Rubber seals and profiles typically use walls of at least 0.030 in (0.8 mm), 15–30% squeeze for static seals and 10–20% for dynamic seals, gland fill under about 85%, and inside radii of at least 0.020 in (0.5 mm). Tolerances start at ISO 3302-1 class M2 for molded parts and E2 for extrusions, with durometer held to ±5 Shore A.
SSG MFG · design guideDG-04 · 12 rules
12 rules, in the order you meet them on a model. Every value is typical; the principal quoting your part confirms its own limits.
| # | Rule | |
|---|---|---|
| 01 | Choose the compound before the geometry70 Shore A for most static seals; 40–60 A for low-force sealing; 80–90 A to resist extrusion; call out by ASTM D2000 line | 70 Shore A for most static seals; 40–60 A for low-force sealing; 80–90 A to resist extrusion; call out by ASTM D2000 line |
| 02 | Keep walls uniformAbout 0.030 in (0.8 mm) minimum, with even thickness around the profile | About 0.030 in (0.8 mm) minimum, with even thickness around the profile |
| 03 | Support thin lips and bulbsLips and fins at least 0.030 in (0.8 mm) at the tip; hollow bulb walls at least 0.040 in (1.0 mm) on extrusions | Lips and fins at least 0.030 in (0.8 mm) at the tip; hollow bulb walls at least 0.040 in (1.0 mm) on extrusions |
| 04 | Size seal squeeze15–30% compression for static seals; 10–20% for dynamic seals | 15–30% compression for static seals; 10–20% for dynamic seals |
| 05 | Limit gland fillSeal volume no more than about 85% of groove volume at maximum material condition | Seal volume no more than about 85% of groove volume at maximum material condition |
| 06 | Limit O-ring stretch and use standard sizesInstalled ID stretch about 1–5%; pick AS568 dash sizes | Installed ID stretch about 1–5%; pick AS568 dash sizes |
| 07 | Radius cornersInside corner radius at least 0.020 in (0.5 mm) | Inside corner radius at least 0.020 in (0.5 mm) |
| 08 | Draft only where the compound needs itUsually none; about 1° on hard compounds over 80 Shore A and deep cavities | Usually none; about 1° on hard compounds over 80 Shore A and deep cavities |
| 09 | Use the rubber's stretch for undercutsStrip grommets and boots off undercut cores where the part can stretch over the core without tearing | Strip grommets and boots off undercut cores where the part can stretch over the core without tearing |
| 10 | Keep parting lines off sealing facesPlace flash and mismatch on non-functional surfaces; on O-rings, at the ID and OD rather than the sealing contact | Place flash and mismatch on non-functional surfaces; on O-rings, at the ID and OD rather than the sealing contact |
| 11 | Specify durometer tolerance±5 Shore A points | ±5 Shore A points |
| 12 | Choose the loosest workable tolerance classStart at ISO 3302-1 M2 for molded parts and E2 for extrusions; allow for 1.5–4% shrink | Start at ISO 3302-1 M2 for molded parts and E2 for extrusions; allow for 1.5–4% shrink |
These build on the short list on the rubber process page, with the failure each rule prevents and a drawing for each.
Change the seal cross-section and the gland depth to see squeeze and gland fill move together. Too little squeeze leaks, too much takes a permanent set, and a gland filled past about 85% has no room left when the rubber swells or heats.
W 0.139 IN · GLAND 0.105 × 0.190 IN · STATIC
Works: seals, with room to swell
24% squeeze and 76% fill: in the typical window for a static seal.
Typical design targets. Fill ignores ring stretch and tolerance stack; check the worst case at maximum ring and minimum gland with the seal maker's handbook.
Each rule with its typical value, the reason behind it and what happens on the floor when it's ignored. Switch any drawing between Do and Avoid; point at a callout to pick it out.
Rule 01
70 Shore A for most static seals; 40–60 A for low-force sealing; 80–90 A to resist extrusion; call out by ASTM D2000 line
Standard: ASTM D2000 / SAE J200
Rule 02
About 0.030 in (0.8 mm) minimum, with even thickness around the profile
Fig. 1Extrusion profile wall
Do. An even wall, at least 0.030 in (0.8 mm), leaves the die, swells and cures the same all the way around, so the profile holds its shape.
Avoid. Thick and thin sections swell and cure at different rates: the bulb pulls off-centre and the profile won't hold its drawn shape.
Rule 03
Lips and fins at least 0.030 in (0.8 mm) at the tip; hollow bulb walls at least 0.040 in (1.0 mm) on extrusions
Rule 04
15–30% compression for static seals; 10–20% for dynamic seals
Standard: AS568
Fig. 2Seal squeeze and gland fill
Do. Squeeze of 15–30% static (10–20% dynamic) seals without overstressing the rubber, and at most about 85% gland fill leaves room for swell and heat.
Avoid. Overfill the gland and the rubber has nowhere to go: it extrudes into the clearance gap and gets nibbled away.
Rule 05
Seal volume no more than about 85% of groove volume at maximum material condition
Rule 06
Installed ID stretch about 1–5%; pick AS568 dash sizes
Standard: AS568
Rule 07
Inside corner radius at least 0.020 in (0.5 mm)
Fig. 3Inside and outside radii
Do. Inside radius at least half the wall and outside radius equal to inside plus T: the corner stays at thickness T and flow turns smoothly.
Avoid. A sharp inside corner concentrates stress about 3× and the corner runs thick: cracks start there, and it cools last.
Rule 08
Usually none; about 1° on hard compounds over 80 Shore A and deep cavities
Fig. 4Draft angle
Do. Walls taper by the draft angle in the line of draw; each side moves in by H × tan θ, and the part slides off the core.
Avoid. With zero draft the part grips the core as it shrinks: drag marks, scuffed texture, and ejector pins that dent or punch through.
Rule 09
Strip grommets and boots off undercut cores where the part can stretch over the core without tearing
Fig. 5Undercuts and side actions
Do. Where an undercut is worth it, a slide pulls sideways before ejection. Better still, move the feature into the line of draw or use a pass-through core.
Avoid. A side hole with no side action locks the part in the steel; every slide or lifter added to fix it adds tool cost and cycle time.
Rule 10
Place flash and mismatch on non-functional surfaces; on O-rings, at the ID and OD rather than the sealing contact
Fig. 6Parting line
Do. A flat parting line at the largest section, off sealing and cosmetic faces, with critical dimensions inside one die half.
Avoid. A dimension across the parting line carries die-half mismatch and flash thickness, typically another ±0.004 in (±0.10 mm) or more.
Rule 11
±5 Shore A points
Rule 12
Start at ISO 3302-1 M2 for molded parts and E2 for extrusions; allow for 1.5–4% shrink
Standard: ISO 3302-1
Fig. 7Shrink and warp
Do. The cavity is cut oversize by the resin's shrink rate, and even walls and gating let the part shrink the same amount everywhere.
Avoid. Different shrink across and along the flow, or between thick and thin areas, bows the part out of flat.
Typical values for 60–80 Shore A compounds. Class M2 and E2 suit most seals. Tighter classes need more die work, slower lines and more inspection, and harder compounds hold size better than soft ones.
| Feature | Typical | Tight | Cost of tight |
|---|---|---|---|
| Molded, under 0.25 in (6.3 mm) | M2: ±0.004–0.006 in (±0.10–0.15 mm) | M1: ±0.003–0.004 in (±0.08–0.10 mm) | Moderate |
| Molded, 0.25–1 in (6.3–25 mm) | M2: ±0.008–0.014 in (±0.20–0.35 mm) | M1: ±0.005–0.008 in (±0.13–0.20 mm) | Moderate |
| Molded, 1–4 in (25–100 mm) | M2: ±0.014–0.020 in (±0.35–0.50 mm) | M1: ±0.008–0.012 in (±0.20–0.30 mm) | High |
| Extruded section, under 0.25 in (6.3 mm) | E2: ±0.010–0.020 in (±0.25–0.50 mm) | E1: ±0.006–0.010 in (±0.15–0.25 mm) | Moderate |
| Extrusion cut length, up to 12 in (300 mm) | ±0.060 in (±1.5 mm) | ±0.030 in (±0.8 mm) | Low |
| O-ring cross-sectionTolerance grows with cross-section. | ±0.003–0.006 in (±0.08–0.15 mm) | Set by AS568 size | Low |
| Hardness | ±5 Shore A | ±3 Shore A | Moderate |
Basis: ISO 3302-1 (M1–M4 molded, E1–E3 extruded); AS568 for O-rings. Put a general tolerance note on the drawing and tighten only the features that need it; every tight callout is priced. See the tolerance guide.
The same rubber rules shift with the alloy or compound.
EPDM handles weather, ozone and hot water to about 300 °F (150 °C) and is the default for extruded weatherstrip. It swells badly in petroleum oils and fuels, so gland fill has no room for it there.
Silicone runs from −75 to 450 °F (−60 to 230 °C) but has low tear strength, so give it larger radii and avoid thin lips in dynamic seals. It often needs an oven post-cure that adds a day to the schedule.
Nitrile is the default for oil and fuel seals and holds 70 Shore A O-rings to AS568 sizes cheaply. It degrades in ozone and sunlight, so keep it out of exposed weatherstrip.
FKM resists fuels and many chemicals to about 400 °F (205 °C) but costs 5–10× more per pound than EPDM. Its higher shrink and post-cure call for looser tolerance classes on small molded parts.
TPE molds on standard plastic injection presses with draft of about 0.5–1° and plastic-style walls. It takes more compression set than thermoset rubber, so use higher squeeze margins or a thermoset for long-term static seals.
Run these against the model before it goes out for quote. The builder's DFM check catches most of them on an uploaded part.
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Static O-rings typically run 15–30% squeeze, with face seals toward the high end and radial static seals near the middle. Dynamic seals typically run 10–20% to limit friction and wear. Check squeeze at both ends of the tolerance stack, after installed stretch, so the seal never drops below about 10% or climbs past 30%.
Gland fill is the seal's cross-sectional area as a percentage of the groove's area. Keep it at or below about 85% at maximum material condition. Rubber is nearly incompressible and swells with heat and fluid, so an overfilled groove forces the seal into the clearance gap, where it extrudes, nibbles or splits.
ISO 3302-1 is the international standard for dimensional tolerances on solid rubber products. It defines classes M1–M4 for molded parts and E1–E3 for extruded sections, among others, with tolerances that widen by size band. Most seals and gaskets work at M2 or E2; tighter classes cost more in tooling and inspection.
About 0.030 in (0.8 mm) is a practical minimum for most compounds, with hollow bulb walls usually at 0.040 in (1.0 mm) or more. Thin, unsupported legs sag or wander between the die and the cure line. Keeping walls even around the profile helps the extruder hold shape and cuts twist and bow.
ASTM D2000, also published as SAE J200, is a line callout that classifies a rubber compound by heat resistance, oil swell, hardness and tensile strength, plus suffix tests such as compression set. A callout like M2BG714 lets any qualified compounder quote the same material without a brand name.
Use a standard AS568 O-ring in a correctly sized gland wherever the geometry allows. O-rings are stocked in common compounds, cost very little and need no mold. A custom molded seal makes sense for non-circular paths, bonded lips or integrated features, and it adds a mold of about $2,000–$15,000 for compression tooling.
Check your part against these rules.
Drop a STEP file in the builder for an instant DFM check and a budgetary estimate, then send it to principals who run rubber every day.
