Silicone seal ring (VMQ O-ring) for static seals

01

Confirm the seal is static before choosing silicone

Polymax's datasheet for its FDA silicone O-ring compound states that silicone rubber has poor mechanical properties compared with other elastomers and that, for this reason, silicone O-rings are not the usual choice for dynamic applications. [1] Angst+Pfister (APSOparts) gives the reason as the high coefficient of friction and relatively low tensile strength, and offers its VMQ O-rings for static sealing in pumps, valves, dosing systems, electrical enclosures and beverage equipment. [2]

Barnwell describes the same trade-off: lower tensile strength and weaker tear and wear resistance than other elastomers, against cold flexibility and resistance to weathering, ozone and UV. [3] Polymax adds a fitting note: the compound "is not suitable for use with silicone O-ring lubrication", and PTFE alternatives should be used instead. [1]

Editorial note: if the joint rotates, reciprocates or is assembled by sliding the ring over threads or edges, raise it with the supplier before ordering; the sources above point to another elastomer or a different seal design for moving duty.

Give the supplier the inner diameter and cross-section (or AS/BS size number), groove dimensions, static or moving duty, medium, temperature range including cleaning, pressure, hardness, colour, approvals needed, moulded or joined construction, quantity and inspection requirements.

02

Check the temperature and media limits stated for the compound

Published operating ranges for silicone O-rings differ by supplier and compound: −60 to 200 °C for the Polymax FDA 70 Shore A grade, −55 to 200 °C for Angst+Pfister's VMQ (with higher or lower temperatures from special materials), and −60 to +250 °C from Barnwell. [1] [2] [3]

Water and steam set lower limits. Barnwell states silicone can endure water up to 100 °C and is ill-suited to superheated steam above 121 °C, acids and alkalis, some chlorinated hydrocarbons, mineral oil, hydrocarbon fuels and aromatic hydrocarbons; it lists good resistance to engine and transmission oil, animal and vegetable oil, non-petroleum brake fluids and fire-resistant hydraulic fluids. [3] Angst+Pfister lists resistance to heat, ozone, ageing, animal and vegetable fats and oils. [2]

Polymax's air-ageing result shows what heat does to its compound: after 70 hours at 225 °C (ASTM D573), hardness changed by +8 points, tensile strength by −25% and elongation by −30%. [1] Editorial note: if the ring will be steam-sterilised or cleaned in place, give the supplier the steam temperature and cycle count, since the Barnwell limit of 121 °C falls well below the dry-heat ratings.

03

Compare published silicone O-ring grades and constructions

04

Compound, cord size, moulded or joined construction and assembly lubricant

Polymax notes that silicone lubricant should not be used with this compound, so name the assembly lubricant in the order.

05

Hardness, FDA grade, temperature range and tensile values for a VMQ O-ring

Tensile strength and elongation are given only on the Polymax datasheet (7.1 MPa, 212%); dielectric strength and volume resistivity are not stated in the reviewed sources.

06

Send the size, duty and compound data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Size: [inner diameter; cross-section; AS/BS number; tolerances]
Groove: [dimensions or mating parts]
Duty: [static; pressure; assembly method]
Service: [medium; temperature range; steam or cleaning]
Compound: [hardness; colour; metal-detectable]
Documents: [compound datasheet; FDA or other approvals]
Construction: [moulded or joined]

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