Is silicone a plastic? Let's clear that up first
No. Silicone is not a plastic. It's a synthetic elastomer with a silicon-oxygen backbone, while most plastics are carbon-based polymers. Once silicone rubber is cured, the polymer chains are crosslinked, so it returns to its original shape and doesn't melt like a thermoplastic.
But if you buy components, the definition matters less than the verification. I'm a material compliance manager. I review roughly 200 lots a year—silicone grease, silicone fluids, silicone egg molds, rubber seals, and a lot of things that claim to be silicone but are not. In Q1 2024, an incoming batch of 'silicone' egg molds passed visual inspection but failed a heat test. The failure caused a $22,000 redo and delayed our launch. After that, I tightened the checklist below.
Use it when a supplier says 'it's silicone,' and you need to prove it before you pay.
The six-point checklist
1. Make them name the material family
Silicone is a family, not one material. Silicone grease, silicone fluids, liquid silicone rubber, high-consistency silicone rubber and silicone resins serve very different purposes. If you're buying Dow Corning silicone fluids, you know the brand; you still need the grade and viscosity. A low-viscosity fluid for damping behaves differently from a high-viscosity fluid used as a rubber processing aid. If a supplier only says 'silicone grease,' ask for the thickener type, base fluid viscosity and temperature range.
2. Verify the polymer, not just the texture
Flexible and rubbery can come from TPE, PVC, latex, or polyurethane. For a molded silicone egg mold, I ask for an FTIR report or a declaration of the elastomer type. If the supplier can't provide one, I send the sample to a lab. It costs less than a bad batch.
A burning tip? Many people try to test silicone by burning it. I don't rely on that. It creates a lot of smoke and can confuse silicone with some plastic formulations. FTIR is faster and safer, and it directly shows a polydimethylsiloxane spectrum.
3. Compare density to the datasheet
Density is the quickest physical clue. Use a simple scale-and-water test for molded parts, or ask for specific gravity on the certificate. The actual value doesn't need to match another manufacturer; it needs to match the product you approved.
When I compared the approved mold sample and production batch side by side, they looked identical. Same hardness, same color. But the approved sample had a specific gravity of about 1.15, while the production batch was 1.5. That gap showed heavy filler loading. The material was still silicone, but it was no longer the same material. That contrast taught me to always compare numbers, not just touch.
4. Request certifications that apply to your end use
A food-contact silicone egg mold is typically expected to meet FDA 21 CFR 177.2600 if sold in the US. For medical parts, ISO 10993 is relevant. In the EU, you might need to show compliance with EU 1935/2004 and the German LFGB requirements.
Don't accept a certificate for 'food grade silicone raw material' if you're buying a finished mold. The finished product may contain pigments, fillers or release agents that move into food. Ask for migration test results on the finished product, with a date and lot number.
5. Run performance tests that match real use
For molds, put them in an oven at 180°C for an hour. Do they smell, discolor, or become sticky? For silicone grease, heat a sample at its rated temperature and watch for oil separation. For silicone fluids, check viscosity before and after heat exposure. The tests don't need to be elaborate—they need to be consistent.
Over the last few years, I've seen this area evolve. In 2019, many QC plans only checked dimensions and hardness. In 2025, we also run application tests on a small sample before every first production order. What was best practice five years ago just isn't enough now; but one fundamental hasn't changed: a part must do its job in the real-world situation, not just on paper.
6. Treat a brand change as an engineering change
If your drawing says 'Dow Corning silicone grease' or 'Dow Corning silicone fluids,' it's not because someone loves the name. It's usually because that grade has a consistent datasheet, and changing to another grade can change sealing behavior, shrinkage, dielectric properties, or heat resistance. A different brand can be perfectly good, but only if you review its datasheet, certifications, and application history before you substitute.
I don't require original-brand materials for every project. I do require a formal note when a manufacturer is substituted. If you find out only after delivery that the material changed, then the QC system failed, not the material.
Common mistakes I still see
The first mistake is asking 'is silicone a plastic?' and stopping there. The second is treating rubber balloons as if they are all the same. Most party balloons are natural latex. Some are synthetic rubber. Latex is an elastomer, not silicone, and it has different allergic and aging properties. If someone asks you to source rubber balloons and claims they're made of silicone, ask whether they mean latex or silicone. Real silicone balloons exist, but they are not the standard party balloon; they cost more and feel heavier.
The next mistake: accepting certificates from raw material suppliers, not finished product test reports. Silicone might start out safe, but additives can change that.
So, no, silicone is not plastic. But the word alone is not a specification. The label is just the beginning. Take it from someone who rejected 8,000 units, then added a material verification step to every vendor contract. (Mental note: review this checklist again after the next thermal aging audit.)