7 Major Mistakes in Silicone & PVC Spec Writing – Rules I Learned the Hard Way

This checklist is for you if...

You've ever received a silicone foam part that looked correct but swelled like a balloon in mild chemical contact—or specified a PVC medical tube with the wrong durometer, only to find it kinked during a simple pump test.

I work in industrial procurement handling orders for rubber and plastic components (including Trelleborg and Minnesota Rubber and Plastics items). Over roughly 10 years, I've made—and documented—at least 40 specification mistakes that together wasted somewhere north of $30K in rework, shipping, and lost production time. This is the checklist I now scan before any custom silicone or thermoplastic order leaves my desk.

It covers 7 steps. Step 3 and Step 6 are the ones most people miss.

Step 1: Don't order by hardness alone

I once approved a prototype order for 250 silicone foam gaskets. The spec sheet said 50 Shore A—which felt fine on the durometer report from the supplier. But Shore A is measured on a solid surface. With foam, it's practically meaningless unless you specify compression deflection or ILD (Indentation Load Deflection).

Correct approach: Write the spec using compression force at 25% or 50% deflection (ASTM D1056 for silicone foam, ASTM D2240 for solid rubber). If the gasket needs to seal against a glass panel under low clamp force, say that. Don't assume the supplier reads your intent.

Real example: In 2022, I specified '30 Shore A silicone sponge.' The factory delivered density 0.35 g/cm³. I needed 0.55 g/cm³. Both were '30 Shore A' by their measurement. That mistake—250 pieces, $1,350, plus 10 days delay—taught me the difference between durometer and firmness.

Step 2: Know the compression set of your gasket—not just the compression force

People often fixate on force. But if the compression set is high, the gasket relaxes over time and leaks after 6 months. In automotive or high-temp applications where Trelleborg EPDM is used, that's a recipe for warranty claims.

What to ask: Request compression set data (ASTM D395 for silicone/EPDM, or ASTM D1056 for closed-cell foams). I'd recommend targeting ≤25% compression set at service temperature if long-term seal is needed. The supplier will normally have this data if they test regularly.

"We don't quote compression set on standard foam profiles—had to learn to demand it."

Step 3: Temperature isn't just high or low—it's where

This is the one I see missed most often. A spec might say 'silicone, rated -60°C to 230°C.' Fine. But if the gasket sits against a hot pipe at one edge and exposed to rain/snow at the other edge—simultaneously—the thermal gradient can delaminate a silicone coating or cause cold-flow inside the foam structure.

In roughly 2019, I ordered a PVC profile (Marketed as 'thermoplastic vs silicone alternative') for a freezer door seal. The spec said -20°C. What I missed: the profile was also exposed to +50°C on the hinge side due to reflected heat from a motor. The PVC turned brittle at the cold end and sticky at the hot end within—literally—2 months.

Rule: Add a 'thermal gradient' line to your spec. Write highest and lowest simultaneous temps, and ask if the material holds performance across both extremes.

Step 4: Chemical resistance—don't rely on generic compatibility charts

I'm not 100% sure who created the first chemical compatibility table, but I know from experience they are dangerously incomplete.

For example, a 'silicone foam product' is typically excellent against water and mild acids. But expose it to concentrated sulfuric acid (say, in a laboratory setup), and the foam will oxidize and crumble. The generic chart might still list 'silicone: good resistance' while ignoring concentration and temperature.

Checklist item for your RFQ: Send a specific chemical, concentration, exposure duration, and temperature before expecting a reliable answer. I've had suppliers say 'resistant' only to reveal later they tested 10% at 20°C when I needed 30% at 50°C.

Also—PVC cardiology applications are a different world. Here, plasticizers leach out if the PVC is not medical-grade (phthalate-free). The mistake: ordering industrial-grade PVC for a medical prototype because 'it's the same polymer.' My team learned this when we noticed clouding in the tubing after two autoclave cycles. $580 worth of test samples went straight to disposal.

Step 5: Don't skip color if it changes anything

This sounds trivial. But black EPDM absorbs heat. White silicone reflects it. If your part seals something that fluctuates with ambient solar load, the color changes the mechanical properties. I've seen white silicone foam expand less than black silicone in July but contract more in winter.

Add to your spec: If the seal or tubing is exposed to UV or visible sunlight, add the specific color code (RAL or Pantone). And don't assume 'natural' equals 'without additives'—natural silicone has trace curing agents that can discolor under sunlight faster than a UV-stabilized formulation.

Step 6: Re-certification loops—what nobody budgets for

Here's the one that wasted my largest single error: ~$2,400. I ordered 1,200 pieces of a custom Trelleborg EPDM profile. It passed initial testing. QC at our facility flagged nothing. We shipped to the end customer. The customer's QA measured a marginal variance of 1.2 mm on the cross-section—not functional, but they rejected it because their tolerance was ±0.5 mm and our spec said '±0.8 mm' (hidden in fine print of a quote appendix).

Lesson: In Q1 2024, I created a 're-certification checklist' that includes: who is the final certifying authority? FDA? USP Class VI? A customer's internal QC? What documents are required at each step? The mistake is assuming your QC standards are the same as the end user's. They rarely are.

Step 7: Prototype? Great. But test under installation conditions, not lab bench

The classic: a silicone gasket seals perfectly on a benchtop fixture. But when assembled by a technician with a pneumatic tool at high torque on an oily surface, it shifts, folds, or extrudes.

Fix: Order 5–10 pieces of test product (not just one sample). Install them exactly as they'll be installed in production. Simulate worst-case operator skill and worst-case surface condition. If the supplier (like Trelleborg or Minnesota Rubber and Plastics) offers a design review, take it. They'll flag things like 'This silicone formulation may not hold the lubricant you specified.'

Common mistakes to avoid

• Assuming 'PVC = PVC.' Not when there are 6 different plasticizer types. Always verify whether the PVC is flexible, rigid, crosslinked, or medical-grade. For PVC cardiology applications, the plasticizer choice is FDA-regulated.
• Ignoring dimensional tolerance buildup on long-run gaskets. A ±1 mm tolerance on a 50-meter length adds up. Specify cumulative vs. incremental tolerance.
• Ordering silicone foam by brand name only. Not all silicone foams meet ASTM D1056 classifications. Request test data.
• Thinking 'thermoplastic vs silicone' is a one-dimensional tradeoff. The real question is: do you need UV resistance (silicone wins) or high tear strength (thermoplastic polyurethane wins)? Both can fail spectacularly if misapplied.

"In my first year (2017), I made the classic mistake of accepting a vendor's generic data sheet as gospel. After the third rejection in Q1 2024, I created our pre-check list."

Prices as of this writing (June 2025—verify current): Custom silicone foam profiles in moderate quantities frequently range $8–25 per meter, tooling $500–2,500 depending on complexity. PVC profiles are typically cheaper (around $5–12 per meter) but can be expensive if you need medical-grade formulation or tight tolerances.

Disclaimer: Material sourcing costs vary widely. Actual prices depend on supplier, volume, formulation, and certification requirements. Regulations (FDA, USP) should be verified on official FDA.gov pages prior to specification.