Silicone VMQ Back-up Ring – Anti-Extrusion Support Ring for O-Rings

Silicone back-up ring, also known as VMQ back-up ring, silicone anti-extrusion ring, is used together with O-rings to prevent extrusion. It does not provide sealing function itself, only serves as support. It is not the mainstream back-up ring for high-pressure hydraulics, mainly for static applications requiring wide temperature range, ozone/UV resistance, food or medical compliance.

1. Function

The back-up ring is installed on the low-pressure side of the O-ring to fill the clearance between mating parts. It protects the O-ring from being squeezed into the gap, damaged or torn under pressure.

• Single-side pressure: one back-up ring on the low-pressure side
• Bidirectional pressure: one back-up ring on each side of the O-ring

Note: Filled PTFE back-up rings are the most common choice for high-pressure hydraulic service. Silicone back-up rings are elastomer type with obvious application limits.

2. Material: VMQ Silicone

Typical working temperature range: -60°C ~ +200°C (special formulations up to 220°C)

Advantages

• Outstanding ozone, UV and weathering resistance, ideal for outdoor and solar applications
• Excellent low-temperature flexibility, no brittleness at low temperature
• Non-toxic, available in FDA food grade and medical LSR grade
• Resistant to hot water, steam, air, weak acid, silicone oil and ethylene glycol

Limitations (critical for customer selection)

• Poor tear resistance and abrasion resistance, low mechanical strength
• Not suitable for dynamic reciprocating or rotary motion, nor mineral oil hydraulic systems
• Swells and fails when exposed to mineral hydraulic oil, gasoline and hydrocarbon fluids
• Limited pressure rating, only recommended for low-pressure static sealing

3. Common Styles

1. Solid / endless: closed ring, preferred for static flange with stable clearance
2. Straight cut: easy to install over threads or shaft shoulders
3. Spiral / scarfed cut: less likely to open and shift after assembly

Size system follows AS568 O-ring groove standard and metric sizes. Common thickness: 1.25 / 1.5 / 1.78 / 2.0 mm.

4. Typical Applications

✅ Suitable

• High-temperature static flanges, ovens, steam piping, heating circuits
• Outdoor equipment, solar thermal collectors
• Food processing and pharmaceutical equipment (FDA VMQ grade)
• Cryogenic equipment, cold boxes, static seals for dry air/gas ❌ Not recommended
• Construction machinery cylinders, hydraulic motors, mineral oil hydraulic circuits, high-pressure dynamic sealing
• Sealing positions with friction and reciprocating movement

5. Material Comparison Table

6. Standards & Certifications

• Groove standards: AS568, ISO 3601
• Common certifications: FDA 21 CFR 177.2600, 3A, USP Class VI for medical grade
• Common hardness: 60 ~ 70 Shore A. Too soft cannot support anti-extrusion; too hard is difficult for installation.

7. FAQ

Q: Can silicone back-up ring match silicone O-ring?

A: Yes. VMQ O-ring plus VMQ back-up ring works for static high-temperature air or steam, but pressure must remain low.

Q: Why is silicone back-up ring less popular than PTFE type?

A: Silicone is a soft elastomer. It will deform and extrude itself under high pressure, so its supporting capacity is much weaker than rigid filled PTFE. That is why it is rarely used in hydraulic systems. Many customers confuse silicone O-rings with silicone back-up rings.