What‘s the Differences Between Precipitated And Fumed Silicone Rubber?

I know the confusion when a compounder tells you “this is fumed silicone” or “this is precipitated silicone” and you wonder what that really changes in your part.

Precipitated and fumed silicone rubber mainly differ in the silica filler they use. Fumed silica gives higher strength, better transparency, and higher cost. Precipitated silica gives easier processing, lower cost, and more general-purpose performance.

When I choose between these two families for a new project, I do not start from price. I start from performance: strength, compression set, transparency, and process stability. After that, the “right” choice is usually very clear.

What is the basic difference between precipitated and fumed silicone rubber?

Many people think “precipitated silicone¹” and “fumed silicone²” are two totally different polymers. They are not. The base polymer is still silicone. The real difference lives in the filler.

Both materials are silicone rubber, but precipitated silicone uses precipitated silica as the main reinforcing filler, while fumed silicone uses fumed silica. The filler structure changes strength, viscosity, clarity, and price.

When I explain this to buyers, I say: “We are not changing the ‘meat’ of the recipe, we are changing the ‘bones’ that give it stiffness and structure.” That simple picture helps non-technical people understand fast.

Filler structure and surface area

Silica is the backbone of reinforcement in most silicone compounds. However, precipitated silica³ and fumed silica⁴ do not look the same under the microscope.

• Precipitated silica comes from a wet-chemical process.
• Fumed silica comes from a flame process at very high temperature.

These different routes create different particle shapes, surface areas, and how those particles form a network inside the rubber.

In practice:

Higher surface area and more complex aggregates mean fumed silica connects more strongly to the silicone polymer. This gives higher strength, but it also makes the compound thicker and harder to process.

Reinforcement level and basic properties

Because of this structure, fumed silicone compounds can reach higher tensile strength⁷, tear strength⁸, and better compression set⁹ at similar hardness. Precipitated silicone can still reach good strength, but usually one level lower at the same hardness.

From my experience:

• Fumed silicone is the “sports version”. It has higher performance, but it is more sensitive and more expensive.
• Precipitated silicone is the “workhorse”. It is easier to mix, easier to mold, and more cost-effective for many general industrial parts.

So when someone asks me “What is the difference?”, my short answer is: same base polymer, different silica network, different balance between strength, processability, and cost.

How do mechanical and processing properties compare?

For a buyer or engineer, names and chemistry matter less than how the material behaves on the press and in the field.

Fumed silicone usually gives higher tensile and tear strength, better compression set, and better clarity. Precipitated silicone usually gives easier mixing, easier demolding, lower viscosity, and lower overall cost.

I learned very quickly that you cannot treat fumed compounds and precipitated compounds the same way on the press. If you do, you get burns, voids, or very unstable dimensions.

Mechanical property comparison

When I present options to a customer, I often summarize the trends like this:

These are trends, not hard rules. A smart compounder can tune many things. However, if I need a strong baseline, I assume fumed silicone² will give me one grade better performance for the same hardness.

Processing and molding behavior

Processing is where buyers sometimes get surprised.

Precipitated silicone compounds:

• Have lower viscosity in the mill and in the mold.
• Flow more easily into complex shapes.
• Are less sensitive to small changes in temperature and shear.

Fumed silicone compounds:

• Feel “stiffer” and more elastic on the mill.
• Need more care with shear and temperature to avoid scorch or flow marks.
• Can trap air more easily due to their high viscosity if the mold and preform are not designed well.

I like to use a quick “traffic light” view when I discuss this with my team:

In my own factory, I plan more careful mold trials when I switch to fumed silicone. I test venting, fill pattern, and cure very carefully. Once the process window is stable, the production can run well, but the path to that window is narrower than for precipitated grades.

Which grade is better for transparent, medical, and food-contact parts¹⁰?

Many customers ask for “very clear silicone” or “medical silicone¹¹” and assume it is always fumed. The story is more nuanced.

Fumed silicone is usually better for high-transparency, high-strength, and premium medical or food-contact parts¹⁰. Precipitated silicone can still be used in many hygienic and food applications, but it is less clear and usually targets less demanding geometries.

When I receive a drawing for a baby product, food valve, or clear gasket, I always ask first about transparency, regulatory needs¹², and expected lifetime. Then I see if fumed silicone² really adds value or if a good precipitated grade is enough.

Optical clarity and surface finish

Fumed silica has very small primary particles and forms a very fine network. This allows the compound to stay clearer when the rest of the recipe is also clean. So:

• Fumed silicone can reach very high transparency or at least excellent translucence.
• Precipitated silicone tends to be more opaque or milky at similar filler loading¹³.

For visible parts, lenses, or products where the end user must see through the material, fumed silicone almost always wins.

For hidden gaskets or non-visual seals, clarity matters less. In those cases, I focus more on mechanical performance and cost.

Regulatory and hygiene considerations

Both precipitated and fumed silicones can be formulated to meet food and medical regulations. The key is the total recipe and the quality of the raw materials, not only the filler type.

Still, in real practice:

• Many premium medical and high-end food-contact base gums are paired with fumed silica.
• Many industrial and mid-range food-contact gums use precipitated silica.

This is not a strict rule, but it is a common pattern. Fumed systems often come with cleaner overall recipes and stricter control. That is why many medical-grade silicones on the market are fumed.

When I work on a project for food or medical use, I do not choose only by filler. I look at:

• Existing test reports for extraction and biocompatibility.
• History of use in similar applications.
• Support from the raw material supplier.

If a fumed system already has a long track record for the target use, it is often worth the higher price.

Typical application map

Here is how I roughly map the two families in my daily work:

So I do not tell buyers “fumed is always better”. I ask: will your customer pay for the extra performance and clarity? If yes, fumed is a good direction. If not, a well-designed precipitated grade can still be a very safe and stable choice.

How should I choose between precipitated and fumed silicone rubber for my project?

In real projects, the question is never just “Which is better?” It is “Which is better for this specific drawing, load, and business case?”

You choose between precipitated and fumed silicone rubber by checking four main factors: temperature and load, mechanical and life requirements, surface and optical demands, and your cost and process window. Then you test critical points with samples.

When I guide a buyer, I use a simple framework that combines technical facts with real-world constraints. This avoids long trial-and-error cycles.

Temperature, load, and lifetime

Both systems handle wide temperature ranges, because the base polymer is silicone. However, fumed systems often hold compression set and strength better at the upper end, especially in demanding seals under constant load.

So I ask:

• What is the normal and peak temperature?
• How long will the seal stay compressed?
• What happens if sealing force drops after one or two years?

If the answer is “failure is expensive”, I lean toward fumed or a higher-grade system even if the immediate cost is higher.

Strength, tear, and dynamic behavior

If the part has thin lips, sharp corners, or must survive repeated flexing, I also lean toward fumed silicone because of its better tear strength and fatigue behavior at the same hardness.

For thick, robust gaskets with simple geometry, a good precipitated system is usually enough.

Surface, optics, and branding

For visible parts, I ask marketing questions too:

• Does the customer want a “premium” look and feel?
• Does the part need high translucence or clarity?
• Will any color differences or flow marks be unacceptable?

For premium consumer and visible medical parts, fumed silicone gives me more room to create a very clean look. For hidden parts, I focus more on process stability.

Cost, process window, and supplier capability

Finally, I put cost and process reality on the table.

• Fumed silicone costs more per kilo and may have a narrower processing window.
• Precipitated silicone costs less, is easier to mix and mold, and gives higher tolerance for operator variation.

If a factory does not have strong process control or if the project budget is tight, I prefer a robust precipitated system with a smart design. That often delivers more value than a “perfect on paper” fumed compound that is hard to control.

Simple decision table you can reuse

I often share this quick table with purchasing and engineering teams:

If you send me a drawing, hardness target, and basic working conditions, I can usually tell within a few minutes which side of this table you are on. Then we use samples to confirm. You can always reach me through www.rubberandseal.com or by email at info@rubberandseal.com if you want to walk through this decision for your own project.

Conclusion

Precipitated and fumed silicone rubber use the same base polymer but different silica networks; the best choice depends on your strength, clarity, lifetime, process window, and how much premium performance your project really needs.