastiline. I cut a piece off the block and put it in the oven. That softened the clay making it easier to push down into the bottom of the mold. Sulfur-free clay is important when molding silicone. There are two different types of silicone; a tin cure and a platinum cure. Sulfur will inhibit the curing of both types. The platinum cure is the most sensitive.

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Creating Mold Box with Building Blocks

I used a brush with some Isopropyl alcohol to help smooth the clay and remove residual clay off of our original. Then built up the sides with additional building blocks to the needed height. I believe this is another advantage of using building blocks for your mold box. On a typical mold box, the sides are in your way while you are working the clay. Depending on how deep the walls are, this can be a real pain. The building blocks allow me to start with low sides. I can build them after I have the clay finished.

The Silicone


I used Mold Max 30 silicone by Smooth-On. I called Smooth-On to ask for some recommendations on materials to use for this project. They recommended the Mold Max 30. After talking about the plan a little bit more, they offered to donate the products. I want to thank them for that. I would have used it based on their recommendation, but I’m certainly happy to have them on board.

Mixing Mold Max 30 Silicone

The Mold Max 30 is a tin cure silicone that you mix by weight. The ratio is 100 parts of bottle A to 10 parts of bottle B. I had to get out my scale, I couldn't just eyeball it this time. I estimated the amount I would need to fill the mold and measured out the correct ratio of parts A & B.

Vacuum Degassing

Then after thoroughly stirring, I placed it in the vacuum chamber to remove the trapped air. The material will rise as the silicone degasses and then will collapse. I had to keep an eye on it. If left alone, it would bubble over. I had to periodically let a small amount of air back into the chamber to halt the overflow. Once the mixture collapsed, I let the pump run for an additional minute.

Like many of the tools in my shop, the vacuum chamber was something I acquired over the years. This particular one was shop-built, but there are several inexpensive options on the market right now.

Degassing Silicone in Vacuum

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Pouring the First Mold

I added a little cone of clay above the part. This clay created what would become a fill spout when we later cast the mold. I poured the silicone into one corner and allowed it to flow across the surface of the piece. This technique helps to minimize air bubble formation in the mold.

Pouring Mold Max 30 Silicone

Mold Max 30 has a 24-hour de-mold time. Some people don't like using Lego blocks for mold boxes. The blocks create a weird leakage pattern on the outside of the mold. This flashing doesn't bother me, and I enjoy picking it off. The mold separated cleanly, and I was happy with the results.

First Half of Silicone Mold

The next step was to clean the remaining clay residue off the first half of the mold. Then I built a box to pour the second half of the mold. I applied a mold release to the first half before adding the silicone for the second half. If you skip the mold release, the silicone could stick to the original silicone and trap our hood ornament piece inside. That's not what we want.

I used the Ease Release 200 by Mann Release Technologies. This spray is another product distributed by Smooth-On. I just sprayed this on the mold then brushed it around. We want it to be a light but thorough coating.

Mann Technologies Ease Release 200

Like the first half of the mold, Parts A & B were measured, mixed, & degassed in the vacuum chamber before pouring into the mold.

Casting the Mold

After another 24 hrs of cure time, it was time to try out the finished mold and cast something. We used Smooth-Cast 66D urethane resin. It is a 1:1 ratio by volume, so pretty easy to mix up. I also applied the Ease Release to the mold. The urethane will not stick to the silicone mold, so this isn't necessary, but it does help the mold last longer.

I inserted a #10-24 machine screw into the mold. The screw head is inside the mold and becomes encapsulated when the mold fills with resin.

Preparing mold for casting

The mold was a little tricky to pour. The spout I created was a bit undersized and tended to clog. I found if I added resin slowly, the air had time to escape out of the mold. This technique worked but required patience. And I still made a big mess. (I would get better at this with practice)

Pouring Smooth Cast 66D Resin into Mold

Designing a new Base

While the Smooth Cast cured, I started working on the next part- the base of the hood ornament. The original design straddled some sort of decorative ridge molding on the hood of the car. We won't have that molding. Rather than modify the original, I took some dimensions from it, then hopped on the computer and designed a new piece. The new piece will be 3D printed and copies will be made from that.

Designing Replacement Base in Fusion 360

Within an hour or so, The first piece was released cleanly from the mold. There were a few air bubbles that formed in the base. I can fill these during the finishing process. I will attempt to "burp" the mold to release the air from these trouble spots in the future.

First Urethane Casting Pulled From Mold

Making the Ring Mold

The ring mold is another 2 part mold. I embedded the ring in clay and built a box up around it with the building blocks. The Mold Max 30 silicone was mixed, degassed, and poured into the box just like the torpedo mold.

Preparing the Chavant Clay Base for the Ring Mold

The mold turned out great, but I realized in hindsight that I had made a small mistake, or at least something I could have done better. I could have taken my own advice. Earlier I mentioned the advantages of using building blocks to form a mold box. One of those advantages is the ability to make a form-fitting box. I should have indented the corners of the mold with blocks. That would have saved quite a bit of silicone.

Original and Mold side by side

The second half of the ring mold was completed just like the torpedo mold. Clean off the excess clay, form a box, use clay to create a pour spout, then mix and vacuum the Mold Max 30.

Pouring the Second half of the Ring Mold

Base Prep

One question that pops up from time to time- "Couldn't you just design and 3D print all these parts and save the time and expense of molding and casting" and the answer is "Yes." However, there are hidden costs with 3D printing - Time. Every 3D printed piece would require extensive sanding, filling, & priming before painting, To obtain the smooth finish I want. This process is OK for making one part but gets out of hand if you need several pieces.

After 3D printing the base, I did several cycles of sanding, filling, & priming to get the finish I desired before making a mold of it. Every piece coming out of the mold will have the same finish I put on the original. The mold for the base was just a one-sided mold.

 

Pouring the Mold for the Hood Ornament Base

Final Resin Castings

With the final two molds complete, it was time to cast them. I tilted the ring mold to prevent trapped air from creating imperfections in the casting. I hoped the Smooth Cast 66D would flow down and pool at the bottom of the mold. As it filled, I wanted the air to escape back out the fill spout. This technique worked, but I had to pour slowly to avoid cutting off the path for the escaping air.

Pouring First Ring Mold Casting

The base mold, being only one piece with an open top, was simple to pour. The Smooth Cast 66D has a demold time of 60 minutes, but I found I needed a bit longer. At the 1 hour mark, the castings were still tacky, and you could leave fingerprints behind.
A great way to know if your casting is ready is to keep the unused portion of resin rather than throwing it away. When the leftover urethane in the mixing cup cures, the resin in the molds will be as well.

Cured resin in mixing container

Final Thoughts

The base looks great, and the ring just needs a little flashing cleaned up at the parting line. I held the mold halves together with electrical tape. I'm going to get rubber bands for future pours. I believe this will create more even pressure throughout the mold. I can also cut backing plates out of 1/4" plywood to clamp the molds. This method should distribute the forces evenly throughout the mold.

Cured Urethane Ring Pulled From Silicone Mold

I'm really please with how these turned out. I will continue casting multiple copies until I run out of Smooth Cast. I also want to try some different finishing options.

Finished Hood Ornament Casting

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