If you're sourcing custom rubber parts—whether it's high-temperature silicone tubing, chemical-resistant fluoroelastomer gaskets, or specific PTFE washers—you're probably already dealing with a ton of technical specs. It's easy to mess up. Trust me, I've done it more than once.

This isn't a theory piece. It's a checklist I built after personally making (and documenting) about 12 significant mistakes over the last 6 years, totaling roughly $15,000 in wasted budget. A single $3,200 order of custom-molded parts went straight to the trash because I got one spec wrong. Here's the step-by-step process I now use to prevent that from happening to anyone else.

When to Use This Checklist

Use this checklist, which has 5 steps, if you're:

  • Ordering custom-molded rubber or plastic parts for the first time.
  • Switching to a new material like Viton or a specific silicone formulation.
  • Quoting a project where performance in extreme environments (heat, chemicals) is critical.
  • Reviewing a supplier's drawing or proposal for PTFE components.

If you've been ordering the same part for 10 years, you probably don't need this. But if anything is new—material, geometry, or application—start here.

Step 1: Lock Down the Material Durometer and Grade

This is where I made my first $890 mistake. I specified "silicone rubber" for a high-temperature gasket. That's it. I didn't specify the durometer (hardness) or the specific grade. The supplier made it from standard 50 Shore A material. It was too soft for the application and failed in a week.

What you need to do:

  • Specify the Durometer: This is measured on the Shore A scale (for flexible materials) or Shore D (for rigid ones like PTFE). For a typical O-ring, 70 Shore A is standard. For a structural gasket, you might need 80-90 Shore A. Industry standard tolerance is ±5 points unless you pay for tighter control.
  • Specify the Grade & Standard: Not all silicone is the same. For medical-grade silicone tubing, you need to reference ISO 10993 or USP Class VI. For high-temperature fluoroelastomer like FKM (e.g., Viton™), you need to specify the fluorine content (e.g., 66%, 68%) which dictates chemical resistance.
  • The Checkpoint: On your print or PO, you need a line that says Material: XXX, Durometer: YY Shore A, Standard: ZZ.

If I remember correctly, the cost for the medical-grade certification paperwork was around $200 more, but it was way cheaper than the alternative.

Step 2: Define the 'Extreme Environment' Accurately

Engineers love saying "high temperature." But what does that mean? 200°F continuous? 400°F intermittent? Chemical resistance to "oil" isn't specific enough. Is it ASTM Oil #1, #2, or #3? The wrong definition can kill a part.

Here's what you need to know:

  • Temperature Range: Always specify continuous operating temp and max intermittent peak temp. For standard silicone rubber, this is -60°C to 200°C (-76°F to 392°F). For fluorosilicone, it's narrower.
  • Chemical Exposure: Name the specific chemical, concentration, temperature, and duration of exposure. "Resists fuel" is useless. "Resistant to ASTM Fuel C at 23°C for 72 hours" is an orderable spec. A 1% change in chemical concentration can change material compatibility.
  • The Checkpoint: Look at your RFQ. If any environmental parameter is a general adjective (hot, cold, corrosive), rewrite it as a number with a unit.

This was accurate as of late 2024. Material science changes fast, so verify current chemical compatibility charts with your supplier before committing.

Step 3: Don't Forget the Tolerances (The Most Common Mistake)

Okay, this is the step that cost me the $3,200. I ordered 1,000 custom-molded PTFE washers. The drawing had the right material. The right dimensions. But I didn't specify the tolerance on the inner diameter (ID). The supplier used a standard ±0.5mm tolerance. My application needed ±0.1mm. Every single washer was out of spec.

What you need to do:

  • Specify Tolerances on Critical Dimensions: This is a non-negotiable. For a gasket, the thickness tolerance is critical. For an O-ring, the cross-section tolerance is critical. For a PTFE washer, the ID and OD tolerances are critical. Don't assume standard tolerances will work.
  • Reference Standards: Use ISO 3302-1 for rubber tolerances or ASTM D2000 for material classification. These give you a standard framework. For example, ISO 3302-1 M1 class gives tighter tolerances than M2.
  • The Checkpoint: On your drawing, every single dimension must have a tolerance. If you see a dimension with no tolerance, stop and add one.

My experience is based on about 200 mid-range orders. If you're working with aerospace or medical-grade tolerances, your experience might differ. Those guys are way tighter.

Step 4: Validate the Mold and Tooling Costs

This isn't about the part cost; it's about the upfront investment. People often compare the unit price from Supplier A vs. Supplier B, and completely ignore the tooling (mold) cost. That's a massive mistake.

So glad I started tracking total cost of ownership (TCO) about 3 years ago. Almost went with a cheap mold from an unverified shop to save $1,000. That mold would have required replacing after 10,000 parts. The more expensive one is rated for 100,000.

Here's the calculation:

  • Supplier A: Mold = $2,000. Part = $1.00. Total for 10,000 parts = $12,000.
  • Supplier B: Mold = $5,000. Part = $0.80. Total for 10,000 parts = $13,000.

See? The lower mold cost doesn't automatically mean the better deal. You have to amortize the tooling over the expected production volume. In my experience managing these projects, the lowest quote has cost us more in 60% of cases when you factor in tool life and rejection rates.

The Checkpoint: Always ask: "What is the expected life of the mold (number of shots)?" and "What is the rejection rate for first-article inspection?"

I want to say the average mold life for a single-cavity silicone mold is around 50,000 shots, but don't quote me on that—it depends heavily on the material and geometry.

Step 5: Order a First-Article Inspection (FAI) Before Production

This step will save your budget. An FAI is a sample run of 1-5 parts before the full production batch. It costs a little extra (maybe $150-$400), but it catches exactly the kind of error I made with the PTFE washers.

What the FAI should include:

  • Dimensional check against your drawing and tolerances.
  • Material verification (density, durometer, hardness).
  • Visual check for flash, sink marks, or incomplete fills.
  • Functional test if possible (fit onto mating part, pressure test).

Honestly, if I had done an FAI on that washer order, I would have caught the tolerance issue. The $200 cost of the FAI would have saved the $3,200 worth of parts. The return on that investment is insane.

The Checkpoint: Make the FAI a non-negotiable line item in the purchase order. Write: "Production cannot proceed until FAI is approved in writing."

Dodged a bullet on a recent order when I insisted on a 2-piece FAI. The first article had a slight edge burr. Fixing that on the sample cost $50. Fixing it on 5,000 parts would have cost... a lot more.

Final Notes & Common Gotchas

A few things that still trip me up:

  • Paperwork is real: For medical or food-grade parts, you need a Certificate of Compliance (CoC) or Certificate of Analysis (CoA). Make sure the supplier knows this before they ship.
  • Color isn't trivial: For brand-critical parts, specify a Pantone color. The industry standard color tolerance is Delta E < 2 for brand-critical colors. A Delta E of 2-4 is noticeable to trained observers.
  • Material shelf life: Silicone and fluoroelastomers age. A gasket made in 2020 might be different than one made in 2025 due to stock material age. Ask for the batch date.

Take it from someone who's wasted $15,000 on avoidable oversights: this checklist is your friend. Print it, use it, and save your next project from becoming my next case study.