FVMQ O-rings: fluorosilicone simply explained

What are FVMQ O-rings?

Silicone stands out from the other elastomer materials for seals through one central property: components made of this material retain their mechanical properties over a very wide temperature range. From the absolute low-temperature range at −60 °C up to the upper service limit at +200 °C, O-rings can provide reliable sealing. This makes silicone ideal for fields of application with strong temperature fluctuations.

FVMQ is one of the two rubbers of the so-called Q group. From a chemical point of view, this group is characterised by carrying silicone in the main chain. FVMQ is a fluorosilicone rubber, a comparatively recent further development of the silicone rubber VMQ.

The developers succeeded in retaining the already very good properties of VMQ and expanding the resistance profile to include fossil fuels. This is precisely the main field of application of FVMQ O-rings. Wherever a wide temperature spectrum and fuels or mineral oils meet as media, FVMQ is ideally suited.

The most important advantages of FVMQ O-rings

Owing to their mechanical properties, FVMQ O-rings, like VMQ O-rings, are suitable exclusively for static sealing. For that, however, their low compression set makes them ideal. Because even in components that are actually stationary, changes in positioning can occur, for example due to external influences, changes in the pressure conditions or vibrations.

The low compression set ensures that FVMQ O-rings relax again and return to their original shape even after prolonged compressive load. This is the most important prerequisite for reliably covering a changed sealing gap. Hardly any other sealing material shows this advantageous behaviour over such a wide temperature range.

In addition, there is the pronounced chemical resistance. Compared with the base material VMQ, FVMQ is additionally durable in contact with oils, fuels and solvents. Together with the very good ozone and weathering resistance, this permits use in extremely demanding applications such as fuel systems in aerospace or automotive engineering.

The most common fields of application of FVMQ O-rings

Automotive sector

Fuel-carrying systems such as lines, the tank system or the injection system are among the particularly critical applications for O-rings. Modern engine technology places high demands on the tightness of the fuel system, while high pressures prevail at the same time.

New types of fuel, which increasingly come from renewable raw materials, also change the requirements for media resistance. Here FVMQ O-rings are used above all for sealing lines and couplings. They are equally applicable to regular fuel, premium fuel and diesel. Compared with the alternative materials NBR and FKM, FVMQ has the advantage of better low-temperature flexibility.

Aerospace

The technical systems in aerospace naturally place particularly high demands on the temperature range of sealing elements. Under the operating conditions, high heat but also very low temperatures can prevail in alternation.

O-rings made of FVMQ are used very frequently in this area. They owe their good suitability above all to their very good low-temperature flexibility and their resistance to many common media.

Oil and gas industry

The sealing applications in the oil and gas industry are also characterised by particularly high requirements. Thanks to their insensitivity in contact with weathering influences and their resistance to mineral-oil-based products, FVMQ O-rings enjoy a very high standing here.

What is the media resistance of FVMQ O-rings?

FVMQ is a fluorosilicone rubber that is usually peroxide cross-linked. The fluorination adds resistance to oils, fuels and solvents to the resistance profile of silicone rubber. In combination with the exceptional low-temperature flexibility, this is precisely what opens up the most important fields of application for FVMQ O-rings.

Compared with VMQ, FVMQ loses resistance above all in contact with hot air. In many applications, however, the gains in media resistance in the area of mineral-oil-based media are the decisive factor for its use. In the past, VMQ was replaced by NBR or FKM when the application involved contact with petrol or oil, at the expense of low-temperature flexibility.

FVMQ now closes this gap. The O-rings combine the silicone-typical low-temperature resistance with the corresponding chemical stability. The decisive factor here is the swelling in media contact, which should be as low as possible. In this respect, FVMQ is on a par with HNBR and NBR. In terms of fuel resistance alone, however, the market offers even higher-performing alternatives with O-rings made of FFKM, FKM or ACM.

What is the temperature resistance of FVMQ O-rings?

FVMQ O-rings have a temperature resistance from −60 °C to +200 °C. However, this figure applies primarily to the surrounding medium air. If the O-rings come into contact with other media during operation, the temperature resistance can be correspondingly lower depending on compatibility.

For example, the medium-specific temperature limit of FVMQ in contact with water is 100 °C. With mineral-oil-based products, however, FVMQ also reaches the upper temperature limit of 200 °C. This gives the material FVMQ a temperature range comparable to EPDM. VMQ, however, permanently withstands hot air at even higher heat of up to 210 °C. Here FVMQ loses out somewhat by comparison.

What are the mechanical properties of FVMQ O-rings?

The mechanical properties of FVMQ are very similar to those of VMQ. Therefore, the dynamic use of FVMQ O-rings must be strictly avoided. The low abrasion resistance leads to rapid wear of the material. The friction between the sealing surfaces causes abrasion, which can contaminate media.

In addition, there is the rapid loss of the sealing effect, because sealing gaps can no longer be adequately covered at their widest point. Apart from that, the mechanical strength of FVMQ O-rings is moderate to medium. The compressive strength is a limiting factor. Under high pressure, O-rings made of FVMQ are easily pressed into the sealing gap and permanently damaged.

One positive point, however, is the low compression set, which the material maintains over an extremely wide temperature range.

What do I need to consider for the groove design of an FVMQ O-ring?

Groove design of FVMQ O-rings

1. Design the groove in accordance with ISO 3601.
2. Reduce the sealing gap by 50 % compared with other elastomers.
3. Ensure a minimum compression of the cross-section of 6 %.
4. Secure the O-ring against excessive pressure with a back-up ring.
5. Choose the manufacturing process with particular focus on the tolerances.

FVMQ and VMQ are poorly suited to pressure applications. Designers should therefore choose a smaller sealing gap than for the other common elastomers.

Installation of FVMQ O-rings

1. Provide lead-in chamfers between 15 and 20 degrees.
2. Deburr bores, even if they are only passed over during installation.
3. Round off sharp edges with a minimum radius of 0.1 to 0.3 mm.
4. Use suitable installation aids such as mounting cones to avoid overstretching.
5. Avoid twisting the O-ring.

Important: FVMQ is not resistant to silicone oils. Therefore, no silicone-based greases or oils should be used during installation, in order to avoid media attack.

Frequently asked questions