What is NBR injection molding used for?

O-rings, seals, gaskets, diaphragms, hydraulic and pneumatic seals, fuel and oil hose, roll covers, vibration mounts and boots. Anywhere a flexible part has to keep oil, fuel, grease or hydraulic fluid contained, or has to survive contact with them without swelling. It is the default rubber in automotive, hydraulic and machinery sealing work.

How is rubber injection molding different from plastic injection molding?

The tool is hot, not cold, and the material reacts rather than solidifying. In plastic molding the melt is injected into a cooler mould and freezes into shape. In rubber molding an uncured compound is injected into a heated mould, and it vulcanises — cross-links — in the cavity. Cure time is therefore driven by heat transfer into the part, which means a thick section takes much longer to cure than a thin one. Injection pressures are much lower than for plastics, so tooling is generally less heavily built, but flash control matters more because rubber flows into every gap.

What is NBR's temperature range?

Roughly -40 °C to +100 °C for continuous service, with short-term excursions above that. The exact figures depend on the compound: the acrylonitrile content that gives NBR its oil resistance also affects its low-temperature flexibility, and a grade formulated for better oil resistance typically performs worse in the cold. Where a part has to work at both temperature extremes, the grade has to be chosen for the balance you need rather than assumed.

Why can rubber parts have undercuts?

Because rubber stretches. A rigid plastic part with an undercut needs a mechanism in the tool — a slider, a lifter or a collapsing core — because the part cannot deform enough to release. A rubber part can be stretched off a simple core, so a shape that would be impossible in plastic is straightforward in rubber. That is a genuine design freedom, and it is why rubber parts often have forms that look unmouldable to someone used to plastics. It comes with the caveat that the part has to be strong enough to survive the stretch without tearing.

What tolerances can a moulded NBR part hold?

Wider than a plastic part, and this surprises buyers who are used to plastics. Rubber shrinkage is larger and less predictable — it depends on the compound, the cure cycle and the section thickness — and the material is flexible, so it deforms under measurement. Rubber dimensioning follows its own conventions, with tolerance bands that widen as the dimension grows. A drawing written to plastic tolerances will not be met by any moulder, so the tolerance is agreed per drawing in DFM rather than carried over from a previous part.

What design rules matter for a moulded rubber part?

Radius everything — rubber tears at sharp corners, and a sharp internal corner is where a part fails. Avoid abrupt changes in section thickness, because thick sections cure much more slowly than thin ones and an uneven part is unevenly cured. Design for the flash line: rubber flashes at the parting line, and where the flash sits matters on a sealing surface. And think about how the part will be stretched off the core, because the demoulding stretch is what limits how severe an undercut can be.

Can NBR parts be moulded with metal inserts?

Yes, and bonded rubber-to-metal parts are common — a metal sleeve inside a rubber bush, a metal insert in a mount, or a metal carrier bonded into a seal. The inserts are machined on our own floor and the rubber is moulded and bonded to them by our partner, from the same drawing, so the fit between rubber and metal is planned before the tool is cut.

When should an NBR part be moulded rather than machined?

Rubber parts are essentially always moulded. Machining rubber is impractical for anything but a one-off, because the material deforms under the cutter, tears easily and cannot be held accurately. So the choice is not between moulding and machining but between a moulded custom part and standard stock — a moulded custom seal versus a catalogue O-ring in a standard size. Where a standard size works, it is cheaper; where the geometry is specific to the design, the tool is cut. The rest of the [materials range](/materials/) covers the machined materials we run.

NBR (Nitrile Rubber)Injection Molding•8 min read

NBR (Nitrile) Injection Molding

NBR — nitrile rubber — moulds into the seals, O-rings, gaskets and diaphragms that keep oil and fuel where they belong. Its oil resistance is the best of any common rubber, which is why it holds the hydraulic, fuel and lubrication seal market almost by default. Moulding rubber is a different process from moulding plastic: the material is injected uncured and vulcanises in the hot tool, the design rules allow shapes a rigid plastic could never release, and the tolerances are much wider than plastics buyers usually expect. Understanding those three differences is most of what it takes to specify an NBR part well.

Specification at a glance

Material family: Synthetic rubber — acrylonitrile butadiene rubber, sulfur-cured
Cure temperature: Roughly 150–180 °C in the mould; cure time scales strongly with section thickness
Mould temperature: Hot — the tool cures the material rather than cooling it
Shrinkage: Roughly 1.5–4%, and dependent on the compound, the cure and the section thickness
Temperature range: Roughly -40 °C to +100 °C; short-term excursions higher
Oil and fuel resistance: Excellent — the defining property of the material
Abrasion resistance: Good
Weather and ozone resistance: Poor — NBR needs protection for outdoor service
Compression set: Moderate; depends on the compound and the cure
Undercuts: Practical — rubber stretches for demoulding, unlike a rigid plastic
Tolerances: Wider than plastics; rubber has its own dimensioning conventions
Molding route: Through our vetted partner network, managed under one quality process; metal inserts machined on our own floor
Mould tooling: Quoted per drawing — cavity count, flash control and cure layout are reviewed before the tool is cut
MOQ: From 50 pieces, samples from a single piece

Rubber molding is a different process

The single most useful thing to understand about moulding rubber is that the tool cures the material rather than cooling it.

	Plastic injection molding	Rubber injection molding
What is injected	Molten polymer	Uncured compound
Mould temperature	Cold relative to the melt	Hot — the tool cures the material
What happens in the cavity	The melt freezes into shape	The material vulcanises — cross-links
What sets the cycle	Cooling time	Cure time, driven by heat transfer
Injection pressure	High	Much lower
Flash	A defect to avoid	A fact of the process to control
Tolerances	Tight	Wider, and dimensioned by their own conventions

Three consequences follow, and each one changes how a part is designed.

Cure time scales with thickness. Heat has to reach the centre of the part for it to cure, so a thick section takes much longer than a thin one. An uneven part is therefore an unequally-cured part, which is why thickness uniformity matters as much for cure as it does for shrinkage.

Injection pressures are low. Rubber compounds flow readily at mould temperature, so the tool does not have to withstand the pressures a plastic tool does. That generally makes rubber tooling less heavily built — but the low viscosity also means the material reaches every gap, so flash control becomes the dominant toolmaking concern, and deflashing is a real cost on a rubber part.

Rubber stretches, so undercuts are practical. A rigid plastic part with an undercut needs a slider, a lifter or a collapsing core. A rubber part can be stretched off a simple core. That is genuine design freedom — and it is why rubber parts often have forms that look unmouldable.

What NBR is for

NBR’s position rests on one property: oil, fuel and hydraulic fluid resistance, the best of the common rubbers.

Where a flexible part has to keep oil or fuel contained, or survive contact with it without swelling and losing its properties, NBR is the default. That is why it holds the hydraulic and fuel sealing market — O-rings, seals, gaskets, diaphragms, hose, roll covers and boots.

The rest of the property set supports that role:

Property	NBR	Note
Oil and fuel resistance	Excellent	The reason for choosing the material
Abrasion resistance	Good	Suits dynamic seals and roll covers
Temperature range	~-40 to +100 °C	Grade-dependent, especially at the cold end
Compression set	Moderate	A specification on long-service seals, not an assumption
Weather and ozone resistance	Poor	Needs protection outdoors
Flame resistance	Poor	Not a fire-rated material

Two of those deserve comment.

Temperature range depends on the grade. The acrylonitrile content that gives NBR its oil resistance also affects its low-temperature flexibility — a grade formulated for better oil resistance typically performs worse in the cold. Where a part has to work at both extremes, the grade is chosen for the balance the application needs.

Outdoor service is the wrong application. Ozone attacks the polymer at stressed points and produces surface cracking, and sunlight and weather accelerate it. A stressed NBR part left outdoors will crack at the points where it is stretched or bent. Where a part has to work outside, neoprene is the usual substitute — much better weather and ozone resistance, at some cost to oil resistance — or the part is shielded. Naming the environment is what allows that choice to be made.

Compression set: the property that decides seal life

A seal works by staying compressed against its mating surfaces. Compression set is the permanent deformation a rubber takes when it is held compressed for a period — the extent to which it fails to return to its original thickness.

A rubber with high compression set takes a set, loses its sealing force and begins to leak. That is how most rubber seals eventually fail, and it is why compression set is a specification on a long-service seal rather than a property to assume.

The compound and the cure both affect it. Where a part is a critical or long-life seal, saying so at the enquiry allows the compound and the cure cycle to be selected for it.

Design rules

Radius everything. Rubber tears at sharp corners, and a sharp internal corner is where a part fails. There is no yielding to redistribute stress the way a ductile material manages it — the tear simply propagates.

Avoid abrupt section changes. They cure unevenly, because heat reaches a thick section more slowly than a thin one. Where a thickness change is unavoidable, blend it with a taper rather than stepping it.

Design for the flash line. Rubber flashes at the parting line, and where the flash sits matters — on a sealing surface, flash is a functional defect, not a cosmetic one. The parting line position is a design decision on a seal.

Think about how the part is stretched off the core. The demoulding stretch is what limits how severe an undercut can be, and a part designed without that in mind can tear on every cycle.

Agree the tolerances. Rubber dimensioning follows its own conventions, with bands that widen as the dimension grows. A drawing written to plastic tolerances will not be met by any moulder. Tolerances are agreed per drawing in DFM.

Molding route

Plastic and rubber molding run through our vetted partner network, managed under one quality process, while metal inserts and machined features are done on our own floor in Qidong City, Jiangsu Province, China. Tooling is quoted per drawing, and on a rubber part the review covers cavity count, flash control, cure layout and the parting line position.

Bonded rubber-to-metal parts — a metal sleeve inside a rubber bush, a metal insert in a mount, a metal carrier bonded into a seal — are handled the same way: the insert is machined on our floor and the rubber is moulded and bonded to it by our partner, from the same drawing. That means the fit between rubber and metal is planned before the tool is cut.

Where molding NBR is the right choice

O-rings, seals and gaskets in oil, fuel and hydraulic service
Diaphragms in pumps and valves
Hydraulic and pneumatic seals — see hydraulic and pneumatic parts
Fuel and oil hose and hose linings
Roll covers where oil resistance and abrasion resistance are both needed
Vibration mounts and bushes in oily environments
Boots and gaiters in mechanical and automotive applications
Bonded rubber-to-metal parts where a machined insert carries the load

Where NBR is the wrong choice

The part works outdoors or in ozone. NBR cracks. Neoprene is the substitute, or the part needs shielding.

The service temperature is above roughly 100 °C continuously. Silicone covers a far wider range.

The part meets ketones, esters, strong acids or aromatic solvents. NBR’s resistance is specific to oils, fuels and hydraulic fluids — it is not a general-purpose chemical barrier, and a part specified for “chemical resistance” without naming the chemical is being specified by hope.

Flame resistance is required. NBR has none. Neoprene is inherently more flame resistant.

The application needs compression set better than the compound delivers. Silicone is markedly better on compression set over a wide temperature range.

A standard catalogue seal would work. Where a standard O-ring or gasket size fits the design, buying it is cheaper than tooling a custom part — a moulded custom seal should exist because the geometry demands it.

What to put on the drawing

Instead of	Write
Rubber	NBR (nitrile), for an oil, fuel or hydraulic seal
NBR, environment	Say indoors or outdoors — NBR has poor ozone and weather resistance
NBR, service temperature	State the low and high ends — the grade is chosen for the balance
NBR, long-service seal	Specify compression set — it is what decides seal life
NBR, sealing surface	Say which surface seals — the flash line must avoid it
NBR, tolerance	Agree it — rubber tolerances are wider than plastics and follow their own conventions
NBR, bonded to metal	Say so — the insert is machined on our floor and bonded by our partner
NBR, meets solvents or acids	Name the chemical — NBR is not a general chemical barrier

The three lines that decide most NBR mouldings are the chemical the part meets, the temperature range, and which surface has to seal. The first is where NBR wins and where it fails. The second decides the grade. The third decides the parting line, and on a seal it is a functional requirement rather than a toolmaking detail.

Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the compound, the cure, the compression set requirement and the parting line where any of them need settling.

Frequently asked

What is NBR injection molding used for?: O-rings, seals, gaskets, diaphragms, hydraulic and pneumatic seals, fuel and oil hose, roll covers, vibration mounts and boots. Anywhere a flexible part has to keep oil, fuel, grease or hydraulic fluid contained, or has to survive contact with them without swelling. It is the default rubber in automotive, hydraulic and machinery sealing work.
How is rubber injection molding different from plastic injection molding?: The tool is hot, not cold, and the material reacts rather than solidifying. In plastic molding the melt is injected into a cooler mould and freezes into shape. In rubber molding an uncured compound is injected into a heated mould, and it vulcanises — cross-links — in the cavity. Cure time is therefore driven by heat transfer into the part, which means a thick section takes much longer to cure than a thin one. Injection pressures are much lower than for plastics, so tooling is generally less heavily built, but flash control matters more because rubber flows into every gap.
What is NBR's temperature range?: Roughly -40 °C to +100 °C for continuous service, with short-term excursions above that. The exact figures depend on the compound: the acrylonitrile content that gives NBR its oil resistance also affects its low-temperature flexibility, and a grade formulated for better oil resistance typically performs worse in the cold. Where a part has to work at both temperature extremes, the grade has to be chosen for the balance you need rather than assumed.
Why is NBR bad outdoors?: Because it has poor ozone and weather resistance. Ozone attacks the polymer at stressed points and produces surface cracking, and sunlight and weather accelerate the process. A stressed NBR part outdoors will crack at the points where it is stretched or bent. Where a part has to work outdoors, neoprene is the usual substitute — it has much better weather and ozone resistance at some cost to oil resistance — or the NBR part is shielded from direct exposure. Naming the environment is what allows that choice to be made.
What is compression set, and why does it matter?: Compression set is the permanent deformation a rubber takes when it is held compressed for a period — the extent to which it fails to return to its original thickness. It is the property that decides how long a seal keeps sealing, because a seal works by staying compressed against its mating surfaces. A rubber with high compression set takes a set, loses its sealing force and begins to leak. The compound and the cure both affect it, so where a part is a long-service seal, compression set is a specification to state rather than a property to assume.
Why can rubber parts have undercuts?: Because rubber stretches. A rigid plastic part with an undercut needs a mechanism in the tool — a slider, a lifter or a collapsing core — because the part cannot deform enough to release. A rubber part can be stretched off a simple core, so a shape that would be impossible in plastic is straightforward in rubber. That is a genuine design freedom, and it is why rubber parts often have forms that look unmouldable to someone used to plastics. It comes with the caveat that the part has to be strong enough to survive the stretch without tearing.
What tolerances can a moulded NBR part hold?: Wider than a plastic part, and this surprises buyers who are used to plastics. Rubber shrinkage is larger and less predictable — it depends on the compound, the cure cycle and the section thickness — and the material is flexible, so it deforms under measurement. Rubber dimensioning follows its own conventions, with tolerance bands that widen as the dimension grows. A drawing written to plastic tolerances will not be met by any moulder, so the tolerance is agreed per drawing in DFM rather than carried over from a previous part.
What design rules matter for a moulded rubber part?: Radius everything — rubber tears at sharp corners, and a sharp internal corner is where a part fails. Avoid abrupt changes in section thickness, because thick sections cure much more slowly than thin ones and an uneven part is unevenly cured. Design for the flash line: rubber flashes at the parting line, and where the flash sits matters on a sealing surface. And think about how the part will be stretched off the core, because the demoulding stretch is what limits how severe an undercut can be.
Can NBR parts be moulded with metal inserts?: Yes, and bonded rubber-to-metal parts are common — a metal sleeve inside a rubber bush, a metal insert in a mount, or a metal carrier bonded into a seal. The inserts are machined on our own floor and the rubber is moulded and bonded to them by our partner, from the same drawing, so the fit between rubber and metal is planned before the tool is cut.
When should an NBR part be moulded rather than machined?: Rubber parts are essentially always moulded. Machining rubber is impractical for anything but a one-off, because the material deforms under the cutter, tears easily and cannot be held accurately. So the choice is not between moulding and machining but between a moulded custom part and standard stock — a moulded custom seal versus a catalogue O-ring in a standard size. Where a standard size works, it is cheaper; where the geometry is specific to the design, the tool is cut. The rest of the [materials range](/materials/) covers the machined materials we run.

Other Rubber & Silicone materials

Have a part in NBR (Nitrile Rubber)?

Send the drawing, the grade and the quantity. You get DFM feedback and a price within 24 hours, and orders start at 50 pieces per style and material.

Request a quote All Rubber & Silicone Injection Molding