ASTM D1418 designates the methyl-vinyl silicone family as VMQ, where V signals the vinyl substituent and MQ identifies a silicone (Q) polymer with methyl (M) groups; the letter code is the baseline filter every specifier should apply before comparing mechanical data [S2][S3].
High consistency silicone rubber (HCR, also called HTV) is the gumstock branch of VMQ supplied as a millable base, used in compression molding, transfer molding, and extrusion, with the alternative liquid branch (LSR) and room-temperature (RTV) branch covered by the same D1418 letter code [S4][S9].
How ASTM D1418 Decodes VMQ
ASTM D1418 is a nomenclature standard, not a property standard; it maps letters onto polymer chemistry so a specifier can read the rubber type from the code without reading the formulation [S2][S3]. For VMQ specifically, M denotes methyl substituents on the siloxane backbone, Q marks the material class as silicone, and V denotes the small vinyl fraction (typically 0.05 to 0.5 mol%) that gives the gum enough unsaturation for peroxide crosslinking [S2][S7]. A phenyl-substituted variant carries the additional letter P (PVMQ) for low-temperature flexibility, and fluorosilicone is FVMQ; both remain under the D1418 family even though their property profiles diverge sharply from base VMQ [S4].
The same D1418 letter can cover radically different processing forms, which is where specifiers get burned: VMQ includes HCR gumstock, LSR two-part pumpable systems, and RTV condensation-cure pastes, all of which the letter code does not distinguish [S4][S7]. When the datasheet says only "VMQ," the specifier still has to ask whether the form is HCR, LSR, or RTV because the same chemistry behaves very differently in compression molding vs. liquid injection molding.
Material Properties: What the VMQ Datasheet Actually Says
Compounded VMQ (HCR) compounds are commonly published with Shore A hardness from 30 to 90, tensile strength 200 to 1500 psi, and maximum elongation around 700%, with low-temperature flexibility near -60°F and continuous high-temperature service around 450°F, with some specialty compounds reaching 600°F [S5]. The same source flags fair-to-poor abrasion resistance, poor tear resistance, and poor solvent and oil resistance, alongside good compression set and resilience, which is the classic trade-off profile for the family [S5].
For high-consistency VMQ specifically, the polymer chains are long enough to give a degree of polymerization (DP) in the 5,000 to 10,000 range, which is roughly an order of magnitude higher than LSR gums and is the reason HCR behaves as a millable, putty-like gum rather than a pumpable liquid [S10]. That long-chain gum is then crosslinked with organic peroxides, with di-(2,4-dichlorobenzoyl) peroxide (DCIBP) and di-(4-methylbenzoyl) peroxide (DMB) typical for pressureless extrusion, and dibenzoyl peroxide or dicumyl peroxide typical for press-cure molding [S1]. The peroxide choice is not cosmetic: it changes the cure by-products, the compression-set rating, and whether the compound can be processed in a hot-air or infrared continuous vulcanization line [S1].
HCR vs. LSR vs. RTV: Same D1418 Code, Different Processing
HCR/HTV VMQ is the millable gum used in compression molding, transfer molding, and extrusion, with extrusion accounting for the largest volume share of silicone rubber processing and relying on hot-air or infrared continuous vulcanization lines [S1][S9]. LSR VMQ is a two-part pumpable system designed for liquid injection molding (LIM) with very short cycle times and no operator handling of the uncured material, which is why medical-grade silicone seals are commonly LIM-LSR rather than HCR [S4][S5]. RTV VMQ covers room-temperature-vulcanizing pastes used in potting, encapsulation, and field-applied seals, where the cure is driven by moisture or a catalyst rather than heat [S5].
On a four-criterion comparison, the three branches line up roughly as: tensile/tear strength: HCR is highest, LSR is high, RTV is lowest; cycle time: HCR is minutes (press cure), LSR is seconds (LIM), RTV is hours; automation fit: HCR is moderate (hand-loaded or extruded), LSR is high (fully automated LIM), RTV is low (manual dispense); and biocompatibility for medical seals: HCR is available in medical grades, LSR is the default for medical seals because of the closed two-part system, and RTV is generally not used for finished medical seals [S4][S5]. The selection question therefore is not "VMQ yes/no" but "which branch of VMQ matches the molding process."
Selection Criteria: When VMQ HCR Is and Is Not the Right Pick
VMQ HCR is the right call when the spec needs a wide temperature window (roughly -60°F to 450°F continuous, with compounds rated up to 600°F), good weathering/ozone/UV resistance, low compression set, and a molding process based on compression, transfer, or extrusion rather than liquid injection [S5][S4]. It is also the right call for extruded profiles (tubing, wire and cable insulation, hoses) where extrusion is the dominant silicone process by volume and HCR is the only VMQ form that extrudes cleanly without a carrier [S1][S5].
VMQ HCR is the wrong call when the seal faces dynamic abrasion or tearing load (tear and abrasion resistance are poor), concentrated solvents, oils, or strong acids and bases, or applications that need gas-tight permeability (gas permeability resistance is low) [S5]. In those cases, the specifier should step sideways to FVMQ for fuel and oil resistance, to EPDM for hot water/steam and caustic service, or to nitrile rubber (NBR) for petroleum-fluid dynamic seals, none of which are substitutes for VMQ's temperature window but all of which outperform VMQ in the property it is weakest at.
Standards, Testing, and Adjacent Specifications
D1418 only names the polymer; the property numbers on a datasheet are generated against separate ASTM test methods, and the specifier who confuses the two will read a Shore A number that was measured to D2240 as if it were a material class [S3]. Hardness is typically D2240 (Durometer A, 0-100 scale), tensile and elongation are D412, compression set is D395, abrasion is D2228, and density/specific gravity is reported against D297 or equivalent [S3]. For high-temperature ratings, the relevant property tests are D573 (heat aging) and D2000 (the SAE/ASTM classification system for rubber products), with D2000 line-callouts such as "FC, FE, GE" being how VMQ is actually ordered on a print rather than "VMQ" alone.
For a broader material-class reference, silicone rubber covers the same VMQ family from the property-and-application side, while industrial rubber gives the cross-family comparison that puts VMQ next to EPDM, NBR, FKM, and natural rubber on temperature, oil, and weathering axes. When the print also requires a specific elastomer class, the D1418 letter (VMQ, FVMQ, PVMQ) is the primary key, the D2000 line-callout is the property key, and the test methods (D412, D2240, D395) are the verification key; all three need to align before a compound is released.
Trackable Signals for Spec Updates
Material datasheets for HCR/HTV VMQ continue to publish the same 30-90 Shore A / 200-1500 psi / 700% elongation / -60°F to 450°F envelope [S5], so the practical spec range is stable; the action item for engineers is to confirm the D2000 callout and the test-method set, not to chase new property numbers.
See also our earlier report, Arc vs TIG welding for pipe root pass: process selection, prep geometry, and code fit.