Fix Cavern-Like Strand Voids in Bunching Machines

Fix Cavern-Like Strand Voids in Bunching Machines | DX Cable Tech

2025-12-23

Cavern-like strand voids are one of the most frustrating defects in bunching and stranding operations. They are hard to spot early, difficult to explain to customers, and often only become visible after insulation extrusion or cross-section cutting. Once detected, the cable is usually already scrap.

These voids are not random. In almost every case, they are the result of mechanical imbalance, tension inconsistency, or process mismatch inside the bunching machine — not raw material defects alone.

This article breaks down what cavern-like strand voids really are, why they happen, and how cable factories fix them without replacing the bunching machine.

1. What Are Cavern-Like Strand Voids (And Why They Matter)

Cavern-like strand voids refer to large internal empty spaces forming inside a bunched conductor, typically near the core. Unlike normal micro-gaps between strands, these voids look like hollow pockets or tunnels when the conductor is cut.

They cause several downstream problems:

Uneven insulation flow during extrusion
Trapped air leading to bubbles or pinholes
Reduced conductor compactness
Higher DC resistance deviation
Increased risk of partial discharge in MV cables

For LV cables, they affect appearance and mechanical strength.
For MV cables, they can become critical electrical defects.

2. Why Cavern Voids Are Different from Normal Strand Gaps

Every bunched conductor has small inter-strand gaps — that’s normal. Cavern-like voids are different because:

They are localized and large
They form systematically, not randomly
They often repeat at similar positions along the length
They persist even with good copper quality

This tells us one thing clearly:

Cavern voids are a process geometry problem, not a material purity problem.

3. The Real Root Causes Inside the Bunching Machine

3.1 Uneven Wire Pay-Off Tension (Primary Cause)

The most common root cause is tension imbalance between individual wires entering the bunching point.

When:

Some wires are under-tensioned
Others are over-tensioned

The loose wires fail to migrate inward properly, creating a hollow zone in the center.

Common reasons:

Mechanical brakes not calibrated evenly
Felt or magnetic tensioners worn unevenly
Pay-off reels with different inertia
Wire path length inconsistency

Key insight:

Even a 5–8% tension difference between wires can create a central void at high bunching speeds.

3.2 Incorrect Lay Length vs Wire Stiffness

If the lay length is too long for the wire diameter and hardness:

Strands do not compact inward
They “bridge” over the center
A cavern void forms naturally

This happens often when:

Switching copper supplier
Switching from annealed to harder copper
Increasing line speed without recalculating lay length

Rule of thumb:

Harder wire = shorter lay length required
Larger diameter = more sensitive to lay length errors

3.3 Over-Speeding the Bunching Rotor

High speed looks good on paper, but it changes strand behavior.

At excessive RPM:

Centrifugal force pushes strands outward
Inner compaction becomes unstable
The core area collapses into a void

This effect increases when:

Using non-compacted bunching
Wire diameter > 0.3 mm
Pay-off tension is already marginal

Many factories chase output and accidentally trade speed for structure.

3.4 Worn or Incorrect Closing Die Geometry

The closing die is often ignored — until defects appear.

Problems include:

Die bore worn into an oval
Die angle too shallow
Die size slightly oversized “to reduce friction”

Result:

Strands close externally
Core remains hollow

Important note:

A die that is 0.05 mm too large can still look “normal” externally but destroy internal structure.

3.5 Inconsistent Wire Pre-Twist or Memory

If incoming wires have:

Different residual twist
Uneven straightness
Storage coil memory

They resist uniform inward migration during bunching.

This is common when:

Mixing wires from different annealing batches
Using rewound wire without tension equalization
Feeding from reels with different winding quality

4. How to Diagnose Cavern Voids Correctly

4.1 Cross-Section Sampling Is Mandatory

Surface inspection will never detect this defect.

Correct method:

Cut conductor at multiple lengths
Polish and etch if needed
Inspect under magnification

Look for:

Central hollow zones
Asymmetric strand distribution
Repeating void position patterns

4.2 Tension Audit on Every Pay-Off

Use:

Handheld tension meters
Or load cells temporarily installed

Check:

Static tension
Dynamic tension at running speed

You’ll almost always find:

One or two wires doing “their own thing”

4.3 Speed-Lay Length Correlation Check

Record:

Actual RPM
Actual lay length
Wire diameter and hardness

Then slow the machine by 15–20%:

If voids shrink or disappear → speed is a factor

5. Proven Fixes (Without Replacing the Machine)

5.1 Equalize Wire Tension — Properly

Not “by feel”.

Actions:

Re-calibrate all tension devices
Replace worn friction pads
Match pay-off reel inertia
Ensure identical wire path length

Best practice:

Tension deviation < ±3%

5.2 Reduce Lay Length Intentionally

Shorten lay length step by step:

Start with 5% reduction
Observe cross-sections
Stop when voids disappear

Yes, torque increases — but structure improves.

5.3 Slow Down (Strategically)

Instead of constant high speed:

Run slightly slower
Maintain stable tension
Improve compaction quality

Many factories discover:

10% speed loss = 30% scrap reduction

5.4 Replace or Re-Grind Closing Dies

Do not guess.

Actions:

Measure die bore with gauges
Re-grind or replace if ovalized
Match die angle to wire size

5.5 Add a Simple Pre-Compaction Step (If Possible)

Some factories add:

Light roller compaction
Pre-forming guides
Controlled wire convergence before closing die

Even simple mechanical guidance helps prevent central collapse.

6. Why This Defect Often Appears “Suddenly”

Factories often say:

“It was fine last month.”

That’s because:

Brake pads wear gradually
Copper hardness drifts between batches
Operators increase speed quietly
Dies wear invisibly

Cavern voids are cumulative failures, not sudden accidents.

7. Real Factory Example

A Southeast Asian LV/MV factory saw unexplained insulation bubbles.

Root cause:

Cavern-like strand voids in 7-wire bunch

Fixes applied:

Rebalanced wire tension
Reduced lay length by 8%
Replaced worn closing die
Reduced speed by 12%