Extra Soft Solid Rubber Stopper

Quick Answer: The Rubber Cork Stopper, Extra Soft, Solid is an unbored rubber stopper made from a softer, more compressible compound than a standard bung. An extra soft rubber stopper seals with light hand pressure and conforms to necks that are slightly out of round.

What Softness Changes

Rubber hardness is usually expressed on the Shore A scale, and a softer compound gives way more under the same push. An extra soft stopper therefore needs less force to seat, which suits thin-walled tubes, older glassware with slightly irregular necks, and younger students who struggle to press a firm bung home. The extra give also spreads contact over a wider band of glass, helping the seal on necks that are not perfectly round.

Softness has costs. The stopper grips the glass strongly and can be harder to pull out, and under vacuum it may be drawn further into the neck than a firmer bung. It also takes a compression set sooner: left in the same neck for weeks, it keeps that shape and may no longer seal elsewhere. For closures that are opened and closed many times a day, a standard solid stopper holds its shape longer.

As with other rubber closures, never use it to seal a vessel that will be heated, and keep hydrocarbon or chlorinated solvents away from it. Where a tube must pass through, the same soft compound is available bored, as in the extra soft one hole stopper.

Handling Tips

Seat the stopper with a gentle twist; pressing hard is unnecessary and makes removal difficult.
Avoid leaving it in one neck for long periods if you want to reuse it on other glassware.
Wash in lukewarm soapy water and let it dry at room temperature, away from heaters.
Inspect for flattened or tacky patches before use, and set aside any stopper that no longer springs back.

Specifications

Type	Solid (unbored) rubber stopper
Compound	Extra soft, highly compressible rubber
Seating force	Low; seals with light hand pressure
Trade-offs	Grips firmly, sets faster, can be drawn in under vacuum
Sizes	Confirm at enquiry

Applications

Closing thin-walled test tubes that could crack under the force needed for a firm bung
Sealing older glassware whose necks are slightly irregular
Junior science activities where small hands need an easy-to-fit closure
Temporary closure of sample tubes in fieldwork kits

Why Choose LabEquip

Soft stoppers suit schools with mixed or older glassware and classes of younger pupils. LabEquip offers them in the General Lab Products range alongside standard-hardness types; mention the glassware and age group on the contact page.

Frequently Asked Questions

When should I choose an extra soft stopper over a standard one?

When the glass is thin-walled, the neck is slightly irregular, or the user cannot apply much force. For frequent opening and closing on well-made glassware, a standard-hardness stopper keeps its shape longer.

Why is the soft stopper hard to remove?

A softer compound presses into the glass more closely and grips it over a wider area. Twist it while pulling, and avoid pushing it in further than needed in the first place.

Can an extra soft stopper be used under vacuum?

It is not a good choice. Suction can pull a soft stopper deeper into the neck, and it may even be drawn inside. Firmer rubber or a purpose-made adapter holds its position better.

What is compression set?

It is the permanent flattening that rubber keeps after being squeezed for a long time. Soft compounds show it sooner, so a stopper left in one neck for weeks may no longer seal a different vessel.

Is an extra soft stopper more chemical resistant?

No. Softness describes how the rubber deforms, not what it resists. Treat it like other general-purpose rubber: fine with water, dilute acids and alkalis, but not with hydrocarbon or chlorinated solvents.

How should soft stoppers be stored?

Keep them clean and dry in a closed box, loose rather than packed tightly, away from sunlight, heat and electric motors, whose ozone hardens rubber over time.

Last Updated: September 2026