Acetal homopolymer (POM-H, sold as DuPont Delrin) rod above roughly 3 inch diameter develops a porous centerline core during extrusion, while acetal copolymer (POM-C, sold as Celcon or Hostaform) rod of the same diameter does not [S4].
The root cause is differential shrinkage: the skin of a homopolymer billet solidifies first, and the still-molten interior contracts as it cools, leaving a low-density voided core that runs the full length of the rod [S3]. Comonomer addition (1,3-dioxolane or ethylene oxide) disrupts perfect crystalline packing in the copolymer, which is the mechanism that suppresses the same defect in POM-C [S4].
Where the Porosity Actually Shows Up
Centerline porosity is most prominent in thick slab and large-diameter rod stock, visible around the center portion of a rod and running its entire length; in sheet stock it appears as a line along the center of each cut edge, with the worst cases looking as if the slab is laminated or glued together [S3]. Multiple distributors converge on the same diameter trigger zone: above approximately 3 inch (75 mm) diameter, Delrin homopolymer rod reliably develops a porous centerline, while copolymer rod of equivalent size does not [S4].
The defect is inherent to the homopolymer's high crystallinity, not a quality-control lapse, so buying "premium grade" Delrin does not buy you porosity-free large-diameter rod [S8]. The practical workaround historically was to order oversize stock and machine off the centerline, but that drives material waste and is not viable for parts where the centerline region is a functional surface (a bore, a seal land, a fluid passage) [S8].
Mechanical Trade-Offs You Are Accepting Either Way
For most precision machined parts under 3 inch diameter, either grade is acceptable, and the property envelope is within roughly 10% on most metrics [S3][S4]. Where they diverge: Delrin homopolymer has approximately 10% higher tensile strength (around 10,000 psi vs 9,000 psi for copolymer) and roughly 7% higher flexural modulus, plus a higher fatigue endurance limit that matters for gears, cams, and pump impellers running millions of cycles [S4].
Copolymer pulls back on the hot-water, steam, alkali, and weld-join axes: it withstands sustained hot-water contact, has a continuous service temperature ceiling of about 80°C (180°F), and welds readily, whereas homopolymer is difficult to weld because its crystalline structure resists fusion bonding [S3][S4]. Both are FDA-compliant in their natural grades, and both machine like a soft brass with standard tooling [S2].
Decision Matrix: Which to Specify
The decision is not about which acetal is "better" but about which failure mode you cannot tolerate. Spec POM-H (Delrin) when the part is small in cross-section (under ~3 inch), thin-walled, high-cycle, and strength-limited: thin bushings, small gears, snap-fit detents, cam followers [S3]. Spec POM-C (copolymer) when any of these conditions apply: large-diameter rod or thick slab, fluid-handling or pressure-containing service, hot-water or caustic exposure, weld-joined assemblies, food-contact parts where bacterial harborage in voids is a concern, and outgassing-sensitive applications [S1][S2][S3][S5].
For ambiguous drawings that simply say "acetal," the engineer should clarify whether homopolymer or copolymer is intended, because substituting one for the other in a hot-water valve seat, a weld-join assembly, or a large-diameter bore can produce failures that are hard to diagnose after the fact [S4]. Material property data for POM homopolymer and copolymer grades confirms that the two resins share the same polyoxymethylene backbone but diverge sharply on shrinkage behaviour, which is exactly what drives the centerline defect.
Why Homopolymer Develops Porosity and Copolymer Does Not
During extrusion, the outside of the shape cools and solidifies before the interior; as the interior material subsequently cools, it contracts, and because the rigid skin restricts bulk volume change, the shrinkage is absorbed by void formation along the centerline [S3]. This is a direct consequence of POM-H's higher degree of crystallinity, which produces a larger specific-volume change on cooling than the comonomer-disrupted POM-C structure [S4][S8].
The comonomer in POM-C acts as a chain interruption that prevents the long-range crystalline order responsible for the high shrinkage; the trade-off is a modest loss in peak tensile strength and stiffness, but the gain is the elimination of the centerline void network in thick sections [S4]. For context on the broader plastics machining landscape, see industrial valve and fitting material selection, where POM-C's porosity-free behaviour is the reason it dominates fluid-handling stock shapes.
Failure Modes and Use Cases
Excessive centerline porosity in finished parts is undesirable on three fronts: aesthetic (inconsistent color appearance, visible voids on machined surfaces), structural (reduced load-bearing cross-section, crack initiation sites under fatigue), and functional (bacterial harborage in food-contact and medical parts, leak paths in pressure-containing parts, outgassing in vacuum or optical assemblies) [S3][S5]. In a fluid-handling bushing or a pump impeller running wet, the porous centerline becomes a wick that can carry process fluid into a region the designer assumed was solid [S2][S5].
Conversely, the homopolymer's higher fatigue endurance is the reason it remains the default for small-diameter, high-cycle, thin-walled bushings and snap-fit components, where centerline porosity simply is not present because the billet is too small to develop the void network [S3]. For applications where the choice genuinely is interchangeable, both grades carry FDA, USDA, NSF, and 3-A Dairy compliance, and both cut cleanly on standard tooling, so the decision should be made on the service environment, not on machining economics [S2][S3].
Sourcing, Standards, and Drawing Callouts
No ISO or ASTM standard currently defines an acceptance limit for centerline porosity in extruded POM stock, so the spec is carried by material grade and trade name on the drawing rather than by a numeric test value [S4]. The standard callout pattern is "POM-H" or "Delrin 150SA" for homopolymer and "POM-C" or "Celcon / Hostaform / Tecaform" for copolymer, with FDA compliance noted where food contact applies [S2][S4].
Distributors stocking both grades note that Delrin homopolymer commands a small price premium as a brand-name material, while copolymer is typically priced at or slightly below the homopolymer; the cost gap is rarely the deciding factor in a correctly specified drawing [S2][S4]. For a wider view of how POM stock shapes compare with other engineering thermoplastics used in precision components, the POM material reference consolidates density, moisture absorption, and continuous-service-temperature data for both grades.
Trackable signals for the next sourcing cycle: confirm rod diameter availability with each supplier for your largest cross-section, request a sample slice from the centerline of any candidate homopolymer billet above 3 inch diameter for visual inspection, and verify FDA/NSF grade certification on a per-lot basis rather than per-resin-family, because compliance varies by specific compound [S2][S4].
The underlying component specifications are covered under wire rod.
For related coverage, see C5 vs C9 Hydrocarbon Tackifier Resins for Hot Melt Adhesives: Spec Decision Map.