Guides / Sourcing

Why PEEK Survives Repeated CIP and SIP Cycles

September 26, 2026

Sources: manufacturer datasheets and published standards. See editorial standards.

Clean-in-place and steam-in-place systems put process equipment through two kinds of stress at once, chemical attack from the cleaning solution and thermal cycling from the steam, and they do it hundreds of times over the life of the equipment. A plastic part in that loop has to survive both, not just one.

What a CIP and SIP cycle actually does to a part

A typical CIP cycle runs a hot caustic solution, sodium hydroxide, often at 60 to 80 degrees C, through the system, followed by an acid rinse using nitric or phosphoric acid, followed by a final rinse. SIP follows with saturated steam, commonly in the 121 to 134 degrees C range, held long enough to sterilize the internal surfaces. A part in the wetted path sees hot alkali, then acid, then steam, then goes cold again before the next production run. That is the cycle repeated daily or more often for years.

Why other plastics struggle here

Most plastics that resist one side of this loop give way on the other. Acetal and nylon degrade under hot caustic. Plastics that tolerate the chemicals poorly handle repeated steam, softening, hydrolyzing or losing strength after enough cycles. PTFE resists both the chemicals and the steam temperature but creeps under load, so a PTFE seal or seat slowly loses its geometry even while the material itself is unaffected chemically. For more on where PTFE holds up and where it does not against PEEK, see PEEK vs PTFE for food and pharma processing parts.

Why PEEK holds up

PEEK combines the two properties this cycle needs together: resistance to hot caustic and acid, and hydrolysis resistance through repeated steam exposure without losing strength. Neither property alone is rare. The combination, plus a continuous service temperature well above what SIP cycles reach, is what keeps PEEK in this kind of equipment after cheaper plastics have been tried and replaced. The chemistry side of this is covered in more depth in what PEEK resists and what attacks it.

Typical parts

• Valve seats and seals in diaphragm and ball valves on the product side of the line
• Scraper blades in scraped surface heat exchangers, where the blade slides against a hot metal wall through every cycle
• Tri-clamp and sanitary fittings where a metal part would corrode under repeated caustic exposure
• Bushings and wear pads inside pumps and mixers that see both product and cleaning chemicals
• Sensor housings and probe bodies mounted in the wetted path

Choosing the grade

Unfilled PEEK is the default for anything that just needs to survive the chemistry and the steam without sliding against another surface: fittings, housings, static seals. Where the part also slides, a bearing grade blend with PTFE and graphite added cuts friction and wear without giving up the chemical and steam resistance of the base resin. Glass filled grades are usually the wrong choice here, since the fiber ends exposed at a machined surface give the caustic and steam a path into the material over enough cycles. The full grade by grade comparison is in which PEEK grade should be on your drawing.

What repeated cycling does to the part itself, not just the material

The chemistry and steam resistance answer whether PEEK survives a single CIP or SIP cycle. What determines whether it survives the thousandth cycle is closer to a machining question than a material one. Residual stress left in a part from aggressive cutting shows up over time as the part is heated and cooled through the same range, day after day, and a stressed part can warp or crack after months of service even though an unstressed part of the same grade would not. Annealing the stock before final machining relieves that stress and is standard practice for parts that will see this kind of repeated thermal cycling, covered in annealing PEEK parts. It is worth asking a shop directly whether their process includes it, since skipping the step is invisible on a new part and only shows up months into service.

What to put on the RFQ

• The cleaning chemical, concentration and temperature for both the caustic and acid steps
• The steam temperature and cycle length for SIP
• Whether the part slides against anything, and against what material
• Whether the part sees product contact between cleaning cycles, which affects whether food contact documentation applies as well

Running CIP and SIP cycles on a line that is chewing through plastic parts faster than it should? Send the drawing with the cleaning conditions and we match you with shops that already machine for this kind of service.

Frequently asked questions

Keep reading

• Clearance, Grade and Fit for PEEK Pump Wear Rings and Bushings
• What a Shop Needs to Machine Spinal Cage Prototypes in PEEK