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PTFE Envelope Gaskets for Corrosive and Thermally Cycled Flange Service
Gansu Juxiang Fluoroplastics Technology Co.,Ltd. is a PTFE Envelope Gasket manufacturer producing machined envelope gaskets from virgin PTFE over compressible cores. The JUXIANG 3510, 3520 and 3530 series cover V-profile, U-profile and flat machined constructions, supplied as finished sealing components for chemical, petrochemical, pharmaceutical, food-processing and water-treatment pipework, reactors and storage vessels.
We operate two production bases, in Zhejiang and Gansu, as a joint venture invested by Zhejiang Jiaxiang Fluoroplastics Co., Ltd. and Gansu Juhua New Materials Co., Ltd. of the Juhua Group. Envelope gaskets sit inside a wider fluoroplastic programme that also covers PTFE sheet, rod and tube, machined seals, PFA and FEP extrusions, PCTFE, PEEK and PI parts, so one package can carry several sealing duties on the same drawing set.
The JUXIANG Envelope Gasket Range
Every construction uses a machined PTFE envelope that keeps the process medium away from the core, while the core supplies the compressibility and recovery that solid PTFE lacks. Profile choice follows the flange type, the medium and the thickness of the joint being sealed.
| Series | Envelope profile | Construction | Application fit |
|---|---|---|---|
| JUXIANG 3510 | V-shaped, machined | Virgin PTFE envelope with non-asbestos or specified filler core | General chemical service where the medium attacks the core but is compatible with PTFE; not intended for thick layers of corrosive fluid |
| JUXIANG 3520 | U-shaped, machined | Virgin PTFE envelope with non-asbestos or specified filler core | Large-diameter gaskets and toxic chemical service, with the envelope carried around both bore and outside edge |
| JUXIANG 3530 | Flat machined profile | Virgin PTFE envelope with non-asbestos or specified filler core | Assemblies with thick layers and corrosive fluids, where the profile is matched to the groove or facing |
Wall thickness, slot width and outside diameter are machined to the flange drawing rather than taken from a fixed catalogue, which lets the same series serve a DN25 instrument connection and a multi-metre vessel opening.
Related sealing components we supply
- Modified PTFE gasket for joints that need higher bolt-load retention and lower creep than a standard envelope construction.
- Expanded PTFE reinforced flange gasket where a fully PTFE sealing face is required across a wide flange face.
- Large integrated molded PTFE sealing frame and gasket for oversized openings and non-circular sealing geometry.
Where Envelope Construction Fits
The envelope carries the chemical duty; the core carries the mechanical duty. PTFE on both sealing faces resists acids, alkalis, solvents and oxidising media, while the compressible core absorbs flange rotation, thermal cycling and surface imperfection that a solid PTFE ring cannot follow without creeping.
- Glass-lined, FRP and plastic-lined flanges where available bolt load is limited and a soft, conformable gasket is essential.
- Chemical storage tanks and tank-mouth connections where gasket change-out is expensive and creep resistance matters more than raw pressure capability.
- Large-diameter pipework and vessel openings where the 3520 U-profile keeps a continuous PTFE barrier around the full sealing perimeter.
- Thick joint assemblies and corrosive fluid layers where the 3530 profile is machined to fill the groove without over-compressing the core.
Envelope gaskets are not a universal replacement for metal or spiral-wound gaskets in high-pressure, high-temperature service, and vacuum duty needs review because permeation through the PTFE skin can occur. We confirm the working envelope against the actual medium, temperature and pressure before a construction is released to production.
Standard Sizes and Machined Configurations
The standard size set below covers common DN and NPS flange bores. Slot width is selected from 1.02 mm, 1.6 mm, 4.8 mm and 6.4 mm according to the flange facing and the core thickness required.
| Outside diameter | Inside diameter | Slot width options |
|---|---|---|
| 48 mm (1-7/8 in) | 12.7 mm (1/2 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 57 mm (2-1/4 in) | 19 mm (3/4 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 67 mm (2-5/8 in) | 25.4 mm (1 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 86 mm (3-3/8 in) | 38 mm (1-1/2 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 105 mm (4-1/8 in) | 51 mm (2 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 136.5 mm (5-3/8 in) | 76 mm (3 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 175 mm (6-7/8 in) | 102 mm (4 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 222 mm (8-3/4 in) | 152 mm (6 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
| 279 mm (11 in) | 203 mm (8 in) | 1.02 / 1.6 / 4.8 / 6.4 mm |
Outside the table, we machine to drawing or sample: raised-face and full-face patterns, tongue-and-groove profiles, oval and non-circular handholes, and diameters beyond the standard set. Drawings are returned with the machined dimensions marked so the customer can confirm the bore, bolt circle and facing before the batch is cut.
Envelope and Core Material Selection
Virgin PTFE is the standard envelope grade. Where the joint sees higher bolt load, tighter compression or a stronger thermal cycle, a modified PTFE envelope gives lower creep and better recovery under sustained load. Both are machined from fully sintered stock, so the sealing face is homogeneous and free of the laminations that can open up in skived-and-wrapped constructions.
- Core: non-asbestos compressed fibre as the standard filler, with other filler types matched to the media list when the process fluid demands it.
- Core thickness: set against the slot width so the envelope closes on the flange faces before the core is over-compressed.
- Media review: the full fluid list, including cleaning and flushing agents, is checked before a core grade is confirmed.
- Envelope integrity: the PTFE skin is inspected for voids, folds and edge damage, since a breach turns the gasket into a core-limited seal.
Machining, Inspection and Documentation
Envelopes are CNC machined from sintered PTFE stock and assembled with the core in a controlled sequence, then checked for outside diameter, inside diameter, slot width, overall thickness and flatness. Batch identity is marked on each gasket so a replacement can be traced to the same material and machining record.
Deliveries are accompanied by material and dimensional records and by installation and storage guidance covering gasket centring, bolt tightening sequence, re-torque practice and protection of the PTFE edge during handling. Gaskets are packed flat, bagged and cartoned so the envelope does not take a set in transit, with labelling matched to the purchase order line items.
Information We Use to Configure Your Solution
A proposal is built from the working point rather than from a size alone. The inputs below are what our engineering team needs to fix the series, envelope grade, core and machining dimensions.
| Project input | Detail we need | How we use it |
|---|---|---|
| Process medium | Fluid list with concentrations, plus cleaning and flushing agents | Confirms PTFE envelope grade and core compatibility |
| Service conditions | Operating and design temperature, pressure, cycling pattern | Sets profile, core density and slot width |
| Flange specification | Standard, DN or NPS, pressure class, facing type, groove dimensions | Drives OD, ID and slot machining |
| Flange material and finish | Steel, glass-lined, FRP or plastic; surface roughness | Limits compression and defines handling advice |
| Bolting | Bolt size, grade and tightening method | Balances gasket thickness against available bolt load |
| Quantities and schedule | First order, commissioning spares, repeat batch forecast | Plans machining batches across the two bases |
| Documentation and packing | Marking, records, inspection hold points, export packing | Sets the inspection plan and delivery scope |
Frequently Asked Questions
What separates the 3510, 3520 and 3530 envelope gaskets?
The 3510 is a V-shaped machined envelope for general chemical service where the medium is aggressive to the core but compatible with PTFE. The 3520 is a U-shaped envelope built for large diameters and toxic chemical duty, where the PTFE skin wraps the bore and outside edge. The 3530 is a flat machined profile for thick-layer assemblies and corrosive fluids, matched to the groove or facing geometry.
Can you machine envelope gaskets outside the standard size table?
Yes. We machine to drawing or to a physical sample, including raised-face, full-face, tongue-and-groove and non-circular patterns. Send the flange standard, bore, bolt circle, facing detail and required slot width, and the machined dimensions are confirmed before production.
Which core material do you supply inside the PTFE envelope?
Non-asbestos compressed fibre is the standard core, with other filler types selected when the media list calls for them. The core grade is confirmed against the process fluid, temperature and pressure, and vacuum service is reviewed separately because permeation through the PTFE skin can limit performance.
What do you need to prepare a quotation?
Media list, operating temperature and pressure, flange drawing or standard with facing type, flange material, bolting details, quantity and required documentation. With those inputs we return the series, envelope grade, core, machined dimensions and delivery scope as a single configuration for approval.
