Custom Molded Seals

Overview

Highly specialized seals for every industry

Custom molded seals are specified when the sealing function requires a geometry that cannot be achieved with standard profiles — complex cross-sections, integrated multiple lips, over-molded metal inserts, bellows geometries, or seals that must perform multiple simultaneous functions in a single molded component.

We collaborate on seal design from project conception through to production approval. If you have an application where every off-the-shelf seal has failed, or you're designing new equipment and need a sealing element that doesn't exist yet, that's the kind of problem custom molding solves.

Materials include nitrile (NBR), EPDM, silicone (VMQ), fluoroelastomers (FKM), urethane, and solid thermoplastics. Two primary manufacturing methods are used depending on part geometry, material, and production volume: injection molding and compression molding.

Manufacturing

Injection molding & compression molding

The injection molding process

Injection molding heats the elastomer or thermoplastic compound to a flowable state, then injects it under high pressure into a multi-cavity mold. The mold geometry defines the part geometry with repeatable precision — every part from the first to the ten-thousandth is dimensionally identical.

Process Step	Description
1. Compound preparation	Raw elastomer compound mixed to exact formulation. Shore hardness, chemical resistance, and temperature range set at this stage.
2. Injection	Compound heated to liquid state and injected under pressure into the mold cavities. Pressure and temperature profile precisely controlled.
3. Cure / vulcanization	Heat initiates crosslinking in the elastomer matrix. This is the step that gives rubber its elastic recovery and mechanical strength.
4. Demolding	Part ejected from mold after full cure cycle. Multi-cavity molds produce multiple identical parts per cycle.
5. Deflashing	Removal of flash at parting lines. Critical for sealing applications — flash in a sealing groove prevents compression.
6. Inspection	Dimensional check, visual inspection, and functional testing per approved inspection plan.

Compression molding — the alternative for complex elastomers

Compression molding places a pre-measured charge of uncured compound directly into an open mold cavity. The mold closes under hydraulic pressure, forcing the material to fill the cavity as heat initiates vulcanization. While slower per cycle than injection molding, compression molding handles compounds with very high viscosity, produces very low residual stress in the finished part, and is the preferred method for large cross-section seals, diaphragms, and parts with undercuts that cannot be injection-molded.

Criterion	Injection Molding	Compression Molding
Material viscosity	Low to medium — compound must flow under injection pressure	High viscosity compounds — material does not need to flow, just compress and fill
Part complexity	High — multi-cavity, tight tolerances, complex geometry	Moderate — large cross-sections, diaphragms, seals with thick sections
Cycle time	Shorter — faster for high-volume production	Longer — preferred for specialty compounds and large-volume parts
Flash	Minimal — parting line flash only	More flash typical — deflashing step required
Tooling cost	Higher — precision injection tooling	Lower — simpler mold construction for moderate volumes
Best for	High-volume precision seals, O-rings, lip seals, multi-lip profiles	Diaphragms, large gaskets, specialty elastomers (FFKM, silicone sponge), prototype quantities

Design flexibility — shape, size, and complexity

Injection molding accommodates geometry that is impossible in extruded or cut gaskets: complex 3D shapes, varying cross-sections, grooves and undercuts, bellows features, and integrated functional elements.

Design Feature	Capability
Cross-section complexity	Multiple thicknesses, grooves, bevels, lips, channels, and bellows in a single part. No limitation from standard profile libraries.
Size range	Miniature (sub-millimeter cross-section) to oversized components (>500 mm diameter). Mold size and injection press capacity are the only constraints.
Tolerances	Tight tolerances achievable with precision molds. Better repeatability than extruded or cut seals for critical dimensions.
Metal inserts	Metal components can be over-molded — integrate metal reinforcement, bonded metal faces, or threaded inserts directly into the molded rubber part.
Multi-material	Co-injection and two-shot molding available for parts requiring different materials in different zones — e.g., rigid thermoplastic body with soft sealing lip.

Collaborative design — example: double-lip urethane axial seal

Custom seal design is a collaborative process. The example below shows a double-lip urethane axial seal developed in partnership with a customer who needed simultaneous radial and axial sealing in a confined space that no standard seal could fill.

The design process typically follows: application analysis → sealing concept development → 2D cross-section drawing → FEA compression simulation (where needed) → prototype mold → prototype validation → production mold → PPAP / first article inspection → production release.

Design tip: Draft angles, parting line placement, and deflashing requirements all affect seal geometry. Involve us at the concept stage — a small change in draft angle or parting line position can eliminate a costly secondary operation and improve dimensional consistency across the production run.

Materials

Molding compound selection