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Vacuum Degassing and Pressurizing

The proper use of vacuum degassing and/or pressurizing can make the difference between a mediocre casting and a perfect casting.

Some materials, by their chemistry, are either very thick or contain a lot of surface tension and therefore entrap air very easily, creating unwanted bubbles in the molds or parts.

Here you see our Freeman 1090 clear urethane. In this first example, we poured the material into a beaker without degassing or applying pressure. In the second example, we vacuum degassed the material before pouring – notice the absence of bubbles in the casting. We placed our third example in a pressure pot while curing. Again, we have no visible bubbles.

Other materials, such as our opaque urethanes and silicone rubber, do benefit from vacuum degassing. However, whether it is essential depends less on the material used and more on the demands of the project itself.

Vacuum Degassing The first step in vacuum degassing is pouring our mixed material in a larger container because it will rise during the degassing process.

Here, we’re using the The Gas Vac II – note the chamber size is large enough to hold a five gallon pail.

We set our material at the bottom of the chamber.

Note the rubber O-Ring gasket and the clear lid, which allow you to know when you are done and also alert you if something is going to overflow.

We start the pump with the valve open – notice we can still lift the lid.

Now with the pump warmed up, we close the valve and the gauge immediately shows the negative pressure created inside the unit.

Notice the mixture start to rise.

You have to pull at least 29 inches of mercury in order to completely degas a polyurethane elastomer or silicone rubber.

There are cheaper degassing units available that only pull 26 or 27 inches of mercury. These units often do more harm than not degassing at all as the bubbles will expand, but they won’t break until at least 28 inches of mercury is reached. This is why Freeman only sells the Gas Vac II, an industrial-grade machine featuring a 6 cubic feeet per minute pump that pulls 29.9 inches of mercury in about 90 seconds. This machine lasts for many years and requires very little maintenance. The unit you are watching has only required two oil changes in over twelve years.

Some materials will break down and then self-level, indicating the degassing process is complete. Other materials will rise and then fall, but not completely self-level. Rather, they will continuously break in a constant motion, indicating they are done.

Make sure you open the valve and release the pressure slowly before turning off the pump.

Our material is now ready to pour.

Pressurizing In some instances, the use of a pressure pot after your casting is poured is enough to eliminate visable air entrapment. While not practical for large castings, a pressure pot connected to an air compressor will squeeze air bubbles in a casting into a virtually undetectable size.

When using a closed mold, we often use both vacuum degassing and pressure since the pressure pot also assists the urethane in filling the entire cavity.

How NOT to Ruin Silicone Rubber

In this video, we are going to ruin some mold-making materials, so you don’t have to. In this round, we are going to focus on how silicone rubber can become ruined.. In another video, we are going to focus how urethane casting resins can become ruined..

Silicone rubber has multiple advantages over urethanes and other materials. Its flexibility, durability, and self-releasing properties make it an especially popular material for making molds.

It is flexible, for easy demolding, even with complex geometries. It is also relatively forgiving, meaning that even when mix ratios are off a bit, the performance doesn’t suffer noticeably. it is durable. A high quality silicone has good tear strength & can be used over and over without fail. And lastly, and most importantly, it is self-releasing, meaning that silicone rubber molds don’t require release agents. unless you are pouring liquid silicone against any cured silicone…like you would in a two part mold. In that case you need to apply Pattern Release 202. Otherwise, your liquid silicone rubber will stick rather aggressively to your cured silicone rubber, resulting in a single block of silicone rubber.

Cure Inhibition The biggest issue with silicone rubber revolves around cure inhibition. There are some substances that interact with some silicone rubbers as they cure and actually prevent the silicone from completely setting up, leaving them wet or sticky. Moreover, some silicone rubbers will inhibit the cure of some liquid urethanes.

So In order to avoid turning your project into a sticky mess, we’ve put together this guide to help you avoid over 99% of the causes of cure inhibition.

The first thing you need to know is which of the two chemistries of silicone rubber you are working with…addition cure rubbers, which are catalyzed with platinum, or condensation cure rubbers, which are catalyzed with tin.

Condensation Cure Now, there’s only one cure inhibition concern with Condensation Cure silicone, but it is a really big one. And that is they don’t play nice with certain casting urethanes.

To demonstrate, we’ve made a mold out of V-1065, our most popular condensation cure silicone, and in it we’ve poured our Freeman 1040 flexible urethane.

The next day, we get a clean release, but the surface of our part is sticky and the surface of our mold is still wet. Even the following day, the part still hasn’t cured, and in fact will never fully cure. Meaning that not only do have an unusable casting, we have also ruined our mold.

So while many urethanes will work fine with condensation cure rubbers, these exceptions do exist and without getting into which raw materials may or may not be the root cause of this phenomenon, there’s no way to know ahead of time whether the urethane you’ll be using is going to be incompatible, thus you risk losing of hours of time and hundreds of dollars if you are wrong. So we normally recommend condensation cure silicone rubber only when working with polyester casting resins.

Addition Cure silicone Addition Cure silicone rubbers, on the other hand, play nice with every casting urethane we’ve ever worked with, which is why they are demonstrated almost exclusively in our instructional videos. They do, however, experience a greater cure inhibition when poured against materials like acrylic, vinyl, wood sap, urethane foam, and sulfur.

Sulphur is a common ingredient in modeling clays, so we’re using it to demonstrate cure inhibition by constructing a circular dam of non-sulphuric clay on top of a block of sulphuric clay. After pouring our V-340 addition cure silicone in this mold, we allow it to cure overnight. Upon demolding, notice how most of the rubber has cured, including that which came into the contact with the non-sulphuric clay. The bottom, however, is still wet. And even after sitting an additional day, it still fails to cure.

So if you are planning to pour addition cure silicone rubber against wood or vinyl, it is best to seal that material ahead of time. As for modeling clay, we simply recommend avoiding the clays that contain sulphur.

Urethane Foam That leaves us with the curious issue of urethane foam. Modeling and styling boards made from urethane foam have become more popular in recent years as they are lighter and less expensive than typical urethane modeling boards, yet they provide a good enough surface finish and edge definition for many projects.

However, we were as surprised as anyone to find out that they can inhibit the curing of addition cure silicone rubber.

Initially, we thought we could treat this issue like we do wood, where a simple application of a sealer would fix the problem. So we ran this experiment. We cut two small cavities in two pieces of RenShape 5030 urethane foam board. On each board, we sealed one cavity with our Wood & Paster Sealer and left the other cavity unsealed. As you can see, it doesn’t really matter whether we sealed the cavities or not, our v-340 addition cure silicone failed to cure and left a mess. However, our V-1065 condensation cure silicone rubber performed well regardless of the application of the sealer.

So if you are planning to machine your model out of urethane foam, it is best to avoid addition cure silicones, or switch to a non-foam urethane modeling board, like RenShape 450, which plays nicely with all silicone rubber.

One additional note on silicone rubber is that you don’t want to switch from one chemistry to the other using the same mold box. So if you create a mold with v1065, and then later want to make the same mold with v340, you’ll need to start over.

Finally, we should note that silicone isn’t the only material available for making flexible molds. Urethane rubber does have some advantages. It is generally less expensive and it is much more abrasion resistant, making it the material of choice for many concrete, ceramic, and architectural applications.

However, since it is a urethane, it requires the proper application of sealers and release agents, which we detail in our other videos.

So there you have it. When it comes to cure inhibition, you now know more than 99% of moldmakers out there. By not only understanding our choices, but also the limitations of each material, we are far less likely to run into trouble once we start using various materials.

As always, if you have any technical questions and concerns, our technical line is open 8-5, Monday through Friday…and you can also submit your questions via our website at freemansupply.com

Thanks for watching.