What Is Sterilization?

So your part needs to be sterile. Great. Which sterilization method? That's the question nobody asks until way too late.

Sterilization Comes After Tooling-Bad Idea

Most people do this backwards. They design the part, pick a resin, get tooling quoted, sometimes even cut steel. Then someone in regulatory says wait, how are we sterilizing this thing?

Now you're stuck. The resin you picked doesn't survive gamma. Or it off-gasses too much for EO. Or it warps in autoclave.

Should've talked to your medical injection molding supplier about this on day one. Not day ninety.

Four Ways to Kill Bugs

EO. Gamma. E-beam. Autoclave. That's basically it for plastics.

EO (ethylene oxide) - Gas sterilization. Low temp, 37-63°C range. The gas creeps into every crack and crevice. Works on almost any thermoplastic. PP, PC, ABS, acetal, whatever. Downside? Takes forever. Two days minimum with aeration, sometimes four. The gas is toxic so it has to off-gas to under 25 ppm before you can ship. Contract sterilizers in the Midwest are backed up for weeks sometimes.

Gamma - Cobalt-60 radiation. Fast. Whole pallet done in an hour. No residue, no aeration. But gamma destroys certain plastics. Acetal? Forget it. Turns to chalk. Standard PP gets brittle and yellow within months. Polycarbonate yellows too, though some customers don't care about cosmetics.

There's radiation-stabilized PP. Costs more. Borealis and LyondellBasell both make grades for this. Your injection molding manufacturer should know which ones. If they're quoting you Pro-fax 6323 for a gamma-sterilized part, find someone else.

E-beam - Electrons instead of gamma rays. Similar material restrictions. Doesn't penetrate as deep, so wall thickness matters. Works great for thin stuff. Catheters, tubing, packaging. Fast throughput. Some facilities run 24/7 now because demand went up after Covid.

Autoclave - Steam at 121°C or 134°C. The oldest method. Hospitals do this in-house. No contract sterilizer needed. But your plastic has to handle the heat. PP softens. HDPE softens. ABS warps. You need PEEK, PPSU, PSU, Ultem, or a glass-filled nylon. And even then, 500 cycles will tell you if the part design was right.

The Resin Compatibility Thing

Look, everyone wants a simple chart. This resin works with this method, that one doesn't. Reality is messier.

Sabic has a polycarbonate that handles gamma fine. Generic PC from some random compounder? Maybe not. The stabilizer package matters. The colorant matters. TiO2 white holds up better than organic pigments under radiation. Carbon black actually protects some polymers.

A part might pass at 25 kGy and fail at 40 kGy. Your dose depends on bioburden and packaging density. So you can't just look at a chart.

Talk to the resin supplier. Get their sterilization compatibility data sheet. Run samples at 1.5x your target dose. This is not optional if gamma or e-beam is the plan.

When EO Makes Sense

Heat-sensitive parts. Complex geometries with internal channels. Anything with a thin membrane or filter. Multi-material assemblies where one component can't take radiation.

EO is forgiving. Cycle after cycle, most plastics don't degrade. That's why hospitals use EO for reusable scopes and instruments.

The EPA has been tightening emissions rules though. Some facilities closed. Capacity is tighter than five years ago. Your custom injection molding company should factor lead time into the project schedule.

When Gamma or E-beam Makes Sense

High volume disposables. Things that ship by the truckload. Syringes, blood collection tubes, IV components. Speed matters when you're making millions.

Gamma facilities are mostly in the Midwest and Southeast. Sterigenics, Nordion, few others. E-beam capacity is growing. Smaller footprint, easier permitting.

Material has to be right though. Get this wrong and you're doing a recall.

Autoclave Is Its Own World

Surgical instruments. Reusable device housings. Anything that goes back into the sterilizer between patients.

PEEK is the gold standard. Expensive but bulletproof. PPSU is cheaper, still good for hundreds of cycles. Ultem (PEI) works but it's brittle. Glass-filled PA66 can handle it if you design the part right.

Wall thickness uniformity matters more for autoclave than other methods. Thin spots warp. Thick spots take longer to heat through and cool down. A good medical injection molding partner will push back on your wall thickness if it's going to cause problems.

Validation Is a Slog

Bioburden testing. Dose mapping. Sterility testing. Stability studies. Then you write it all up for your 510(k) or your Design History File.

Change the resin? Revalidate. Change the sterilization facility? Revalidate. Change the packaging? Might need to revalidate.

Budget six months minimum. Probably more. The contract sterilizer does validation runs on their schedule, not yours.

The Actual Question

Before you talk to any injection molding service provider, figure out where your device gets used.

Single-use disposable going to hospitals that don't have autoclaves? Gamma or e-beam probably.

Reusable device? Autoclave-compatible materials, higher cost.

Low volume, complex geometry, heat-sensitive? EO.

High volume, simple geometry, radiation-stable material? Gamma for cost, e-beam if the facility is closer.

Your plastic injection molding vendor can help narrow this down if they've done medical work before. The ones who haven't will just quote whatever resin is cheapest and let you figure out sterilization later. That's how projects go sideways.

Wrap Up

Sterilization isn't complicated. Four methods, each with tradeoffs. Pick your resin based on what the part needs to do and what sterilization it needs to survive. Not the other way around.

Talk to your tooling supplier early. Talk to the contract sterilizer early. Don't wait until you've already cut steel. That's when the expensive surprises happen.