Preparing Models, Molds, & Materials
Sealing & Releasing
Models and Molds
We demonstrate proper procedures for sealing your model or mold and then applying release agents to allow for easy and clean part release.
Preparing Models and Molds
One of the most common frustrations in making molds and casting parts comes from parts sticking to molds. This is the number one source of problems cited by toolmakers and why we constantly investigate newer and better ways to seal and release models and molds to ensure easy part release.
And nothing has yet matched the following combination of Freeman Wood and Plaster Sealer, Freeman Wax Release, and, with a notable exception of casting a urethane against a urethane, which we’ll address at the end of this video segment.
Sealing your pattern/model
Sealing is required anytime you work with wood, plaster or sheet wax, since these materials are known to interact with tooling plastics. Applying a sealer, such as what we are doing here with this pure bristle brush, will not only prevent the moisture in the wood or plaster from reacting with the tooling plastics, it also smoothes the surface.
Here we are applying our Freeman Wood and Plaster Sealer, a fairly thin viscosity, lacquer-based paint, on a piece of wood.
And here is what the first application of the sealer looks like.
After the first coat has dried (which will take about half an hour), you’ll notice that the sealer has swelled the grain and made it rough. So you’ll want to take sand paper or scotch-brite and lightly sand it down to make it smooth again. Sanding is not necessary when working with plaster or sheet wax.
When you’re done, wipe it off with a cloth and then apply a second coat of sealer.
After allowing the second coat to dry, you will again want to sand the wood very lightly and then wipe it off with a cloth.
We are now ready to apply the release agents.
Applying Release Agents
After the sealer has been applied, it is time to cover the entire surface with Freeman Wax Release - a semi-paste, typically applied with a brush.
You may allow this coat to dry or immediately wipe the off excess with a cloth.
We suggest at least two coats of Wax Release to make sure your entire part is covered evenly.
Next, you’ll apply two layers of Partall PVA Mold Release, which is a polyvinyl alcohol that you can apply with brush or a spray.
PVA forms a thin film – almost like a plastic wrap – which serves as a barrier coat for any of the active ingredients in the epoxies, urethanes, or polyesters.
The green color ensures complete coverage.
Each coat will require a half hour of drying time unless you use a fan or air hose.
Here you see the second coat being applied. Note how the material self-levels, which makes using a spray unnecessary, although some people prefer to use a spray in order to prevent brush marks.
After the second coat of PVA has dried thoroughly, you will apply a final coat of Freeman Wax Release.
Be very careful when buffing this last coat – do so very gently so as not to break through the layers of the PVA.
Your part or model is now ready for casting
Casting Urethane Against A Urethane
The only exception to this releasing procedure is when casting a urethane against another urethane, such as we’re doing here, pouring our Repro fast-cast urethane into a mold made out of Repro. In these cases, you should only apply the PVA once – to either the model or the mold, but not both.
Here, since we used the wax, PVA, wax procedure on the original model, we do not want to use PVA on the mold itself. Instead, we only apply three coats of wax release to the mold before casting our part. Otherwise, we risk having the solvents in the urethane react with the PVA in a way that forms a bond, rather than enabling easy part release.
Weighing & Mixing
Liquid Tooling Materials
We demonstrate proper agitating, weighing, & mixing procedures for both 1:1 ratio and uneven ratio materials. We also explain how to calculate your material requirements.
Weighing and Mixing Liquid Tooling Plastics
In order to guarantee the performance of any tooling plastic, you must weigh and mix the materials properly before using them. Poorly prepared liquid tooling materials is our number two source of tech calls and is usually the cause of materials not performing to their specifications.
Calculating Material Requirements
One question new users often ask is “how do I know how much material is needed to make a mold or fill a mold cavity” because if we mix too much, we end up wasting material, and if we mix too little, we may ruin our mold.
To provide a useful estimate of the amount of material needed, we need two pieces of information.
First, we need to know the volumetric yield of the material we’ll be using. This number is available on our website and is expressed In cubic inches per pound.
Second, we need to know the volume of the mold or casting. This is measured in cubic inches and can be obtained by using some simple math.
• For rectangular box shapes, we simply multiply the length by the width by the height. For example, if your mold box is 6” x 5” x 1.5” inches, the total volume would be 45 cubic inches
• For cylindlical shapes, we simply multiply the radius of the circle, by the radius of the circle, by the height of the cylinder by 3.14. For example, if your radius is 2 inches and your height is 1.5 inches, then your volume would be 18.8 cubic inches
Once we have both the volumetric yield and the volume, we simply divide the volume by the volumetric yield. This answer gives us the total amount of material in pounds that we’ll need to fill our mold box, or mold. Therefore, our mixture of both parts A and B together should equal or exceed this amount.
There are two additional considerations. First, if we are measuring the volume of a mold box, then we can assume there will be a model inside which will take up some of the volume, therefore decreasing the amount of material needed.
Secondly, because models and molds rarely have perfect angles or circles, we always over-estimate the volume to ensure we have enough material.
Agitating Materials
Before being weighed and mixed together, most materials need to be mixed individually in their can. Unless they ship far or sit for a long time, most materials won’t settle very much in the can. Therefore, they can be mixed manually with a paint paddle.
Some materials, such as our Repro 83 fast-cast urethane, contain fillers which reduce shrink and add to the wear and machining characteristics. These fillers often settle during shipment and must be agitated mechanically with a plunge mixer shown here attached to a drill, or better yet, a Red Devil Paint mixer, shown here.
After 6 minutes in a Red Devil Mixer, the fillers are in suspension and the material is ready to be weighed and mixed together. Note that our three newest Repro formulations are all non-settling, meaning they can be mixed thoroughly without a mechanical aid.
Weighing and Mixing 1:1 Ratio Materials
One of the advantages of a 1 to 1 mix ratio material is that it can be measured without a scale. All of our Repro and a few of our Freeman Polyurethane Elastomers can be weighed and mixed using this three-cup procedure.
Here, we poured enough material of each side into two lined cups and eyeballed it so that each cup had about equal amounts.
Then, we poured the material from one side into the other and mixed it with a paint paddle, making sure we scrape all of the material along the sides of the cup.
Finally, we poured the material into a third container because it is physically impossible to completely scrape the sidewalls of the cup and it is very important that all material is mixed before using it.
When we’re done with our material, we add a quick spray of Magic Blanket into each can before resealing the lids. This shot of nitrogen helps to preserve the material for later use.
Weighing and Mixing Plastics with an uneven ratio
The process for mixing tooling plastics with uneven mix ratios, such as most epoxies and polyurethane elastomers, is different than a 1 to 1 mix ratio urethane like Repro.
Here we’ll demonstrate the proper mixing procedure with Freeman 705 Epoxy Surface Coat, but this procedure applies to all plastics that have an uneven, or non 1 to 1 ratio.
The mix ratio we’re following here is 100 parts resin to 14 parts hardener by weight.
Once we put the lined cup on the scale, we zeroed the scale and then pour 100 grams of the epoxy resin in the cup.
We use a paddle to add or remove material towards the end since is it easier to control.
Most uneven ratio materials are much more sensitive to their ratio than a 1:1 system like Repro. Since we will want to be within 1% of the required ratio, we are using an electronic scale.
Once the resin is poured, we re-tared the scale to zero and added the harder directly on top of the resin.
It is important that you don’t use a separate cup for the hardener or you may lose too much material when you combine them.
After mixing the two materials together thoroughly, we pour the material into a second cup because it is physically impossible to completely scrape the sidewalls of the first cup and it is very important that all material is mixed before using it.
Vacuum Degassing
& Pressurizing
We demonstrate the use of vacuum degassing equipment and pressure pots in order to achieve virtually perfect, void-free castings.
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
We decided to ruin some mold making materials so you won't have to. Here, we demonstrate and explain cure inhibition of silicone rubber.
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.
