CNC plastic processing corrosion solution: how plastics fail and how to design around it

• 1
  What corrosion actually means for a machined plastic part
• 2
  Chemical attack: which bonds break first
• 3
  Environmental stress cracking: when load and chemistry combine
• 4
  Geometry decisions that decide service life
• 5
  How the machining process itself controls corrosion resistance
• 6
  Resin vs typical aggressive media: rough ranking
• 7
  Symptom, cause and process fix

Engineers from metal backgrounds ask the wrong first question. They ask whether the resin corrodes. It does not, at least not in the electrochemical sense. A machined POM gear will never grow red oxide. What it will do is absorb a solvent, swell by 2 percent, and lose the interference fit that keeps a shaft from slipping.

So when we talk about a CNC plastic processing corrosion solution, we mean controlling four failure paths: chemical attack at the polymer chain, environmental stress cracking under load, dimensional change from moisture or solvent uptake, and surface breakdown that releases filler particles into a process line.

Each path has a different root cause, and each one is attacked differently. A single material swap rarely fixes all four. This page separates them so the fix matches the mechanism.

Aggressive media attack polymer chains in predictable places. Strong bases hydrolyze ester linkages, which is why PET and PC struggle with caustic cleaning agents. Strong acids attack amide groups, so PA (nylon) and PAI lose strength in acid service. Aromatic rings resist oxidation but are vulnerable to aromatic solvents by the like-dissolves-like rule.

Semi-crystalline resins behave better than amorphous ones in most chemical service. The crystalline regions block solvent diffusion, so only the amorphous fraction absorbs. PEEK at roughly 30 percent crystallinity will hold up in steam and many solvents where PC would craze. POM, PP and PVDF sit in similar territory for common industrial fluids.

Fillers change the picture. Glass fiber improves stiffness and creep resistance but creates wicking paths along the fiber-matrix interface. Carbon fiber is chemically inert and adds electrical conductivity, useful when static charge would otherwise attract dust or ignite solvent vapor. PTFE is close to inert but cold-flows under load, so it needs mechanical support.

Temperature accelerates everything. A resin rated for 20 °C service may lose half its strength at 60 °C in the same bath. Always check the chemical resistance chart at the actual service temperature, not at room temperature.

• 1
  Bases + estersHydrolysis of PC and PET in caustic wash. Switch to PP or PVDF.
• 2
  Acids + amidesPA and PAI degrade. PEEK or PVDF holds.
• 3
  Aromatic solventsAttack amorphous PS, PC. Semi-crystalline PEEK resists.
• 4
  Glass fillersWicking along fibers. Consider unfilled or carbon-filled grades.

Environmental stress cracking (ESC) is the failure mode that surprises engineers most. The part passes a 30-day immersion test with no load, then cracks in two weeks in service. The resin is fine. The combination of tensile stress and a mild chemical is not.

ESC starts at a surface flaw, often a machining mark, a sharp internal corner or a tool witness line. Solvent diffuses into the stressed polymer, plasticizes the crack tip, and the crack grows at loads far below the yield strength. Residual stress from the machining operation adds to this. A rough face milled at aggressive feed leaves subsurface damage that becomes the initiation site.

Three controls reduce ESC risk. First, keep tensile stress below roughly 20 percent of the yield strength in chemical contact. Second, radius every internal corner to at least 0.5 mm. Third, specify a finishing pass that removes subsurface damage, typically 0.2–0.3 mm radial depth of cut at Ra 0.8–1.6 μm.

Annealing after machining relieves residual stress. For PEEK and POM, a controlled ramp to just below the glass transition or melting range, held and slow-cooled, cuts internal stress significantly. Talk to us before annealing thin walls, because the same heat can warp them.

Wall thickness drives both diffusion time and internal stress. A thick wall takes longer for solvent to reach the core, which sounds good, but it also locks in more molded-in or machined-in stress. For parts in chemical contact, keep walls between 2 mm and 6 mm where the design allows. Above 10 mm, consider a cored or split design.

Sharp corners concentrate stress. Every internal corner on a part exposed to chemicals should carry a radius of at least 0.5 mm, and 1 mm is safer on load-bearing features. The same rule applies to the bottom of blind holes. A flat-bottom drill leaves a sharp corner that becomes a crack origin, so we often add a corner radius tool or relieve the bottom.

Surface finish matters more than most drawings show. As-machined Ra 3.2 μm leaves tool marks that trap media and start cracks. For chemical service, specify Ra 0.8–1.6 μm on wetted surfaces; for sealing faces or FDA paths, Ra 0.2–0.8 μm. Bead blasting after machining can hide tool marks but it also opens surface pores, so it is not a substitute for a proper finishing pass.

Threads and press fits are common failure points. Cut threads hold media in the root; rolled or formed threads are smoother but not always possible in plastics. For press fits, remember that plastic relaxes over time, so a light interference that works on day one may lose grip after thermal cycling.

• 1
  Wall thickness2–6 mm typical for chemical service. Thicker walls hold more residual stress.
• 2
  Internal radii≥0.5 mm minimum; 1 mm on load-bearing corners.
• 3
  Wetted finishRa 0.8–1.6 μm standard; Ra 0.2–0.8 μm for seals.
• 4
  ThreadsAvoid sharp-root cut threads in chemical contact.

Machining is not just shaping. It sets the surface state that chemistry later attacks. A PEEK seal machined with a dull tool leaves a smeared layer with different crystallinity than the bulk. That layer absorbs solvent faster and cracks sooner. Sharp tooling and a controlled finishing pass produce a surface that behaves like the base resin.

Coolant choice also matters. Water-based coolants can be absorbed by PA and POM, causing dimensional change after machining. For those resins we run dry or with a minimum quantity of lubricant, and we check dimensions after a stabilization period. For PEEK and PVDF, water-based coolant is normally fine, but we dry the parts before inspection.

Five-axis machining helps on complex chemical hardware. A ported manifold or a valve body with angled passages can be cut in one setup, which keeps wall thickness consistent and avoids the re-fixturing marks that become crack origins. Our 16 simultaneous five-axis centers handle parts up to 4,000 × 400 × 150 mm, and the Ø400 mm rotary table covers round valve and pump components.

After machining, we inspect 100 percent of parts before shipment and can supply dimensional reports on request. For chemical service parts, we also record the finishing pass parameters so the surface state is repeatable across production lots.