VAD Core Deposition Burner Parts: Custom Manufacturing Specification Guide

Specifying VAD core deposition burner parts is a different engineering exercise from ordering general torch consumables. In vapor axial deposition (VAD), the core burner builds the porous preform axially — soot particles are deposited onto the growing end face of the target rod while the burner and rod move relative to each other. The core burner largely determines axial deposition rate, radial dopant distribution, and the centering of the core region within the preform. For that reason, custom VAD core deposition burner parts cannot be treated as drop-in mechanical substitutes for an OVD burner or an off-the-shelf torch tip. They have to be specified around the deposition function they perform.

This guide is written for engineers and purchasing teams preparing a manufacturing specification for custom VAD core deposition burner parts. It covers the parameters that should appear in a specification package, the materials and fabrication decisions that follow from them, and the inspection criteria that determine whether a delivered VAD core deposition burner part is acceptable. It does not replace validation of the part on your own deposition equipment.

Why VAD core deposition burner parts require their own specification

VAD and outside vapor deposition (OVD) both use flame hydrolysis to form silica soot, but the deposition geometry and burner duty differ. In OVD, soot is deposited radially onto a rotating mandrel, and the burner array is arranged along the length of the target. In VAD, deposition occurs at the end face of the growing boule, and the VAD core deposition burner part is typically surrounded by additional burners that shape the flame and manage the thermal environment.

That difference changes what matters in the burner specification. For a VAD core burner, axial symmetry of the flame matters more directly, because asymmetry translates into core offset or ovality of the deposited core region. Channel-to-channel flow balance affects the mixing zone and therefore the particle formation environment. Face geometry and orifice spacing affect where the soot stream lands on the growing end. None of these are interchangeable with OVD burner design assumptions — the OVD, VAD, and MCVD nozzle comparison explains where the design priorities diverge across the three processes.

A practical consequence: when you write a specification for custom VAD core deposition burner parts, start from the deposition outcome you need (core diameter target, radial uniformity, stable deposition rate) and work back to mechanical requirements.

Copying an existing OVD drawing and adjusting dimensions is a common shortcut that fails, because the deposition geometry drives the specification, not the other way around. The functional requirement — a stable, centered soot stream landing on the growing end face — has to be decomposed into mechanical, flow, thermal, and material constraints before any drawing is issued for VAD core deposition burner parts.

What belongs in a VAD burner specification package

Interface and envelope dimensions

The specification should fix the mounting interface completely: overall length, body diameter, mounting flange or thread geometry, and the position of the burner face relative to the mounting datum. Datum selection should be explicit. A burner face located from a different reference than the original will shift the flame position even if every individual dimension is within tolerance.

When the burner mounts into a fixture that has its own datum chain, it is worth documenting the stack-up. A few hundredths of a millimetre of variation at the mount can translate into a visible offset at the deposition end face, especially where the VAD core deposition burner part is long relative to its diameter. Specifying face position as a basic dimension with a geometric tolerance, instead of a simple linear dimension, removes most of the ambiguity.

Channel architecture and gas connections

List every gas channel with its function (for example, core precursor delivery, fuel, oxygen, and any shroud or shaping gas), and specify the connection type, port size, and port clocking for each. Port angular position matters when the burner mounts into a manifold block — a burner with the correct port pattern rotated by a few degrees may not connect without stressing the feed lines.

Where the burner interfaces with a distribution manifold, it is worth specifying the two as a system. Components such as an oxygen distribution manifold influence channel balance upstream of the burner, and mismatch between manifold and burner pressure characteristics is a common source of hard-to-diagnose asymmetry. The principles behind this interaction are covered in the guide to l'équilibrage de pression et la dynamique des flux de gaz dans les torches OVD et VAD multicanaux.

Channel numbering deserves more attention than it usually receives. Assign each channel a stable identifier on the drawing, and use the same identifier in the flow test table that ships with the VAD core deposition burner part. When incoming inspection later detects an imbalance, a common channel name lets the supplier trace the issue to a specific machining operation instead of re-testing the whole assembly.

Face and orifice geometry

The burner face specification should define orifice count and arrangement, individual orifice diameter with tolerance, orifice-to-orifice pitch, countersink or chamfer geometry, and face flatness. If the face is not flat, orifice exit planes sit at different distances from the deposition zone, which can distort the soot stream even when the orifices themselves are machined correctly.

Face flatness and perpendicularity interact with the mounting datum. A flat face that is also tilted relative to the mount will still produce an asymmetric flame, so it helps to specify perpendicularity of the face relative to the mounting axis in addition to face flatness itself. For small orifices, define which dimension governs — diameter, position, or edge condition — because manufacturing processes that hold one cannot always hold all three to the same grade for VAD core deposition burner parts.

Materials and surface condition

Material selection is not a checkbox exercise; it follows from the burner’s thermal and chemical duty. A VAD core burner face sees high temperature and thermal cycling, so the material must retain dimensional stability over the expected service interval. Where the core channel carries halide precursors, chemical resistance to those species and their decomposition products is also relevant.

Surface finish on internal channels affects flow behavior, and finish on sealing surfaces affects leak integrity. Because requirements differ by channel and by position within the burner, a mixed-material construction is sometimes appropriate — for example, where a specific alloy or machinable grade offers better performance in one region of the assembly. The material selection logic is covered in more detail in the guide to custom OVD and VAD burner sourcing, and the specific case for titanium alloy components is treated in the article on titanium OVD torch parts.

Flow and pressure requirements

Rather than specifying a single flow number, define the acceptable flow range per channel and the pressure drop expected across the VAD core deposition burner part at representative conditions. This gives the supplier a measurable target for channel sizing and lets you verify the finished part against the same criteria. If your process uses a particular balance between channels, state it — the burner geometry can be optimized around it.

State the reference gas as well. Flow coefficients depend on gas properties, and a flow target expressed in nitrogen will not correspond to the same reading on hydrogen or oxygen. Supplying the reference conditions prevents a supplier from testing to a different basis and delivering a VAD core deposition burner part that meets the drawing but not the process requirement.

Fabrication decisions that affect the finished part

Once the specification is fixed, the manufacturing route still leaves decisions that affect performance of VAD core deposition burner parts:

• Machining strategy for internal channels. Deep, small-diameter passages require either drilled or assembled construction. Assembled (multi-piece) burners allow complex internal routing but introduce joints that must be leak-tight and dimensionally stable.
• Joining method. Brazing, welding, and diffusion bonding each impose different thermal limits and produce different levels of distortion. The joining method should be agreed in the specification because it affects what the burner can withstand in service.
• Orifice formation. Drilling, electrical discharge machining, and laser processes produce different edge quality and diameter repeatability. For small orifices, the achievable tolerance band narrows the choice.
• Post-machining treatment. Cleaning, deburring, and any stress relief should be defined, because residual burrs or contamination in a gas channel can change flow characteristics after installation.
• Dimensional verification sequencing. Deciding which features are measured before and after joining prevents a situation where distortion from the joining step is discovered only at final inspection.

The relationship between orifice quality and deposition behavior is not marginal. Small deviations in orifice geometry propagate into the soot stream and ultimately into preform characteristics, as discussed in the article on la précision des buses de brûleur et le rendement des préformes.

An assembled construction is often the only way to reach internal routing that cannot be machined from solid. The trade-off is that every joint becomes a potential leak path, so the specification for VAD core deposition burner parts should state the leak test method and acceptance limit explicitly. Where the assembly is joined by brazing, the filler alloy should be listed alongside the base material, because a filler with a lower melting point can limit the maximum service temperature of the finished burner.

It is also worth specifying whether the supplier must submit first-article inspection data for VAD core deposition burner parts before volume production, because that single requirement converts an ambiguous acceptance conversation into a documented pass/fail decision.

Inspection criteria for delivered VAD core deposition burner parts

A specification is only useful if acceptance criteria are attached. For custom VAD core deposition burner parts, a practical incoming inspection set includes:

1. Dimensional verification of critical features. Mounting interface, face location from datum, orifice diameters, and port positions. Orifice diameter should be verified by a method that resolves the tolerance you specified — pin gauges, optical measurement, or flow-based methods, agreed in advance.
2. Flow verification per channel. Measure flow or pressure drop against the specified range. Channel-to-channel comparison is often more informative than absolute values, because it exposes imbalance that absolute measurements can hide.
3. Leak and pressure integrity. Confirm that internal channels and any joints hold pressure without measurable loss. This is especially important for assembled burners.
4. Visual and dimensional check of the face. Look for burrs, dents, incomplete orifice break-through, and contamination. Verify face flatness if it was specified.
5. Geometric symmetry check. Where your equipment permits, verify that the burner’s channel pattern and face features are symmetric about the intended axis and correctly clocked relative to the mount.

If a VAD core deposition burner part fails inspection, document which criterion failed before returning it. Flow imbalance and dimensional error are different problems with different root causes, and the distinction helps the supplier correct the process rather than guess.

Retaining the inspection record for each VAD core deposition burner part also builds a service history. Over several replacement cycles, a trend in flow drift or orifice wear can justify a specification change, and the original acceptance data is the baseline for that decision.

A short acceptance checklist that travels with the part, signed at each stage, is often more effective than a comprehensive procedure that stays in a binder. If a VAD core deposition burner part passes dimensional checks but fails the flow test, the checklist makes the failure visible before the burner reaches the deposition lathe.

Troubleshooting logic before you blame the burner

When deposition results drift, the VAD core deposition burner part is a candidate cause but rarely the only one. A structured check sequence prevents unnecessary part replacement:

• Core offset or ovality. Possible causes include burner misalignment in the mount, face contamination, upstream flow imbalance, or a genuinely off-axis burner. Verify alignment and clean the face first; compare channel flows second; inspect the burner geometry third.
• Deposition rate decline. Soot accumulation on the face, gradual orifice wear, and changes in gas delivery all produce this symptom. Rate decline is not by itself proof of burner wear — the mechanisms behind soot collection on a deposition surface, including the role of thermophoresis as described in the NIST publication record, are worth understanding before replacing hardware.
• Radial profile changes. Profile drift is often traced to concentricity or symmetry, which connects back to burner centering and face geometry rather than to material choice.

In each case, the point is to separate causes you can confirm from causes you are inferring. Replace the VAD core deposition burner part only after the mechanical and flow-related causes have been ruled out.

It also helps to change one variable at a time. Replacing a VAD core deposition burner part while also adjusting gas flows or rod speed makes it impossible to attribute the resulting change, and the next time the symptom appears you will have no reliable reference point.

Preparing the sourcing request

A complete request for VAD core deposition burner parts shortens the path from inquiry to quotation and reduces the risk of a part that does not match your process. Include the following with your enquiry:

• Drawings or sketches with the mounting interface, overall envelope, and datum references.
• Channel list with function, connection type, and port clocking.
• Orifice geometry: count, diameter with tolerance, pitch, and face requirements.
• Material requirements by region, with any service temperature or chemical exposure information you can share.
• Flow or pressure-drop targets per channel, including the reference gas.
• Inspection and documentation expectations.

Where an original drawing is unavailable, reverse engineering from a sample is often feasible, but the specification for VAD core deposition burner parts still needs the functional requirements above — dimensions alone do not define a burner’s behavior. Comparing replacement options against original drawings before committing to a route is the subject of OVD burner head replacement and drawing comparison, and OEM/ODM machining allows the geometry and materials to be defined around your process rather than around a catalogue part number.

A documented upgrade can also be considered where service life is the limiting factor. The experience of moving to titanium alloy consumables for a global fiber producer is set out in the titanium OVD burner parts case study.

For any VAD core deposition burner parts program, the quotation stage is also the right time to confirm which tolerances the supplier can actually measure, not just machine. A tolerance that cannot be verified in inspection is a risk carried into production.

To move a VAD core burner project forward, contact our team with your drawings, dimensions, material requirements, and operating conditions. A specification review before machining is the cheapest place to catch a mismatch.

FAQ

Can VAD core deposition burner parts be replaced with an OVD burner of similar dimensions?

Not as a general practice for VAD core deposition burner parts. Even where mounting dimensions are similar, VAD and OVD burners are designed around different deposition geometries and different channel arrangements. Substituting one for the other would require verification against your equipment documentation and process requirements, and would normally change deposition behavior.

How do I specify orifice tolerance if I don’t have the original drawing?

Start from the deposition requirement — the soot stream characteristics your process needs — and set tolerances that the chosen orifice formation method can actually hold. Then confirm the achievable band with the supplier before committing. Measuring orifice diameters on a sample VAD core deposition burner part can provide a useful reference point, but it reflects one part, not the design intent.

Should the burner and oxygen manifold be specified together?

In many cases, yes. Channel balance depends on the interaction between manifold pressure characteristics and burner channel resistance. Specifying one without the other leaves the interface unverified.

What information is needed for a reverse-engineered VAD burner?

A physical sample or detailed drawings, plus the functional data: gas types, approximate flow ranges, expected thermal exposure, and any known failure history of the original part. Reverse engineering reproduces geometry; the functional data determines whether that geometry is right for your process.

Conclusion

A manufacturing specification for VAD core deposition burner parts earns its value at the point of inspection, not on the drawing sheet. When the specification fixes the datum, the channel architecture, the face geometry, and the acceptance criteria together, both the supplier and the incoming inspection team are working from the same standard. That shared standard is what keeps a custom VAD core deposition burner part aligned with the deposition outcome it was ordered to produce.