Molded vs. Spliced Solid and Hollow Extrusion O-Ring Tolerances

Molded O-Rings (RMA A1–A3)

Molded O-rings are produced by injection, transfer, or compression molding in a closed cavity.
The mold cavity defines both the inside diameter and the cross-section, which makes this method the most precise.
The main tolerance drivers are tool precision, cavity balance, thermal expansion, material flow, and shrinkage.
Tolerance classes range from RMA A1 for precision aerospace applications to RMA A2 for standard industrial use.
Molded O-rings provide highly consistent dimensions because the cavity controls the final part shape.

Spliced Solid Extrusion (ISO 3302-1 M2 / ISO 3601-3 N)

Spliced solid rings are manufactured from extruded cord stock that is cut to length and bonded into a ring.
Dimensional control is influenced by extrusion variability, diameter drift along the cord, cut-length tolerance,
shrinkage, and the added distortion created at the splice joint.
Because extrusion and cutting are inherently less controlled than molding, the resulting tolerances are wider.
Cross-section variation typically ranges from ±0.15 to ±0.30 mm or more, while cut-length tolerance is usually
dependent on circumference, often ±1 to ±2 mm.

Spliced Hollow Extrusion

Spliced hollow O-rings are created from extruded tubing that is cut and spliced in the same way as solid cord.
Hollow constructions show additional dimensional variability due to fluctuations in wall thickness, difficulty
maintaining concentricity, and increased ovality during handling or splice curing.
Aligning the inner and outer walls at the splice is more challenging, resulting in weaker splice strength and
greater dimensional scatter. Because ID, OD, and wall thickness are not independently controlled, tolerances
are even looser than in solid extrusions, and dimensional stack-up becomes the dominant factor.

Relative Tolerance Precision

From tightest to loosest tolerance capability: molded O-rings in RMA A1 or ISO Class A, molded O-rings in
RMA A2, spliced solid extrusions under ISO 3601-3 N or ISO 3302-1 M2, and spliced hollow extrusions, which
typically require project-specific tolerance agreements.

Why Molded Tolerances Do Not Apply to Spliced Solid or Hollow Rings

Molded rings rely on a cavity that defines the final geometry with high repeatability.
Spliced solid rings add variation from extrusion, cutting, and splicing.
Spliced hollow rings add further variation from wall thickness drift, ovality, and handling effects.
For these reasons, molded tolerances must not be applied to spliced products.
Instead, extrusion-based ISO tolerances should be used, and hollow constructions should include an additional
allowance.

Dimensional Comparison Example (AS568-277: 11.480 × 0.139″)

Cut Length vs. Derived ID Tolerance

Summary of Dimensional Drivers by Construction Type

Molded rings achieve the tightest tolerances because the tool cavity defines both the inside diameter and the cross-section.
Spliced solid rings introduce variability from extrusion drift, cut-length tolerance, and splice-joint distortion.
Spliced hollow rings exhibit the widest variation because they also include wall-thickness fluctuation, concentricity issues,
and ovality. These combined factors make molded parts the most precise and hollow spliced rings the least dimension-stable.

Key Takeaway

Molded parts can maintain tight, repeatable tolerances.
Spliced rings, whether solid or hollow, must allow looser dimensional limits because extrusion and splicing introduce more
variability. Hollow spliced rings should be treated as functional sealing elements rather than precision-dimension components
unless a custom tolerance agreement is established.