Key Takeaways
- A custom PP duct is a polypropylene ventilation section made to order — diameter, wall thickness, section length, material grade, color and connection are confirmed per project instead of taken from a catalog.
- Custom covers size, material, color and connection—not material limits or system responsibility. Operating temperature, chemical load and static pressure still determine wall thickness and reinforcement.
- Tolerances are specified, not assumed. Put an ISO 2768-1 general tolerance class or explicit ± values on the drawing and into the inquiry, and both sides measure against the same target.
- An OEM custom order runs through visible checkpoints — duct schedule + drawings → section-by-section confirmation → drawing approval → manufacture → MOQ and lead-time confirmation.
- Use the 10-point RFQ checklist in the last section — to send a complete custom duct inquiry in one pass, so the first quote is close to the final order.
When your duct schedule calls for a diameter that sits between catalog sizes, a wall thickness the standard range does not list, or a connection detail that must match existing plant pipework, a standard PP duct will not close the gap. A custom PP duct closes it—but only when the specification is complete. The most common mistake is treating “custom” as “just give us your size”: without a stated tolerance class, a confirmed section length, and the operating data behind wall thickness, the delivered duct can miss the design intent and the correction cycle costs weeks. This guide covers what you can actually customize, when custom is the right call, how the OEM order process works, how to express tolerances, and what to send with the inquiry.
What Can Be Customized on a PP Duct?
A custom PP duct starts from the same material as a catalog section—polypropylene sheet, welded into a round or rectangular section—but every dimension and detail is fixed per project. Six parameters are open to specification, and two are not.
| Parameter | Available options | How it is confirmed |
|---|---|---|
| Diameter | Any non-standard size; XICHENG’s standard round range is φ20–500 mm, custom sizes extend it per drawing | Against your duct schedule or drawings |
| Wall thickness | Per project, based on duty and diameter | Confirmed with static pressure and gas data |
| Section length | 3 m standard, or project-confirmed | Per duct schedule |
| Material grade | Standard PP or flame-retardant PPs | Specify grade where fire-rated ductwork is required |
| Color | Beige or grey | Per project |
| Connection | Collar (socket) or flange; hot-air welding for sealed runs | Per joint, to match the rest of the run |
Diameter, Wall Thickness and Section Length
Diameter is the first custom decision because it is the dimension that most often falls outside a catalog. XICHENG’s standard round duct range covers φ20–500 mm; a custom order extends that range with dimensions confirmed against your drawings, and the full product line covers φ20–600 mm overall. Wall thickness is rarely a catalog item at all—it is an engineering output that depends on operating pressure, temperature and diameter, which is why a custom order asks for duty data before fixing the wall. Section length follows the duct schedule: 3 m sections are the standard shipping unit, but a run can specify shorter or longer sections where layout, handling or joint positions require it.
Material Grade: Standard PP vs Flame Retardant PP
Custom orders also decide the polymer grade. Standard polypropylene handles the corrosive fume duty most ventilation runs are built for, with chemical resistance across pH 1-14 and a working temperature range of -15°C to 80°C. Where fire-rated ductwork is required, a flame-retardant PPs grade is available—the grade itself must be specified, because the flame-retardant masterbatch changes the material’s burning behavior, and a building authority will ask which rating the duct is certified to. This is a material selection, not a marketing option: state the grade in the inquiry if the project requires it.
Color and Connection Details
Color (beige or grey) and connection type (collar or flange) are project-level details confirmed per joint. They look minor, but they determine whether the custom section mates with the existing run without field adapters. A retrofit run that mixes old and new sections should specify the connection on every joint, not assume one default.
What is not customizable: the material’s physical limits and the system’s responsibility boundary. Working temperature stays within -15°C to 80°C, chemical resistance within pH 1-14, and a duct section does not replace the fan, damper or gas treatment equipment that the system design assigns to it. After this module, you can list the six customizable parameters and the two fixed boundaries for your project.
Custom PP Duct vs Standard Catalog Duct: When to Order Custom
Standard catalog ducts exist for a reason: fixed tooling, repeatable production, shorter lead times and lower unit cost. Custom is not a quality upgrade—it is a fit decision. Use the catalog when a standard diameter and wall thickness match the schedule; order custom when the fit fails. The volume math also matters: a project with two or three non-standard sections pays a custom setup cost that a large run absorbs easily, while a run of twenty or more sections spreads that cost thin. Treat the first question as a fit decision and the second as a quantity decision.
When a Catalog Size Is Enough
If your run uses diameters inside the standard range (φ20–500 mm for XICHENG round duct), a standard wall thickness, and 3 m sections, the catalog section is the faster and cheaper choice. Standard sections are produced continuously, which keeps MOQ low and delivery predictable. The sealing performance of a standard collar or flange joint is identical to a custom joint because both are welded the same way. Choose catalog when every line in your duct schedule maps to a standard item.
When Non-Standard Sizes Justify Custom
Custom becomes the right call when the schedule contains a non-standard diameter, a wall thickness no standard item lists, or a section length that does not divide the run evenly. Three situations recur: a required diameter that sits between catalog steps (for example φ450 mm between φ400 mm and φ500 mm), a wall thickness dictated by higher static pressure than the standard range covers, and existing plant pipework that must be matched exactly rather than adapted. In these cases the custom section is the only way to keep the whole run on one material and one joint system.
Catalog ranges are also manufacturer-specific: one fabricator’s standard round range may cover 63 mm to 1200 mm while another’s stops at φ500 mm, so “standard” is not a universal constant—it is a line your supplier draws. What matters is whether your schedule fits the range you are being quoted against, not whether the range is large in absolute terms. Compare your duct schedule against the quoted catalog before deciding; a single non-standard diameter in an otherwise standard run can sometimes be solved with a reducer and a catalog section, which is cheaper than custom fabrication for one piece.
Retrofit Runs and Drawing-Based Projects (OEM)
Two more cases justify custom regardless of size: retrofit runs and OEM projects. A retrofit must match duct sizes already installed years earlier, and no catalog guarantees that match. An OEM project—equipment manufacturers who integrate ducting into a machine or packaged system—usually works from their own drawings, where every dimension is fixed by the equipment envelope. Both cases share one requirement: the replacement or integration geometry is defined on paper, and the supplier builds to that paper. If your project has a drawing with fixed dimensions, custom fabrication is the standard path, and the OEM process in the next section applies directly. After reading this module, you can decide whether your run calls for catalog or custom sections.
How a Custom PP Duct OEM Order Works
A custom order is a small engineering project, and its process is designed to remove guesswork before steel—or in this case PP sheet—is cut. The sequence below is the one XICHENG uses, and it matches how industrial ductwork fabricators run drawing-based orders generally.
| Step | Buyer action | Supplier action | Confirmation point |
|---|---|---|---|
| 1 | Send duct schedule + drawings + application data | Review and check against standard range | Scope of custom work |
| 2 | Confirm diameter, wall thickness, length, material, color, connection per section | Issue section-by-section specification | Text specification |
| 3 | Approve drawings / specification | Freeze dimensions and tolerances | Drawing approval |
| 4 | Confirm material grade (PP or flame-retardant PPs) | Manufacture: PP sheet welding, flange reinforcement for large diameters | Material and fabrication record |
| 5 | (Optional) arrange inspection | Quality check on dimensions and welds | Inspection report |
| 6 | Confirm MOQ and lead time | Commit to quantity and schedule | Commercial confirmation |
| 7 | Receive and verify sections | Ship with packing per export requirements | Goods receipt |
Send Duct Schedule and Drawings with the Inquiry
The process starts with information, not a quote. The duct schedule names every section: diameter, length, quantity and connection. Drawings add the geometry that a schedule cannot express—offsets, transitions and positions relative to equipment. Suppliers price custom work from this package; without it, the quote contains assumptions that become change orders later. Sending the schedule and drawings with the first inquiry is the single highest-impact step in the whole process.
Section-by-Section Confirmation Before Manufacture
After review, each section is confirmed individually: approved dimensions, wall thickness and connection detail. This step exists because custom runs rarely repeat a single size—a typical exhaust run mixes straight sections, reducing diameters and flange transitions, and each one needs its own confirmation. The buyer’s task is to verify that the confirmed specification matches the design intent section by section, because approval here freezes what gets manufactured.
MOQ, Lead Time and Drawing Approval
Two commercial parameters and one quality parameter close the loop. MOQ and lead time are confirmed per project: custom fabrication is batch work, so minimums and delivery dates depend on total quantity, section count and current production load—estimate them from the supplier’s commitment rather than from standard product lead times. Drawing approval is the quality checkpoint: the buyer signs off the frozen specification before fabrication starts, which anchors every later claim about “what we agreed.” After this module, you can recognize the six checkpoints of a custom order and name the one where your written approval matters most.
Custom Duct Sizes: Diameter, Length and Wall Thickness
Sizes are where custom orders are won or lost, because a dimension error cannot be corrected on site. Specify three numbers per section: diameter, section length and wall thickness—each with its own confirmation rule. The same three numbers also tell the fabricator how the section ships and handles, so they are the first thing a quotation is built on.
Diameter Range and Non-Standard Sizes
The standard XICHENG round duct range covers φ20–500 mm in diameter; custom sections extend beyond that per drawing, and the overall product line spans φ20–600 mm. A non-standard diameter is the most common reason to order custom, so state it exactly: φ450 mm, not “about 18 inches” (450 mm ≈ 17.7 in). Give the dimension, the unit and whether it is inside or outside diameter; HVAC convention typically treats duct diameter as the inside clearance dimension, so say which one your schedule uses. For rectangular duct, the convention is long side × short side in mm (for example 500 × 320 mm), and the same rule applies: state both faces and which is the clearance dimension.
Section Length: 3 m Standard or Project-Confirmed
Sections ship at 3 m as standard, with project-confirmed lengths available. Length selection balances three constraints: transport limits, on-site handling weight, and joint count. Longer sections reduce the number of collar or flange joints—fewer joints mean fewer leak points—but they are heavier to lift into position and harder to get through doorways and shafts. State the section length in the schedule; if the run has no reason to deviate, 3 m is the sensible default. For runs through confined shafts, shorter sections (for example 1.5 m or 2 m) are a legitimate custom choice even when the diameter is standard, because handling constraint outweighs joint count.
Wall Thickness Depends on Duty, Diameter and Static Pressure
Wall thickness is the dimension that most buyers under-specify, because it is an output, not an input. It depends on operating temperature (within -15°C to 80°C), chemical load, diameter and the static pressure the duct must withstand. A large-diameter section under higher static pressure gets a heavier wall and, in fabrication, flange reinforcement welded to the body to prevent deformation. Industry guidance on duct system pressure classes gives the pattern: low-pressure systems (up to about 500 Pa) can use thinner panels, while high-pressure systems (above roughly 1500 Pa) need thicker panels and reinforcement measures. Your static pressure number is the single most useful input for the wall-thickness conversation, which is why the inquiry asks for it. Use this module to assemble the three size numbers per section and the duty data that fixes wall thickness.
Duct Tolerances: How to Specify Them
Tolerance is the most under-specified item in custom duct orders, and the least contested once written down. The rule is simple: state the tolerance class or the ± value on the drawing and in the inquiry, and both sides measure against the same target. Three reference layers are available.
General Tolerances: ISO 2768-1 and Drawing Notes
When a drawing does not carry individual tolerance callouts, general tolerances apply. ISO 2768-1 (Geometrical product specifications—Dimensional tolerancing—Tolerance limits for general specification of linear and angular sizes) provides four classes—fine, medium, coarse, very coarse—that cover un-dimensioned features. A drawing note such as “general tolerances ISO 2768-m” (medium) or “ISO 2768-c” (coarse) gives every dimension a measurable limit without cluttering the drawing. For ductwork, the class selection reflects the couple: structural duct dimensions tolerate medium-to-coarse grades, while flange and connection faces where sealing depends on the fit warrant a tighter explicit callout.
Industry Fabrication References: SMACNA Thermoplastic Manual
Fabrication-side references give the shop its own benchmarks. SMACNA’s Thermoplastic Duct (PVC) Construction Manual covers polypropylene, polyethylene and other thermoplastics, and is the standard fabrication reference U.S. fabricators use for thermoplastic ductwork construction. Its guidance is construction-oriented: wall thickness by diameter, reinforcement spacing and connection practice—not a single universal ± value, because tolerance follows size. For reference, metal ductwork tables (SMACNA rectangular standards) show the pattern tolerances follow: 0-4 mm for side lengths up to 1000 mm, 0-5 mm for 1001-4000 mm—in other words, the permitted deviation grows with the dimension. The same size-scaling logic applies to PP ductwork; common industry practice cites ±1 mm to ±3 mm depending on dimension. Treat these as screening references to discuss with the fabricator, not as XICHENG commitment values.
What to Write in Your Inquiry
Put tolerance into the inquiry in one of three forms:
– Drawing note: “General tolerances ISO 2768-m for all duct dimensions.”
– Explicit callout: “φ450 mm inside diameter, tolerance +2/-0 mm at the flange face.”
– Reference to a fabrication standard: “fabrication per SMACNA Thermoplastic Duct Manual, deviation per size class.”
A written tolerance also protects the buyer: the same clause that guides the manufacturer is the basis for acceptance inspection, and it pairs naturally with the inspection step in the OEM process above. After this module, you can state the tolerance class and ± values your project needs in one sentence.
Collar vs Flange Connections on Custom Ducts
The connection method decides how the run goes together, how sealed it is, and whether sections come apart again. Both collar and flange joints work on custom PP duct; the choice follows the sealing and maintenance requirements of each joint. System static pressure influences the decision too: at low system pressure (up to about 500 Pa) the collar’s interference fit is usually adequate, while higher duties push the specification toward flanged joints where the seal is positive and bolted.
Collar (Socket) Connection for Fast Field Assembly
A collar connection carries a collar on the section end that receives the next section, forming a stable joint that is fast to assemble on site. It is the economical default for long straight runs where the joint does not need periodic opening, and it keeps field labor low—the section is pushed into the collar and secured rather than bolted up. Choose collar where assembly speed matters and the joint will not be opened again.
Flange Connection for High-Sealing and Dismantling Needs
A flange connection welds flanges to the duct and joins them with bolts, giving two advantages: a higher-sealing joint for demanding duties, and the ability to dismantle the run for maintenance. Flanged joints are standard where the duct accesses equipment that needs periodic service—fans, scrubbers and heat exchangers—or where the system operates at higher static pressure and a positive seal matters. The trade-off is component cost and assembly time, which is why flanged joints are specified where they earn their keep.
Hot-Air Welding for Continuous Sealed Runs
Where a run should have no mechanical joint at all, hot-air welding joins sections into a continuous sealed assembly. Welding quality follows the same discipline as any PP weld: hot-air string-bead welding of PP sheet and fittings is covered by DVS 2207-3, and extrusion welding by DVS 2207-4, giving the fabricator a documented procedure and the buyer a reference for inspecting weld continuity. Welded runs trade future accessibility for maximum seal integrity—use it where the run is permanent and leak-free operation is the priority.
| Connection | Best for | Seal level | Maintenance access | Assembly speed |
|---|---|---|---|---|
| Collar | Long straight runs, low to medium pressure (up to about 500 Pa) | Good interference fit | No (closed joint) | Fastest |
| Flange | Equipment access points, higher static pressure, dismantling needs | Positive bolted seal | Yes (unbolt and open) | Slower (bolting) |
| Hot-air weld | Permanent runs, leak-free priority | Continuous fused joint | No (permanent) | Slowest on site |
The system’s pressure class sets the practical lower bound for the connection. Fabrication references such as SMACNA construct duct classes with static-pressure windows—a low-pressure design from roughly -400 Pa to +1000 Pa and a medium-pressure design to about 2500 Pa—and the connection choice follows that pressure story: interference joints for the low end, bolted and welded joints as pressure climbs. Gasket practice in the same references uses strips on the order of 10 mm wide and 4.5 mm thick for lower-pressure flanges, stepping up to about 15 mm by 6 mm for higher classes, which gives you a concrete detail to put on the drawing instead of a vague “sealed flange.” After this module, you can choose the connection type per joint against your sealing, maintenance and assembly priorities, and state the gasket dimension where the drawing needs one.
Custom Ducts as Part of a Complete PP Duct System
A custom duct is one component of a ventilation system, and it earns its value only when the rest of the system matches it. The custom section connects to fans, fittings and airflow dampers; it does not replace them. If you are starting from scratch rather than matching an existing run, the complete PP air duct guide connects duct types, sizing and materials into one system picture.
| System element | Its job | What it does not do |
|---|---|---|
| Custom duct section | Carries exhaust within material limits | Creates airflow or treats gas |
| Elbow / tee / reducer | Turns, branches and transitions the run | Fixes an undersized main |
| Fan | Moves the air volume | Sets duct corrosion resistance |
| Damper / grille | Balances and distributes flow | Prevents condensation alone |
| Gas treatment (scrubber etc.) | Removes contaminants | Replaces the extraction system |
Matching Fittings: Elbows, Tees, Reducers and Flanges
A non-standard diameter is only useful if the run can turn, branch and reduce around it. Matching elbows, tees, reducers and flanges should be specified from the same supplier as the custom sections, so every joint shares one material, one wall thickness logic and one connection system. Mixing suppliers on a custom run invites mismatched wall thicknesses at welded transitions and connection details that need field adapters. Name the fittings on the duct schedule at the same time as the custom sections. As a planning rule of thumb, budget roughly two to three fittings for every ten straight sections—a run rarely stays straight for long, and every transition needs its own confirmation like the sections do.
What a Custom Duct Does Not Replace
The custom section does not create airflow, balance branches or treat the gas. Fans, airflow balancing devices, room controls and gas treatment equipment are separate system elements, selected by the system design—the duct’s job is to carry the exhaust within its temperature and chemical limits. Recognizing this boundary keeps the custom order realistic: the duct is made to the system’s duty, not to fix the system’s deficiencies.
Airflow itself is the product of area and velocity, and the sizing target follows common HVAC practice—main ducts around 8 to 12 m/s, branch ducts 3 to 6 m/s—so a duct carrying, for example, 4000 m³/h at 10 m/s needs roughly 0.11 m² of cross-section. A section that is too small throttles the system, and one that is too large wastes material; both outcomes are decided by the sizing calculation, not by the fabricator. After this module, you can name the fittings your run needs and the system components the custom duct does not replace.
Custom PP Duct RFQ Checklist: What to Send a Manufacturer
The final step turns everything above into one message. A complete custom duct inquiry contains ten fields plus operating data, and a supplier can confirm specifications and return a quote on the first pass when all of them are present. For the full procurement sequence behind the quote—from specification freeze to payment terms—see the complete PP duct buying guide.
The 10-Point Inquiry Checklist
| # | Field | Why it is needed | Example |
|---|---|---|---|
| 1 | Diameter(s) | Core dimension; state ID or OD and unit | φ450 mm inside diameter |
| 2 | Wall thickness | Duty-dependent; confirm with supplier | 5 mm (confirm against static pressure) |
| 3 | Section length | Joint count and transport | 3 m per section |
| 4 | Quantity | MOQ and production planning | 24 sections |
| 5 | Material grade | PP or flame-retardant PPs | Flame-retardant PPs, grade per spec |
| 6 | Color | Cosmetic and identification | Grey |
| 7 | Connection | Collar, flange, or welded | Flange, both ends |
| 8 | Application | Context for material selection | Plating line exhaust |
| 9 | Destination port | Freight and packaging | Los Angeles, CA |
| 10 | Duct schedule / drawings | The geometry itself | Schedule + DWG attached |
Operating Data That Confirms Wall Thickness and Reinforcement
Beyond the ten fields, four operating values let the supplier engineering the wall: gas composition and concentration (corrosive load), temperature (within -15°C to 80°C), moisture, and available static pressure. High static pressure pushes wall thickness up and reinforcement in—large diameters get flange welding to prevent deformation exactly for this reason. If you do not know every value, state which ones are unknown rather than omitting them; the supplier can then quote with a clearly marked assumption instead of a silent guess.
Worked example: an electroplating line needs a φ450 mm exhaust branch, between the φ400 mm and φ500 mm catalog steps. The retrofit decision says custom (non-standard size). The size module fixes φ450 mm inside diameter, 3 m sections, wall 5 mm subject to static pressure confirmation. The tolerance module adds “ISO 2768-m, flange face +2/-0 mm.” The connection module picks flanges because the branch disconnects for plating-tank maintenance. The inquiry then reads:
> “Custom PP round duct: φ450 mm ID, 5 mm wall (0.2 in, confirm vs 800 Pa static), 3 m (9.8 ft) sections × 8, flame-retardant PPs grade, grey, flanged both ends, plating line exhaust, destination Port of Long Beach. Duct schedule and DWG attached. General tolerances ISO 2768-m.”
That message contains every decision this guide covers, and it is ready to send to any PP duct manufacturer, including the XICHENG custom duct team. When the sections arrive, the supplier-quality checks in the PP duct quality inspection guide apply to custom orders as well. After this module, you can compile the complete custom PP duct RFQ for your non-standard run—and decide whether the first reply matches your expectations well enough to move to drawing approval.
